X-ray Surveys Research Articles

Euclid preparation. LVII. Observational expectations for redshift z<7 active galactic nuclei in the Euclid Wide and Deep surveys

We forecast the expected population of active galactic nuclei (AGN) observable in the Euclid Wide Survey (EWS) and Euclid Deep Survey (EDS). Starting from an X-ray luminosity function (XLF), we generated volume-limited samples of the AGN expected in the survey footprints. Each AGN was assigned a spectral energy distribution (SED) appropriate for its X-ray luminosity and redshift, with perturbations sampled from empirical distributions. The photometric detectability of each AGN was assessed via mock observations of the assigned SED. We estimate 40 million AGN will be detectable in at least one band in the EWS and 0.24 million in the EDS, corresponding to surface densities of $ and $. The relative uncertainty on our expectation for detectable AGN is 6.7<!PCT!> for the EWS and 12.5<!PCT!> for the EDS, driven by the uncertainty of the XLF. Employing colour selection criteria on our simulated data we select a sample of $) AGN in the EWS and $) in the EDS, amounting to 10<!PCT!> and 8<!PCT!> of the AGN detectable in the EWS and EDS. Including ancillary Rubin/LSST bands improves the completeness and purity of AGN selection. These data roughly double the total number of selected AGN to comprise 21<!PCT!> and 15<!PCT!> of the detectable AGN in the EWS and EDS. The total expected sample of colour-selected AGN contains 6.0\,times \,106 (74<!PCT!>) unobscured AGN and 2.1\,times \,106 (26<!PCT!>) obscured AGN, covering $0.02 z 5.2$ and $43 (L_ bol erg s 47$. With these simple colour cuts expected surface densities are already comparable to the yield of modern X-ray and mid-infrared surveys of similar area. The EWS sample is most comparable to the WISE C75 AGN selection and the EDS sample is most similar to the yield of the collated Spitzer cryogenic surveys when considering bands alone, or the XXL-3XLSS survey AGN sample when also considering selection with ancillary optical bands We project that 15<!PCT!> (7.6<!PCT!>) of the total detectable population in the EWS (EDS) will exhibit X-ray fluxes that could be detected in the XMM-COSMOS survey, showing that the vast majority of AGN would not be detectable in modern medium-depth X-ray surveys.

The ultraviolet luminosity function of star-forming galaxies between redshifts of 0.4 and 0.6

Abstract We combine ultraviolet imaging of the 13H survey field, taken with the XMM-Newton Optical Monitor telescope (XMM-OM) and the Neil Gehrels Swift Observatory Ultraviolet and Optical Telescope (UVOT) in the UVM2 band, to measure rest-frame ultraviolet 1,500 Å luminosity functions of star-forming galaxies with redshifts z between 0.4 and 0.6. In total the UVM2 imaging covers a sky area of 641 arcmin2, and we detect 273 galaxies in the UVM2 image with 0.4 < z < 0.6. The luminosity function is fit by a Schechter function with best-fit values for the faint end slope $\alpha =-1.8^{+0.4}_{-0.3}$ and characteristic absolute magnitude $M^{*} = -19.1^{+0.3}_{-0.4}$. In common with XMM-OM based studies at higher redshifts, our best-fitting value for M* is fainter than previous measurements. We argue that the purging of active galactic nuclei from the sample, facilitated by the co-spatial X-ray survey carried out with XMM-Newton is important for the determination of M*. At the brightest absolute magnitudes (M1500 < −18.5) the average UV colour of our galaxies is consistent with that of minimal-extinction local analogues, but the average UV colour is redder for galaxies at fainter absolute magnitudes, suggesting that higher levels of dust attenuation enter the sample at absolute magnitudes somewhat fainter than M*.

Catalog of Cataclysmic Variables Around the Period-bounce: New Systems

A large portion of cataclysmic variables (CVs), between 40% and 80%, are predicted to be in the last stage of their evolution, becoming period-bouncers. This sub-class of CVs is characterized by having gone through a reversal in the evolution of their orbital periods as well as having a late-type donor. The large predicted population of period-bouncers has, however, not been observed so far. We have led an effort to reveal the missing population of period-bouncers though the use the X-ray survey eROSITA, which has increased the population of period-bouncers by 63% through the use of eROSITA. This study has established eROSITA as a promising path for the identification of the new members of this underrepresented sub-class of CVs.

Towards a complete census of luminous Compton-thick active galactic nuclei in the Local Universe

X-ray surveys provide the most efficient means for the detection of active galactic nuclei (AGNs). However, they do face difficulties in detecting the most heavily obscured Compton-thick AGNs. The BAT detector on board the mission, operating in the very hard 14-195 keV band, has provided the largest samples of Compton-thick AGN in the Local Universe. However, even these flux-limited samples may miss the most obscured sources among the Compton-thick AGN population. A robust way to find these local sources is to systematically study volume-limited AGN samples detected in the IR or the optical part of the spectrum. Here, we utilise a local sample (<100 Mpc) of mid-IR-selected AGNs, unbiased against obscuration, to determine the fraction of Compton-thick sources in the Local Universe. When available, we acquired X-ray spectral information for the sources in our sample from previously published studies. In addition, to maximise the X-ray spectral information for the sources in our sample, we analysed eleven unexplored and observations, for the first time. In this way, we identified four new Compton-thick sources. Our results reveal an increased fraction of Compton-thick AGNs among the sources that have not been detected by BAT of 44 <!PCT!>. Overall, we have estimated a 25-30<!PCT!> share of Compton-thick sources in the Local Universe among mid-IR-selected AGNs. We find no evidence for any evolution of the AGN Compton-thick fraction with luminosity.

The cosmological analysis of X-ray cluster surveys. VI. Inference based on analytically simulated observable diagrams

The number density of galaxy clusters across mass and redshift has been established as a powerful cosmological probe, yielding important information on the matter components of the Universe. Cosmological analyses with galaxy clusters traditionally employ scaling relations, which are empirical relationships between cluster masses and their observable properties. However, many challenges arise from this approach as the scaling relations are highly scattered, maybe ill-calibrated, depend on the cosmology, and contain many nuisance parameters with low physical significance. For this paper, we used a simulation-based inference method utilizing artificial neural networks to optimally extract cosmological information from a shallow X-ray survey, solely using count rates, hardness ratios, and redshifts. This procedure enabled us to conduct likelihood-free inference of cosmological parameters $ m $ and $ To achieve this, we analytically generated several datasets of 70\,000 cluster samples with totally random combinations of cosmological and scaling relation parameters. Each sample in our simulation is represented by its galaxy cluster distribution in a count rate (CR) and hardness ratio (HR) space in multiple redshift bins. We trained convolutional neural networks (CNNs) to retrieve the cosmological parameters from these distributions. We then used neural density estimation (NDE) neural networks to predict the posterior probability distribution of $ m $ and $ given an input galaxy cluster sample. Using the survey area as a proxy for the number of clusters detected for fixed cosmological and astrophysical parameters, and hence of the Poissonian noise, we analyze various survey sizes. The 1 sigma errors of our density estimator on one of the target testing simulations are 1\,000\,deg$^2$, 15.2<!PCT!> for $ m $ and 10.0<!PCT!> for $ and 10\,000\,deg$^2$, 9.6<!PCT!> for $ m $ and 5.6<!PCT!> for $ We also compare our results with a traditional Fisher analysis and explore the effect of an additional constraint on the redshift distribution of the simulated samples. We demonstrate, as a proof of concept, that it is possible to calculate cosmological predictions of $ m $ and $ from a galaxy cluster population without explicitly computing cluster masses and even the scaling relation coefficients, thus avoiding potential biases resulting from such a procedure.

An X-ray flaring event and a variable soft X-ray excess in the Seyfert LCRS B040659.9–385922 as detected with eROSITA

Context. Extreme continuum variability in extragalactic nuclear sources can indicate extreme changes in accretion flows onto supermassive black holes. Aims. We explore the multiwavelength nature of a continuum flare in the Seyfert LCRS B040659.9−385922. The all-sky X-ray surveys conducted by the Spectrum Roentgen Gamma (SRG)/extended ROentgen Survey with an Imaging Telescope Array (eROSITA) showed that its X-ray flux increased by a factor of roughly five over six months, and concurrent optical photometric monitoring with the Asteroid Terrestrial-impact Last Alert System (ATLAS) showed a simultaneous increase. Methods. We complemented the eROSITA and ATLAS data by triggering a multiwavelength follow-up monitoring program (X-ray Multi-Mirror Mission: XMM-Newton, Neutron star Interior Composition Explorer: NICER; optical spectroscopy) to study the evolution of the accretion disk, broad-line region, and X-ray corona. During the campaign, X-ray and optical continuum flux subsided over roughly six months. Our campaign includes two XMM-Newton observations, one taken near the peak of this flare and the other taken when the flare had subsided. Results. The soft X-ray excess in both XMM-Newton observations was power law-like (distinctly nonthermal). Using a simple power law, we observed that the photon index of the soft excess varies from a steep value of Γ ∼ 2.7 at the flare peak to a relatively flatter value of Γ ∼ 2.2 as the flare subsided. We successfully modeled the broadband optical/UV/X-ray spectral energy distribution at both the flare peak and post-flare times with the AGNSED model, incorporating thermal disk emission into the optical/UV and warm thermal Comptonization in the soft X-rays. The accretion rate falls by roughly 2.5, and the radius of the hot Comptonizing region increases from the flaring state to the post-flare state. Additionally, from the optical spectral observations, we find that the broad He IIλ4686 emission line fades significantly as the optical/UV/X-ray continuum fades, which could indicate a substantial flare of disk emission above 54 eV. We also observed a redshifted broad component in the Hβ emission line that is present during the high flux state of the source and disappears in subsequent observations. Conclusions. A sudden strong increase in the local accretion rate in this source manifested itself via an increase in accretion disk emission and in thermal Comptonized emission in the soft X-rays, which subsequently faded. The redshifted broad Balmer component could be associated with a transient kinematic component distinct from that comprising the rest of the broad-line region.

The eROSITA Final Equatorial Depth Survey (eFEDS). The hard X-ray selected sample

During its calibration and performance verification phase, the eROSITA instrument aboard the Spectrum-RG satellite performed a uniform wide-area X-ray survey of approximately 140 deg2, known as the eROSITA Final Equatorial Depth Survey (eFEDS). The primary aim of eFEDS is to demonstrate the scientific performance to be expected at the end of the eight--pass eROSITA all--sky survey. This survey will provide the first focussed image of the whole sky in the hard X-ray ($>2$ keV) bandpass. The expected source population in this energy range is thus of great interest, particularly for AGN studies. We used a 2.3--5 keV selection to construct a sample of 246 point-like hard X-ray sources for further study and characterisation. These sources are classified as either extragalactic ($ 90$ <!PCT!>) or Galactic ($ 10$ <!PCT!>), with the former consisting overwhelmingly of AGN and the latter active stars. We concentrated our further analysis on the extragalactic AGN sample, describing their X-ray and multi-wavelength properties and comparing them to the eFEDS main AGN sample selected in the softer 0.2--2.3 keV band. The eROSITA hard band selects a subsample of sources that is a factor of more than ten brighter than the eFEDS main sample. The AGN within the hard population reach up to $z=3.2$ but on the whole, they are relatively nearby, with median $z$=0.34 compared to $z$=0.94 for the main sample. The hard survey probes typical luminosities in the range $ X = 43-46$. The X-ray spectral analysis shows significant intrinsic absorption (with $ H >21$) in $ 20$ <!PCT!> of the sources, with a hard X-ray power law continuum with mean $< which is typical of AGN, but slightly harder than the soft-selected eROSITA sample. Around $10$ <!PCT!> of the hard sample show a significant `soft excess' component. The sampled black hole mass distribution in the eFEDS broad-line AGN population is consistent with that of the deeper COSMOS survey that probes a higher redshift population. On the other hand, the Eddington ratios appear systematically lower, which is consistent with the idea that the decline in SMBH activity since $z 1$ is due to a reduction in the typical accretion rate, rather than a shift towards activity in lower-mass black holes. The eFEDS hard sample provides a preview of what can be expected from the eRASS final survey in terms of data quality. This pilot survey indicates the power of eROSITA to shed new light on the demographics and evolution of AGN, and the potential for discovery of new and rare populations.

Stripe 82-XL: The ∼54.8 deg2 and ∼18.8 Ms Chandra and XMM-Newton Point-source Catalog and Number of Counts

We present an enhanced version of the publicly available Stripe 82X catalog (S82-XL), featuring a comprehensive set of 22,737 unique X-ray point sources identified with a significance ≳4σ. This catalog is four times larger than the original Stripe 82X catalog, by including additional archival data from the Chandra and XMM-Newton telescopes. Now covering ∼54.8 deg2 of nonoverlapping sky area, the S82-XL catalog roughly doubles the area and depth of the original catalog, with limiting fluxes (half-area fluxes) of 3.4 × 10−16 (2.4 × 10−15), 2.9 × 10−15 (1.5 × 10−14), and 1.4 × 10−15 (9.5 × 10−15) erg s−1 cm−2 across the soft (0.5–2 keV), hard (2–10 keV), and full (0.5–10 keV) bands, respectively. S82-XL occupies a unique region of flux-area parameter space compared to other X-ray surveys, identifying sources with rest-frame luminosities from 1.2 × 1038 to 1.6 × 1047 erg s−1 in the 2–10 keV band (median X-ray luminosity, 7.2 × 1043 erg s−1), and spectroscopic redshifts up to z ∼ 6. By using hardness ratios, we derived the obscuration of active galactic nuclei (AGNs), obtaining a median value of log(NH/cm−2)=21.6−1.6+1.0 and an overall, obscured fraction ( log(NH/cm−2)>22 ) of ∼36.9%. S82-XL serves as a benchmark in X-ray surveys and, with its extensive multiwavelength data, is especially valuable for comprehensive studies of luminous AGNs.

Stripe 82X Data Release 3: Multiwavelength Catalog with New Spectroscopic Redshifts and Black Hole Masses

We present the third catalog release of the wide-area (31.3 deg2) Stripe 82 X-ray survey. This catalog combines previously published X-ray source properties with multiwavelength counterparts and photometric redshifts, presents 343 new spectroscopic redshifts, and provides black hole masses for 1297 Type 1 active galactic nuclei (AGN). With spectroscopic redshifts for 3457 out of 6181 Stripe 82X sources, the survey has a spectroscopic completeness of 56%. This completeness rises to 90% when considering the contiguous portions of the Stripe 82X survey with homogeneous X-ray coverage at an optical magnitude limit of r < 22. Within that portion of the survey, 23% of AGN can be considered obscured by being either a Type 2 AGN, reddened (R − K > 4, Vega), or X-ray obscured with a column density of N H > 1022 cm−2. Unlike other surveys, there is only an 18% overlap between Type 2 and X-ray obscured AGN. We calculated black hole masses for Type 1 AGN that have Sloan Digital Sky Survey spectra using virial mass estimators calibrated on the Hβ, Mg ii, Hα, and C iv emission lines. We find wide scatter in these black hole mass estimates, indicating that statistical analyses should use black hole masses calculated from the same formula to minimize bias. We find that the AGN with the highest X-ray luminosities are accreting at the highest Eddington ratios, consistent with the picture that most black hole mass accretion happens in the phase when the AGN is luminous (L 2−10keV > 1045 erg s−1).

X-Ray Flashes on Helium Novae

A helium nova occurs on a white dwarf (WD) accreting hydrogen-deficient matter from a helium star companion. When the mass of a helium envelope on the WD reaches a critical value, unstable helium burning ignites to trigger a nova outburst. A bright soft X-ray phase appears in an early outbursting phase of a helium nova before it optically rises toward maximum. Such an X-ray bright phase is called the X-ray flash. We present theoretical light curves of X-ray flashes for 1.0, 1.2, and 1.35 M ☉ helium novae with mass accretion rates of (1.6–7.5) × 10−7 M ☉ yr−1. Long durations of the X-ray flashes (100 days–10 yr) and high X-ray luminosities (∼1038 erg s−1) indicate that X-ray flashes are detectable as a new type of X-ray transient or persistent X-ray sources. An X-ray flash is a precursor of optical brightening, so that the detection of X-ray flashes on helium novae enables us to plan arranged observation for optical premaximum phases that have been one of the frontiers of the study of novae. We found a candidate object of helium-burning X-ray flash from the literature on extragalactic X-ray surveys. This X-ray transient source is consistent with our X-ray flash model of a 1.35 M ⊙ WD.

A first systematic characterization of cataclysmic variables in SRG/eROSITA surveys

We present an account of known cataclysmic variables (CVs) that were detected as X-ray sources in eROSITA X-ray surveys and have gai DR3 counterparts. We address standard CVs with main sequence donors and white dwarfs accreting via Roche-lobe overflow (RLOF) and related objects, the double degenerates (DDs), and the symbiotic stars (SySts). We discern between nonmagnetic (dwarf novae and nova-like objects) and magnetic CVs (polars and intermediate polars (IPs)). In the publically available eROSITA catalog from the recent DR1, typically 65&lt;!PCT!&gt; of known cataloged and classified CVs are detected. This fraction rises to over 90&lt;!PCT!&gt; if the stack of all X-ray surveys (called S45 in this paper) is considered and the search volume is restricted to a radius of 500\,pc. We examine the various classes of CVs in various diagnostic diagrams relating X-ray and optical properties (luminosity, absolute magnitude, color, X-ray spectral hardness, and optical variability) and establish their average class properties. We derive spectral properties for the 22 brightest polars and confirm an increase in the ratio of soft to hard X-rays with increasing magnetic field in the accretion region. We report three new soft IPs and present a spectral analysis of all soft IPs. Their blackbody temperatures agree well with published values. The DDs represent the bluest and faintest subcategory but reach the same identification fraction as the standard CVs. The SySts are the most distant systems; only 20 (13&lt;!PCT!&gt;) were detected as X-ray sources in S45, and 7 of those are first-time detections. We investigate their mean properties using an upper limit on the flux of the nondetected CVs. Their X-ray nondetection is indeed a distance effect. We used all properties combined to select candidate CVs for all-sky optical identification programs, with the ultimate aim being to compose large CV samples in order to better constrain the impact of magnetic fields on the evolution of CVs, to derive space densities and luminosity functions, and to quantify the contribution of white-dwarf accreting systems to the Galactic Ridge X-ray emission (GRXE). The results of the optical identification program will be presented in forthcoming papers.

EROSITA narrowband maps at the energies of soft X-ray emission lines

Hot plasma plays a crucial role in regulating the baryon cycle within the Milky Way, flowing from the energetic sources in the Galactic center and plane, to the corona and the halo. This hot plasma represents an important fraction of the Galactic baryons, plays a key role in galactic outflows and is an important ingredient in galaxy evolution models. Taking advantage of the Spectrum-Roentgen-Gamma (SRG first all-sky survey (eRASS1), in this work our aim is to provide a panoramic view of the hot circumgalactic medium (CGM) of the Milky Way. Compared to the previous all-sky X-ray survey performed by ROSAT, the improved energy resolution of enabled us to map, for the first time, the sky within the narrow energy bands characteristic of soft X-ray emission lines. These lines provide essential information on the physical properties of the hot plasma. Here we present the eRASS1\ half sky maps in narrow energy bands corresponding to the most prominent soft X-ray lines and which allowed us to constrain the distribution of the hot plasma within and surrounding the Milky Way. We corrected the maps by removing the expected contribution associated with the cosmic X-ray background, the time-variable solar wind charge exchange, and the local hot bubble. We applied corrections to mitigate the effect of absorption, therefore highlighting the emission from the CGM of the Milky Way. We used the line ratio of the oxygen lines as a proxy to constrain the temperature of the warm-hot CGM, and we defined a pseudo-temperature $ T $ map. The map highlights how different regions are dominated by different thermal components. Toward the outer halo, the temperature distribution of the CGM on angular scales of 2--20 deg is consistent with being constant $ T T 4&lt;!PCT!&gt;$ with a marginal detection of $ T T = 2.7 &lt;!PCT!&gt; 0.2&lt;!PCT!&gt;$ (statistical) $ 0.6&lt;!PCT!&gt;$ (systematic) in the southern hemisphere. Instead significant variations of $ 12&lt;!PCT!&gt;$ are observed on scales of many tens of degrees when comparing the northern and southern hemispheres. The pseudo-temperature map shows significant variations across the borders of the bubbles, suggesting temperature variations, possibly linked to shocks, between the interior of the Galactic outflow and the unperturbed CGM. In particular, a shell characterized by a lower line ratio appears close to the edge of the bubbles.

XMM-Newton and NuSTAR discovery of a likely IP candidate XMMU J173029.8--330920 in the Galactic disc

We aim to characterise the population of low-luminosity X-ray sources in the Galactic plane by studying their X-ray spectra and periodic signals in the light curves. We are performing an X-ray survey of the Galactic disc using and the source XMMU J173029.8--330920 was serendipitously discovered in our campaign. We performed a follow-up observation of the source using our pre-approved target of opportunity time. We used various phenomenological models in xspec for the X-ray spectral modelling. We also computed the Lomb-Scargle periodogram to search for X-ray periodicity. A Monte Carlo method was used to simulate 1000 artificial light curves in order to estimate the significance of the detected period. We also searched for X-ray, optical, and infrared counterparts of the source in various catalogues. The spectral modelling indicates the presence of an intervening cloud with $N_ H $ that partially absorbs the incoming X-ray photons. The X-ray spectra are best fit by a model representing emission from a collisionally ionised diffuse gas with a plasma temperature of $kT=26^ $ keV. Furthermore, an Fe $K_ alpha $ line at $6.47^ $ keV was detected with an equivalent width of the line of $312 eV. We discovered a coherent pulsation with a period of $521.7 s. The 3--10 keV pulsed fraction of the source is around sim 50--60&lt;!PCT!&gt;. The hard X-ray emission with plasma temperature $kT=26^ $ keV, iron $K_ alpha $ emission at 6.4 keV, and a periodic behaviour of $521.7 s suggest XMMU J173029.8--33092 to be an intermediate polar. We estimated the mass of the central white dwarf to be $0.94-1.4\ odot $ by assuming a distance to the source of $ kpc.

SRG/ART-XC Galactic Bulge deep survey. I. Maximum likelihood source detection algorithm for X-ray surveys

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

SRG/ART-XC all-sky X-ray survey: Catalog of sources detected during the first five surveys

We present an updated catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope aboard the Spektrum-Roentgen-Gamma (SRG) observatory during its all-sky survey. It is based on the data of the first four and the partially completed fifth scans of the sky (ARTSS1-5). The catalog comprises 1545 sources detected in the 4–12 keV energy band. The achieved sensitivity ranges between ~4 × 10−12 erg s−1 cm−2 near the ecliptic plane and ~7 × 10−13 erg s−1 cm−2 near the ecliptic poles, which is a ~30–50% improvement over the previous version of the catalog based on the first two all-sky scans (ARTSS12). There are ~130 objects, excluding the expected contribution of spurious detections, that were not known as X-ray sources before the SRG/ART-XC all-sky survey. We provide information, partly based on our ongoing follow-up optical spectroscopy program, on the identification and classification of the majority of the ARTSS1-5 sources (1463), of which 173 are tentative at the moment. The majority of the classified objects (964) are extragalactic, a small fraction (30) are located in the Local Group of galaxies, and 469 are Galactic. The dominant classes of objects in the catalog are active galactic nuclei (911) and cataclysmic variables (192).

Discovery of the Optical and Radio Counterpart to the Fast X-Ray Transient EP 240315a

Fast X-ray Transients (FXTs) are extragalactic bursts of soft X-rays first identified ≳10 yr ago. Since then, nearly 40 events have been discovered, although almost all of these have been recovered from archival Chandra and XMM-Newton data. To date, optical sky surveys and follow-up searches have not revealed any multiwavelength counterparts. The Einstein Probe, launched in 2024 January, has started surveying the sky in the soft X-ray regime (0.5–4 keV) and will rapidly increase the sample of FXTs discovered in real time. Here we report the first discovery of both an optical and radio counterpart to a distant FXT, the fourth source publicly released by the Einstein Probe. We discovered a fast-fading optical transient within the 3′ localization radius of EP 240315a with the all-sky optical survey ATLAS, and our follow-up Gemini spectrum provides a redshift, z = 4.859 ± 0.002. Furthermore, we uncovered a radio counterpart in the S band (3.0 GHz) with the MeerKAT radio interferometer. The optical (rest-frame UV) and radio luminosities indicate that the FXT most likely originates from either a long gamma-ray burst or a relativistic tidal disruption event. This may be a fortuitous early mission detection by the Einstein Probe or may signpost a mode of discovery for high-redshift, high-energy transients through soft X-ray surveys, combined with locating multiwavelength counterparts.

Compact white dwarf binaries in the combined SRG/eROSITA/SDSS eFEDS survey

Context. X-ray surveys combined with optical follow-up observations are used to generate complete flux-limited samples of the main X-ray emitting source classes. eROSITA on the Spectrum-Roentgen-Gamma mission provides sufficient sensitivity to build significantly enhanced samples of rare X-ray emitting sources. Aims. We strive to identify and classify compact white dwarf binaries, cataclysmic variables (CVs), and related objects, which were detected in the sky area of eFEDS, the eROSITA Final Equatorial Depths Survey, and they were observed in the plate program of SDSS-V. Methods. Compact white dwarf binaries were selected from spectra obtained in the early SDSS-V plate program. A dedicated set of SDSS plate observations were carried out in the eFEDS field, providing spectroscopic classifications for a significant fraction of the optically bright end (r &lt; 22.5) of the X-ray sample. The identification and subclassification rests on visual inspections of the SDSS spectra, spectral variability, color-magnitude and color-color diagrams involving optical and X-ray fluxes, optical variability, and literature work. Results. Upon visual inspection of SDSS spectra and various auxiliary data products, we have identified 26 accreting compact white dwarf binaries (aCWDBs) in eFEDS, of which 24 are proven X-ray emitters. Among those 26 objects, there are 12 dwarf novae, three WZ Sge-like disk-accreting nonmagnetic CVs with low accretion rates, five likely nonmagnetic high accretion rate nova-like CVs, two magnetic CVs of the polar subcategory, and three double degenerates (AM CVn objects). Period bouncing candidates and magnetic systems are rarer than expected in this sample, but it is too small for a thorough statistical analysis. Fourteen of the systems are new discoveries, of which five are fainter than the Gaia magnitude limit. Thirteen aCWDBs have measured or estimated orbital periods, of which five were presented here. Through a Zeeman analysis, we revise the magnetic field estimate of the polar system J0926+0105, which is likely a low-field polar at B = 16 MG. We quantified the success of X-ray versus optical/UV selection of compact white dwarf binaries which will be relevant for the full SDSS-V survey. We also identified six white dwarf main sequence (WDMS) systems, among them there is one confirmed pre-CV at an orbital period of 17.6 h and another pre-CV candidate. Conclusions. This work presents successful initial work in building large samples of all kinds of accreting and X-ray emitting compact white dwarf binaries that will be continued over the full hemisphere in the years to come.

Radiation tolerance test and performance verification of pnCCD at high temperatures for future satellite mission HiZ-GUNDAM

A wide-field X-ray survey in the soft X-ray band is crucial for future satellite missions in the astronomical observations. HiZ-GUNDAM, currently under development, is a proposed satellite designed to observe soft X-ray transients including gamma-ray bursts. This satellite employs wide-field X-ray monitors consisting of lobster-eye optics and focal-plane pixel sensors in the soft X-ray band of 0.4–4 keV. A pnCCD is a candidate for focal-plane Si pixel detectors, featuring a back-illuminated X-ray CCD, large pixel size (70–100 µm), and a large active image area of approximately 55 × 55 mm2 for the flight model. Here, we investigated the basic characteristics and performance of the small-size pnCCD with 128 × 256 pixels, a pixel size of 75 µm, and a detector thickness of 450 µm. High-energy cosmic rays such as protons can degrade the performance of pnCCDs by increasing dark current and charge transfer inefficiency due to ionizing and displacement damage. These factors may affect soft X-ray observations, potentially causing the degradation of lower-detectable energy thresholds and an increase in the number of hot pixels. Therefore, we conducted a radiation tolerance test at room temperature using a proton beam. After irradiating the pnCCD with 10-MeV protons equivalent to three years of nominal operation for HiZ-GUNDAM, we found that the operation temperatures of the pnCCD should be lower than -35 °C. This requirement will be incorporated into the design of the mission operation system.

Active galactic nucleus X-ray luminosity function and absorption function in the Early Universe (3 ≤ z ≤ 6)

The X-ray luminosity function (XLF) of active galactic nuclei (AGN) offers a robust tool to study the evolution and the growth of the supermassive black-hole population over cosmic time. Owing to the limited area probed by X-ray surveys, optical surveys are routinely used to probe the accretion in the high-redshift Universe z ≥ 3. However, optical surveys may be incomplete because they are strongly affected by dust redenning. In this work we derive the XLF and its evolution at high redshifts (z ≥ 3) using a large sample of AGN selected in different fields with various areas and depths covering a wide range of luminosities. Additionally, we put the tightest yet constraints on the absorption function in this redshift regime. In particular, we used more than 600 soft X-ray selected (0.5 − 2 keV) high-z sources in the Chandra deep fields, the Chandra COSMOS Legacy survey, and the XMM-XXL northern field. We derived the X-ray spectral properties for all sources via spectral fitting, using a consistent technique and model. To model the parametric form of the XLF and the absorption function, we used a Bayesian methodology, allowing us to correctly propagate the uncertainties for the observed X-ray properties of our sources and also the absorption effects. The evolution of XLF is in agreement with a pure density evolution model similar to what is witnessed at optical wavelengths, although a luminosity-dependent density evolution model cannot be securely ruled out. A large fraction (∼60%) of our sources are absorbed by column densities of NH ≥ 1023 cm−2, while ∼17% of the sources are Compton-Thick. Our results favour a scenario where both the interstellar medium of the host and the AGN torus contribute to the obscuration. The derived black hole accretion rate density is roughly in agreement with the large-scale cosmological hydrodynamical simulations, if one takes into account the results that the X-ray AGN are hosted by massive galaxies, while it differs from that derived using JWST data. The latter could be due to the differences in the AGN and host-galaxy properties.

The SRG/eROSITA All-Sky Survey

Clusters of galaxies can be used as powerful probes to study astrophysical processes on large scales, test theories of the growth of structure, and constrain cosmological models. The driving science goal of the SRG/eROSITA All-Sky Survey is to assemble a large sample of X-ray clusters with a well-defined selection function to determine the evolution of the mass function and, hence, the cosmological parameters. We present here a catalog of 12 247 optically confirmed galaxy groups and clusters detected in the 0.2–2.3 keV as extended X-ray sources in a 13 116 deg2 region in the western Galactic half of the sky, which eROSITA surveyed in its first six months of operation. The clusters in the sample span the redshift range 0.003 &lt; z &lt; 1.32. The majority (68%) of these clusters, 8361 sources, represent new discoveries without known counterparts in the literature. The mass range of the sample covers three orders of magnitude from 5 × 1012 Msun to 2 × 1015Msun. We construct a sample for cosmology with a higher purity level (~95%) than the primary sample, comprising 5259 securely detected and confirmed clusters in the 12791 deg2 common footprint of eRASS1 and the DESI Legacy Survey DR10. We characterize the X-ray properties of each cluster, including their flux, luminosity and temperature, the total mass, gas mass, gas mass fraction, and mass proxy YX. These are determined within two apertures, 300 kpc, and the overdensity radius R500, and are calculated by applying a forward modeling approach with a rigorous X-ray background treatment, K-factor, and the Galactic absorption corrections. Population studies utilizing log N-log S, the number of clusters detected above a given flux limit, and the luminosity function show overall agreement with the previous X-ray surveys after accounting for the survey completeness and purity through the selection function. The first eROSITA All-Sky Survey provides an unprecedented sample of galaxy groups and clusters selected in the X-ray band. The eRASS1 cluster catalog demonstrates the excellent performance of eROSITA for extended source detection, consistent with the pre-launch expectations for the final all-sky survey, eRASS:8.