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XTE J1701−462 is a neutron star low-mass X-ray binary (NS LMXB) discovered in 2006 as the first system to demonstrate unambiguously that the 'Atoll' and 'Z' classes of accreting neutron stars are separated by accretion rate. Radio observations during the 2006/7 outburst provided evidence for the formation of a relativistic jet, as now expected for all accreting neutron star and black hole X-ray binaries at high-accretion rates. The source entered a new outburst in 2022, and we report 29 observations made with the MeerKAT radio telescope. The first radio detection was on the 16th September 2022, we continued detecting the source until mid-December 2022. Thereafter, establishing radio upper limits till 25 March 2023. We present the radio analysis alongside analysis of contemporaneous X-ray observations from MAXI. The radio light curve shows evidence for at least three flare-like events over the first hundred days, the most luminous of which has an associated minimum energy of |$1\times 10^{38}$|  erg. We provide a detailed comparison with the 2006/7 outburst, and demonstrate that we detected radio emission from the source for considerably longer in the more recent outburst, although this is probably a function of sampling. We further constrain the radio emission from the source to have a polarization of less than 9 per cent at the time of 2022 IXPE detection of X-ray polarization. Finally, we place the source in the radio–X-ray plane, demonstrating that when detected in radio it sits in a comparable region of parameter space to the other Z-sources. [ABSTRACT FROM AUTHOR]

We describe an X-ray source detection method entirely based on the maximum likelihood analysis, in application to observations with the ART-XC telescope onboard the Spectrum Roentgen Gamma observatory. The method optimally combines the data taken at different conditions, a situation commonly found in scanning surveys or mosaic observations with a telescope with a significant off-axis PSF distortion. The method can be naturally extended to include additional information from the X-ray photon energies, detector grades, etc. The likelihood-based source detection naturally results in an optimal use of available information for the sources detection and stable and uniform definition of detection thresholds under different observing conditions (PSF, background level). This greatly simplifies the statistical calibration of the survey needed to, e.g. obtain the |$\log N - \log S$| distribution of detected sources or their luminosity function. The method can be applied to the data from any imaging X-ray telescope. [ABSTRACT FROM AUTHOR]

In this work, we use 8 years of deep near-infrared imaging to select and study a new set of 601 active galaxies identified through long-term near-infrared (NIR) variability in the UKIDSS Ultra Deep Survey (UDS). These objects are compared to 710 X-ray bright Active Galactic Nuclei (AGN) detected by the Chandra X-ray observatory. We show that infrared variability and X-ray emission select distinct sets of active galaxies, finding only a 37 per cent overlap of galaxies detected by both techniques and confirming NIR-variable AGN to be typically X-ray quiet. Examining the mass functions of the active galaxies shows that NIR variability detects AGN activity in galaxies over a significantly wider range of host stellar mass compared to X-ray detection. For example, at z ∼ 1, variable AGN are identified among approximately 1 per cent of galaxies in a roughly flat distribution above the stellar mass completeness limit (⁠|$\gt 10^{9}\rm \, {\rm M}_{\odot }$|⁠), while X-ray detection primarily identifies AGN in galaxies of higher mass (⁠|$\gt 10^{10}\rm \, {\rm M}_{\odot }$|⁠). We conclude that long-term near-infrared variability provides an important new tool for obtaining more complete samples of AGN in deep survey fields. [ABSTRACT FROM AUTHOR]

We report on the SRG /eROSITA detection of ultra-soft (⁠|$kT=47^{+5}_{-5}$|  eV) X-ray emission (L X = |$2.5^{+0.6}_{-0.5} \times 10^{43}$|  erg s−1) from the tidal disruption event (TDE) candidate AT 2022dsb ∼14 d before peak optical brightness. As the optical luminosity increases after the eROSITA detection, then the 0.2–2 keV observed flux decays, decreasing by a factor of ∼39 over the 19 d after the initial X-ray detection. Multi-epoch optical spectroscopic follow-up observations reveal transient broad Balmer emission lines and a broad He  ii 4686 Å emission complex with respect to the pre-outburst spectrum. Despite the early drop in the observed X-ray flux, the He  ii 4686  Å complex is still detected for ∼40 d after the optical peak, suggesting the persistence of an obscured hard ionizing source in the system. Three outflow signatures are also detected at early times: (i) blueshifted H α emission lines in a pre-peak optical spectrum, (ii) transient radio emission, and (iii) blueshifted Ly α absorption lines. The joint evolution of this early-time X-ray emission, the He  ii 4686 Å complex, and these outflow signatures suggests that the X-ray emitting disc (formed promptly in this TDE) is still present after optical peak, but may have been enshrouded by optically thick debris, leading to the X-ray faintness in the months after the disruption. If the observed early-time properties in this TDE are not unique to this system, then other TDEs may also be X-ray bright at early times and become X-ray faint upon being veiled by debris launched shortly after the onset of circularization. [ABSTRACT FROM AUTHOR]

We report the first detection of evolving low-frequency quasi-periodic oscillation (LFQPO) frequencies in hard X-rays upto 100 keV with AstroSat/ LAXPC during 'unusual' outburst phase of Swift J1727.8−1613 in hard intermediate state (HIMS). The observed LFQPO in 20–100 keV has a centroid |$\nu _{_{\rm QPO}}=1.43$| Hz, a coherence factor Q  = 7.14 and an amplitude |${\rm rms_{_{\rm QPO}}} = 10.95{{\ \rm per\ cent}}$| with significance σ = 5.46. Type-C QPOs (1.09–2.6 Hz) are found to evolve monotonically during HIMS of the outburst with clear detection in hard X-rays (80−100 keV), where |${\rm rms_{_{\rm QPO}}}$| decreases (⁠|$\sim 12\!-\!3{{\ \rm per\ cent}}$|⁠) with energy. Further, |$\nu _{_{\rm QPO}}$| is seen to correlate (anticorrelate) with low- (high-) energy flux in 2–20 keV (15–50 keV). Wide-band (0.7−40 keV) energy spectrum of NICER/ XTI and AstroSat/ LAXPC is satisfactorily described by the 'dominant' thermal Comptonization contribution (∼88 per cent) in presence of a 'weak' signature of disc emissions (kT in ∼ 0.36 keV) indicating the harder spectral distribution. Considering source mass |$M_{\rm BH}=10\, \mathrm{M}_\odot$| and distance 1.5 < d (kpc) < 5, the unabsorbed bolometric luminosity is estimated as |$\sim 0.03\!-\!0.92{{\ \rm per\ cent}}\, L_{\rm Edd}$|⁠. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries. [ABSTRACT FROM AUTHOR]

The Snake is a remarkable Galactic Centre radio filament with a morphology characterized by two kinks along its ∼20 arcmin extent. The major and minor kinks are located where the filament is most distorted from a linear magnetized structure running perpendicular to the Galactic plane. We present Chandra , VLA, and MeerKAT data and report the detection of an X-ray and radio source at the location of the major kink. High-resolution radio images of the major kink reveal a compact source with a steep spectrum with spectral index α ∼ −2.7 surrounded by extended emission. The radio luminosity and steep spectrum of the compact source are consistent with a pulsar. We also show flattening of the spectrum and enhanced synchrotron emissivity away from the position of the major kink along the Snake, which suggests injection of relativistic particles along the Snake. We argue that the major kink is created by a fast-moving (∼500–1000 km s−1) object punching into the Snake, distorting its magnetic structure, and producing X-ray emission. X-ray emission pinpoints an active acceleration site where the interaction is taking place. A secondary kink is argued to be induced by the impact of the high-velocity object producing the major kink. [ABSTRACT FROM AUTHOR]

The automatic classification of X-ray detections is a necessary step in extracting astrophysical information from compiled catalogues of astrophysical sources. Classification is useful for the study of individual objects, statistics for population studies, as well as for anomaly detection, that is, the identification of new unexplored phenomena, including transients and spectrally extreme sources. Despite the importance of this task, classification remains challenging in X-ray astronomy due to the lack of optical counterparts and representative training sets. We develop an alternative methodology that employs an unsupervised machine learning approach to provide probabilistic classes to Chandra Source Catalog sources with a limited number of labelled sources, and without ancillary information from optical and infrared catalogues. We provide a catalogue of probabilistic classes for 8756 sources, comprising a total of 14 507 detections, and demonstrate the success of the method at identifying emission from young stellar objects, as well as distinguishing between small- and large-scale compact accretors with a significant level of confidence. We investigate the consistency between the distribution of features among classified objects and well-established astrophysical hypotheses such as the unified active galactic nucleus model. This provides interpretability to the probabilistic classifier. Code and tables are available publicly through GitHub. We provide a web playground for readers to explore our final classification at https://umlcaxs-playground.streamlit.app. [ABSTRACT FROM AUTHOR]

Hyperluminous infrared galaxies (HyLIRGs) are the most extreme star-forming systems observed in the early Universe, and their properties still elude comprehensive understanding. We have undertaken a large XMM – Newton observing programme to probe the total accreting black hole population in three HyLIRGs at z = 2.12, 3.25, and 3.55, gravitationally lensed by foreground galaxies. Selected from the Planck All-Sky Survey to Analyse Gravitationally lensed Extreme Starbursts (PASSAGES), these HyLIRGs have apparent infrared luminosities >1014 L⊙. Our observations revealed X-ray emission in each of them. PJ1336+49 appears to be dominated by high-mass X-ray binaries (HMXBs). Remarkably, the luminosity of this non-AGN X-ray emission exceeds by a factor of about 3 the value obtained by calibration with local galaxies with much lower star formation rates. This enhanced X-ray emission most likely highlights the efficacy of dynamical HMXB production within compact clusters, which is an important mode of star formation in HyLIRGs. The remaining two (PJ0116−24 and PJ1053+60) morphologically and spectrally exhibit a compact X-ray component in addition to the extended non-AGN X-ray emission, indicating the presence of Active Galactic Nuclei (AGNs). The AGN appears to be centrally located in the reconstructed source plane images of PJ0116−24, which manifests its star-forming activity predominantly within an extended galactic disc. In contrast, the AGN in the field of PJ1053+60 is projected 60 kpc away from the extreme star-forming galaxy and could be ejected from it. These results underline the synergistic potential of deep X-ray observations with strong lensing for the study of high-energy astrophysical phenomena in HyLIRGs. [ABSTRACT FROM AUTHOR]

We report on IXPE, NICER, and XMM–Newton observations of the magnetar 1E 2259+586. We find that the source is significantly polarized at about or above 20  per cent for all phases except for the secondary peak where it is more weakly polarized. The polarization degree is strongest during the primary minimum which is also the phase where an absorption feature has been identified previously. The polarization angle of the photons are consistent with a rotating vector model with a mode switch between the primary minimum and the rest of the rotation of the neutron star. We propose a scenario in which the emission at the source is weakly polarized (as in a condensed surface) and, as the radiation passes through a plasma arch, resonant cyclotron scattering off of protons produces the observed polarized radiation. This confirms the magnetar nature of the source with a surface field greater than about 1015 G. [ABSTRACT FROM AUTHOR]

The study of jet-inflated X-ray cavities provides a powerful insight into the energetics of hot galactic atmospheres and radio-mechanical AGN feedback. By estimating the volumes of X-ray cavities, the total energy and thus also the corresponding mechanical jet power required for their inflation can be derived. Properly estimating their total extent is, however, non-trivial, prone to biases, nearly impossible for poor-quality data, and so far has been done manually by scientists. We present a novel machine-learning pipeline called Cavity Detection Tool (CADET), developed as an assistive tool that detects and estimates the sizes of X-ray cavities from raw Chandra images. The pipeline consists of a convolutional neural network trained for producing pixel-wise cavity predictions and a DBSCAN clustering algorithm, which decomposes the predictions into individual cavities. The convolutional network was trained using mock observations of early-type galaxies simulated to resemble real noisy Chandra -like images. The network's performance has been tested on simulated data obtaining an average cavity volume error of 14 per cent at an 89 per cent true-positive rate. For simulated images without any X-ray cavities inserted, we obtain a 5 per cent false-positive rate. When applied to real Chandra images, the pipeline recovered 93 out of 97 previously known X-ray cavities in nearby early-type galaxies and all 14 cavities in chosen galaxy clusters. Besides that, the CADET pipeline discovered seven new cavity pairs in atmospheres of early-type galaxies (IC 4765, NGC 533, NGC 2300, NGC 3091, NGC 4073, NGC 4125, and NGC 5129) and a number of potential cavity candidates. [ABSTRACT FROM AUTHOR]

The recent serendipitous discovery of a new population of short duration X-ray transients, thought to be associated with collisions of compact objects or stellar explosions in distant galaxies, has motivated efforts to build up statistical samples by mining X-ray telescope archives. Most searches to date, however, do not fully exploit recent developments in the signal and imaging processing research domains to optimize searches for short X-ray flashes. This paper addresses this issue by presenting a new source detection pipeline, STATiX (Space and Time Algorithm for Transients in X-rays), which directly operates on three-dimensional X-ray data cubes consisting of two spatial and one temporal dimension. The algorithm leverages wavelet transforms and the principles of sparsity to denoise X-ray observations and then detect source candidates on the denoised data cubes. The light curves of the detected sources are then characterized using the Bayesian blocks algorithm to identify flaring periods. We describe the implementation of STATiX in the case of XMM–Newton data, present extensive validation and performance verification tests based on simulations and also apply the pipeline to a small subset of seven XMM–Newton observations, which are known to contain transients sources. In addition to known flares in the selected fields, we report a previously unknown short duration transient found by our algorithm that is likely associated with a flaring Galactic star. This discovery demonstrates the potential of applying STATiX to the full XMM–Newton archive. [ABSTRACT FROM AUTHOR]

The paper presents the analysis of optically selected GAMA groups and clusters in the SRG/eROSITA X-ray map of eROSITA Final Equatorial Depth Survey, in the halo mass range 1013−5 × 1014 M⊙ and at z < 0.2. All X-ray detections have a clear GAMA counterpart, but most of the GAMA groups in the halo mass range 1013−1014 M⊙ remain undetected. We compare the X-ray surface brightness profiles of the eROSITA detected groups with the mean stacked profile of the undetected low-mass haloes at fixed halo mass. Overall, we find that the undetected groups exhibit less concentrated X-ray surface brightness, dark matter, and galaxy distributions with respect to the X-ray-detected haloes. The mean gas mass fraction profiles are consistent in the two samples within 1.5σ, indicating that the gas follows the dark matter profile. The low-mass concentration and the magnitude gap indicate that these systems are young. They reside with a higher probability in filaments while X-ray-detected groups favour the nodes of the Cosmic Web. Because of the lower central emission, the undetected systems tend to be X-ray underluminous at fixed halo mass and to lie below the LX − M halo relation. Interestingly, the X-ray-detected systems inhabiting the nodes scatter the less around the relation, while those in filaments tend to lie below it. We do not observe any strong relationship between the system X-ray appearance and the active galactic nucleus (AGN) activity. We cannot exclude the role of the past AGN feedback in affecting the gas distribution over the halo lifetime. However, the data suggests that the observed differences might be related to the halo assembly bias. [ABSTRACT FROM AUTHOR]

In this paper, we performed a spectro-polarimetric analysis of the transient NS-LMXB XTE J1701−462 using IXPE, NICER , and NuSTAR data during its 2022 outburst. We report significant detection of energy-dependent polarization in the X-ray signal from the source on 2022 September 29 in the 2–4, 4–8, and 2–8 keV energy bands with a polarization degree of 3.9 ± 0.3, 5.5 ± 0.6, and 4.5 ± 0.4 per cent, respectively. The polarization angle in the overall 2–8 keV band was ∼143° ± 2°. The spectra were modelled using a combination of thermal emission from an accretion disc, Comptonized emission from a hot electron plasma (or corona) and a Gaussian line. From spectro-polarimetric analysis, the polarization degree due to the disc emission had an upper limit of ∼11.5 per cent, and that of the Comptonized emission was constrained at 7.7 ± 2.5 per cent (at the 3σ level). The results suggest that the Comptonized component probably originates from a spreading-layer/boundary-layer above the neutron star surface. IXPE observation of the source on 2022 October 8 does not show significant polarization which can be attributed to a weakening of the coronal Comptonized emission. The implication of these results are discussed. [ABSTRACT FROM AUTHOR]

A complete demographic of active galactic nuclei (AGNs) is essential to understand the evolution of the Universe. Optical surveys estimate the population of AGNs in the local Universe to be of ∼ 4 per cent. However, these results could be biased towards bright sources, not affected by the host galaxy attenuation. An alternative method for detecting these objects is through the X-ray emission. In this work, we aim to complement the AGN population of the optical CALIFA survey (941 sources), by using X-ray data from Chandra , which provides the best spatial resolution to date, essential to isolate the nuclear emission from the host galaxy. We study a total of 138 sources with available data. We find 34 new bona fide AGNs and 23 AGN candidates, which could increase the AGN population to 7–10  per cent among the CALIFA survey. X-rays are particularly useful for low-luminosity AGNs since they are excluded by the criterion of large equivalent width of the |$\rm {H\alpha }$| emission line when applied to optical selections. Indeed, placing such a restrictive criteria might cause a loss of up to 70 per cent of AGN sources. X-ray detected sources are preferentially located in the right side of the [ |${\mathrm{O}}\, {\small {\rm III}}$| ]/Hβ versus [ |${\mathrm{N}}\, {\small {\rm II}}$| ]/Hα diagram, suggesting that this diagram might be the most reliable at classifying AGN sources. Our results support the idea that multiwavelength studies are the best way to obtain a complete AGN population. [ABSTRACT FROM AUTHOR]

We present observations from X-ray to mid-infrared wavelengths of the most energetic non-quasar transient ever observed, AT2021lwx. Our data show a single optical brightening by a factor >100 to a luminosity of 7 × 1045 erg s−1 and a total radiated energy of 1.5 × 1053 erg, both greater than any known optical transient. The decline is smooth and exponential and the ultraviolet–optical spectral energy distribution resembles a blackbody with a temperature of 1.2 × 104 K. Tentative X-ray detections indicate a secondary mode of emission, while a delayed mid-infrared flare points to the presence of dust surrounding the transient. The spectra are similar to recently discovered optical flares in known active galactic nuclei but lack some characteristic features. The lack of emission for the previous 7 yr is inconsistent with the short-term, stochastic variability observed in quasars, while the extreme luminosity and long time-scale of the transient disfavour the disruption of a single solar-mass star. The luminosity could be generated by the disruption of a much more massive star, but the likelihood of such an event occurring is small. A plausible scenario is the accretion of a giant molecular cloud by a dormant black hole of 108–109 solar masses. AT2021lwx thus represents an extreme extension of the known scenarios of black hole accretion. [ABSTRACT FROM AUTHOR]

Detached eclipsing binaries are the primary tool used to measure precise masses and radii of stars. In our previous paper estimating the parameters of more than 30 000 detached eclipsing binaries, we identified 766 eclipsing binaries with additional features in their All-Sky Automated Survey for Supernovae (ASAS-SN) and Transiting Exoplanet Survey Satellite light curves. Here, we characterize these 'extra-physics' systems, identifying eclipsing binaries with spotted stars, pulsating components, and candidate triple/quadruple systems. We use the Gaia , Asteroid Terrestrial-impact Last Alert System (ATLAS), Zwicky Transient Facility (ZTF), and ASAS-SN variable star catalogues to search for possible blends. We use MIST (MESA Isochrones and Stellar Tracks) isochrones and evolutionary tracks to identify systems with main-sequence, subgiant, and giant primaries and highlight systems in sparsely populated regions of the colour–magnitude diagram. We find that the orbital period distribution of spotted binaries is divided by evolutionary state and find 68 with X-ray detections. For the candidate triple/quadruples and pulsating systems, we calculate the extra orbital/pulsational period and identify systems with resonances. Finally, we highlight a number of exotic systems, including eclipsing cataclysmic variables, subdwarfs, and binaries with discs. [ABSTRACT FROM AUTHOR]

We present a study of optically selected dual Active Galactic Nuclei (AGN) with projected separations of 3–97 kpc. Using multiwavelength (MWL) information (optical, X-ray, mid-IR), we characterized the intrinsic nuclear properties of this sample and compared them with those of isolated systems. Among the 124 X-ray-detected AGN candidates, 52 appear in pairs and 72 as single X-ray sources. Through MWL analysis, we confirmed the presence of the AGN in >80 per cent of the detected targets in pairs (42 out of 52). X-ray spectral analysis confirms the trend of increasing AGN luminosity with decreasing separation, suggesting that mergers may have contributed to triggering more luminous AGN. Through X-ray/mid-IR ratio versus X-ray colours, we estimated a fraction of Compton-thin AGN (with 1022 cm−2 < N H < 1024 cm−2) of about 80 per cent, while about 16 per cent are Compton-thick sources (with N H > 1024 cm−2). These fractions of obscured sources are larger than those found in samples of isolated AGN, confirming that pairs of AGN show higher obscuration. This trend is further confirmed by comparing the de-reddened [O  iii ] emission with the observed X-ray luminosity. However, the derived fraction of Compton-thick sources in this sample at the early stages of merging is lower than that reported for late-merging dual-AGN samples. Comparing N H from X-rays with that derived from E (B − V) from narrow-line regions, we found that the absorbing material is likely to be associated with the torus or broad-line regions. We also explored the X-ray detection efficiency of dual-AGN candidates, finding that, when observed properly (at on-axis positions and with long exposures), X-ray data represent a powerful way to confirm and investigate dual-AGN systems. [ABSTRACT FROM AUTHOR]

We present multiwavelength observations of supernova (SN) 2017hcc with the Chandra X-ray telescope and the X-ray telescope onboard Swift (Swift-XRT) in X-ray bands, with the Spitzer and the TripleSpec spectrometer in near-infrared (IR) and mid-IR bands and with the Karl G. Jansky Very Large Array (VLA) for radio bands. The X-ray observations cover a period of 29 to 1310 d, with the first X-ray detection on day 727 with the Chandra. The SN was subsequently detected in the VLA radio bands from day 1000 onwards. While the radio data are sparse, synchrotron-self absorption is clearly ruled out as the radio absorption mechanism. The near- and the mid-IR observations showed that late time IR emission dominates the spectral energy distribution. The early properties of SN 2017hcc are consistent with shock breakout into a dense mass-loss region, with |$\dot{M} \sim 0.1$| M⊙ yr−1 for a decade. At few 100 d, the mass-loss rate declined to ∼0.02 M⊙ yr−1, as determined from the dominant IR luminosity. In addition, radio data also allowed us to calculate a mass-loss rate at around day 1000, which is two orders of magnitude smaller than the mass-loss rate estimates around the bolometric peak. These values indicate that the SN progenitor underwent an enhanced mass-loss event a decade before the explosion. The high ratio of IR to X-ray luminosity is not expected in simple models and is possible evidence for an asymmetric circumstellar region. [ABSTRACT FROM AUTHOR]

In this paper, we present the X-ray analysis of Sloan Digital Sky Survey DR8 redMaPPer (SDSSRM) clusters using data products from the XMM Cluster Survey (XCS). In total, 1189 SDSSRM clusters fall within the XMM – Newton footprint. This has yielded 456 confirmed detections accompanied by X-ray luminosity (LX) measurements. Of these clusters, 381 have an associated X-ray temperature measurement (TX). This represents one of the largest samples of coherently derived cluster TX values to date. Our analysis of the X-ray observable to richness scaling relations has demonstrated that scatter in the TX –λ relation is roughly a third of that in the LX –λ relation, and that the LX –λ scatter is intrinsic, i.e. will not be significantly reduced with larger sample sizes. Analysis of the scaling relation between LX and TX has shown that the fits are sensitive to the selection method of the sample, i.e. whether the sample is made up of clusters detected 'serendipitously' compared to those deliberately targeted by XMM. These differences are also seen in the LX –λ relation and, to a lesser extent, in the TX –λ relation. Exclusion of the emission from the cluster core does not make a significant impact on the findings. A combination of selection biases is a likely, but yet unproven, reason for these differences. Finally, we have also used our data to probe recent claims of anisotropy in the LX – TX relation across the sky. We find no evidence of anistropy, but stress this may be masked in our analysis by the incomplete declination coverage of the SDSS. [ABSTRACT FROM AUTHOR]

XMM–Newton observations of the middle-aged radio-quiet γ-ray pulsar J0554+3107 allowed us, for the first time, firmly identify it in X-rays by detection of pulsations with the pulsar period. In the 0.2–2 keV band, the pulse profile shows two peaks separated by about a half of the rotation phase with the pulsed fraction of 25 ± 6 per cent. The profile and spectrum in this band can be mainly described by thermal emission from the neutron star with the hydrogen atmosphere, dipole magnetic field of ∼1013 G, and non-uniform surface temperature. Non-thermal emission from the pulsar magnetosphere is marginally detected at higher photon energies. The spectral fit with the atmosphere+power-law model implies that J0554+3107 is a rather heavy and cool neutron star with the mass of 1.6–2.1 M⊙, the radius of ≈13 km, and the redshifted effective temperature of ≈50 eV. The spectrum shows an absorption line of unknown nature at ≈350 eV. Given the extinction–distance relation, the pulsar is located at ≈2 kpc and has the redshifted bolometric thermal luminosity of ≈2 × 1032 erg s−1. We discuss cooling scenarios for J0554+3107 considering plausible equations of state of superdense matter inside the star, different compositions of the heat-blanketing envelope, and various ages. [ABSTRACT FROM AUTHOR]

Short rise times of fast blue optical transients (FBOTs) require very light ejected envelopes, |$M_{\rm ej} \le 10^{-1} \, \mathrm{M}_\odot$|⁠ , much smaller than of a typical supernova. The detection by Chandra of X-ray emission in AT2020mrf of L X ∼ 1042 erg s−1 after 328 d implies total, overall dominant, X-ray energetics at the gamma-ray burst level of ∼6 × 1049 erg. We further develop a model of Lyutikov and Toonen, whereby FBOTs are the results of a late accretion-induced collapse of the product of double white dwarf (WD) merger between ONeMg WD and another WD. Small ejecta mass, and the rarity of FBOTs, results from the competition between mass-loss from the merger product to the wind, and ashes added to the core, on a time-scale of ∼103–104 yr. FBOTs proper come from central engine-powered radiation-dominated forward shock as it propagates through ejecta. All the photons produced by the central source deep inside the ejecta escape almost simultaneously, producing a short bright event. The high-energy emission is generated at the highly relativistic and highly magnetized termination shock, qualitatively similar to pulsar wind nebulae. The X-ray bump observed in AT2020mrf by SRG /eROSITA, predicted by Lyutikov and Toonen, is coming from the breakout of the engine-powered shock from the ejecta into the preceding wind. The model requires total energetics of just few × 1050 erg, slightly above the observed X-rays. We predict that the system is hydrogen poor. [ABSTRACT FROM AUTHOR]

We present the results of a radio observing campaign on GRB 201216C, combined with publicly available optical and X-ray data. The detection of very high energy (VHE, >100 GeV) emission by MAGIC makes this the fifth VHE GRB at the time of publication. Comparison between the optical and X-ray light curves show that GRB 201216C is a dark GRB, i.e. the optical emission is significantly absorbed and is fainter than expected from the X-ray detections. Our e -MERLIN data also shows evidence of diffractive interstellar scintillation. We can study the column density along the line of sight to the GRB in both the host galaxy, from the damped optical light curve, and the Milky Way, via scintillation studies. We find that the afterglow is best modelled using a jet-cocoon geometry within a stellar wind environment. Fitting the data with a multicomponent model, we estimate that the optical, X-ray, and higher frequency radio data before ∼25 d originates from an ultrarelativistic jet with an isotropic equivalent kinetic energy of (0.6–10) × 1052 erg and an opening angle of ∼1–9°. The lower frequency radio emission detected by MeerKAT, from day 28 onwards, is produced by the cocoon with a kinetic energy that is between two and seven orders of magnitude lower (0.02–50) × 1048 erg. The energies of the two components are comparable to those derived in simulations of such scenarios. [ABSTRACT FROM AUTHOR]

We examine Chandra observations of the powerful Fanaroff–Riley class II (FR II) radio galaxy Cygnus A for an X-ray counterpart to the radio transient Cygnus A-2 that was first detected in 2011. Observations are performed using the High-Resolution Camera (HRC) instrument in order to spatially resolve Cygnus A-2 and the central active galactic nucleus (AGN) at a separation of  0.′′42. Simulated images are generated of the emission region, and radial profiles for the region of interest are extracted. A comparison between the simulations and observations reveals no X-ray detection of Cygnus A-2 to a 0.5–7.0 keV flux upper limit of |$1.04 \times 10^{-12}\rm \, erg\, cm^{-2}\, s^{-1}$|⁠ , or a rest-frame 2–10 keV luminosity of |$8.6\times 10^{42}\rm \, erg\, s^{-1}$|⁠. We estimate the black hole mass of Cygnus A-2 based on our X-ray flux limit and find it to be consistent with a flaring black hole rather than a steadily accreting source. The HRC observations are additionally compared with archival Advanced CCD Imaging Spectrometer (ACIS) data from 2016 to 2017, and both the overall morphology and the flux limits of the AGN complex agree between the two data sets. This consistency is despite the pile-up effect in ACIS which was previously considered to bias the observed morphology of the AGN. The agreement between the data sets demonstrates the viability of utilizing the archival Chandra data of Cygnus A to analyse its AGN at an unprecedented level of precision. [ABSTRACT FROM AUTHOR]

Reionization-era galaxies often display intense nebular emission lines, both in rest-frame optical ([O  iii ] + H β) and ultraviolet (UV; C  iii ], C  iv). How such strong nebular emission is powered remains unclear, with both active galactic nuclei (AGNs) and hot stars considered equally viable. The UV continuum slopes of these early systems tend to be very blue (β < −2), reflecting minimal dust obscuration, young ages, and low metallicities. This contrasts with narrow-lined AGNs at |$z$| ∼ 2−3, whose UV slopes are significantly redder (β > −1) than typical star-forming systems in the reionization era. To investigate the properties of AGNs in the reionization era, we have conducted a search for potential examples of rare analogues with blue continua at intermediate redshift (⁠|$z$| ∼ 2−3). Our goals are to determine whether AGNs with intense line emission and blue continua exist and thereby to establish the range of rest-frame UV and optical line ratios in this population. In this paper, we report the detection of an X-ray luminous AGN at |$z$|  = 3.21 (UDS-24561) with extreme [O  iii ] + H β line emission (equivalent width = 1300 Å) and a blue UV continuum slope (β = −2.34). MMT/Binospec and Keck/MOSFIRE spectra indicate rest-frame UV line ratios consistent with AGN photoionization models and rest-frame optical lines with both a narrow component [full width at half-maximum (FWHM) |$=154\, {\rm km\, s}^{-1}$| ] and extended broad wings (FWHM |$=977\, {\rm km\, s}^{-1}$|⁠), consistent with outflowing gas. We describe how such objects can be identified in future James Webb Space Telescope emission line surveys in the reionization era, thereby providing a valuable census of AGN activity at |$z$| > 6 and understanding their contribution to cosmic reionization. [ABSTRACT FROM AUTHOR]

Neutron stars in low mass X-ray binaries (LMXBs) are hypothesised to emit continuous gravitational waves that may be detectable by ground-based observatories. The torque balance model predicts that a higher accretion rate produces larger-amplitude gravitational waves, hence LMXBs with high X-ray flux are promising targets for gravitational wave searches. The detection of X-ray pulsations would identify the spin frequency of these neutron stars, and thereby improve the sensitivity of continuous gravitational-wave searches by reducing the volume of the search parameter space. We perform a semi-coherent search for pulsations in the two LMXBs Scorpius X-1 (Sco X-1) and Cygnus X-2 (Cyg X-2) using X-ray data from the Rossi X-ray Timing Explorer Proportional Counter Array. We find no clear evidence for pulsations, and obtain upper limits (at 90 per cent confidence) on the fractional pulse amplitude, with the most stringent being 0.034 per cent for Sco X-1 and 0.23 per cent for Cyg X-2. These upper limits improve upon those in previous work by factors of ∼8.2 and ∼1.6, respectively. [ABSTRACT FROM AUTHOR]