Search

X-ray observations of active galactic nuclei (AGNs) reveal relativistic reflections from the innermost regions of accretion disks, which contain general-relativistic footprints caused by spinning supermassive black holes (SMBH). We anticipate the spin of a SMBH to be stable over the human timeframe, so brightness changes in the high-energy corona above the SMBH should slightly alter relativistic reflection. In this brief review, we discuss the latest developments in modeling relativistic reflection, as well as the rapid small variation in relativistic emission disclosed by the principal component analysis (PCA) of X-ray variability in AGN. PCA studies of X-ray spectra from AGNs have shown that relativistically blurred reflection has negligible fluctuations over the course of observations, which could originate from rapid (intrahour) intrinsic variations in near-horizon accretion flows and photon rings. The PCA technique is an effective way to disclose relativistic reflection from X-ray observations of AGNs, simplifying the complexity of largely variable X-ray data for automated spectral analysis with machine learning algorithms. [ABSTRACT FROM AUTHOR]

Aims. Cataclysmic variables (CVs) are compact binary systems in which a white dwarf accretes matter from a Roche-lobe-filling companion star. For this study we searched for new CVs in the Milky Way in the Chandra Source Catalog v2.0, cross-matched with Gaia Data Release 3 (DR3). Methods. We identified new CV candidates by combining X-ray and optical data in a color-color diagram called the X-ray main sequence. We used two different cuts in this diagram to compile pure and optically variable samples of CV candidates. We undertook optical spectroscopic follow-up observations with the Keck and Palomar Observatories to confirm the nature of these sources. Results. We assembled a sample of 25 887 Galactic X-ray sources and found 14 new CV candidates. Seven objects show X-ray and/or optical variability. All sources show X-ray luminosity in the 1029 − 1032 erg s−1 range, and their X-ray spectra can be approximated by a power-law model with photon indices in the Γ ∼ 1 − 3 range or an optically thin thermal emission model in the kT ∼ 1 − 70 keV range. We spectroscopically confirmed four CVs, discovering two new polars, one low accretion rate polar and a WZ Sge-like low accretion rate CV. X-ray and optical properties of the other nine objects suggest that they are also CVs (likely magnetic or dwarf novae), and one other object could be an eclipsing binary, but revealing their true nature requires further observations. Conclusions. These results show that a joint X-ray and optical analysis can be a powerful tool for finding new CVs in large X-ray and optical catalogs. X-ray observations such as those by Chandra are particularly efficient at discovering magnetic and low accretion rate CVs, which could be missed by purely optical surveys. [ABSTRACT FROM AUTHOR]

While much has been learned in recent decades about the X-ray emission of the extragalactic ultraluminous X-ray sources (ULXs), their radiative output in the ultraviolet (UV) band remains poorly constrained. Understanding of the full ULX spectral energy distribution (SED) is imperative to constrain the accretion flow geometry powering them, as well as their radiative power. Here we present constraints on the UV emission of the pulsating ULX (PULX) NGC 1313 X-2 based on the absence of nebular He  ii  λ4686 emission in its immediate environment. To this end, we first perform multiband spectroscopy of the ULX to derive three realistic extrapolations of the SED into the inaccessible UV, each predicting varying levels of UV luminosity. We then perform photoionization modelling of the bubble nebula and predict the He  ii  λ4686 fluxes that should have been observed based on each of the derived SEDs. We then compare these predictions with the derived upper limit on He  ii  λ4686 from the Multi-Unit Spectroscopic Explorer data, which allows us to infer a UV luminosity L UV ≲ 1 × 1039 erg s−1 in the PULX NGC 1313 X-2. Comparing the UV luminosity inferred with other ULXs, our work suggests there may be an intrinsic difference between hard and soft ULXs, either related to different mass-transfer rates and/or the nature of the accretor. However, a statistical sample of ULXs with inferred UV luminosities is needed to fully determine the distinguishing features between hard and soft ULXs. Finally, we discuss ULXs ionizing role in the context of the nebular He  ii  λ4686 line observed in star-forming metal-poor galaxies. [ABSTRACT FROM AUTHOR]

We studied the multiband properties of two ultraluminous X-ray sources (2CXO J225728.9−410211 = X-1 and 2CXO J225724.7−410343 = X-2) and their surroundings, in the spiral galaxy NGC 7424. Both sources have approached X-ray luminosities L X ∼ 1040 erg s−1 at some epochs. Thanks to a more accurate astrometric solution (based on Australia Telescope Compact Array and Gaia data), we identified the point-like optical counterpart of X-1, which looks like an isolated B8 supergiant (M ≈ 9 M ⊙, age ≈30 Myr). Instead, X-2 is in a star-forming region (size of about 100 pc × 150 pc), near young clusters and ionized gas. Very large telescope long-slit spectra show a spatially extended region of He  ii λ4686 emission around the X-ray position, displaced by about 50 pc from the brightest star cluster, which corresponds to the peak of lower ionization line emission. We interpret the He  ii λ4686 emission as a signature of X-ray photoionization from the ULX, while the other optical lines are consistent with UV ionization in an ordinary He ii region. The luminosity of this He++ nebula puts it in the same class as other classical photoionized ULX nebulae such as those around Holmberg II X-1 and NGC 5408 X-1. We locate a strong (5.5 GHz luminosity |$\nu \, L_{\nu } \approx 10^{35}$|  erg s−1), steep-spectrum, unresolved radio source at the peak of the low-ionization lines, and discuss alternative physical scenarios for the radio emission. Finally, we use WISE data to obtain an independent estimate of the reddening of the star-forming clump around X-2. [ABSTRACT FROM AUTHOR]

SMC X-2 exhibits X-ray outburst behaviour that makes it one of the most luminous X-ray sources in the Small Magellanic Cloud. In the last decade it has undergone two such massive outbursts – in 2015 and 2022. The first outburst is well reported in the literature, but the 2022 event has yet to be fully described and discussed. That is the goal of this paper. In particular, the post-peak characteristics of the two events are compared. This reveals clear similarities in decay profiles, believed to be related to different accretion mechanisms occurring at different times as the outbursts evolve. The H α emission line indicates that the Be disc undergoes complex structural variability, with evidence of warping as a result of its interaction with the neutron star. The detailed observations reported here will be important for modelling such interactions in this kind of binary systems. [ABSTRACT FROM AUTHOR]

High-mass X-ray binaries hosting red supergiant (RSG) donors are a rare but crucial phase in massive stellar evolution, with only one source previously known in the Milky Way. In this letter, we present the identification of the second Galactic RSG X-ray binary SWIFT  J0850.8−4219. We identify the source 2MASS 08504008−4211514 as the likely infrared counterpart with a chance coincidence probability ≈5 × 10−6. We present a |$1.0{\!-\!}2.5\, \mu$| m spectrum of the counterpart, exhibiting features characteristic of late-type stars and an exceptionally strong He  i emission line, corroborating the identification. Based on i) the strength of the 12CO(2,0) band, ii) strong CN bandheads and absent TiO bandheads at ≈1.1 µm and iii) equivalent width of the Mg  i |$1.71\, \mu$| m line, we classify the counterpart to be a K3–K5 type RSG with an effective temperature of 3820 ± 100 K, located at a distance of ≈12 kpc. We estimate the source X-ray luminosity to be (4 ± 1) × 1035 erg s−1, with a hard photon index (Γ < 1), arguing against a white dwarf accretor but consistent with a magnetized neutron star in the propeller phase. Our results highlight the potential of systematic near-infrared spectroscopy of Galactic hard X-ray sources in completing our census of the local X-ray binary population. [ABSTRACT FROM AUTHOR]

We present a systematic search for periodic X-ray sources in 10 Galactic globular clusters (GCs) utilizing deep archival Chandra observations. By applying the Gregory–Loredo algorithm, we detect 28 periodic signals among 27 independent X-ray sources in 6 GCs, which include 21 newly discovered ones in the X-ray band. The remaining 4 GCs exhibit no periodic X-ray sources, mainly due to a relatively lower sensitivity of the data. Through analysis of their X-ray timing and spectral properties, complemented with available optical and ultraviolet information, we identify 21 of these periodic sources as cataclysmic variables (CVs). Combining with 11 periodic CVs in 47 Tuc similarly identified in the X-ray band, we compile the most comprehensive sample to date of GC CVs with a probable orbital period. The scarcity of old, short-period CVs in GCs compared to the Galactic inner bulge and solar neighbourhood, can be attributed to both a selection effect favouring younger, dynamically formed systems and the hindrance of CV formation through primordial binary evolution by stellar dynamical interactions common to the GC environment. Additionally, we identify a significant fraction of the GC CVs, most with an orbital period below or within the CV period gap, as probable magnetic CVs, but in the meantime there is a deficiency of luminous intermediate polars in the GC sample compared to the solar neighbourhood. [ABSTRACT FROM AUTHOR]

We present a methodology for modeling the joint ionizing impact due to a "simple X-ray population" (SXP) and its corresponding simple stellar population (SSP), where "simple" refers to a single age and metallicity population. We construct composite spectral energy distributions (SEDs) including contributions from ultraluminous X-ray sources and stars, with physically meaningful and consistent consideration of the relative contributions of each component as a function of instantaneous burst age and stellar metallicity. These composite SEDs are used as input for photoionization modeling with Cloudy, from which we produce a grid for the time- and metallicity-dependent nebular emission from these composite populations. We make the results from the photoionization simulations publicly available. We find that the addition of the SXP prolongs the high-energy ionizing output from the population—and correspondingly increases the intensity of nebular lines such as He ii λ 1640,4686, [Ne v ] λ 3426,14.3 μ m, and [O iv ] 25.9 μ m by factors of at least two relative to models without an SXP spectral component. This effect is most pronounced for instantaneous bursts of star formation on timescales >10 Myr and at low metallicities (∼0.1 Z ⊙), due to the imposed time- and metallicity-dependent behavior of the SXP relative to the SSP. We propose nebular emission line diagnostics accessible with JWST suitable for inferring the presence of a composite SXP + SSP, and we discuss how the ionization signatures compare to models for sources such as intermediate-mass black holes. [ABSTRACT FROM AUTHOR]

In this paper we significantly improve estimates of distance to the X-ray binary systems Cyg X-3 and GRS 1915+105. We report a highly accurate trigonometric parallax measurement for Cyg X-3 using the Very Long Baseline Array at 43 GHz, placing the source at a distance of 9.67 − 0.48 + 0.53 kpc. We also use Galactic proper motions and line-of-sight radial velocity measurements to determine three-dimensional (3D) kinematic distances to both systems, under the assumption that they have low peculiar velocities. This yields distances of 8.95 ± 0.96 kpc for Cyg X-3 and 9.4 ± 0.6 (statistical) ± 0.8 (systematic) for GRS 1915+105. The good agreement between parallax and 3D kinematic distances validates the assumption of low peculiar velocities, and hence small natal kicks, for both of the systems. For a source with a low peculiar velocity, given its parallax distance, Cyg X-3 should have a V LSR near −64 ± 5 km s−1. Our measurements imply a slightly higher inclination angle, and hence lower black hole mass, for GRS 1915+105 than found from previous work by Reid et al. and strengthen arguments from X-ray polarization that Cyg X-3 would be an ultraluminous X-ray source if viewed face-on. [ABSTRACT FROM AUTHOR]

We show that magnetar models for ultraluminous X-ray sources (ULXs) have serious internal inconsistencies. The magnetic fields required to increase the limiting luminosity for radiation pressure above the observed (assumed isotropic) luminosities are completely incompatible with the spin-up rates observed for pulsing ULXs. We note that at least one normal Be-star + neutron star system, with a standard (non-magnetar) field, is observed to become a ULX during a large outburst and return to its previous Be-star binary state afterwards. We note further that recent polarimetric observations of the well-studied binary Cyg X-3 reveal that it produces strong emission directed away from the observer, in line with theoretical predictions of its total accretion luminosity from evolutionary arguments. We conclude that the most likely explanation for ULX behaviour involves radiation beaming by accretion disc winds. A large fraction of X-ray binaries must pass through a ULX state in the course of their evolution. [ABSTRACT FROM AUTHOR]

We have conducted long-slit spectroscopic observations and analyzed archival radio data for the ultraluminous X-ray source (ULX) NGC 4861 X-1. Our spectral line analysis unveils NGC 4861 X-1 as the fourth ULX situated within an X-ray photoionized nebula, following three previous findings made approximately two decades ago. Remarkably, we discover NGC 4861 X-1 also possesses a radio nebula emitting optically thin synchrotron radiation, which contradicts its X-ray photoionization and raises doubts about the four ULXs being a mere coincidence. Instead of gradually accumulating from different bands bit by bit, our multiband discovery is made all at once. Moreover, we tentatively perceive a faint continuum spectrum of the optical nebula. Further observations are needed to ascertain its radio structures and verify the optical continuum. [ABSTRACT FROM AUTHOR]

Complex Fe-K emission/absorption line features are commonly observed in the 6–11 keV band from Active Galactic Nuclei (AGN). These features are formed in various physical components surrounding the black holes. The Narrow-Line Seyfert 1 (NLS1) galaxy Mrk 766, in particular, exhibits characteristic blue-shifted Fe-K absorption lines caused by the ultrafast outflow (UFO), and a broad Fe-K emission line, as well as variable absorbers partially covering the X-ray emitting region. We re-analyse the Mrk 766 archival data of XMM-Newton, NuSTAR , and Swift to investigate the origin of the Fe-K line feature and the 0.3–79 keV energy spectral variation. We have found that the spectral variation in ≲10 keV is primarily explained by the variable partial covering of the central X-ray source by multilayer absorbing clouds. The Fe-K line feature consists of the blue-shifted absorption lines due to the UFO, a narrow emission line from the distant material, a broad emission line from the inner-disc reflection, and a slightly broadened weak emission line at around 6.4–6.7 keV whose equivalent width is ∼0.05 keV. The last one is presumably due to the resonance scattering in the UFO out of the line-of-sight, as predicted by a Monte Carlo simulation based on the hydrodynamical UFO modelling. We suggest that the seemingly complex Fe-K line features and the X-ray energy spectra of Mrk 766 are explained by a moderately extended central X-ray source around a Schwarzschild black hole, an optically thick accretion disc with a truncated inner-radius, the UFO, multilayer partial covering clouds, and a torus. [ABSTRACT FROM AUTHOR]

This paper studies the ionizing radiation effects on functionalized single-walled carbon nanotube (SWCNT)/poly(methyl methacrylate) (PMMA) thin-film nanocomposites [SWNT/PMMA]. The functionalized thin-film devices are made of ferrocene-doped SWCNTs, SWCNTs functionalized with carboxylic acid (COOH), and SWCNTs coated/ modified with copper. The nanocomposite was synthesized by the solution blending method and the resulting nanocomposite was spin-cast on interdigitated electrodes (IDEs). A 160 kV X-ray source was used to irradiate the thin film and changes in the electrical resistance of the nanocomposites due to X-rays were measured using a semiconductor device analyzer. Carboxylic acid functionalized and copper-coated SWCNT/PMMA nanocomposite showed a reduced response to X-rays compared to unfunctionalized SWCNT/PMMA nanocomposite. Ferrocene-doped SWCNT showed a higher sensitivity to X-rays at lower dose rates. This is in contrast to a previous study that showed that similar nanocomposites using functionalized multi-walled CNTs (MWCNTs) had demonstrated an improved response to X-rays ionizing radiation compared to unfunctionalized MWCNTs for all dose rates. Electrical measurements were also performed using the Arduino Nano microcontroller. The result showed that a relatively economical, lightweight-designed prototype radiation sensor based on SWCNT/PMMA thin-film devices could be produced by interfacing the devices with a modest microcontroller. This work also shows that by encapsulating the SWCNT/PMMA thin-film device in a plastic container, the effect of ambient humidity can be reduced and the device can still be used to detect X-ray radiation. This study further shows that the sensitivity of SWCNT to X-rays was dependent on both the functionalization of the SWCNT and the dose rate. [ABSTRACT FROM AUTHOR]

The spatial distribution of linear defects in cuboctahedral single crystals of synthetic diamonds, grown under laboratory conditions by the high-pressure high-temperature (HPHT) method near the diamond–graphite equilibrium line, has been studied. Synthetic diamonds of this type have been studied for the first time by the X-ray topo-tomography using a laboratory X-ray source. [ABSTRACT FROM AUTHOR]

We perform general relativistic radiation magnetohydrodynamics simulations of super-Eddington accretion flows around a neutron star with a dipole magnetic field for modeling the Galactic ultraluminous X-ray source exhibiting X-ray pulsations, Swift J0243.6+6124. Our simulations show the accretion columns near the magnetic poles, the accretion disk outside the magnetosphere, and the outflows from the disk. It is revealed that the effectively optically thick outflows, consistent with the observed thermal emission at ∼107 K, are generated if the mass accretion rate is much higher than the Eddington rate M ̇ Edd and the magnetospheric radius is smaller than the spherization radius. In order to explain the blackbody radius (∼100–500 km) without contradicting the reported spin period (9.8 s) and spin-up rate ( P ̇ = − 2.22 × 10 − 8 s s − 1 ), a mass accretion rate of (200 – 1200) M ̇ Edd is required. Since the thermal emission was detected in two observations with P ̇ of −2.22 × 10−8 s s−1 and −1.75 × 10−8 s s−1 but not in another with P ̇ = − 6.8 × 10 − 9 s s − 1 , the surface magnetic field strength of the neutron star in Swift J0243.6+6124 is estimated to be between 3 × 1011 G and 4 × 1012 G. From this restricted range of magnetic field strength, the accretion rate would be (200 – 500) M ̇ Edd when the thermal emission appears and (60 – 100) M ̇ Edd when it is not detected. Our results support the hypothesis that the super-Eddington phase in the 2017–2018 giant outburst of Swift J0243.6+6124 is powered by highly super-Eddington accretion flows onto a magnetized neutron star. [ABSTRACT FROM AUTHOR]

This paper is an extension of the study by Khamitov et al. (2022) with regard to the catalog and the astrophysical interpretation of the imitation of significant proper motions in active galactic nuclei (AGNs) and quasars based on data from the Gaia space observatory. We present a sample of SRG/eROSITA X-ray sources in the eastern Galactic hemisphere () having significant proper motions in the Gaia EDR3 measurements with the confirmed extragalactic nature of the objects. The catalog consists of 248 extragalactic sources with spectroscopically measured redshifts. The catalog includes all of the objects available in the SIMBAD database and coincident with the identified optical counterpart within 0.5 arcsec. Eighteen sources with spectroscopically measured redshifts from observations with the Russian–Turkish 1.5-m telescope RTT-150 (Khamitov et al. 2022) have been additionally included in the catalog. The sources in the catalog are AGNs of various types (Sy1, Sy2, LINER), quasars, radio galaxies, and star-forming galaxies. The imitation of significant proper motions can be explained (by the VIM effect previously known in astrometry) by the presence of transient events on the line of sight in the vicinity of AGNs and quasars (within the Gaia optical resolution element). Among such astrophysical events are supernova outbursts, tidal disruption events in binary AGNs, the variability of high-mass supergiants, the presence of OB associations against the background of AGNs with a variable brightness, etc. The model of outbursts with a fast rise–exponential decay profile allows the variable positional parameters of most sources observed in Gaia to be described. This approach can be used as an independent way of detecting transient events in the vicinity of AGNs (on scales of several hundred parsecs in the plane of the sky) based on data from the SRG/eROSITA catalogs of X-ray sources and the optical Gaia catalog. [ABSTRACT FROM AUTHOR]

Although the use of a monochromated silver X‐ray source for X‐ray photoelectron spectroscopy (XPS) analysis was first reported in the 1980s, it has found limited application. Two important advances have been made in recent years that now enable it to be used more routinely for surface analysis. These advancements are the development of relative sensitivity factors for quantification and the fully motorized dual silver/aluminum X‐ray source, which enables the change to monochromated X‐ray energies in a few minutes versus hours. Silicones are an important class of materials that have unique chemical properties that can vary from liquids to glass‐like materials. XPS, with monochromatic Al Kα X‐rays, is used routinely to characterize silicon‐containing materials and to identify different silicone functional groups. This was aided by the work published on the analysis of Si 2p peaks acquired from polysiloxane materials. The binding energy increases nonlinearly when methyl groups on a silicon atom are replaced by oxygen atoms. This work will now be extended to determine the binding energy shifts in Si 1s, 2s, and Auger spectra, which will allow the identification of silicon chemistry when using monochromated silver X‐rays. The Auger parameters of the different polysiloxanes will be compared to those of other silicon‐containing materials. Comparison of the depth of analysis in silicon‐containing materials when changing from aluminum to silver X‐rays will also be addressed. [ABSTRACT FROM AUTHOR]

We present a multiwavelength study of the active nucleus and the off-nuclear X-ray sources in the nearby spiral galaxy, NGC 1365 using three simultaneous UV/X-ray observations by AstroSat over a two months period and archival IR observations performed with Spitzer and Herschel. Utilizing the data from the Soft X-ray Telescope (SXT) onboard AstroSat , we find spectral variability mainly caused by the variation in the X-ray column density, (N H ∼ 1022–1023 cm−2). With the accurate spatial resolution of the UVIT onboard AstroSat , we separate the intrinsic AGN flux from the host galaxy emission and then correct for the Galactic and the internal reddening. We detect no significant variation in the NUV emission over the observation period. The AGN in FUV band is undetectable due to heavy intrinsic extinction. Further, the multiwavelength IR/UV/X-ray AGN SED reveals that the AGN is in a low-luminosity phase with accretion rate ∼0.01 L Edd. The steady UV emission and strong X-ray absorption variability suggest that the obscuring clouds are likely compact and affect the compact X-ray source only and do not possibly cover the extended UV emitting region. In addition, the UVIT is able to resolve two bright spots at a radius of 7 arcsec (∼6.3 Kpc) from the central nucleus in the South-West (SW) direction. In the UVIT image of the entire galaxy, we identify UV counterparts to four Chandra identified bright X-ray sources. One well-known ultra-luminous X-ray source (ULX) NGC 1365 X2 is identified with its UV counterpart at 86 arcsec from the nucleus in the North-East (NE) direction from the active nucleus. [ABSTRACT FROM AUTHOR]

Context. Particle-accelerating colliding-wind binaries (PACWBs) are systems that are formed by two massive and hot stars and produce nonthermal radiation. The key elements of these systems are fast winds and the shocks that they create when they collide. Binaries with nonaccreting young pulsars have also been detected as nonthermal emitters, again as a consequence of the wind–wind interaction. Black holes might produce nonthermal radiation by this mechanism if they accrete at super-Eddington rates. In such cases, the disk is expected to launch a radiation-driven wind, and if this wind has an equatorial component, it can collide with the companion star yielding a PACWB. These systems are supercritical colliding wind binaries. Aims. We aim to characterize the particle acceleration and nonthermal radiation produced by the collision of winds in binary systems composed of a superaccreting black hole and an early-type star. Methods. We estimated the terminal velocity of the disk-driven wind by calculating the spatial distribution of the radiation fields and their effect on disk particles. We then found the location of the wind collision region and calculated the timescales of energy gain and losses of relativistic particles undergoing diffusive particle acceleration. With this information, we were able to compute the associated spectral energy distribution of the radiation. We calculated a number of specific models with different parameters to explore this scenario. Results. We find that the interaction of winds can produce nonthermal emission from radio up to tens of GeV, with luminosities in the range of ∼1033–1035 erg s−1, which for the most part are contributed by electron synchrotron and inverse Compton radiation. Conclusions. We conclude that supercritical colliding wind binaries, such as some ultraluminous X-ray sources and some Galactic X-ray binaries, are capable of accelerating cosmic rays and producing nonthermal electromagnetic emission from radio to γ-rays, in addition to the thermal components. [ABSTRACT FROM AUTHOR]

Context. Tidal disruption events (TDEs) are usually discovered at X-ray or optical wavelengths through their transient nature. A characteristic spectral feature of X-ray detected TDEs is a 'supersoft' X-ray emission, not observed in any other extragalactic source, with the exception of a few rapidly variable hyperluminous X-ray sources (HLXs) or supersoft active galactic nuclei (AGN) that are distinguishable by their optical emission. Aims. The goal of our work is to find extragalactic supersoft sources associated with galactic centres. We expect this category to include overlooked TDEs, supersoft AGN, and nuclear HLXs. Finding such sources would allow for the study of extreme regime accretion on different black hole mass scales. Methods. We searched for supersoft X-ray sources (SSSs) by cross-correlating optical and X-ray catalogues to select extragalactic near-nuclear sources and we then filtered for very steep spectra (photon index Γ > 3) and high X-ray luminosities (LX > 1041 erg s−1). Results. With our blind search we retrieved about 60 sources, including 15 previously known supersoft AGN or TDEs, thus demonstrating the efficiency of our selection. Of the remaining sample, 36 sources are optically classified as AGN, although they show steeper-than-usual spectra. The remaining nine previously unknown sources show spectral properties consistent with emission by extremely soft-excess dominated AGN (five sources) or TDEs (four sources). An XMM-Newton follow-up observation of one of these sources confirmed its likely TDE nature. Conclusions. Our work is the first attempt to discover TDEs by their spectral features rather than their variability, and it has been successful in retrieving known TDEs and in discovering new extreme ultrasoft sources, including four new TDE candidates, one of which is confirmed via follow-up observations. [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]

Context. Nowadays, we know that the origin of the cosmic X-ray background (CXB) is mainly due to the integrated emission of active galactic nuclei. Therefore, in order to obtain a precise estimate of the contribution of different source classes to the CXB, it is crucial to achieve full characterization of the hard-X ray sky. Aims. We present a multifrequency analysis of all sources listed in the third release of the Palermo Swift-BAT hard X-ray catalog (3PBC) with the goal of (i) identifying and classifying the largest number of sources adopting multifrequency criteria, with particular emphasis on extragalactic populations and (ii) extracting sources belonging to the class of Seyfert galaxies to present here the release of the second version of the Turin-SyCAT. Methods. We outline a classification scheme based on radio, infrared (IR), and optical criteria that allows us to distinguish between unidentified and unclassified hard X-ray sources, as well as to classify those sources belonging to the Galactic and the extragalactic populations. Results. Our revised version of the 3PBC lists 1176 classified, 820 extragalactic, and 356 Galactic sources, as well as 199 unclassified and 218 unidentified sources. According to our analysis, the hard X-ray sky is mainly populated by Seyfert galaxies and blazars. For the blazar population, we report trends between the hard X-ray and the gamma-ray emissions based on the fact that a large fraction of them also have a counterpart detected by the Fermi satellite. These trends are all in agreement with the expectations of inverse Compton models which are widely adopted to explain the blazar broadband emission. For the Seyfert galaxies, we present the second version of the Turin-SyCAT, including a total of 633 Seyfert galaxies, with 282 new sources corresponding to an increase of ~80% with respect to the previous release. Comparing the hard X-ray and the infrared emissions of Seyfert galaxies, we confirm that there is no clear difference between the flux distribution of the infrared-to-hard X-ray flux ratio of Seyfert galaxies Type 1 and Type 2. However, there is a significant trend between the mid-IR flux and hard X-ray flux, confirming previous statistical results in the literature. Conclusions. We provide two catalog tables. The first is the revised version of the 3PBC catalog based on our multifrequency analyses. The second catalog table is a release of the second version of the Turin-SyCAT catalog. Finally, we highlight that extensive soft X-ray data are already available in the form of the Swift archive which can be used to search for potential counterparts of unidentified hard X-ray sources. All these datasets will be reduced and analyzed in a forthcoming analysis to determine the precise position of low-energy counterparts in the 0.5–10 keV energy range for 3PBC sources that can be targets of future optical spectroscopic campaigns; this is necessary to obtain their precise classification. [ABSTRACT FROM AUTHOR]

Stanford Synchrotron Radiation Lightsource serves a wide scientific community with its variety of X‐ray capabilities. Recently, a wiggler X‐ray source located at beamline 10‐2 has been employed to perform high‐resolution rocking curve imaging (RCI) of diamond and silicon crystals. X‐ray RCI is invaluable for the development of upcoming cavity‐based X‐ray sources at SLAC, including the cavity‐based X‐ray free‐electron laser and X‐ray laser oscillator. In this paper, the RCI apparatus is described and experimental results are provided to validate its design. Future improvements of the setup are also discussed. [ABSTRACT FROM AUTHOR]

Introduction: The study focused on the assessment of quality control tests in X-ray units from the southern regions of Tanzania to yield the data required to create and implement quality control policies and strategies. Material and Methods: Quality control tests were conducted on twenty-six diagnostic X-ray units in private and government hospitals in the southern regions of Tanzania during 2020 - 2021. The tests focused on the reproducibility of accelerating tube potential, time reproducibility, the accuracy of accelerating tube potential, half-value layer, beam alignment, and collimation. The statistical analyses were done using the Microsoft Excel spreadsheet of 2013. The results were compared with the tolerance limits. Results: Of all X-ray units evaluated, 92.31% had kVp accuracy within the tolerance limit of 5% and 92% of the X-ray units had acceptable HVL values = 2.3 mm Al at 80 kVp. Moreover, results have shown that 86.96% and 82.61% of X-ray units had acceptable beam collimation (= ±2 cm) and beam alignment (=3% of the X-ray source and X-ray table), respectively. Conclusion: Comprehensive regulatory inspections and equipment maintenance plans in southern Tanzania are significantly required due to the high patient workload which attributes to frequent breakdowns of X-ray units. Moreover, radiographers need to be trained on how to minimize and detect beam misalignment and collimation failure since the most unacceptable results were observed from these tests. [ABSTRACT FROM AUTHOR]

We report X-ray spectral variability in an ultraluminous X-ray source NGC 1042 ULX1, using archival XMM-Newton and recent NuSTAR observations. In long-term evolution, the source has shown a trend of variation in spectral hardness. The variability in different XMM-Newton observations is prominent above ∼1 keV. Cool thermal disc component with a characteristic temperature of ∼0.2 keV manifests that the spectral state of NGC 1042 ULX1 in all epochs is similar to that of the ultraluminous state sources. An apparent anti-correlation between luminosity and power-law index demonstrates that the source becomes spectrally harder when it is in a brighter state. That is conceivably related to variation in accretion rate, strength of Comptonization, wind/outflow in the system or a manifestation of varying disc occultation. Typical hard ultraluminous-type spectra indicate that NGC 1042 ULX1 is a low inclination system in general. Spectral properties suggest that, like many other ULXs which show spectral curvature around ∼6–10 keV, NGC 1042 ULX1 could be another stellar-mass super-Eddington accretor. [ABSTRACT FROM AUTHOR]

We present results from analysis of the XMM–Newton observation (2021 February) of the enigmatic object WX Cen: a presumed massive Wolf–Rayet (WR) binary, (WN3 + WC4), or a low‐mass binary. WX Cen was not detected in X‐rays. Its UV emission shows periodic variability with a period (0.405–0.434 day) similar to that found in the optical for this object (0.417 day). The X‐ray nondetection of WX Cen is inconsistent with the fact that WR binaries are X‐ray sources. Also, the X‐ray emission from its early WN‐type component would have been clearly detected in the XMM–Newton exposure. Thus, the X‐ray nondetection does not support the physical picture of WX Cen as a massive WR binary. Alternatively, WX Cen might be a low‐mass binary, presumably a supersoft X‐ray source (a V Sagittae star), and the X‐ray nondetection of WX Cen could be consistent with such a physical picture. [ABSTRACT FROM AUTHOR]

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We present the results of eight epochs of simultaneous UV and X-ray observations of the highly variable ultraluminous X-ray source (ULX) Holmberg II X-1 with AstroSat—Indian multiwavelength space satellite. During the entire observation period from late 2016 to early 2020, Holmberg II X-1 showed a moderate X-ray luminosity about of erg s and a hard power-law spectrum with . Due to low variability of the object in X-rays (by a factor 1.5) and insignificant variability in the UV range (upper limit approximately 25%) we could not find reliable correlation between flux changes in these ranges. Inside each particular observation, the X-ray variability amplitude is higher, reaching a factor of 2–3 respect to the mean level, however, it is observed in the form of relatively short stochastic bursts. We discussed our results in terms of three models of a heated donor star, a heated disk and a heated wind, and estimated the lower limit to the variability which would allow to reject at least part of them. [ABSTRACT FROM AUTHOR]

Some ultraluminous X-ray sources (ULXs) exhibit X-ray pulses, and their central sources are thought to be neutron stars. It has also been suggested that some are transient sources with Be-type donors. In this study, we use the mass accretion model of a Be-type high-mass X-ray binary (BeHMXB) to estimate the conditions under which a giant X-ray burst caused by a BeHMXB exceeds the Eddington luminosity. Moreover, we investigate the duration for which BeHMXBs can be observed as transient ULXs with bursts above the Eddington luminosity during binary evolutions. The results indicate that BeHMXBs could be ULXs for a typical duration of approximately 1 Myr. Comparisons with nearby observed BeHMXBs indicate that many binary systems have the potential to become ULXs during their evolution. Particularly, a BeHMXB system tends to become a ULX when the Be donor has a dense deccretion disc aligned with the orbital plane. Because BeHMXBs are very common objects and a significant number of them can become ULXs, we conclude that a reasonable fraction of the observed ULXs could consist of evolved BeHMXBs. [ABSTRACT FROM AUTHOR]

Insight-HXMT is the first Chinese X-ray astronomical mission. With a 1–250 keV bandpass, an energy resolution of 150 eV at 6 keV, and without being subject to pile-up distortions, it is suitable to study bright X-ray sources like Galactic black holes. In this paper, we study Insight-HXMT observations of the X-ray binaries MAXI J1535–571 and 4U 1630–472 during their outbursts in 2017 and 2020, respectively. From the analysis of the reflection features, we are able to infer the black hole spin parameter in the two sources. For MAXI J1535–571, we find a spin parameter very close to 1, a * = 0.9916 ± 0.0012 (90 per cent C.L. statistical error), which is consistent with the previous NuSTAR measurement. For 4U 1630–472, we find a moderately high value of the black hole spin parameter, a * = 0.817 ± 0.014 (90 per cent C.L. statistical error), which is lower than the almost extremal value found with NuSTAR data. [ABSTRACT FROM AUTHOR]

NGC 1313 X-2 is one of the few known pulsating ultraluminous X-ray sources (PULXs), and so is thought to contain a neutron star that accretes at highly super-Eddington rates. However, the physics of this accretion remains to be determined. Here, we report the results of two simultaneous XMM–Newton and HST observations of this PULX taken to observe two distinct X-ray behaviours as defined from its Swift light curve. We find that the X-ray spectrum of the PULX is best described by the hard ultraluminous regime during the observation taken in the lower flux, lower variability amplitude behaviour; its spectrum changes to a broadened disc during the higher flux, higher variability amplitude epoch. However, we see no accompanying changes in the optical/UV fluxes, with the only difference being a reduction in flux in the near-infrared (NIR) as the X-ray flux increased. We attempt to fit irradiation models to explain the UV/optical/IR fluxes but they fail to provide meaningful constraints. Instead, a physical model for the system leads us to conclude that the optical light is dominated by a companion O/B star, albeit with an IR excess that may be indicative of a jet. We discuss how these results may be consistent with the precession of the inner regions of the accretion disc leading to changes in the observed X-ray properties, but not the optical, and whether we should expect to observe reprocessed emission from ULXs. [ABSTRACT FROM AUTHOR]

We study a large sample of dwarf galaxies using archival Chandra X-ray observations, with the aim of detecting accreting intermediate-mass black holes (IMBHs). IMBHs are expected to inhabit dwarf galaxies and to produce specific signatures in terms of luminosity and X-ray spectra. We report the discovery of an X-ray source associated with an A85 dwarf galaxy that fits the IMBH description. The stellar mass of the host galaxy is estimated to be 2 Ă— 108 M ⊙, which makes it one of the least massive galaxies to potentially host an accreting black hole. The source is detected in the soft band, under 1 keV, and is undetected at higher energies. The X-ray luminosity is ≈1041 erg sâˆ'1, making it almost three orders of magnitude more luminous than the most luminous stellar-mass supersoft emitters. From the galaxy stellar mass versus black hole mass relation, we estimate the mass to be within the intermediate regime. Another method that resulted in an intermediate mass relies on the fact that supersoft emission is expected to be associated with high accretion rates, approaching the Eddington limit. We suggest that the observed offset of the X-ray source from the galactic center (≈1.8 kpc) is due to galaxy interactions, and we present evidence from the literature that supports the relation between black hole activity and galaxy interactions. [ABSTRACT FROM AUTHOR]

Supernovae explosions (SNe) in a stratified interstellar disc can create large scale structures and outflows perpendicular to the disc. Their morphology and emission characteristics depend on interrelations between the number of exploded SNe, the ISM vertical scale height, the height above the ISM disc at which SNe are located, and the mid-plane density of ambient gas. When observed such out-of-plane gas structures do characterize properties of the driving SNe and reveal the presence of underlying stellar clusters. Even though themorphology and emission properties of such structures are sensitive to characteristics of ambient gas they might be used for rough estimates of the clusters' mass. [ABSTRACT FROM AUTHOR]

Extraplanar structures of ionized gas are detected in numerous edge-on galaxies. Whether HII regions or supernovae (SNe) responsible for its ionization is still debated. Here, we consider the Hα emissivity from shells of superbubbles driven by SNe explosions in a stellar cluster located either in the midplane or above the midplane of the disc. We found that these shells form filamentary structures emitting brightly in Hα line at height of ~0.6-1.6 kpc above the disc depending on cluster location and gas metallicity. The Hα surface brightness ranges within - ~10-17-10-16 erg s-1 cm-2 arcsec-2 are close to the values observed in edge-on galaxies. We conclude that the Hα emission of extraplanar structures can be explained by recombination in collisionally ionized gas of unstable shells driven by multiple SNe. [ABSTRACT FROM AUTHOR]

Most ultraluminous X-ray sources (ULXs) are thought to be powered by super-Eddington accretion on to stellar-mass compact objects. Accretors in this extreme regime are naturally expected to ionize copious amounts of plasma in their vicinity and launch powerful radiation-driven outflows from their discs. High spectral resolution X-ray observations [with reflection grating spectrometer (RGS) gratings onboard XMM–Newton ] of a few ULXs with the best data sets indeed found complex line spectra and confirmed such extreme (0.1–0.3c) winds. However, a search for plasma signatures in a large ULX sample with a rigorous technique has never been performed, thereby preventing us from understanding their statistical properties such as the rate of occurrence, to constrain the outflow geometry, and its duty cycle. We developed a fast method for automated line detection in X-ray spectra and applied it to the full RGS ULX archive, rigorously quantifying the statistical significance of any candidate lines. Collecting the 135 most significant features detected in 89 observations of 19 objects, we created the first catalogue of spectral lines detected in soft X-ray ULX spectra. We found that the detected emission lines are concentrated around known rest-frame elemental transitions and thus originate from low-velocity material. The absorption lines instead avoid these transitions, suggesting they were imprinted by blueshifted outflows. Such winds therefore appear common among the ULX population. Additionally, we found that spectrally hard ULXs show fewer line detections than soft ULXs, indicating some difference in their accretion geometry and orientation, possibly causing overionization of plasma by the harder spectral energy distributions of harder ULXs. [ABSTRACT FROM AUTHOR]

The first two papers of this Special Issue are devoted to the numerical modeling of the wind/disk outflow from Be stars and its collision with the pulsar wind. Gamma-ray binaries are a subclass of high-mass binary systems whose energy spectrum peaks at high energies (MeV-GeV energy range) and extends to very high energy (GeV-TeV) HT ht -rays. PSR B1259-63 is a gamma-ray binary system hosting a radio pulsar orbiting around a O9.5Ve star, LS 2883, with a period of 3.4 years. [Extracted from the article]

The neutron imaging system at the National Ignition Facility is an important diagnostic tool for measuring the two-dimensional size and shape of the source of neutrons produced in the burning deuterium-tritium plasma during the stagnation phase of inertial confinement fusion implosions. Very few two-dimensional projections of neutron images are available to reconstruct the threedimensional neutron source. In this paper, we present a technique that has been developed for the 3D reconstruction of neutron and x-ray sources from a minimal number of 2D projections. We present the detailed algorithms used for this characterization and the results of reconstructed sources from experimental data collected at Omega. [ABSTRACT FROM AUTHOR]

Previous detections of an X-ray emission line near 3.5 keV in galaxy clusters and other dark-matter-dominated objects have been interpreted as observational evidence for the decay of sterile neutrino dark matter. Motivated by this, we report on a search for a 3.5 keV emission line from the Milky Way's galactic dark matter halo with HaloSat. As a single pixel, collimated instrument, HaloSat observations are impervious to potential systematic effects due to grazing incidence reflection and CCD pixelization, and thus may offer a check on possible instrumental systematic errors in previous analyses. We report nondetections of a ∼3.5 keV emission line in four HaloSat observations near the Galactic center. In the context of the sterile neutrino decay interpretation of the putative line feature, we provide 90% confidence level upper limits on the 3.5 keV line flux for a field centered 18.6 degrees from the Galactic center and the corresponding 7.1 keV sterile neutrino mixing angle: F ≤ 0.077 ph cm−2 s−1 sr−1 and. The HaloSat mixing angle upper limit was calculated using a modern parameterization of the Milky Way's dark matter distribution, and in order to compare with previous limits, we also report the limit calculated using a common historical model. The HaloSat mixing angle upper limit places constraints on a number of previous mixing angle estimates derived from observations of the Milky Way's dark matter halo and galaxy clusters, and excludes several previous detections of the line. The upper limits cannot, however, entirely rule out the sterile neutrino decay interpretation of the 3.5 keV line feature. [ABSTRACT FROM AUTHOR]

Ultraluminous X-ray sources are considered amongst the most extremely accreting objects in the local Universe. The recent discoveries of pulsating neutron stars in ULXs strengthened the scenario of highly super-Eddington accretion mechanisms on stellar mass compact objects. In this work, we present the first long-term light curve of the source NGC 4559 X7 using all the available Swift/XRT, XMM–Newton, Chandra , and NuSTAR data. Because of the high quality 2019 XMM–Newton and NuSTAR observations, we investigated in an unprecedented way the spectral and temporal properties of NGC 4559 X7. The source displayed flux variations of up to an order of magnitude and an unusual flaring activity. We modelled the spectra from NGC 4559 X7 with a combination of two thermal components, testing also the addition of a further high energy cut-off power law. We observed a spectral hardening associated with a luminosity increase during the flares, and a spectral softening in the epochs far from the flares. Narrow absorption and emission lines were also found in the RGS spectra, suggesting the presence of an outflow. Furthermore, we measured hard and (weak) soft lags with magnitudes of a few hundreds of seconds whose origin is possibly due to the accretion flow. We interpret the source properties in terms of a super-Eddington accretion scenario assuming the compact object is either a light stellar mass black hole or a neutron star. [ABSTRACT FROM AUTHOR]

Recently, it has been discovered that the transition of X-ray pulsars to the low luminosity state (⁠|$L\lesssim 10^{35}\, {\rm erg\ \rm s^{-1}}$|⁠) is accompanied by a dramatic spectral change. That is, the typical power-law-like spectrum with high-energy cut-off transforms into a two-component structure with a possible cyclotron absorption feature on top of it. It was proposed that these spectral characteristics can be explained qualitatively by the emission of cyclotron photons in the atmosphere of the neutron star caused by collisional excitation of electrons to upper Landau levels and further Comptonization of the photons by electron gas. The electron gas are expected to be overheated in a thin top layer of the atmosphere. In this paper, we perform Monte Carlo simulations of the radiative transfer in the atmosphere of an accreting neutron star while accounting for a resonant scattering of polarized X-ray photons by thermally distributed electrons. The spectral shape is shown to be strongly polarization-dependent in soft X-rays (⁠|$\lesssim 10\, {\rm keV}$|⁠) and near the cyclotron scattering feature. The results of our numerical simulations are tested against the observational data of the X-ray pulsar A 0535+262 in the low-luminosity state. We show that the spectral shape of the pulsar can be reproduced by the proposed theoretical model. We discuss applications of the discovery to the observational studies of accreting neutron stars. [ABSTRACT FROM AUTHOR]

We investigate the radiation fields around super-Eddington accretion flow, in which multiple inverse-Compton scattering plays a principal role, by using newly developed code describing Boltzmann radiation transfer in the Schwarzschild space-time. We apply this code to post-processing spectral calculations based on general relativistic, radiation magnetohydrodynamic simulation data to obtain X-ray spectra seen from various viewing angles. The radiation fields are distinctively separated into a funnel region with an opening angle of ∼30°, which is full with hot (gas temperature of T > 108 K), tenuous, and high-velocity plasmas, and surrounding cooler (T ∼ 107 K) and optically thick outflow regions. Accordingly, there is a clear tendency that the smaller the viewing angle, the harder the spectra. In particular, hard photons with several tens of keV are observable only by observers at viewing angles less than ∼30°, consistent with past spectral studies based on simulations. Further, we investigate how the spectra are varied by a flare occurring in the innermost region, finding that the variation amplitude grows as the photon energy increases and that the harder photons emerge more quickly than softer photons. The observational implications on long-term spectral variability of ultra-luminous X-ray sources are briefly discussed. [ABSTRACT FROM AUTHOR]

One of the mainstreams in modern X-ray astronomy is research into extragalactic X-ray sources on the basis of the data acquired at the X-ray Multi-Mirror Newton (XMM-Newton) space observatory. According to observations, X-rays coming from galaxies are mainly radiated from their central regions, i.e., active galactic nuclei and groups of X-ray sources in the galactic disks. In this paper, we consider the cross-correlation between the 4XMM-DR9 catalog and the Hyper-Linked Extragalactic Databases and Archives (HyperLeda) of galaxies. The 4XMM-DR9 catalog is a large, up-to-date catalog of observations, which contains 550 124 unique sources and covers 2.85% of the sky, while the HyperLeda database comprises 1.5 million galaxies. Our analysis resulted in a sample of more than 5000 X-ray galaxies, most of which are active galactic nuclei of low luminosity. From this sample, we selected galaxies whose the X-ray flux exceeds F = 10–20 J/cm2s. The sources of this kind are of particular interest since it is easier to construct an informative spectrum for them. The identified and classified catalog of 1172 manually verified galaxies—the X-ray galaxy catalog named Xgal—was created. In the Xgal catalog, most galaxies have an active X-ray nucleus; Seyfert galaxies predominate among them at short distances, while quasars are prevalent at large distances. We revealed 169 galaxies that exhibit extended nuclei with a visible surface brightness distribution and 173 galaxies with more than one X-ray source. Based on the Xgal catalog, we created a catalog of elongated X-ray galaxies (the optical angular sizes of which are a > 60″) that have X-ray sources outside the nucleus. Both catalogs are freely accessible. The Xgal catalog may serve to construct the spectra of objects of a certain class in different ranges, to develop or improve the theory of their emission, and to survey bright and extended quasars. Moreover, the entire cross-sample may be used to study active galactic nuclei with low luminosity and a large-scale structure of the universe in the X-ray range. [ABSTRACT FROM AUTHOR]

We present quasi-simultaneous radio, (sub-)millimetre, and X-ray observations of the Galactic black hole X-ray binary GX 339−4, taken during its 2017–2018 outburst, where the source remained in the hard X-ray spectral state. During this outburst, GX 339−4 showed no atypical X-ray behaviour that may act as an indicator for an outburst remaining within the hard state. However, quasi-simultaneous radio and X-ray observations showed a flatter than expected coupling between the radio and X-ray luminosities (with a best-fitting relation of |$L_{\rm radio} \propto L_{\rm X}^{0.39 \pm 0.06}$|⁠), when compared to successful outbursts from this system (⁠|$L_{\rm radio} \propto L_{\rm X}^{0.62 \pm 0.02}$|⁠). While our 2017–2018 outburst data only span a limited radio and X-ray luminosity range (∼1 order of magnitude in both, where more than 2 orders of magnitude in L X is desired), including data from other hard-only outbursts from GX 339−4 extends the luminosity range to ∼1.2 and ∼2.8 orders of magnitude, respectively, and also results in a flatter correlation (where |$L_{\rm radio} \propto L_{\rm X}^{0.46 \pm 0.04}$|⁠). This result is suggestive that for GX 339−4 a flatter radio–X-ray correlation, implying a more inefficient coupling between the jet and accretion flow, could act as an indicator for a hard-only outburst. However, further monitoring of both successful and hard-only outbursts over larger luminosity ranges with strictly simultaneous radio and X-ray observations is required from different single sources to explore if this applies generally to the population of black hole X-ray binaries, or even GX 339−4 at higher hard-state luminosities. [ABSTRACT FROM AUTHOR]

The recurring transient outbursts in low-mass X-ray binaries (LMXBs) provide us with strong test-beds for constraining the poorly understood accretion process. While impossible to image directly, phase-resolved spectroscopy can provide a powerful diagnostic to study their highly complex, time-dependent accretion discs. We present an 8-month long multiwavelength (UV, optical, X-ray) monitoring campaign of the new candidate black hole LMXB MAXI J0637–430 throughout its 2019/2020 outburst, using the Neil Gehrels Swift Observatory , as well as three quasi-simultaneous epochs of Gemini/GMOS optical spectroscopy. We find evidence for the existence of a correlation between the X-ray irradiation heating the accretion disc and the evolution of the He  ii 4686 Å emission line profiles detected in the optical spectra. Our results demonstrate a connection between the line emitting regions and physical properties of the X-ray irradiation heating the discs during outburst cycles of LMXBs. Further, we are able to show that changes in the physical properties of the irradiation heating the disc in outburst can be imprinted within the H/He emission line profiles themselves in these systems. [ABSTRACT FROM AUTHOR]

We present in this paper a first step toward a semi-analytical treatment of the accretion process in wind-fed neutron star (NS) supergiant X-ray binaries with eccentric orbits. We consider the case of a spherically symmetric wind for the supergiant star and a simplified model for the accretion on to the compact object. A self-consistent calculation of the photoionization of the stellar wind by the X-rays from the accreting NS is included. This effect is convolved with the modulation of the mass accretion rate induced by the eccentric orbit to obtain the expected X-ray luminosity of a system along the orbit. As part of our results, we first show that the bi-modality of low- and high-X-ray luminosity solutions for supergiant X-ray binaries reported in previous papers is likely to result from the effect of the NS approaching first and then moving away from the companion (without coexisting simultaneously). We propose that episodes of strong wind photoionization can give rise to off-states of the sources. Our calculations are applied to the case of a few classical supergiant X-ray binary systems with known eccentricities (Vela X-1, 4U 1907+09, GX 301-2) and to the case of the only supergiant fast X-ray transient with a confirmed eccentric orbit, IGR J08408-4503. The results are compared with observational findings on these sources. We also discuss the next steps needed to expand the calculations toward a more comprehensive treatment in future publications. [ABSTRACT FROM AUTHOR]

We report on the discovery of a new, transient ultraluminous X-ray source (ULX) in the galaxy NGC 7090. This new ULX, which we refer to as NGC 7090 ULX3, was discovered via monitoring with Swift during 2019–2020, and to date has exhibited a peak luminosity of L X ∼ 6 × 1039 erg s−1. Archival searches show that, prior to its recent transition into the ULX regime, ULX3 appeared to exhibit a fairly stable luminosity of L X ∼ 1038 erg s−1. Such strong long-time-scale variability may be reminiscent of the small population of known ULX pulsars, although deep follow-up observations with XMM–Newton and NuSTAR do not reveal any robust X-ray pulsation signals. Pulsations similar to those seen from known ULX pulsars cannot be completely excluded, however, as the limit on the pulsed fraction of any signal that remains undetected in these data is ≲20 per cent. The broad-band spectrum from these observations is well modelled with a simple thin disc model, consistent with sub-Eddington accretion, which may instead imply a moderately large black hole accretor (M BH ∼ 40 M⊙). Similarly, though, more complex models consistent with the super-Eddington spectra seen in other ULXs (and the known ULX pulsars) cannot be excluded given the limited signal-to-noise ratio of the available broad-band data. The nature of the accretor powering this new ULX therefore remains uncertain. [ABSTRACT FROM AUTHOR]

We present neutral hydrogen (H  i) absorption spectra of the black hole candidate X-ray binary (XRB) MAXI J1348–630 using the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT. The ASKAP H  i spectrum shows a maximum negative radial velocity (with respect to the local standard of rest) of −31 ± 4 km s−1 for MAXI J1348–630, as compared to −50 ± 4 km s−1 for a stacked spectrum of several nearby extragalactic sources. This implies a most probable distance of |$2.2^{+0.5}_{-0.6}$|  kpc for MAXI J1348–630, and a strong upper limit of the tangent point distance at 5.3 ± 0.1 kpc. Our preferred distance implies that MAXI J1348–630 reached 17 ± 10  per cent of the Eddington luminosity at the peak of its outburst, and that the source transited from the soft to the hard X-ray spectral state at 2.5 ± 1.5  per cent of the Eddington luminosity. The MeerKAT H  i spectrum of MAXI J1348–630 (obtained from the older, low-resolution 4k mode) is consistent with the re-binned ASKAP spectrum, highlighting the potential of the eventual capabilities of MeerKAT for XRB spectral line studies. [ABSTRACT FROM AUTHOR]

In this work, we present detailed constraints on the metallicity dependence of the high-mass X-ray binary (HMXB) X-ray luminosity function (XLF). We analyze ≈5 Ms of Chandra data for 55 actively star-forming galaxies at D ≲ 30 Mpc, with gas-phase metallicities spanning ≈ 7–9.2. Within the galactic footprints, our sample contains a total of 1311 X-ray point sources, of which ≈49% are expected to be HMXBs, with the remaining sources likely to be low-mass X-ray binaries (LMXBs; ≈22%) and unrelated background sources (≈29%). We construct a model that successfully characterizes the average HMXB XLF over the full metallicity range. We demonstrate that the SFR-normalized HMXB XLF shows clear trends with metallicity, showing steadily increasing numbers of luminous and ultraluminous X-ray sources ((erg s−1) = 38–40.5) with declining metallicity. However, we find that the low-luminosity ((erg s−1) = 36–38) HMXB XLF appears to show a nearly constant SFR scaling and slope with metallicity. Our model provides a revised scaling relation of integrated LX/SFR versus , and a new characterization of its SFR-dependent stochastic scatter. The general trend of this relation is broadly consistent with past studies based on integrated galaxy emission; however, our model suggests that this relation is driven primarily by the high-luminosity end of the HMXB XLF. Our results have implications for binary population synthesis models, the nature of super-Eddington accreting objects (e.g., ultraluminous X-ray sources), recent efforts to identify active galactic nucleus candidates in dwarf galaxies, and the X-ray radiation fields in the early universe during the epoch of cosmic heating at z ≳ 10. [ABSTRACT FROM AUTHOR]