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A systematic comparison of the models of the circumgalactic medium (CGM) and their observables is crucial to understanding the predictive power of the models and constraining physical processes that affect the thermodynamics of CGM. This paper compares four analytic CGM models: precipitation, isentropic, cooling flow, and baryon pasting models for the hot, volume-filling CGM phase, all assuming hydrostatic or quasi-hydrostatic equilibrium. We show that for fiducial parameters of the CGM of a Milky Way (MW)-like galaxy (⁠|$M_{\rm vir} \sim 10^{12}~{\rm M}_{\odot }$| at |$z\sim 0$|⁠), the thermodynamic profiles – entropy, density, temperature, and pressure – show most significant differences between different models at small (⁠|$r\lesssim 30$|  kpc) and large scales (⁠|$r\gtrsim 100$|  kpc) while converging at intermediate scales. The slope of the entropy profile, which is one of the most important differentiators between models, is |$\approx 0.8$| for the precipitation and cooling flow models, while it is |$\approx 0.6$| and 0 for the baryon pasting and isentropic models, respectively. We make predictions for various observational quantities for an MW mass halo for the different models, including the projected Sunyaev–Zeldovich effect, soft X-ray emission (0.5–2 keV), dispersion measure, and column densities of oxygen ions (O  vi , O  vii , and O  viii) observable in absorption. We provide Python packages to compute the thermodynamic and observable quantities for the different CGM models. [ABSTRACT FROM AUTHOR]

The origins of quasi-periodic eruptions (QPEs) are poorly understood, although most theoretical explanations invoke an accretion disc around a supermassive black hole. The gas and stellar environments in the galactic nuclei of these sources are also poorly constrained. In this paper, we present an analysis of archival Hubble Space Telescope (HST) images to study the narrow-line [O  iii ] emission in the QPE source GSN 069. We find strong evidence for a compact nuclear [O  iii ] emission region of size ≲ 35 pc, overlaid on top of extended [O  iii ] emission up to 1 kpc away from the nucleus. The age of the accretion system is estimated to be between 10 and 100 yr. The [O  iii ] luminosity of the compact region was measured to be |$(2.1 \pm 0.3) \times 10^{40}\, \rm erg\, s^{-1}$|⁠. Based on cloudy simulations, we constrain that the [O  iii ] emitting gas has a hydrogen number density in the range |$2 \times 10^{3} \lt n_{\rm H} \lesssim 10^{8}\, \rm cm^{-3}$| and volume filling factor f V < 4 × 10−3. We suggest that the dense gas in the nuclear region of GSN 069 originates from molecular clouds (with total mass |$\gtrsim 3 \times 10^{3}\, {\rm M}_{\odot }$|⁠), which are freshly ionized by the soft X-ray photons from the accretion disc. We predict possible evolution of the compact narrow-line region on emission-line diagnostic diagrams, and hence future HST or integral-field unit observations can be used to further pin down the age of this puzzling system. [ABSTRACT FROM AUTHOR]

We analyse the X-ray spectrum of the black hole (BH) X-ray binary MAXI J1820 + 070 using observations from XMM-Newton and NuSTAR during 'hard' states of its 2018–2019 outburst. We take a fully Bayesian approach, and this is one of the first papers to present a fully Bayesian workflow for the analysis of an X-ray binary X-ray spectrum. This allows us to leverage the relatively well-understood distance and binary system properties (like inclination and BH mass), as well as information from the XMM-Newton RGS data to assess the foreground X-ray absorption. We employ a spectral model for a 'vanilla' disc-corona system: the disc is flat and in the plane perpendicular to the axis of the jet and the BH spin, the disc extends inwards to the innermost stable circular orbit around the BH, and the (non-thermal) hard X-ray photons are up-scattered soft X-ray photons originating from the disc thermal emission. Together, these provide tight constraints on the spectral model and, in combination with the strong prior information about the system, mean we can then constrain other parameters that are poorly understood such as the disc colour correction factor. By marginalizing over all the parameters, we calculate a posterior density for the BH spin parameter, a. Our modelling suggests a preference for low or negative spin values, although this could plausibly be reproduced by higher spins and a modest degree of disc truncation. This approach demonstrates the efficacy and some of the complexities of Bayesian methods for X-ray spectral analysis. [ABSTRACT FROM AUTHOR]

One of the most prominent problems of optical/ultraviolet (UV) tidal disruption events (TDEs) is the origin of their optical/UV emission. It has been proposed that the soft X-rays produced by the stellar debris accretion disc can be reprocessed into optical/UV photons by a surrounding optically thick envelope or outflow. However, there is still no detailed models for this mechanism. In this paper, by performing hydrodynamic simulations with radiative transfer, we calculate the optical/UV emission of the circularized stellar debris accretion flow/outflow system. We find that the optical/UV photons can be generated by reprocessing the emission of the accretion flow in the optically thick outflows. The model can well interpret the observed emission properties of optical/UV TDEs, including the emission radius, the radiation temperature, and the blackbody luminosity, as well as the evolution of these quantities with time, providing a strong theoretical basis for understanding the origin of optical/UV TDEs. [ABSTRACT FROM AUTHOR]

Research into OJ 287 has been ongoing for many years. In 2020 April–June, this source underwent the second highest X-ray outburst (second only to the 2016–2017 outburst) and the mechanism of this outburst is still under debate. In this paper, we discuss two scenarios to explore the origin of the outburst: an after-effect of a black hole–disc impact and a tidal disruption event (TDE). We present the weak correlations of the spectral index versus X-ray flux and the hardness ratio (HR) versus the soft X-ray flux during the outburst, and these features are different from the case in the quiescent state. The correlations are compared with those of the 2016–2017 outburst with the highest X-ray flux in monitoring history. Analysis of the outbursts in 2016–2017 and 2020 shows that the expected time of the X-ray outburst, based on the theory of the after-effect of the black hole–disc impact and the estimation of available data, is inconsistent with historical observations. The soft X-ray spectra, the barely temporal evolution of colour, and the evolution of the HR mean that the 2020 outburst shares similar features with the 2016–2017 outburst, which was considered as a possible candidate for a TDE. Additionally, we find that the predictions of full TDEs (t −5/3) and partial TDEs (t −9/4) for the soft X-ray decay light curve are well fitted. Our analysis suggests that the 2020 outburst in OJ 287 is probably related to the TDE candidate. [ABSTRACT FROM AUTHOR]

Star formation accretion on to the super massive black holes (SMBHs) and feedback processes link the evolution of galaxies with their SMBHs. X-ray observations in the imaging and spectral regime have revealed to be an important tool to study the connection between nuclear activity and circumnuclear star formation in nearby active galactic nuclei (AGN). X-ray spectral diagnostics can lead to disentangle, which is the ionization mechanism in the central kiloparsecs of nearby AGN. In this paper, we investigate, by means of XMM-Newton observations, the nuclear X-ray emission of the changing-look AGN NGC 1566 during its quiescent state. European Photon Imaging Camera (EPIC) soft X-ray images show an extended arm-like morphology. A detailed analysis of the high-resolution Reflection Grating Spectrometer (RGS) and medium resolution EPIC spectra shows that the material concentrated in the central kiloparsecs region is not in the photoionization-dominated regime while the presence of collisionally ionized plasma is needed to explain the observed spectral features. In the inner region (∼1.5 kpc), the brightness of the collisionally ionized gas is comparable to the brightness of the photoionized gas: the inner region hosts a luminous starburst whose ionized gas shows up bright in the X-rays when the AGN is in quiescence. [ABSTRACT FROM AUTHOR]

This paper provides a new analysis of ROSAT observations of the Coma cluster, to determine the amount of soft X-ray radiation in excess of the contribution from the hot intracluster medium (ICM). The re-analysis is made possible by a high-resolution study of the hot ICM with the XMM–Newton and Planck telescopes out to the cluster's virial radius. The analysis confirms the original findings of a strong excess of soft X-ray radiation, which is likely to be of thermal origin. We find quantitative agreement between the detected soft excess and the physical characteristics of warm–hot intergalactic medium (WHIM) filaments seen in hydrodynamical simulations. We conclude that the most plausible explanation for the soft excess is the presence of ∼10 Mpc long filaments at log  T (K) ≃ 6, with a baryon overdensity of ∼300, converging towards the Coma cluster. This interpretation therefore provides support for the identification of the missing low-redshift baryons with WHIM filaments, as predicted by numerical simulations. [ABSTRACT FROM AUTHOR]

We present results from radio and X-ray observations of the X-ray transient MAXI J1810−222. The nature of the accretor in this source has not been identified. In this paper, we show results from a quasi-simultaneous radio and X-ray monitoring campaign taken with the Australia Telescope Compact Array, the Neil Gehrels Swift Observatory X-ray Telescope (XRT) , and the Swift Burst Alert Telescope. We also analyse the X-ray temporal behaviour using observations from the Neutron star Interior Composition Explorer. Results show a seemingly peculiar X-ray spectral evolution of MAXI J1810−222 during this outburst, where the source was initially only detected in the soft X-ray band for the early part of the outburst. Then, ∼200 d after MAXI J1810−222 was first detected the hard X-ray emission increased and the source transitioned to a long-lived (∼1.5 yr) bright, harder X-ray state. After this hard state, MAXI J1810−222 returned back to a softer state, before fading and transitioning again to a harder state and then appearing to follow a more typical outburst decay. From the X-ray spectral and timing properties, and the source's radio behaviour, we argue that the results from this study are most consistent with MAXI J1810−222 being a relatively distant (≳6 kpc) black hole X-ray binary. A sufficiently large distance to source can simply explain the seemingly odd outburst evolution that was observed, where only the brightest portion of the outburst was detectable by the all-sky XRTs. [ABSTRACT FROM AUTHOR]

Symbiotic X-ray binaries are systems hosting a neutron star accreting form the wind of a late-type companion. These are rare objects and so far only a handful of them are known. One of the most puzzling aspects of the symbiotic X-ray binaries is the possibility that they contain strongly magnetized neutron stars. These are expected to be evolutionary much younger compared to their evolved companions and could thus be formed through the (yet poorly known) accretion induced collapse of a white dwarf. In this paper, we perform a broad-band X-ray and soft γ-ray spectroscopy of two known symbiotic binaries, Sct X−1 and 4U 1700+24, looking for the presence of cyclotron scattering features that could confirm the presence of strongly magnetized NSs. We exploited available Chandra ,  Swift , and NuSTAR data. We find no evidence of cyclotron resonant scattering features (CRSFs) in the case of Sct X−1 but in the case of 4U 1700+24 we suggest the presence of a possible CRSF at ∼16 keV and its first harmonic at ∼31 keV, although we could not exclude alternative spectral models for the broad-band fit. If confirmed by future observations, 4U 1700+24 could be the second symbiotic X-ray binary with a highly magnetized accretor. We also report about our long-term monitoring of the last discovered symbiotic X-ray binary IGR J17329−2731 performed with Swift /XRT. The monitoring revealed that, as predicted, in 2017 this object became a persistent and variable source, showing X-ray flares lasting for a few days and intriguing obscuration events that are interpreted in the context of clumpy wind accretion. [ABSTRACT FROM AUTHOR]

Young solar-type stars are known to be strong X-ray emitters and their X-ray spectra have been widely studied. X-rays from the central star may play a crucial role in the thermodynamics and chemistry of the circumstellar material as well as in the atmospheric evolution of young planets. In this paper, we present model spectra based on spectral parameters derived from the observations of young stars in the Orion nebula cluster from the Chandra Orion Ultradeep Project (COUP). The spectra are then used to calculate new photoevaporation prescriptions that can be used in disc and planet population synthesis models. Our models clearly show that disc wind mass loss rates are controlled by the stellar luminosity in the soft (⁠|$100\, \mathrm{eV}$| to |$1\, \mathrm{keV}$|⁠) X-ray band. New analytical relations are provided for the mass loss rates and profiles of photoevaporative winds as a function of the luminosity in the soft X-ray band. The agreement between observed and predicted transition disc statistics moderately improved using the new spectra, but the observed population of strongly accreting large cavity discs can still not be reproduced by these models. Furthermore, our models predict a population of non-accreting transition discs that are not observed. This highlights the importance of considering the depletion of millimetre-sized dust grains from the outer disc, which is a likely reason why such discs have not been detected yet. [ABSTRACT FROM AUTHOR]

Charge exchange has been recognized as a primary source of soft X-ray emission in many astrophysical outflow environments, including cometary and planetary exospheres impacted by the solar wind. Some models have been set up by using different data collections of charge-exchange cross-sections. However, multiple-electron transfer has not been included in these models. In this paper, we set up a charge-exchange model with the inclusion of double-electron capture (DEC), and make a detailed investigation of this process on X-ray emissions of highly charged carbon, nitrogen, oxygen, and neon ions by using available experimental cross-sections. We also study the effect of different n- selective cross-sections on soft X-ray emission by using available experimental n -distributions. This work reveals that DEC enhancement on line intensity is linearly proportional to the ratio of ion abundance in the solar wind. It is more obvious for soft X-rays from carbon ions (C4+) in collision with CO2, and the enhancement on line intensity can be up to 53 per cent with typical ion abundances [ Advanced Composition Explorer (ACE)] in the solar wind. The synthetic spectra with parameters from the Ulysses mission for the solar wind reveal velocity dependence, target dependence, as well as the non-negligible contribution from the DEC. [ABSTRACT FROM AUTHOR]

Symbiotic stars are long-period interacting binaries where the compact object, most commonly a white dwarf, is embedded in the dense stellar wind of an evolved companion star. Ultraviolet and soft X-ray emission of the accretion disc and the nuclear-burning white dwarf plays a major role in shaping the ionization balance of the surrounding wind material, giving rise to the rich line emission. In this paper, we employ two-dimensional photoionization calculations based on the cloudy code to study the ionization state of the circumbinary material in symbiotic systems and to predict their emission-line spectra. Our simulations are parametrized via the orbital parameters of the binary and the wind mass-loss rate of the donor star, while the mass accretion rate, temperature and luminosity of the white dwarf are computed self-consistently. We explore the parameter space of symbiotic binaries and compute luminosities of various astrophysically important emission lines. The line ratios are compared with traditional diagnostic diagrams used to distinguish symbiotic binaries from other types of sources, and it is shown how the binary system parameters shape these diagrams. In the significant part of the parameter space, the wind material is nearly fully ionized, except for the 'shadow' behind the donor star, so the white dwarf emission is typically freely escaping the system. [ABSTRACT FROM AUTHOR]

θ Car is a bright well studied B0 type star in a close binary (P orb = 2.2 d) with an unseen companion. The system has enjoyed extensive spectroscopic characterization at multiple wavelengths, revealing narrow X-ray lines, a soft X-ray tail, and chemical peculiarities. The star was recently observed in high-cadence photometric time-series by the NASA TESS satellite. In this paper, we investigate the photometric variability and demonstrate that it is likely associated with surface brightness inhomogeneities rather than orbital modulation. We investigate the potential distribution of surface features required to produce the observed variability, and report a new rotational period of P rot = 2.2034 d to be commensurate with the orbital period. [ABSTRACT FROM AUTHOR]

IGR J17091−3624 is a transient galactic black hole which has a distinct quasi-periodic variability known as 'heartbeat', similar to the one observed in GRS 1915 + 105. In this paper, we report the results of ∼125 ks AstroSat observations of this source during the 2016 outburst. For the first time, a double-peaked QPO (DPQ) is detected in a few time segments of this source with a difference of δ f   ∼ 12 mHz between the two peaks. The nature of the DPQ was studied based on hardness ratios and using the static as well as the dynamic power spectrum. Additionally, a low-frequency (25–48 mHz) 'heartbeat' single-peak QPO (SPQ) was observed at different intervals of time along with harmonics (50–95 mHz). Broad-band spectra in the range 0.7–23 keV, obtained with Soft X-ray Telescope and Large Area X-ray Proportional Counter , could be fitted well with combination of a thermal Comptonization and a multicolour disc component model. During AstroSat observation, the source was in the soft-intermediate state (SIMS) as observed with Swift /XRT. We present a comparative study of the 'heartbeat' state variability in IGR J17091−3624 with GRS 1915 + 105. Significant difference in the timing properties is observed although spectral parameters (Γ ∼ 2.1–2.4 and T max ∼ 0.6–0.8 keV) in the broad energy band remain similar. Spectral properties of segments exhibiting SPQ and DPQ are further studied using simple phase-resolved spectroscopy which does not show a significant difference. Based on the model parameters, we obtain the maximum ratio of mass accretion rate in GRS 1915 + 105 to that in IGR J17091−3624 as ∼25: 1. We discuss the implications of our findings and comment on the physical origin of these exotic variabilities. [ABSTRACT FROM AUTHOR]

The North Polar Spur (NPS) is one of the largest structures observed in the Milky Way in both the radio and soft X-rays. While several predictions have been made regarding the origin of the NPS, modelling the structure is difficult without precise distance constraints. In this paper, we determine accurate distances to the southern terminus of the NPS and towards latitudes ranging up to 55°. First, we fit for the distance and extinction to stars towards the NPS using optical and near-infrared photometry and Gaia Data Release 2 astrometry. We model these per-star distance–extinction estimates as being caused by dust screens at unknown distances, which we fit for using a nested sampling algorithm. We then compare the extinction to the Spur derived from our 3D dust modelling with integrated independent measures from XMM–Newton X-ray absorption and H  i column density measures. We find that we can account for nearly 100 per cent of the total column density of the NPS as lying within 140 pc for latitudes >26° and within 700 pc for latitudes <11°. Based on the results, we conclude that the NPS is not associated with the Galactic Centre or the Fermi bubbles. Instead, it is likely associated, especially at higher latitudes, with the Scorpius–Centaurus association. [ABSTRACT FROM AUTHOR]

The great power offered by photoionization models of active galactic nuclei emission line regions has long been mitigated by the fact that very little is known about the spectral energy distribution (SED) between the Lyman limit, where intervening absorption becomes a problem, and  0.3 keV, where soft X-ray observations become possible. The emission lines themselves can, to some degree, be used to probe the SED, but only in the broadest terms. This paper employs a new generation of theoretical SEDs that are internally self-consistent, energy conserving, and tested against observations, to infer properties of the emission-line regions. The SEDs are given as a function of the Eddington ratio, allowing emission-line correlations to be investigated on a fundamental basis. We apply the simplest possible tests, based on the foundations of photoionization theory, to investigate the implications for the geometry of the emission-line region. The SEDs become more far-ultraviolet bright as the Eddington ratio increases, so the equivalent widths of recombination lines should also become larger, an effect that we quantify. The observed lack of correlation between Eddington ratio and equivalent width shows that the cloud covering factor must decrease as Eddington ratio increases. This would be consistent with recent models proposing that the broad-line region is a failed dusty wind off the accretion disc. [ABSTRACT FROM AUTHOR]

Accretion discs around stellarmass black holes (BHs) emit radiation peaking in the soft X-rays when the source is in the thermal state. The emerging photons are polarized and, for symmetry reasons, the polarization integrated over the source is expected to be either parallel or perpendicular to the (projected) disc symmetry axis, because of electron scattering in the disc. However, due to general relativity effects photon polarization vectors will rotate with respect to their original orientation, by an amount depending on both the BH spin and the observer's inclination. Hence, X-ray polarization measurements may provide important information about strong gravity effects around these sources. Along with the spectral and polarization properties of radiation which reaches directly the observer once emitted from the disc, in this paper we also include the contribution of returning radiation, i.e. photons that are bent by the strong BH gravity to return again on the disc, where they scatter until eventually escaping to infinity. A comparison between our results and those obtained in previous works by different authors shows an overall good agreement, despite the use of different code architectures. We finally consider the effects of absorption in the disc material by including more realistic albedo profiles for the disc surface. Our findings in this respect show that considering also the ionization state of the disc may deeply modify the behaviour of polarization observables. [ABSTRACT FROM AUTHOR]

In the context of time domain astronomy, we present an offline detection search of gamma-ray transients using a wild binary segmentation analysis called F-WBSB targeting both short and long gamma-ray bursts (GRBs) and covering the soft and hard gamma-ray bands. We use NASA Fermi /GBM archival data as a training and testing data set. This paper describes the analysis applied to the 12 NaI detectors of the Fermi /GBM instrument. This includes background removal, change-point detection that brackets the peaks of gamma-ray flares, the evaluation of significance for each individual GBM detector, and the combination of the results among the detectors. We also explain the calibration of the ∼ 10 parameters present in the method using one week of archival data. Finally, we present our detection performance result for 60 d of a blind search analysis with F-WBSB by comparing to both the onboard and offline GBM search as well as external events found by others surveys such as Swift -BAT. We detect 42/44 onboard GBM events but also other gamma-ray flares at a rate of 1 per hour in the 4–50 keV band. Our results show that F-WBSB is capable of recovering gamma-ray flares, including the detection of soft X-ray long transients. FWBSB offers an independent identification of GRBs in combination with methods for determining spectral and temporal properties of the transient as well as localization. This is particularly useful for increasing the GRB rate and that will help the joint detection with gravitational-wave events. [ABSTRACT FROM AUTHOR]

In the last years, a peculiarity of some Be stars – their association with unusually hard and intense X-ray emission – was shown to extend beyond a mere few cases. In this paper, we continue our search for new cases by performing a limited survey of 18 Be stars using XMM–Newton. The targets were selected either on the basis of a previous X-ray detection (exosat,rosat , XMM -slew survey) without spectral information available or because of the presence of a peculiar spectral variability. Only two targets remain undetected in the new observations and three other stars display only faint and soft X-rays. Short-term and/or long-term variations were found in one third of the sample. The spectral characterization of the X-ray brightest 13 stars of the sample led to the discovery of three new γ Cas (HD 44458, HD 45995, and V558 Lyr), bringing the total to 25 known cases, and another γ Cas candidate (HD 120678), bringing the total to two. [ABSTRACT FROM AUTHOR]

Observationally, the X-ray spectrum (0.5–10 keV) of low-level accreting neutron stars (NSs) (⁠|$L_{\rm 0.5{-}10\,\rm kev}\lesssim 10^{36}\ \rm erg \ s^{-1}$|⁠) can generally be well fitted by the model with two components, i.e., a thermal soft X-ray component plus a power-law component. Meanwhile, the fractional contribution of the power-law luminosity η (⁠|$\eta \equiv L^{\rm power\ law}_{\rm 0.5{-}10\,\rm kev}/L_{\rm 0.5{-}10\,\rm kev}$|⁠) varies with the X-ray luminosity |$L_{\rm 0.5{-}10\,\rm kev}$|⁠. In this paper, we systematically investigate the origin of such X-ray emission within the framework of the advection-dominated accretion flow (ADAF) around a weakly magnetized NS, in which the thermal soft X-ray component arises from the surface of the NS and the power-law component arises from the ADAF itself. We test the effects of the viscosity parameter α in the ADAF and thermalized parameter f th (describing the fraction of the ADAF energy released at the surface of the NS as thermal emission) on the relation of η versus |$L_{\rm 0.5{-}10\,\rm kev}$|⁠. It is found that η is nearly a constant (∼zero) with |$L_{\rm 0.5{-}10\,\rm kev}$| for different α with f th = 1, which is inconsistent with observations. Meanwhile, it is found that a change of f th can significantly change the relation of η versus |$L_{\rm 0.5{-}10\,\rm kev}$|⁠. By comparing with a sample of non-pulsating NS-low mass X-ray binaries probably dominated by low-level accretion on to NSs, it is found that a small value of f th ≲ 0.1 is needed to match the observed range of |$\eta \gtrsim 10{{\ \rm per\ cent}}$| in the diagram of η versus |$L_{\rm 0.5{-}10\,\rm kev}$|⁠. Finally, we argue that the small value of f th ≲ 0.1 implies that the radiative efficiency of NSs with an ADAF accretion may not be as high as the predicted result previously of |$\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$| despite the existence of the hard surface. [ABSTRACT FROM AUTHOR]

Quasiperiodic eruptions (QPEs) from low-mass galaxy centres may result from accretion from a white dwarf in a very eccentric orbit about the central massive black hole. Evolution under gravitational radiation losses reduces the separation and eccentricity. I note that below a critical eccentricity e crit ≃ 0.97, the accretion disc's viscous time-scale at pericentre passage is probably longer than the orbital period P , and periodic eruptive behaviour is no longer possible. These QPE descendant systems (QPEDs) are then likely to produce quasiperiodic oscillations rather than eruptions, varying more smoothly over the orbital cycle, with duty cycles ∼1. I identify 2XMM J123103.2+110648 (P ≃ 3.9 h) and (more tentatively) RE J1034+396 (P ≃ 1 h) as candidate systems of this type, and find agreement with their deduced eccentricities e < e crit. The absence of eruptions and the lower accretion luminosities resulting from the smaller gravitational radiation losses may make QPED systems harder to discover. Ultimately they must evolve to have viscous times much longer than the orbital period, and either remain steady, or possibly have infrequent but large outbursts. The latter systems would be massive analogues of the soft X-ray transients produced by low stellar-mass X-ray binaries. [ABSTRACT FROM AUTHOR]

We present measurements of the soft X-ray background emission for 130 Suzaku observations at 75° < l < 285° and | b | > 15° obtained from 2005 to 2015, covering nearly one solar cycle. In addition to the standard soft X-ray background model consisting of the local hot bubble and the Milky Way Halo (MWH), we include a hot collisional-ionization-equilibrium component with a temperature of ∼0.8 keV to reproduce spectra of a significant fraction of the lines of sight. Then, the scatter in the relation between the emission measure vs. temperature of the MWH component is reduced. Here, we exclude time ranges with high count rates to minimize the effect of the solar wind charge exchange (SWCX). However, the spectra of almost the same lines of sight are inconsistent. The heliospheric SWCX emissions likely contaminate and give a bias in measurements of temperature and the emission measure of the MWH. Excluding the data around the solar maximum and using the data taken before the end of 2009, at | b | > 35° and 105° < l < 255°, the temperature (0.22 keV) and emission measure (2 × 10−3 cm−6 pc) of the MWH are fairly uniform. The increase of the emission measure toward the lower Galactic latitude at | b | < 35° indicates the presence of a disk-like morphology component. A composite model which consists of disk-like and spherical-morphology components also reproduces the observed emission measure distribution of MWH. In this case, the hydrostatic mass at a few tens of kiloparsec from the Galactic center agrees with the gravitational mass of the Milky Way. The plasma with the virial temperature likely fills the Milky Way halo in nearly hydrostatic equilibrium. Assuming a gas metallicity of 0.3 solar, the upper limit of the gas mass of the spherical component out to 250 kpc, or the virial radius, is ∼ a few × 1010  M ⊙. [ABSTRACT FROM AUTHOR]

This paper is a sequel to the extensive study of warm absorber (WA) in X-rays carried out using high-resolution grating spectral data from XMM-Newton satellite (WAX-I). Here we discuss the global dynamical properties as well as the energetics of theWA components detected in the WAX sample. The slope of WA density profile (n ∝ r−α) estimated from the linear regression slope of ionization parameter ξ and column density NH in the WAX sample is α = 1.236 ± 0.034. We find that the WA clouds possibly originate as a result of photoionized evaporation from the inner edge of the torus (torus wind). They can also originate in the cooling front of the shock generated by faster accretion disc outflows, the ultrafast outflows, impinging on to the interstellar medium or the torus. The acceleration mechanism for the WA is complex and neither radiatively driven wind nor MHD-driven wind scenario alone can describe the outflow acceleration. However, we find that radiative forces play a significant role in accelerating the WA through the soft X-ray absorption lines, and also with dust opacity. Given the large uncertainties in the distance and volume filling factor estimates of the WA, we conclude that the kinetic luminosity ĖK ofWA may sometimes be large enough to yield significant feedback to the host galaxy. We find that the lowest ionization states carry the maximum mass outflow, and the sources with higher Fe M UTA absorption (15−17 Å) have more mass outflow rates. [ABSTRACT FROM AUTHOR]

The TJ-II stellarator presently in operation at the National Laboratory of Fusion is subject to radiological surveillance in compliance with the Spanish Nuclear Regulation. During its pulsed operation hard X-rays are emitted by the runaway electrons, and soft X-rays are generated by bremsstrahlung of thermalized electrons. Inside the experimental hall, the plasma heating systems and some active diagnostics are additional sources of radiation. Outside the experimental hall, some auxiliary systems, such as high-voltage power supplies, must be checked for radiological influence. In order to verify the adequate radiological classification of the different working areas, 15 dosemeters are distributed in radiologically controlled areas close to the machine, as well as in areas of public use in the TJ-II building. The integrated dose equivalent is analyzed to help establish the range of radiological influence of the stellarator. The results of the period 2015–20 are presented and analyzed. [ABSTRACT FROM AUTHOR]

Utilizing Chandra, XMM-Newton, and NuSTAR, a wide-band X-ray spectrum from 0.2 to 20 keV is reported from the western hotspot of Pictor A. In particular, the X-ray emission is significantly detected in the 3 to 20 keV band at 30σ by NuSTAR. This is the first detection of hard X-rays with energies above 10 keV from a jet termination hotspot of active galactic nuclei. The hard X-ray spectrum is well described with a power-law model with a photon index of Γ = 1.8 ± 0.2, and the flux is obtained to be (4.5 ± 0.4) × 10−13 erg s−1 cm−2 in the 3 to 20 keV band. The obtained spectrum is smoothly connected with those soft X-ray spectra observed by Chandra and XMM-Newton. The wide-band spectrum shows a single power-law spectrum with a photon index of Γ = 2.07 ± 0.03, excluding any cut-off/break features. Assuming the X-rays to be synchrotron radiation of the electrons, the energy index of the electrons is estimated as p = 2Γ − 1 = 3.14 ± 0.06 from the wide-band spectrum. Given that the X-ray synchrotron-emitting electrons quickly lose their initial energies via synchrotron radiation, the energy index of electrons at acceleration sites is estimated as p acc = p − 1 = 2.14 ± 0.06. This is consistent with the prediction of the diffusive shock acceleration. Since the spectrum has no cut-off feature up to 20 keV, the maximum electron energy is estimated to be no less than 40 TeV. [ABSTRACT FROM AUTHOR]

We report the results of AstroSat observations of Cygnus X-2 during 2016 February. The source's power density spectrum generated using Large Area X-ray Proportional Counter (LAXPC) data revealed the presence of a prominent quasi-periodic oscillation (QPO) at ∼42 Hz with broad-band continuum noise at lower frequencies at ∼10 Hz. The large effective area of LAXPC at ≳30 keV allowed for an unprecedented study of the energy dependence of the QPO and the broad noise continuum. The fractional rms increases with energy, and its shape is similar for both the QPO and the continuum noise, suggesting a common radiative origin. However, while the QPO exhibits hard time-lags, with the high-energy photons lagging the low ones by a few milliseconds, the continuum noise shows the opposite behaviour. The photon spectrum from Soft X-ray Imaging Telescope and LAXPC in 0.7–30 keV band comprises the soft component from a disc and a hard Comptonized component from a hot corona. While the energy dependence of the rms shows that the QPO and the continuum noise variability are dominated by the Comptonized component, the change in sign of the time-lag suggests that the dynamic origin of the QPO may be in the disc while the noise continuum may originate from the corona. [ABSTRACT FROM AUTHOR]

We report on the results of our simultaneous observations of three large stellar flares with soft X-rays (SXRs) and an Hα emission line from two binary systems of RS CVn type. The energies released in the X-ray and Hα emissions during the flares were 1036–1038 and 1035–1037 erg, respectively. This renders the set of the observations as the first successful simultaneous X-ray/Hα observations of the stellar flares with energies above 1035 erg; although the coverage of the Hα observations was limited, with |$\sim\! 10\%$| of the e -folding time in the decay phase of the flares, that of the SXR ones was complete. Combining the obtained physical parameters and those in literature for solar and stellar flares, we obtained a good proportional relation between the emitted energies of X-ray and Hα emissions for a flare energy range of 1029–1038 erg. The ratio of the Hα-line to bolometric X-ray emissions was ∼0.1, where the latter was estimated by converting the observed SXR emission to that in the 0.1–100 keV band according to the best-fitting thin thermal model. We also found that the e -folding times of the SXR and Hα light curves in the decaying phase of a flare are in agreement for a time range of 1–104 s. Even very large stellar flares with energies of six orders of magnitude larger than the most energetic solar flares follow the same scaling relationships with solar and much less energetic stellar flares. This fact suggests that their physical parameters can be estimated on the basis of the known physics of solar and stellar flares. [ABSTRACT FROM AUTHOR]

Aql X-1 is the most prolific low-mass X-ray binary transient hosting a neutron star. In this paper we focus on the return to quiescence following the 2010 outburst of the source. This decay was monitored thanks to 11 pointed observations taken with XMM–Newton, Chandra and Swift. The decay from outburst to quiescence is very fast, with an exponential decay characteristic time-scale of ∼2 d. Once in quiescence the X-ray flux of Aql X-1 remained constant, with no further signs of variability or decay. The comparison with the only other well-monitored outburst from Aql X-1 (1997) is tale-telling. The luminosities at which the fast decay starts are fully compatible for the two outbursts, hinting at a mechanism intrinsic to the system and possibly related to the neutron star rotation and magnetic field (i.e. the propeller effect). In addition, for both outbursts, the decay profiles are also very similar, likely resulting from the shut-off of the accretion process on to the neutron star surface. Finally, the quiescent neutron star temperatures at the end of the outbursts are well consistent with one another, suggesting a hot neutron star core dominating the thermal balance. Small differences in the quiescent X-ray luminosity among the two outbursts can be attributed to a different level of the power-law component. [ABSTRACT FROM AUTHOR]

This is the first report on analyzing the chemical state of Li-ion battery electrodes at different states of charge by using a wavelength-dispersive spectrometer, which has a two-order improved energy resolution in the soft X-ray energy region compared with that of a conventional energy-dispersive X-ray analyzer. Electrodes containing LiMn1.5Ni0.5O4 were charged to prepare Li0.5Mn1.5Ni0.5O4 and λ-Mn0.75Ni0.25O2. The soft X-ray emission spectra obtained from those materials show that the O-K emission signal was drastically decreased throughout the charging process. This suggests that O-2p electron contributed to the electrochemical oxidation. The density of states and Bader charge evaluated from ab initio calculation support this result. [ABSTRACT FROM AUTHOR]

We report results of an analysis of the black hole (BH) candidate source X1755−338 made in 1989, 1990, and 1991 with Ginga, and in 1995 with ASCA. The spectra were well represented by a model consisting of a soft thermal emission from an accretion disk and a hard X-ray tail. The normalization of the multi-color disk, related to the inner disk radius, was similar to each other. The unabsorbed X-ray fluxes from the disk component in the 0.01–10 keV band were estimated to be 1.3 × 10−9, 3.0 × 10−9, 9.8 × 10−10, and 2.4 × 10−9 erg s−1 cm−2 in 1989, 1990, 1991, and 1995, respectively, and are proportional to |$kT_{\rm in}^4$|⁠ , where kT in is a temperature at the inner disk radius. Based on the standard accretion disk model for a non-rotating BH, our results suggest either a small BH mass or a large inclination angle. Otherwise, X1755−338 is a rotating BH. The hard X-ray intensity was found to be variable, while the soft X-ray intensity was stable. Although previous work showed the existence of an iron line at 6.7 keV, no clear iron line feature was found in any of the spectra. We infer that most of the iron line flux reported in the previous work was due to contamination of the Galactic diffuse X-ray emission. [ABSTRACT FROM AUTHOR]

We report the first detection of an Fe Kα line and soft X-ray lag in the ultraluminous X-ray (ULX) source NGC 7456 ULX-1. The XMM–Newton spectra show the presence of the 6.4 keV Fe line at 2.6σ confidence and an upper limit on the FWHM of 32900 km s−1. Assuming that the line arises by reflection from a Keplerian disk, it must originate beyond 85 r g from the compact object. As a result of Fourier timing analysis, we found that the soft X-ray photons lag behind the hard X-ray photons with a ∼1300 s delay. The covariance spectra indicate that the hard spectral component is responsible for the correlated variability and the soft X-ray lag. This is the second ULX in which an Fe Kα line is found, the fifth with a soft X-ray lag, and the first with both features detected. [ABSTRACT FROM AUTHOR]

I report the discovery of a transient broad-H α point source in the outskirts of the giant elliptical galaxy NGC 1404, discovered in archival observations taken with the Multi-Unit Spectroscopic Explorer (MUSE) integral field spectrograph. The H α line width of 1950 km s−1 full width at half-maximum, and luminosity of (4.1 ± 0.1) × 1036 erg s−1, are consistent with a nova outburst, and the source is not visible in MUSE data obtained 9 months later. A transient soft X-ray source was detected at the same position (within <1 arcsec), 14 yr before the H α transient. If the X-ray and H α emission are from the same object, the source may be a short-time-scale recurrent nova with a massive white dwarf accretor, and hence a possible Type-Ia supernova progenitor. Selecting broad-H α point sources in MUSE archival observations for a set of nearby early-type galaxies, I discovered 12 more nova candidates with similar properties to the NGC 1404 source, including five in NGC 1380 and four in NGC 4365. Multi-epoch data are available for four of these twelve sources; all four are confirmed to be transient on ∼1 yr time-scales, supporting their identification as novae. [ABSTRACT FROM AUTHOR]

We present results from the Hitomi X-ray observation of a young composite-type supernova remnant (SNR) G21.5-0.9, whose emission is dominated by the pulsar wind nebula (PWN) contribution. The X-ray spectra in the 0.8-80 keV range obtained with the Soft X-ray Spectrometer (SXS), Soft X-ray Imager, and Hard X-ray Imager (HXI) show a significant break in the continuum as previously found with the NuSTAR observation. After taking into account all known emissions from the SNR other than the PWN itself, we find that the Hitomi spectra can be fitted with a broken power law with photon indices of Γ1 = 1.74 ± 0.02 and Γ2 = 2.14 ± 0.01 below and above the break at 7.1 ± 0.3 keV, which is significantly lower than the NuSTAR result (~9.0 keV). The spectral break cannot be reproduced by time-dependent particle injection one-zone spectral energy distribution models, which strongly indicates that a more complex emission model is needed, as suggested by recent theoretical models. We also search for narrow emission or absorption lines with the SXS, and perform a timing analysis of PSR J1833-1034 with the HXI and the Soft Gamma-ray Detector. No significant pulsation is found from the pulsar. However, unexpectedly, narrow absorption line features are detected in the SXS data at 4.2345 keV and 9.296 keV with a significance of 3.65 σ. While the origin of these features is not understood, their mere detection opens up a new field of research and was only possible with the high resolution, sensitivity, and ability to measure extended sources provided by an X-ray microcalorimeter. [ABSTRACT FROM AUTHOR]

A detailed search for emission and absorption lines and an assessment of their upper limits are performed for Suzaku data. The method utilizes a matched-filtering approach to maximize the signal-to-noise ratio for a given energy resolution, which could be applicable to many types of line search. We first applied it to well-known active galactic nuclei spectra that have been reported to have ultra-fast outflows, and find that our results are consistent with previous findings at the ∼3σ level. We proceeded to search for emission and absorption features in two bright magnetars 4U 0142+61 and 1RXS J1708-4009, applying the filtering method to Suzaku data. We found that neither source showed any significant indication of line features, even using long-term Suzaku observations or dividing their spectra into spin phases. The upper limits on the equivalent width of emission/absorption lines are constrained to be a few eV at ∼1 keV and a few hundreds of eV at ∼10 keV. This strengthens previous reports that persistently bright magnetars do not show proton cyclotron absorption features in soft X-rays and, even if they exist, they would be broadened or much weaker than below the detection limit of X-ray CCD. [ABSTRACT FROM AUTHOR]

X-ray observations play a crucial role in understanding the emission mechanism and relevant physical phenomena of magnetars. We report on X-ray observations made in 2016 of a young magnetar, SGR 1900+14, which is famous for a giant flare in 1998 August. Simultaneous observations were conducted with XMM-Newton and NuSTAR on 2016 October 20 with 23 and 123 ks exposures, respectively. The NuSTAR hard X-ray coverage enabled us to detect the source up to 70 keV. The 1–10 keV and 15–60 keV fluxes were |$3.11(3)\times 10^{-12} \, {\rm erg \, s^{-1} \, cm^{-2}}$| and |$6.8(3)\times 10^{-12} \, {\rm erg \, s^{-1} \, cm^{-2}}$|⁠ , respectively. The 1–70 keV spectra were fitted well by a blackbody plus power-law model with a surface temperature of |$kT=0.52(2) \, {\rm keV}$|⁠ , a photon index of the hard power-law of Γ = 1.21(6), and a column density of |$N_{\,\rm H}=1.96(11)\times 10^{22} \, {\rm cm^{-2}}$|⁠. Compared with previous observations with Suzaku in 2006 and 2009, the 1–10 keV flux showed a decrease by 25%–40%, while the spectral shape did not show any significant change with differences of kT and N H being within 10% of each other. Through timing analysis, we found that the rotation period of SGR 1900+14 on 2016 October 20 was |$5.22669(3) \, {\rm s}$|⁠. The long-term evolution of the rotation period shows a monotonic decrease in the spin-down rate |$\dot{P}$| lasting for more than 15 yr. We also found characteristic behavior of the hard-tail power-law component of SGR 1900+14. The energy-dependent pulse profiles vary in morphology with a boundary of 10 keV. The phase-resolved spectra show the differences between photon indices (Γ = 1.02–1.44) as a function of the pulse phase. Furthermore, the photon index is positively correlated with the X-ray flux of the hard power-law component, which could not be resolved by the previous hard X-ray observations. [ABSTRACT FROM AUTHOR]

Reliable results when fitting X-ray data require proper consideration of the statistics involved. We probe the impact of Gaussian versus Poisson statistics at low count levels using both the standard χ2 method and maximum likelihood based on Poisson (C) statistics. The difference is studied and quantified through simulated spectra with known properties. We then test the results through analysis of Mn Kα calibration data taken with the flight spare microcalorimeter for the Hitomi soft X-ray spectrometer. Through comparison with simulations, our results show that the χ2 method tends to give overly optimistic estimates of the detector energy resolution, in particular when there are few counts. Given an energy resolution of ∼5 eV and a line with about 100 photons, the line width becomes ∼10% lower in the χ2 method than in Poisson statistics. This is a consequence of the uncertainties being dominated by counting statistics, and therefore highlights the need to choose the appropriate fit statistic. [ABSTRACT FROM AUTHOR]