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Broad absorption line (BAL) quasars are characterized by gas clouds that absorb flux at the wavelength of common quasar spectral features, although blueshifted by velocities that can exceed |$0.1c$|⁠. BAL features are interesting as signatures of significant feedback, yet they can also compromise cosmological studies with quasars by distorting the shape of the most prominent quasar emission lines, impacting redshift accuracy and measurements of the matter density distribution traced by the Lyman |$\alpha$| forest. We present a catalogue of BAL quasars discovered in the Dark Energy Spectroscopic Instrument (DESI) survey Early Data Release, which were observed as part of DESI Survey Validation, as well as the first two months of the main survey. We describe our method to automatically identify BAL quasars in DESI data, the quantities we measure for each BAL, and investigate the completeness and purity of this method with mock DESI observations. We mask the wavelengths of the BAL features and re-evaluate each BAL quasar redshift, finding new redshifts which are |$243\, {\rm km}\, {\rm s}^{-1}$| smaller on average for the BAL quasar sample. These new, more accurate redshifts are important to obtain the best measurements of quasar clustering, especially at small scales. Finally, we present some spectra of rarer classes of BALs that illustrate the potential of DESI data to identify such populations for further study. [ABSTRACT FROM AUTHOR]

We investigate the photometric properties of 35 well-observed asteroids using dense ground-based and sparse ATLAS survey data. Focusing on two-colour photometric phase functions, derived using inverse methods, we explore the wavelength dependence. Our study reveals distinct (G 1, G 2) domains for cyan and orange filters, especially among some S-complex asteroids. For other asteroids, substantial uncertainties prevail, or their distributions of phase curve parameters overlap, precluding definitive conclusions on wavelength dependence. Notably, for S-complex objects, the effect appears systematic, characterized by lower G 2 values in the cyan filter. The effect can be explained by considering the known geometric albedo correlation: higher albedo corresponds to flatter, whereas lower albedo corresponds to steeper phase curves. In the case of equal albedo, asteroids with red spectral slopes have a more pronounced opposition effect in red and asteroids with blue spectral slopes in blue filters. We explore the variation of slope differences of orange and cyan phase curves with phase angle. For most asteroids, the largest nominal variation is observed at phase angles < 10°. This suggests that the phase colouring for the orange–cyan colour slope is more pronounced at small phase angles. Through meticulous analysis of the opposition effect amplitudes, we also pinpoint preferred rotational pole solutions. We identify inconsistencies between phase curve parameters and spectral types in specific cases. The shape- and geometry-corrected phase curves signify an important advancement in studying asteroid photometric behaviour and may offer a deeper understanding of surface and regolith properties previously obscured by these effects, such as surface roughness. [ABSTRACT FROM AUTHOR]

Intensive broad-band reverberation mapping campaigns have shown that AGN variability is significantly more complex than expected from disc reverberation of the variable X-ray illumination. The UV/optical variability is highly correlated and lagged, with longer lags at longer wavelengths as predicted, but the observed time-scales are longer than expected. Worse, the UV/optical light curves are not well correlated with the X-rays, which should drive them. Instead, we consider an intrinsically variable accretion disc, where slow mass accretion rate fluctuations are generated in the optical-UV disc, propagating down to modulate intrinsically faster X-ray variability from the central regions. We match our model to Fairall 9, a well-studied AGN with L ∼ 0.1 L Edd, where the spectrum is dominated by the UV/EUV. Our model produces light curves where the X-rays and UV have very different fast variability, yet are well correlated on longer time-scales, as observed. It predicts that the intrinsic variability has optical/UV leading the X-rays, but including reverberation of the variable EUV from an inner wind produces a lagged bound-free continuum that matches the observed UV-optical lags. We conclude that optical/UV AGN variability is likely driven by intrinsic fluctuations within the disc, not X-ray reprocessing: the observed longer than expected lags are produced by reverberation of the EUV illuminating a wind, not by X-ray illumination of the disc: the increasing lag with increasing wavelength is produced by the increased contribution of the (constant lag) bound-free continuum to the spectrum, rather than indicating intrinsically larger reverberation distances for longer wavelengths. [ABSTRACT FROM AUTHOR]

Achieving a truly accurate wavelength calibration of high-dispersion echelle spectrographs is a challenging task but crucially needed for certain science cases, e.g. to test for a possible variation of the fine-structure constant in quasar spectra. One of the spectrographs best suited for this mission is Very Large Telescope/Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observation (ESPRESSO). Nevertheless, previous studies have identified significant discrepancies between the classical wavelength solutions and the one derived independently from the laser frequency comb. The dominant parts of these systematics were intra-order distortions, most-likely related to a deviation of the instrumental line-spread function from the assumed Gaussian shape. Here, we therefore present a study focused on a detailed modelling of the ESPRESSO instrumental line-spread function. We demonstrate that it is strongly asymmetric, non-Gaussian, different for the two slices and fibres, and varies significantly along the spectral orders. Incorporating the determined non-parametric model in the wavelength calibration process drastically improves the wavelength calibration accuracy, reducing the discrepancies between the two independent wavelength solutions from |$50\,\rm{m\,s^{-1}}$| to about |$10\, \rm{m\,s^{-1}}$|⁠. The most striking success is, however, that the different fibres and slices now provide fully consistent measurements with a scatter of just a couple m s−1. This demonstrates that the instrument-related systematics can be nearly eliminated over most of the spectral range by properly taking into account the complex shape of the instrumental line-spread function and paves the way for further optimizations of the wavelength calibration process. [ABSTRACT FROM AUTHOR]

The vertical shear instability (VSI) is a promising mechanism to drive turbulence in protoplanetary discs. Numerical simulations in the literature demonstrate that the VSI non-linear saturation is predominated by the linear corrugation modes. These modes possess vertical wavelengths crucially longer than radial wavelengths. This paper aims to investigate the natural radial wavelength of corrugation modes upon VSI saturation, by a series of numerical simulations conducted in Athena++ at different grid resolutions, disc aspect ratios, and viscosity parametrized by ν. We find a sign of convergence emerges at 64 cells per gas scale height for fiducial simulations, below which a continuous reduction of wavelengths with grid resolution is observed. Synthetic ALMA molecular line observations of |$^{12}\rm CO(2-1)$| are performed to inspect the observability of the corrugation modes feature, which is significantly diminished with a resolution of 32 cells per scale height or above. Flared and viscous discs exhibiting longer saturation wavelengths may mitigate the observational difficulty. [ABSTRACT FROM AUTHOR]

The redshift evolution of bars is an important signpost of the dynamic maturity of disc galaxies. To characterize the intrinsic evolution safe from band-shifting effects, it is necessary to gauge how bar properties vary locally as a function of wavelength. We investigate bar properties in 16 nearby galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) at ultraviolet, optical, and mid-infrared wavebands. Based on the ellipticity and position angle profiles from fitting elliptical isophotes to the two-dimensional light distribution, we find that both bar length and ellipticity – the latter often used as a proxy for bar strength – increase at bluer wavebands. Bars are 9 per cent longer in the B band than at 3.6 μ m. Their ellipticity increases typically by 8 per cent in the B band, with a significant fraction (>40 per cent) displaying an increase up to 35 per cent. We attribute the increase in bar length to the presence of star-forming knots at the end of bars: these regions are brighter in bluer bands, stretching the bar signature further out. The increase in bar ellipticity could be driven by the apparent bulge size: the bulge is less prominent at bluer bands, allowing for thinner ellipses within the bar region. Alternatively, it could be due to younger stellar populations associated with the bar. The resulting effect is that bars appear longer and thinner at bluer wavebands. This indicates that band-shifting effects are significant and need to be corrected for high-redshift studies to reliably gauge any intrinsic evolution of the bar properties with redshift. [ABSTRACT FROM AUTHOR]

The interband correlations between optical/ultraviolet (UV) and X-ray luminosities of active galactic nuclei (AGNs) are important for understanding the disc–coronal connection, as well as using AGN as standard candles for cosmology. It is conventional to measure the X-ray luminosity at rest-frame 2 keV and compare to the UV luminosity at the rest-frame 2500 Å, but the wavelength dependence was never well explored. In this work, we adopt a well-defined sample of 1169 unobscured quasars in the redshift range 0.13–4.51, and apply the direct-correlation method to explore how the correlation with the 2 keV luminosity changes at different optical/UV wavelengths, from 1280 to 5550 Å where the spectral quality is high. We find that the luminosity at all UV continuum wavelengths correlates with the X-ray luminosity similarly to that at 2500 Å, and that these correlations are better than at the optical wavelengths. Strong self-correlation is also found in the broad-band optical/UV continuum, supporting the scenario that it is dominated by the disc emission. Correlations of various emission lines are also investigated (e.g. C iv , C iii ], Mg ii , Hβ, and [O iii ]λλ4959/5007), including the Baldwin effect and correlations involving linewidths. We find the forms of these line correlations are different, and they are also different from their underlying continua, suggesting various complexities in the line-generation process. We discuss these results in the disc-wind scenario. Our study confirms that the rest-frame 2500 Å is a good wavelength to represent the optical/UV continual properties of quasars, and shows the advantages of the direct-correlation method. [ABSTRACT FROM AUTHOR]

The most striking characteristics of the mysterious 2175 |$\mathring{\rm A}$| extinction bump, the strongest spectroscopic absorption feature seen on the interstellar extinction curve, are the invariant central wavelength and variable bandwidth: its peak position at 2175 |$\mathring{\rm A}$| is remarkably constant, while its bandwidth varies from one line of sight to another. However, recent studies of the lines of sight towards a number of Herbig Ae/Be stars have revealed that the extinction bump exhibits substantial shifts from the canonical wavelength of 2175 |$\mathring{\rm A}$|⁠. In this work, we revisit these lines of sight and take a physical approach to determine the ultraviolet (UV) extinction curve for each line of sight. It is found that the wavelengths of the derived UV extinction bumps are around 2200 |$\mathring{\rm A}$| and the scatters are considerably smaller than those of the previous study based on the same set of Herbig Ae/Be stars, consistent with the conventional wisdom of a stable bump position. Nevertheless, the scatters are still appreciably larger than those associated with the nominal bump position of 2175 |$\mathring{\rm A}$|⁠. This is discussed in the context that Herbig Ae/Be stars are not well suited for interstellar extinction studies. [ABSTRACT FROM AUTHOR]

The astronomical transient AT2018cow is the closest example of the new class of luminous, fast blue optical transients (FBOTs). Liverpool telescope RINGO3 observations of AT 2018cow are reported here, which constitute the earliest polarimetric observations of an FBOT. At |$5.7\, \mathrm{days}$| post-explosion, the optical emission of AT2018cow exhibited a chromatic polarization spike that reached |$\sim 7{{\ \rm per\ cent}}$| at red wavelengths. This is the highest intrinsic polarization recorded for a non-relativistic explosive transient and is observed in multiple bands and at multiple epochs over the first night of observations, before rapidly declining. The apparent wavelength dependence of the polarization may arise through depolarization or dilution of the polarized flux, due to conditions in AT 2018cow at early times. A second 'bump' in the polarization is observed at blue wavelengths at |$\sim 12\, \mathrm{days}$|⁠. Such a high polarization requires an extremely aspherical geometry that is only apparent for a brief period (<1 d), such as shock breakout through an optically thick disk. For a disk-like configuration, the ratio of the thickness to radial extent must be |$\sim 10{{\ \rm per\ cent}}$|⁠. [ABSTRACT FROM AUTHOR]

We report accurate Ritz wavelengths for parity-forbidden [Ni  ii ] transitions, derived from energy levels determined using high-resolution Fourier transform spectroscopy. Transitions between the 18 lowest Ni  ii energy levels of even-parity produced Ritz wavelengths for 126 parity-forbidden lines. Uncertainties for the Ritz wavelengths derived in this work are up to two orders of magnitude lower than previously published values. Transition probabilities were calculated using the semi-empirical orthogonal operator method, with uncertainties ranging from approximately 1 per cent for strong M1 lines and up to 10 per cent for weak E2 lines. Accurate forbidden line wavelengths and transition probabilities, particularly for lines in the infrared, are important in the analyses of low-density astrophysical plasmas, such as supernova remnants, planetary nebulae, and active galactic nuclei. [ABSTRACT FROM AUTHOR]

We present optical spectropolarimetry of the tidal disruption event (TDE) AT 2019qiz on days +0 and +29 relative to maximum brightness. Continuum polarization, which informs the shape of the electron-scattering surface, was found to be consistent with 0 per cent at peak brightness. On day +29, the continuum polarization rose to ∼1 per cent, making this the first reported spectropolarimetric evolution of a TDE. These findings are incompatible with a naked eccentric disc that lacks significant mass outflow. Instead, the spectropolarimetry paints a picture wherein, at maximum brightness, high-frequency emission from the accretion disc is reprocessed into the optical band by a nearly spherical, optically thick, electron-scattering photosphere located far away from the black hole. We estimate the radius of the scattering photosphere to be |$\sim 100\rm \, au$| at maximum brightness – significantly larger than the tidal radius (⁠|$\sim 1\rm \, au$|⁠) and the thermalization radius (⁠|$\sim 30\rm \, au$|⁠) where the optical continuum is formed. A month later, as the fallback rate drops and the scattering photosphere recedes, the continuum polarization increases, revealing a moderately aspherical interior. We also see evidence for smaller scale density variations in the scattering photosphere, inferred from the scatter of the data in the Stokes q − u plane. On day +29, the H α emission-line peak is depolarized to ∼0.3 per cent (compared to ∼1 per cent continuum polarization), and displays a gradual rise towards the line's redder wavelengths. This observation indicates the H α line formed near the electron-scattering radius. [ABSTRACT FROM AUTHOR]

We present an analytical model for the free–free emission at radio wavelengths produced by the interaction of a Coronal Mass Ejection (CME) with the ambient solar wind. Using our previous models, we show that a dense shell bounded by two shock fronts is formed from this interaction, whose dynamical evolution can be calculated based on considerations of the mass and momentum conservation for the shell. This structure undergoes two stages in its dynamical evolution: (1) A first one of constant velocity, when the shell is bounded by two shock fronts, and (2) a second one, in which a one-shock structure (the leading shock) is decelerated. Here, we compute the emission produced by these shocks, and present a comparison with synthetic observations of the 2011 March 7 CME. Our simplified model gives a physical insight into the free–free emission produced by shocks of CMEs. [ABSTRACT FROM AUTHOR]

The paper describes profile broadening and peak wavelength variation of diffuse interstellar bands (DIBs) measured for 46 lines of sight, probably caused by the physical properties of intervening clouds. The full width at half-maximum of four studied DIBs (5780, 5797, 6196 and 6614 Å) demonstrates strong variability, sometimes doubling the widths of the features. Despite the high magnitude of the effect, our current analysis is restricted to the strongest DIBs because the weaker ones require a much higher signal-to-noise ratio. The profile broadening in the studied DIBs moves the profile's centres towards longer wavelengths, probably due to the excitation of higher levels of the P branch of the unknown molecular carrier. Moreover, DIBs are broader in clouds with abundantly populated vibrationally excited states of hydrogen molecules; that is, the broadening of DIBs correlates with the rotational temperature estimated on the H2 ν = 2 vibrational level. However, objects that demonstrate extremely broadened profiles of DIBs are scarce. The extreme peculiarity of the DIB profiles was detected in Herschel 36. Here we show the gradual growths of the widths of DIBs, confirmed in spectra from different instruments. [ABSTRACT FROM AUTHOR]

Selection of extreme objects in data from large-scale sky surveys is a powerful tool for the detection of new classes of astrophysical objects or rare stages of their evolution. The cross-matching of catalogues and analysis of the colour indices of their objects is the usual approach to this problem, which has already provided a lot of interesting results. However, the analysis of objects that are found in only one of the surveys and are absent from all others should also attract close attention, as it may lead to the discovery of both transients and objects with extreme colour values. Here we report on an initial study aimed at the detection of objects with a significant UV excess in their spectra by cross-matching of the GALEX all-sky catalogue with several other surveys in different wavelength ranges and analysing those visible in GALEX only, or with extreme UV–optical colours (ultraviolet luminous objects). We describe the methodology for such an investigation, explain the selection of surveys for this study, and show the initial results based on a search in a small fraction of the sky. We uncovered several prominent UV-only objects lacking counterparts in catalogues of longer wavelengths, and discuss their possible nature. We also detected a single source showing an extreme UV–optical colour corresponding to UV flare on a cool sdM subdwarf star. Finally, we discuss the possible populations of objects that may be revealed in a future larger-scale analysis of this kind. [ABSTRACT FROM AUTHOR]

To study the resolution required for simulating gravitational fragmentation with newly developed Lagrangian hydrodynamic schemes, meshless finite-volume method (MFV) and meshless finite-mass method, we have performed a number of simulations of the Jeans test and compared the results with both the expected analytical solution and results from the more standard Lagrangian approach: smoothed particle hydrodynamics (SPH). We find that the different schemes converge to the analytical solution when the diameter of a fluid element is smaller than a quarter of the Jeans wavelength, λJ. Among the three schemes, SPH/MFV shows the fastest/slowest convergence to the analytical solution. Unlike the well-known behaviour of Eulerian schemes, none of the Lagrangian schemes investigated displays artificial fragmentation when the perturbation wavelength, λ, is shorter than λJ, even at low numerical resolution. For larger wavelengths (λ > λJ), the growth of the perturbation is delayed when it is not well resolved. Furthermore, with poor resolution, the fragmentation seen with the MFV scheme proceeds very differently compared to the converged solution. All these results suggest that, when unresolved, the ratio of the magnitude of hydrodynamic force to that of self-gravity at the sub-resolution scale is the largest/smallest in MFV/SPH, the reasons for which we have discussed in detail. These tests are repeated to investigate the effect of kernels of higher order than the fiducial cubic spline. Our results indicate that the standard deviation of the kernel is a more appropriate definition of the 'size' of a fluid element than its compact support radius. [ABSTRACT FROM AUTHOR]

Measuring the star-forming properties of active galactic nucleus (AGN) hosts is key to our understanding of galaxy formation and evolution. However, this topic remains debated, partly due to the difficulties in separating the infrared (i.e. 1–1000  |${\rm \mu m}$|⁠) emission into AGN and star-forming components. Taking advantage of archival far-infrared data from Herschel , we present a new set of AGN and galaxy infrared templates and introduce the spectral energy distribution fitting code iragnsep. Both can be used to measure infrared host galaxy properties, free of AGN contamination. To build these, we used a sample of 100 local (z  < 0.3), low-to-high luminosity AGNs (i.e. L bol |$\ \sim \ 10^{42-46}$|  erg s−1), selected from the 105-month Swift –BAT X-ray survey, which have archival Spitzer –IRS spectra and Herschel photometry. We first built a set of seven galaxy templates using a sample of 55 star-forming galaxies selected via infrared diagnostics. Using these templates, combined with a flexible model for the AGN contribution, we extracted the intrinsic infrared emission of our AGN sample. We further demonstrate that we can reduce the diversity in the intrinsic shapes of AGN spectral energy distributions down to a set of three AGN templates, of which two represent AGN continuum, and one represents silicate emission. Our results indicate that, on average, the contribution of AGNs to the far-infrared (λ ≳ 50  |${\rm \mu m}$|⁠) is not as high as suggested by some recent work. We further show that the need for two infrared AGN continuum templates could be related to nuclear obscuration, where one of our templates appears dominated by the emission of the extended polar dust. [ABSTRACT FROM AUTHOR]

Wavelengths of absorption lines in the spectra of galaxies along the line of sight to distant quasars can be used to probe the variablility of the fine structure constant, α, at high redshifts, provided that the laboratory wavelengths are known to better than 6 parts in 108, corresponding to a radial velocity of ≈20 ms−1. For several lines of Si  ii , C  ii , Fe  i , and Ni  ii , previously published wavelengths are inadequate for this purpose. Improved wavelengths for these lines were derived by re-analysing archival Fourier transform (FT) spectra of iron hollow cathode lamps (HCL), a silicon carbide Penning discharge lamp, and with new spectra of nickel HCLs. By reoptimizing the energy levels of Fe  i , the absolute uncertainty of 13 resonance lines has been reduced by over a factor of 2. A similar analysis for Si  ii gives improved values for 45 lines with wavelength uncertainties over an order of magnitude smaller than previous measurements. Improved wavelengths for eight lines of Ni  ii were measured and Ritz wavelengths from optimized energy levels determined for an additional three lines at shorter wavelengths. Three lines of C  ii near 135 nm were observed using FT spectroscopy and the wavelengths confirm previous measurements. [ABSTRACT FROM AUTHOR]

Debris discs around main-sequence stars are belts of planetesimals – asteroids and comets – formed in the protoplanetary discs around young stars. Planetesimals comprise both the building blocks of planets around young stars and the source of dusty debris around older stars. Imaging observations of dust continuum emission and scattered light reveal the location of these planetesimal belts around their host stars. Analysis of debris discs observed at millimetre wavelengths revealed a trend between the discs' radii and the host star luminosities. This trend was tentatively linked to the preferential formation of dust-producing planetesimals near snow lines (specifically CO) in the protoplanetary discs around the host stars. Here, we perform a homogeneous analysis of 95 debris discs observed at far-infrared wavelengths by the Herschel Space Observatory and fit the obtained distribution of radii and widths as a function of stellar luminosity with a power-law relation. We identify a trend in disc radius as a function of stellar luminosity similar to that identified at millimetre wavelengths, but cannot convincingly recover it from the available data set due to the large uncertainties on disc radius and width inherent in the marginally spatially resolved data, and the bias of smaller discs around more distant stars (which are also the more luminous) being omitted from our analysis. We see a trend in disc temperature as a function of stellar luminosity, consistent with previous findings from similar analyses. [ABSTRACT FROM AUTHOR]

Active regions were observed with different instruments covering the spectral band from 17 to 405 GHz. The observations were made with the Nobeyama Radioheliograph (17 GHz), the Atacama Large Millimetre Array (107 and 238 GHz), and the Solar Submillimeter Telescope (212 and 405 GHz). A procedure was developed that allows the comparison between observations taken with telescopes of different operational characteristics and mainly of different spatial resolution. The brightness temperature and density flux spectra of several active regions corresponding to a different phase of its lifetime were obtained. The flux density invariably increases in all cases from 107 to 405 GHz and the mean spectral index is ∼2 showing that the dominant emission mechanism at submillimeter frequencies is still thermal. We show that Solar Submillimeter Telescope (SST) and Atacama Large Millimeter/submillimeter Array (ALMA) observations are compatible within the uncertainties, a result of great interest for future joint observations. [ABSTRACT FROM AUTHOR]

Cross-matching catalogues from radio surveys to catalogues of sources at other wavelengths is extremely hard, because radio sources are often extended, often consist of several spatially separated components, and often no radio component is coincident with the optical/infrared host galaxy. Traditionally, the cross-matching is done by eye, but this does not scale to the millions of radio sources expected from the next generation of radio surveys. We present an innovative automated procedure, using Bayesian hypothesis testing, that models trial radio-source morphologies with putative positions of the host galaxy. This new algorithm differs from an earlier version by allowing more complex radio-source morphologies, and performing a simultaneous fit over a large field. We show that this technique performs well in an unsupervised mode. [ABSTRACT FROM AUTHOR]

Using the first station of the Long Wavelength Array (LWA1), we examine polarized pulsar emission between 25 and 88 MHz. Polarized light from pulsars undergoes Faraday rotation as it passes through the magnetized interstellar medium. Observations from low-frequency telescopes are ideal for obtaining precise rotation measures (RMs) because the effect of Faraday rotation is proportional to the square of the observing wavelength. With these RMs, we obtained polarized pulse profiles to see how polarization changes in the 25–88 MHz range. The RMs were also used to derive values for the electron-density-weighted average Galactic magnetic field along the line of sight. We present RMs and polarization profiles of 15 pulsars acquired using data from LWA1. These results provide new insight into low-frequency polarization characteristics and pulsar emission heights, and complement measurements at higher frequencies. [ABSTRACT FROM AUTHOR]

We report on the results of a multiwavelength monitoring campaign of the bright, nearby Seyfert galaxy Ark 120, using a ∼50-d observing programme with Swift and a ∼4-month co-ordinated ground-based observing campaign, pre-dominantly using the Skynet Robotic Telescope Network. We find Ark 120 to be variable at all optical, UV, and X-ray wavelengths, with the variability observed to be well correlated between wavelength bands on short time-scales. We perform cross-correlation analysis across all available wavelength bands, detecting time delays between emission in the X-ray band and the Swift V, B , and UVW1 bands. In each case, we find that the longer wavelength emission is delayed with respect to the shorter wavelength emission. Within our measurement uncertainties, the time delays are consistent with the τ ∼ λ4/3 relation, as predicted by a disc reprocessing scenario. The measured lag centroids are τcent = 11.90 ± 7.33, 10.80 ± 4.08, and 10.60 ± 2.87 d between the X-ray and V, B , and UVW1 bands, respectively. These time delays are longer than those expected from standard accretion theory and, as such, Ark 120 may be another example of an active galaxy whose accretion disc appears to exist on a larger scale than predicted by the standard thin-disc model. Additionally, we detect further inter-band time delays: most notably between the ground-based I and B bands (τcent = 3.46 ± 0.86 d), and between both the Swift XRT and UVW1 bands and the I band (τcent = 12.34 ± 4.83 and 2.69 ± 2.05 d, respectively), highlighting the importance of co-ordinated ground-based optical observations. [ABSTRACT FROM AUTHOR]

Astrocombs are ideal spectrograph calibrators whose limiting precision can be derived using a second, independent, astrocomb system. We therefore analyse data from two astrocombs (one 18 GHz and one 25 GHz) used simultaneously on the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph at the European Southern Observatory. The first aim of this paper is to quantify the wavelength repeatability achieved by a particular astrocomb. The second aim is to measure wavelength calibration consistency between independent astrocombs, that is to place limits or measure any possible zero-point offsets. We present three main findings, each with important implications for exoplanet detection, varying fundamental constant and redshift drift measurements. First, wavelength calibration procedures are important: using multiple segmented polynomials within one echelle order results in significantly better wavelength calibration compared to using a single higher order polynomial. Segmented polynomials should be used in all applications aimed at precise spectral line position measurements. Secondly, we found that changing astrocombs causes significant zero-point offsets (⁠|${\approx}60\, {\rm cm\, s}^{-1}$| in our raw data) which were removed. Thirdly, astrocombs achieve a precision of |${\lesssim }4\, {\rm cm\, s}^{-1}$| in a single exposure (⁠|${\approx }10{{\,\rm per\,cent}}$| above the measured photon-limited precision) and 1 cm s−1 when time-averaged over a few hours, confirming previous results. Astrocombs therefore provide the technological requirements necessary for detecting Earth–Sun analogues, measuring variations of fundamental constants and the redshift drift. [ABSTRACT FROM AUTHOR]

A small subset of young stellar objects exhibit 'see-saw' temporal variations in their mid-infrared spectral energy distribution (SED); as the flux shortwards of a fulcrum wavelength (λ f ) increases, the flux longwards of this wavelength decreases (and vice versa) over time-scales of weeks to years. While previous studies have shown that an opaque, axisymmetric occulter of variable height can cause this behaviour in the SED of these objects, the conditions under which a single λ f occurs have not previously been determined, nor the factors determining its value. Using radiative transfer modelling, we conduct a parametric study of the exemplar of this class, LRLL 31 to explore this phenomenon, and confirm that the cause of this flux variation is likely due to the change in height of the optically thick inner rim of the accretion disc at the dust sublimation radius, or some other phenomenon that results in a similar appearance. We also determine that a fulcrum wavelength only occurs for high inclinations, where the line of sight intersects the accretion disc. Accepting that the disc of LRLL 31 is highly inclined, the inner rim radius, radial and vertical density profiles are independently varied to gauge what effect this had on λ f and its position relative to the silicate feature near |$10\, {\mu \rm{m}}$|⁠. While λ f is a function of each of these parameters, it is found to be most strongly dependent on the vertical density exponent β. All other factors being held constant, only for flatter discs (β < 1.2) did we find a λ f beyond the silicate feature. [ABSTRACT FROM AUTHOR]

In this paper, I show that the concept of relativistic beaming – the process by which light emitted by fast-moving sources is lensed towards the direction of motion – can be easily extended to model the signal from both the star and any secondary companions. Most companions will be cooler and less massive than their host star. Their lower mass leads to faster orbital velocities, and thus a potentially larger beaming effect. The lower temperature will mean that most of their light is emitted at longer wavelengths, where the relative photometric dominance of the primary is reduced. Thus, for some systems, the secondary companion can be the main contributor to observed relativistic beaming signals at long wavelengths. Furthermore, if the system is observed over a range of wavelengths we can independently constrain the temperature of the companion, and the mass and radius ratio of the binary. To conclude, I discuss the current and future observational prospects of this signal, using the properties of known exoplanets to show that such a signal may be observable by upcoming surveys. [ABSTRACT FROM AUTHOR]

The Palomar Fiber Nuller (PFN) is a rotating-baseline nulling interferometer that enables high-accuracy near-infrared (NIR) nulling observations with full azimuth coverage. To achieve NIR null-depth accuracies of several x 10−4, the PFN uses a common-mode optical system to provide a high degree of symmetry, single-mode-fibre beam combination to reduce sensitivity to pointing and wavefront errors, extreme adaptive optics to stabilize the fibre coupling and the cross-aperture fringe phase, rapid signal calibration and camera readout to minimize temporal effects, and a statistical null-depth fluctuation analysis to relax the phase stabilization requirement. Here, we describe the PFN's final design and performance and provide a demonstration of faint-companion detection by means of nulling-baseline rotation, as originally envisioned for space-based nulling interferometry. Specifically, the K s-band null-depth rotation curve measured on the spectroscopic binary η Peg reflects both a secondary star 1.08 ± 0.06 × 10−2 as bright as the primary, and a null-depth contribution of 4.8 ± 1.6 × 10−4 due to the size of the primary star. With a 30 mas separation at the time, η Peg B was well inside both the telescope's diffraction-limited beam diameter (88 mas) and typical coronagraphic inner working angles. Finally, we discuss potential improvements that can enable a number of small-angle nulling observations on larger telescopes. [ABSTRACT FROM AUTHOR]

WASP-121b is a transiting gas giant exoplanet orbiting close to its Roche limit, with an inflated radius nearly double that of Jupiter and a dayside temperature comparable to a late M dwarf photosphere. Secondary eclipse observations covering the 1.1– |$1.6\, \mu{\rm m}$| wavelength range have revealed an atmospheric thermal inversion on the dayside hemisphere, likely caused by high-altitude absorption at optical wavelengths. Here we present secondary eclipse observations made with the Hubble Space Telescope Wide Field Camera 3 spectrograph that extend the wavelength coverage from |$1.1\, \mu{\rm m}$| down to |$0.8\, \mu{\rm m}$|⁠. To determine the atmospheric properties from the measured eclipse spectrum, we performed a retrieval analysis assuming chemical equilibrium, with the effects of thermal dissociation and ionization included. Our best-fitting model provides a good fit to the data with reduced |$\chi ^2_\nu =1.04$|⁠. The data diverge from a blackbody spectrum and instead exhibit emission due to H− shortward of |$1.1\, \mu{\rm m}$|⁠. The best-fitting model does not reproduce a previously reported bump in the spectrum at |$1.25\,\mu{\rm m}$|⁠ , possibly indicating this feature is a statistical fluctuation in the data rather than a VO emission band as had been tentatively suggested. We estimate an atmospheric metallicity of |$[{\rm M}/{\rm H}]= {1.09}_{-0.69}^{+0.57}$|⁠ , and fit for the carbon and oxygen abundances separately, obtaining |$[{\rm C}/{\rm H}]= {-0.29}_{-0.48}^{+0.61}$| and |$[{\rm O}/{\rm H}]= {0.18}_{-0.60}^{+0.64}$|⁠. The corresponding carbon-to-oxygen ratio is |${\rm C/O} = 0.49_{-0.37}^{+0.65}$|⁠ , which encompasses the solar value of 0.54, but has a large uncertainty. [ABSTRACT FROM AUTHOR]

Accretion onto protostars may occur in sharp bursts. Accretion bursts during the embedded phase of young protostars are probably most intense, but can only be inferred indirectly through long-wavelength observations. We perform radiative transfer calculations for young stellar objects (YSOs) formed in hydrodynamic simulations to predict the long wavelength, sub-mm and mm, flux responses to episodic accretion events, taking into account heating from the young protostar and from the interstellar radiation field. We find that the flux increase due to episodic accretion events is more prominent at sub-mm wavelengths than at mm wavelengths; e.g. a factor of ∼570 increase in the luminosity of the young protostar leads to a flux increase of a factor of 47 at 250  |$\mu$| m but only a factor of 10 at 1.3 mm. Heating from the interstellar radiation field may reduce further the flux increase observed at longer wavelengths. We find that during FU Ori-type outbursts the bolometric temperature and luminosity may incorrectly classify a source as a more evolved YSO due to a larger fraction of the radiation of the object being emitted at shorter wavelengths. [ABSTRACT FROM AUTHOR]

The study of minor planets is motivated both by fundamental science of Solar system origins (some of these bodies contain the most pristine materials from the early ages of the planetary nebula) and by practical reasons concerning space exploration and impact frequency with Earth. Among minor bodies, near-Earth asteroids are a particularly important group: these objects are nearby the Earth's orbit and they represent both resources and hazards to humans. This is the case of 2014 JO25. The encounter of this potentially hazardous asteroid with the Earth at 0.011 75 au on 2017 April 19 was a good opportunity to study its properties through photometric and spectral analyses. The work we present here has been carried out thanks to a worldwide observational campaign that included time-series photometry and spectroscopy in the visible and near-infrared wavelengths. The optical images for photometric analysis were collected at different phase angles using small telescopes (<0.5 m) and medium telescopes (from 0.6 to 1.5 m). Spectral analysis was performed by 2–4 m telescopes. The light curve of 2014 JO25 indicates a synodic rotational period of 4.5286 ± 0.0004 h. Although rotational period had been previously obtained by other authors, this work confirms it with a better accuracy. The obtained reflectance spectrum of this asteroid indicates that it belongs to the S-complex and its surface is most likely composed of a mixture of pyroxenes and olivine. From the comparison of its spectrum to those of meteorite samples, as well as from the wavelength position of the first absorption band (close to 0.9 |$\mu$| m), we suggest that this asteroid might contain a large fraction of low-calcium pyroxene and, tentatively, some amounts of metal. [ABSTRACT FROM AUTHOR]

We present a method to estimate the physical parameters of nova systems using an extensive grid of photoionization models for novae. We use the photoionization code cloudy to construct a grid of models covering a wide range of parameters, for example the total hydrogen density (n H), the source temperature (T BB) and luminosity (L), the inner radius (R in) and the thickness of the ejecta (Δ R), keeping other elements other than hydrogen at solar values. In this way, a total of 1792 models were generated. From the model-generated spectra, which cover a wide wavelength region from the ultraviolet to the infrared, we calculate the ratios of hydrogen and helium emission-line fluxes, which are generally strong in nova spectra. We show that the physical parameters associated with nova systems can be estimated by comparing these line ratios with those obtained from observed spectra. We illustrate the idea with examples and estimate the parameter values for a few other novae. The results of the grid model are available online. [ABSTRACT FROM AUTHOR]

The properties of dust in the interstellar medium (ISM) nearest the Sun are poorly understood because the low column densities of dust toward nearby stars induce little photometric reddening, rendering the grains largely undetectable. Stellar polarimetry offers one pathway to deducing the properties of this diffuse material. Here we present multi-wavelength aperture polarimetry measurements of seven bright stars chosen to probe interstellar polarization near the edge of the Local Hot Bubble (LHB) – an amorphous region of relatively low-density interstellar gas and dust extending ∼70–150 pc from the Sun. The measurements were taken using the HIgh Precision Polarimetric Instrument (HIPPI) on the 3.9-m Anglo-Australian Telescope. HIPPI is an aperture stellar polarimeter with a demonstrated sensitivity of 4.3 parts-per-million (ppm). Of the stars observed two are polarized to a much greater degree than the others; they have a wavelength of maximum polarization (λmax) of ∼550 ± 20 nm – similar to that of stars beyond the LHB – and we conclude that they are in the wall of the LHB. The remaining five stars have polarizations of ∼70–160 ppm, of these four have a much bluer λmax, ∼350 ± 50 nm. Bluer values of λmax may indicate grains shocked during the evolution of the Loop I Superbubble. The remaining star, HD 4150 is not well fit by a Serkowski curve, and may be intrinsically polarized. [ABSTRACT FROM AUTHOR]

The unidentified TeV γ-ray source HESS J1702–420 has no firm associations with any counterparts at other wavelengths. We present an analysis of the interstellar medium using results from the Mopra Southern Galactic Plane CO survey and archival H  i data from the Southern Galactic Plane Survey. We find significant components of molecular and atomic gas overlapping the TeV γ-ray source at several velocities along the line of sight, with masses ∼102–104 M⊙ and densities up to ∼103 cm−3. Potentially interesting and favourable morphological correspondences between the ISM and the TeV γ-ray emission were found, and in a hadronic scenario, we find that cosmic ray (CR) enhancement values of ∼100 times the local solar values are required to produce the TeV γ-rays. We investigate the separate scenarios in which HESS J1702–420 is powered by the nearby SNR G344.7−0.1, PSRJ1702−4128, and an unknown accelerator. No evidence in the ISM was found to link the SNR with the TeV source. While the morphology of the gas located at the dispersion distance of PSRJ1702−4128 (5.2 kpc, |$v$| LSR ∼ −70 km s−1) did not discount the possibility of a leptonic origin, the required energetics and the broad and hard TeV spectrum would likely disfavour such a scenario. Given the presence of potentially favourable ISM overlapping HESS J1702–420, we additionally speculate on the plausibility of scenarios involving undetected CR accelerators. [ABSTRACT FROM AUTHOR]

We present photometric redshifts for ∼4 million galaxies detected in the Spitzer 3.6 or 4.5 |$\mu$| m bands of the Spitzer Extragalactic Representative Volume Survey (SERVS). The 18 deg2 area of SERVS covers five extragalactic fields with a variety of ancillary, optical, and near-infrared (IR) photometry. We evaluate the accuracy of our photometric redshifts with ∼90.000 publicly available, spectroscopic redshifts. Overall, we find an average of ∼0.038 for the normalized median absolute deviation (σNMAD), a measure similar to the standard deviation yet more robust against outliers, and outlier fraction η of 3.7 per cent for the sources with the widest wavelength coverage. On the example of the XMM field, we quantify the quality and reliability of photometric redshifts as a function of (1) the number of photometric bands available in the fitting, (2) i AB magnitude, (3) Infrared Array Camera (IRAC) 3.6 |$\, \mu$| m magnitude, (4) spectroscopic redshift, and (5) the survey origin of the spectroscopic redshift. The best results are achieved when the photometry available for the fitting covers rest-frame optical and near-IR wavelengths. σNMAD and η are smallest for the brightest objects. Similarly, we find σNMAD and η to be smaller on average at z < 1.5 than at z > 1.5. Photometric redshifts derived without the two IRAC filter bands, but detections in all other bands are slightly underestimated. Approximately 76 per cent of SERVS sources with at least five available filter bands lie between redshift 0 and ∼1.5. We find a tail of high-redshift galaxies, i.e. |${\sim }7\,$|  per cent of all objects with at least five available filter bands for the fitting lie at z > 3. We discuss ways to improve upon the photometric redshifts for SERVS galaxies in the future. [ABSTRACT FROM AUTHOR]

We present new Hubble Space Telescope (HST) ultraviolet and ground-based optical observations of the hot, metal-rich white dwarf GD 394. Extreme-ultraviolet (EUV) observations in 1992–1996 revealed a 1.15 d periodicity with a 25 per cent amplitude, hypothesized to be due to metals in a surface accretion spot. We obtained phase resolved HST /Space Telescope Imaging Spectrograph high resolution far-ultraviolet spectra of GD 394 that sample the entire period, along with a large body of supplementary data. We find no evidence for an accretion spot, with the flux, accretion rate, and radial velocity of GD 394 constant over the observed time-scales at ultraviolet and optical wavelengths. We speculate that the spot may have no longer been present when our observations were obtained, or that the EUV variability is being caused by an otherwise undetected evaporating planet. The atmospheric parameters obtained from separate fits to optical and ultraviolet spectra are inconsistent, as is found for multiple hot white dwarfs. We also detect non-photospheric, high ionisation absorption lines of multiple volatile elements, which could be evidence for a hot plasma cocoon surrounding the white dwarf. [ABSTRACT FROM AUTHOR]

We present a multiwavelength analysis of the core of the massive galaxy cluster MACS J0417.5−1154 (⁠|$z$|  = 0.441). Our analysis takes advantage of Very Large Telescope/Multi-Unit Spectroscopic Explorer observations which allow the spectroscopic confirmation of three strongly lensed systems. System #1, nicknamed The Doughnut, consists of three images of a complex ring galaxy at |$z$|  = 0.8718 and a fourth, partial and radial image close to the brightest cluster galaxy (BCG) only discernible thanks to its strong [O  ii ] line emission. The best-fitting mass model (rms of 0.38 arcsec) yields a two-dimensional enclosed mass of |$M({R \lt 200\, \rm kpc}) = (1.77\pm 0.03)\times 10^{14}\, {\rm M}_{\odot}$| and almost perfect alignment between the peaks of the BCG light and the dark matter of (0.5 ± 0.5) arcsec. We observe a significant misalignment when system #1 radial image is omitted. The result serves as an important caveat for studies of BCG–dark-matter offsets in galaxy clusters. Using Chandra to map the intracluster gas, we observe an offset between gas and dark matter of (1.7 ± 0.5) arcsec, and excellent alignment of the X-ray peak with the location of optical emission line associated with the BCG. We interpret all observational evidences in the framework of ongoing cluster merger activity, noting specifically that the coincidence between the gas and optical line peaks may be evidence of dense, cold gas cooled directly from the intracluster gas. Finally, we measure the surface area, σμ, above a given magnification factor μ, a metric to estimate the lensing power of a lens, σ(μ > 3) = 0.22 arcmin2, which confirms MACS J0417 as an efficient gravitational lens. [ABSTRACT FROM AUTHOR]

The large crescents imaged by ALMA in transition discs suggest that azimuthal dust trapping concentrates the larger grains, but centimetre–wavelengths continuum observations are required to map the distribution of the largest observable grains. A previous detection at ∼1 cm of an unresolved clump along the outer ring of MWC 758 (Clump 1), and buried inside more extended sub-mm continuum, motivates followup VLA observations. Deep multiconfiguration integrations reveal the morphology of Clump 1 and additional cm-wave components that we characterize via comparison with a deconvolution of recent 342 GHz data (∼1 mm). Clump 1, which concentrates ∼1/3 of the whole disc flux density at ∼1 cm, is resolved as a narrow arc with a deprojected aspect ratio χ > 5.6, and with half the azimuthal width than at 342 GHz. The spectral trends in the morphology of Clump 1 are quantitatively consistent with the Lyra-Lin prescriptions for dust trapping in an anticyclonic vortex, provided with porous grains (f ∼ 0.2 ± 0.2) in a very elongated (χ ∼ 14 ± 3) and cold (⁠|$T\sim 23\pm 2\,$| K) vortex. The same prescriptions constrain the turbulence parameter α and the gas surface density Σg through |$\log _{10}\left(\alpha \times \Sigma _g / \mathrm{g\, cm}^{-2} \right) \sim -2.3\pm 0.4$|⁠, thus requiring values for Σg larger than a factor of a few compared to that reported in the literature from the CO isotopologues, if α ≲ 10−3. Such physical conditions imply an appreciably optically thick continuum even at cm-wavelengths (⁠|$\tau _{33\, \mathrm{GHz}}\sim 0.2$|⁠). A secondary and shallower peak at 342 GHz is about twice fainter relative to Clump 1 at 33 GHz. Clump 2 appears to be less efficient at trapping large grains. [ABSTRACT FROM AUTHOR]

We have investigated the properties of gravito-turbulent discs in 3D using high-resolution shearing-box simulations. For large enough domain sizes, L y ≳ 60 H, the disc settles down into a quasi-steady state, showing no long-term trends in properties or variation with box size. For smaller boxes, we find that the azimuthal wavelength of the dominant spiral modes is limited to the domain size. This is associated with a bursty behaviour that differs from the quasi-steady dynamics at larger sizes. We point out that a similar transition may be expected in global simulations at the point where the range of azimuthal wavelengths is limited by the finite disc size. This condition (i.e. when |$60 H \sim 2 {\pi} R$|⁠, i.e. H / R  ∼ 0.1) correctly predicts the transition to bursty behaviour previously found in global simulations for disc-to-star mass ratios in excess of 0.25. We recover a transition in the dynamics from 2D to 3D behaviour, characterized by a turbulence that becomes more isotropic on small scales. This turbulence likely plays an important role in the evolution of dust in self-gravitating discs, potentially dominating the collision velocity for particles with Stokes number < 1. Finally, we consider the stability of gravito-turbulence against fragmentation, finding that discs that cool faster than a few dynamical times fragment immediately, supporting previous results. We also find hints of stochastic fragmentation at marginally longer cooling times, in which a fragment forms from a quasi-equilibrium state. However, this makes little practical difference to region where planet formation via gravitational instability may occur. [ABSTRACT FROM AUTHOR]

The goal of this paper is to find a method to yield solar surface magnetic field measurements while inflicting minimal prior assumptions on the acceptable solutions. A small spacecraft placed at least 0.1 radian away from the Earth–Sun line can, with vector field observations from the Earth, offer unambiguous determinations of magnetic fields from the Sun. A telescope of between 10 and 40 cm aperture operating at ultraviolet (UV) wavelengths can, with a simple Stokes-V polarimeter, enhance observations made with the suite of large telescopes on the Earth. A 40 cm aperture can achieve the same diffraction limit at 400 nm that Daniel K. Innouye Solar Telescope (DKIST) might achieve at 4 microns. Even a 10 cm aperture offers higher signal to noise in Stokes V than DKIST can Q and U in magnetic fields outside sunspots, transverse to the line of sight, offering enormous benefits for understanding magnetic components transverse to the Earth–Sun line. We identify UV and infrared (IR) lines of Fe  i that are useful for joint spacecraft–DKIST observations. Benefits of space observations also include line-of-sight fields measured in strong chromospheric lines in the UV that are free of seeing, such as Ca  ii H&K and Mg  ii h&k. A simple mission devoted to such space measurements will enhance our understanding of solar magnetism and hence, space weather and space climate, for decades to come. [ABSTRACT FROM AUTHOR]

Weather has a critical impact on observations at optical and infrared wavelengths, and is a key factor in assessing the performance and productivity of future facilities. As a reference for comparison, we present an analysis of cloud cover, 200-hPa wind speed, precipitable water vapour, vertical wind velocity, and aerosol index, for 15 observatory sites around the world. Our study employs ERA-40 and Total Ozone Mapping Spectrometer climatological data covering a 45-yr time period. We have examined this data set for long-term trends. Several sites are found to show significant long-term changes in some parameters, possibly due to climate change. Four sites show significant increases in cloud cover (99 per cent confidence level). Four sites show changes in 200-hPa wind speed. Eight sites show increases or decreases in precipitable water vapour. The largest changes are found for Antarctic sites, which have clear trends of increasing cloud cover and water vapour. These results should help to characterize the relative astronomical potential of the sites and highlight the influence of both seasonal variations and long-term trends. [ABSTRACT FROM AUTHOR]

We present the dynamical status of the Galactic globular cluster NGC 6656 using the spatial distribution of blue straggler stars (BSSs). A combination of multi-wavelength high-resolution space- and ground-based data are used to cover a large cluster region. We determine the centre of gravity (C grav) and construct the projected density profile of the cluster using the probable cluster members selected from the Hubble Space Telescope and Gaia Data Release 2 proper-motion data sets. The projected density profile in the investigated region is nicely reproduced by a single-mass King model, with core (r c) and tidal (r t) radii of 75″.2 ± 3″.1 and 35′.6 ± 1′.1 respectively. In total, 90 BSSs are identified on the basis of proper-motion data in the region of radius 625″. The average mass of the BSSs is determined as 1.06 ± 0.09 M |$\odot$| and with an age range of 0.5 to 7 Gyr. The BSS radial distribution shows a bimodal trend, with a peak in the centre, a minimum at r ∼ r c and a rising tendency in the outer region. The BSS radial distribution shows a flat behaviour in the outermost region of the cluster. We also estimate the |$A^{+}_{r_\mathrm{h}}$| parameter as an alternative indicator of the dynamical status of the cluster; this is found to be 0.038 ± 0.016. Based on the radial distribution and |$A^{+}_{r_\mathrm{h}}$| parameter, we conclude that NGC 6656 is an intermediate dynamical age cluster. [ABSTRACT FROM AUTHOR]

We present a comprehensive multiwavelength analysis of the merging galaxy cluster MACS J0417.5–1154 at a redshift of |$z$|  = 0.44, using available images red obtained with Chandra in X-ray, Subaru, Hubble Space Telescope (HST) in optical, Giant Metrewave Radio Telescope (GMRT) in radio, and Bolocam at 2.1 mm wavelength. This is an example of a complex merging galaxy cluster also hosting a steep-spectrum Mpc scale radio halo. The mass distribution obtained by weak-lensing reconstruction shows that MACS J0417.5–1154 belongs to the dissociative class of mergers, where one of its substructures has had its gas content detached after the pericentric passage. We find the main cluster mass |$M_{200} = 11.5^{+3.0}_{-3.5} \times 10^{14}\ {\rm M}_\odot$| and the smaller second (sub)cluster mass to be |$M_{200} = 1.96^{+1.60}_{-0.95} \times 10^{14}\ {\rm M}_\odot$|⁠, leading to a large total mass of |$M_{200} = 13.8^{+2.6}_{-2.8} \times 10^{14}\ {\rm M}_\odot$|⁠. The overall structure, surface brightness profile, temperature, and metal abundance of the intracluster medium all point towards the presence of a cold front and merger induced gas-sloshing motion near the core. We detect a surface brightness edge to the southeast direction at a projected distance of ∼45 arcsec (∼255 kpc) from the centre of this cluster. The X-ray spectral analysis across the inner and outer edges allows us to confirm the detected edge as a cold front. The GMRT 235 MHz observation shows a comet-like extended sychrotron radio halo emission trailing behind the cold front. The peak of the Sunyaev–Zel'dovich decrement is found displaced from the centre of X-ray emission, which is interpreted as consequence of the merger dynamics. The optical HST imaging analysis of the cluster reveals the complex morphology of the brightest cluster galaxy, with three surrounding ring-shaped structures with bright knots, which appear to be images of a multiply imaged strongly lensed background galaxy. In addition, two previously unknown giant arcs are found which are all indications of strong gravitational lensing in this massive system. [ABSTRACT FROM AUTHOR]

Supermassive black holes can launch powerful jets which can be some of the most luminous multiwavelength sources; decades after their discovery their physics and energetics are still poorly understood. The past decade has seen a dramatic improvement in the quality of available data, but despite this improvement the semi-analytical modelling of jets has advanced slowly: simple one-zone models are still the most commonly employed method of interpreting data, in particular for active galactic nucleus (AGN) jets. These models can roughly constrain the properties of jets but they cannot unambiguously couple their emission to the launching regions and internal dynamics, which can be probed with simulations. However, simulations are not easily comparable to observations because they cannot yet self-consistently predict spectra. We present an advanced semi-analytical model which accounts for the dynamics of the whole jet, starting from a simplified parametrization of relativistic magnetohydrodynamics in which the magnetic flux is converted into bulk kinetic energy. To benchmark the model, we fit six quasi-simultaneous, multiwavelength spectral energy distributions of the BL Lac PKS 2155–304 obtained by the TANAMI (Tracking Active Galactic Nuclei with Austral Milliarc-second Interferometry) program, and we address the degeneracies inherent to such a complex model by employing a state-of-the-art exploration of parameter space, which so far has been mostly neglected in the study of AGN jets. We find that this new approach is much more effective than a single-epoch fit in providing meaningful constraints on model parameters. [ABSTRACT FROM AUTHOR]

Stark widths and shifts of Mn ii lines with wavelengths in the range 2300–3500 Å have been measured by laser-induced breakdown spectroscopy. The use of fused glass samples with varying Mn concentrations has allowed minimization of the error due to self-absorption in the Stark width measurements, which has made it possible to include resonance lines in the study. The laser-induced plasmas are characterized along their temporal evolution, showing electron densities in the range (0.60–6.75) × 1017cm−3and temperatures in the range 11 000–13 900 K. The results are compared with experimental and theoretical values available in the literature. [ABSTRACT FROM AUTHOR]

Doublet line emission and absorption are common in astronomical sources (e.g. [O iii ], [O ii ], NaD, Mg ii). In many cases, complex kinematics in the emitting source can cause the doublet lines to merge, making characterization of the source kinematics challenging. Here, we present a non-parametric method for resolving merged doublet lines when the line ratio and wavelength difference are known. The method takes as input only the line ratio and wavelength difference, using these quantities to resolve the components of the doublet without resorting to fitting (e.g. using multiple Gaussians) or making any assumptions about the components' line profiles (save that they are the same for both components). The method is simple, fast, and robust. It is also ideal for visualization. We show that the method recovers line profiles of merged emission lines in simulated data. We also show, using simulated data and mathematical analysis, that the method does not significantly increase noise levels in the extracted lines, and is robust to background contamination. We demonstrate the strength of the method by applying it to strongly merged [O iii ] 5007/4959 Å in Active Galactic Nuclei. A python implementation of the method is provided in the appendix. [ABSTRACT FROM AUTHOR]

Fast Radio Bursts (FRBs) are a class of short-duration transients at radio wavelengths with inferred astrophysical origin. The prototypical FRB is a broadband signal that occurs over the extent of the receiver frequency range, is narrow in time, and is highly dispersed, following a ν−2relation. However, some FRBs appear band-limited, and show apparent scintillation, complex frequency-dependent structure, or multi-component pulse shapes. While there is sufficient evidence that FRBs are indeed astrophysical, their one-off nature necessitates extra scrutiny when reporting a detection as bona fide and not a false positive. Currently, there is no formal validation framework for FRBs, rather a set of community practices. In this article, we discuss potential sources of false positives, and suggest a framework in which FRB-like events can be evaluated as real or otherwise. We present examples of false-positive events in data from the Arecibo, LOFAR, and Nanshan telescopes, which while FRB-like, are found to be due to instrumental variations, noise, and radio-frequency interference. Differentiating these false-positive detections from astrophysical events requires knowledge and tests beyond thresholded single-pulse detection. We discuss post-detection analyses, verification tests, and data sets which should be provided when reporting an FRB detection. [ABSTRACT FROM AUTHOR]

Fast radio bursts (FRBs) are bright flashes observed typically at GHz frequencies with millisecond duration, whose origin is likely extragalactic. Their nature remains mysterious, motivating searches for counterparts at other wavelengths. FRB 121102 is so far the only source known to repeatedly emit FRBs and is associated with a host galaxy at redshift |$z$| ≃ 0.193. We conducted simultaneous observations of FRB 121102 with the Arecibo and MAGIC telescopes during several epochs in 2016–2017. This allowed searches for millisecond time-scale burst emission in very-high-energy (VHE) gamma-rays as well as the optical band. While a total of five FRBs were detected during these observations, no VHE emission was detected, neither of a persistent nature nor burst-like associated with the FRBs. The average integral flux upper limits above 100 GeV at 95 per cent confidence level are 6.6 × 10−12photons cm−2s−1(corresponding to luminosity L VHE≲ 1045erg s−1) over the entire observation period, and 1.2 × 10−7photons cm−2s−1(L VHE≲ 1049erg s−1) over the total duration of the five FRBs. We constrain the optical U -band flux to be below 8.6 mJy at 5σ level for 1-ms intervals around the FRB arrival times. A bright burst with U -band flux 29 mJy and duration ∼12 ms was detected 4.3 s before the arrival of one FRB. However, the probability of spuriously detecting such a signal within the sampled time space is 1.5 per cent (2.2, post-trial), i.e. consistent with the expected background. We discuss the implications of the obtained upper limits for constraining FRB models. [ABSTRACT FROM AUTHOR]

Disc reverberation mapping of a handful of nearby active galactic nuclei (AGNs) suggests accretion disc sizes that are a factor of a few too large for their luminosities, apparently at odds with the standard model. Here, we investigate the likely contribution to the measured delay signature of diffuse continuum emission arising from broad-line region gas. We start by constructing spherically symmetric pressure-law BLR models (i.e. P (r)∝ r − s ) that approximately reproduce the observed emission line fluxes of the strong UV–optical emission lines in the best-studied source, NGC 5548. We then determine the contribution of the diffuse continuum to the measured continuum flux and inter-band delays, accounting for the observed variability behaviour of the ionizing nuclear continuum. Those pressure-law models that approximately reproduce the observed emission-line luminosities unavoidably produce substantial diffuse continuum emission. This causes a significant contamination of the disc reverberation signature (i.e. wavelength-dependent continuum delays). Qualitatively, the diffuse continuum delay signatures produced by our models resemble that observed for NGC 5548, including the deviation of the lag spectrum above that of a simple power law in wavelength, short-ward of the Balmer and Paschen jumps. Furthermore, for reasonable estimates of the BLR covering fraction, the delay induced by diffuse continuum emission causes elevated inter-band delays over the entire UV–optical regime; for these pressure-law models, there are no 'disc-dominated' wavelength intervals. Thus, the diffuse continuum contribution must be taken into account in order to correctly infer AGN accretion disc sizes based on inter-band continuum delays. [ABSTRACT FROM AUTHOR]

The presence of a debris disc around the Gyr-old solar-type star |$\zeta ^2\, \mathrm{Reticuli}\,$| was suggested by the Spitzer infrared excess detection. Follow-up observations with Herschel /PACS revealed a double-lobed feature that displayed asymmetries both in brightness and position. Therefore, the disc was thought to be edge-on and significantly eccentric. Here we present ALMA/ACA observations in Band 6 and 7 which unambiguously reveal that these lobes show no common proper motion with |$\zeta ^2\, \mathrm{Reticuli}\,$|⁠. In these observations, no flux has been detected around |$\zeta ^2\, \mathrm{Reticuli}\,$| that exceeds the 3σ levels. We conclude that surface brightness upper limits of a debris disc around |$\zeta ^2\, \mathrm{Reticuli}\,$| are |$5.7\, \mathrm{\mu Jy\, arcsec^{-2}}$| at 1.3 mm, and |$26\, \mathrm{\mu Jy\, arcsec^{-2}}$| at 870 microns. Our results overall demonstrate the capability of the ALMA/ACA to follow-up Herschel observations of debris discs and clarify the effects of background confusion. [ABSTRACT FROM AUTHOR]

The most intensively star-forming galaxies are extremely luminous at far-infrared (FIR) wavelengths, highly obscured at optical and ultraviolet wavelengths, and lie at |$z$| ≥ 1–3. We present a programme of FIR spectroscopic observations with the SPIRE FTS, as well as photometric observations with PACS, both on board Herschel, towards a sample of 45 gravitationally lensed, dusty starbursts across |$z$| ∼ 1–3.6. In total, we detected 27 individual lines down to |$3\, \sigma$|⁠, including nine [C  ii ] 158  |$\mu$| m lines with confirmed spectroscopic redshifts, five possible [C  ii ] lines consistent with their FIR photometric redshifts, and in some individual sources a few [O  iii ] 88  |$\mu$| m, [O  iii ] 52  |$\mu$| m, [O  i ] 145  |$\mu$| m, [O  i ] 63  |$\mu$| m, [N  ii ] 122  |$\mu$| m and OH 119  |$\mu$| m (in absorption) lines. To derive the typical physical properties of the gas in the sample, we stack all spectra weighted by their intrinsic luminosity and by their 500 |$\mu$| m flux densities, with the spectra scaled to a common redshift. In the stacked spectra, we detect emission lines of [C  ii ] 158  |$\mu$| m, [N  ii ] 122  |$\mu$| m, [O  iii ] 88  |$\mu$| m, [O  iii ] 52  |$\mu$| m, [O  i ] 63  |$\mu$| m and the absorption doublet of OH at 119  |$\mu$| m, at high fidelity. We find that the average electron densities traced by the [N  ii ] and [O  iii ] lines are higher than the average values in local star-forming galaxies and ULIRGs, using the same tracers. From the [N  ii ]/[C  ii ] and [O  i ]/[C  ii ] ratios, we find that the [C  ii ] emission is likely dominated by the photodominated regions (PDR), instead of by ionized gas or large-scale shocks. [ABSTRACT FROM AUTHOR]

At radio wavelengths, solar-type stars emit thermal free–free and gyroresonance, gyrosynchrotron, and impulsive coherent emission. Thermal free–free emission originates at layers where the optical depth is close to unit, while high brightness temperature and variable emission can be due to flares via gyrosynchrotron emission. We observed the |$\alpha \,$| Cen  system with the Australian Telescope Compact Array at 2 GHz for three days and 17 GHz for one day. Both stars have been detected at 17 GHz, while only an upper limit has been obtained at low frequency despite the longer integration time. The brightness temperatures are consistent with the temperature of the upper chromosphere of the Sun. Inverting the formulae of the free–free emission, the average electron density of the plasma has been inferred. The same procedure was applied to the data in the millimetre recently acquired with ALMA. A comparison with the atmospheric solar models reveals a higher level of activity in |$\alpha \,$| Cen B rather than in |$\alpha \,$| Cen A, even if still at quiescent level. The non-detection at 2 GHz allows us to put a lower limit in the filling factor of active regions. The claimed detection of an Earth-sized planet in close orbit to |$\alpha \,$| Cen B, although doubtful, opens the opportunity to check the existence of Star–Planet Magnetic Interaction. We constructed dynamic spectra in the 1.3–2.9GHz of the two stars to search for time-variable coherent emission but obtained a null result. [ABSTRACT FROM AUTHOR]