Context. After nearly a decade of quiescence, the transient Be/X-ray binary pulsar RX J0520.5–6932 underwent an outburst in 2024. We performed X-ray monitoring of the source with NICER and AstroSat near the peak of the event. Aims. Our primary objective was to investigate the energy and luminosity dependence of the pulsed emission, characterize the spin evolution, and study the broadband X-ray spectral properties of RX J0520.5–6932 during the outburst. Methods. We extracted light curves and spectra from NICER and AstroSat observations carried out during the outburst. Pulsations were detected using epoch-folding techniques, enabling a detailed study of pulse-profile evolution as a function of energy and intensity. Broadband spectral modeling was performed using simultaneous data from SXT, LAXPC, and NICER. The spectra from individual NICER observations were used to study spectral variability. Results. The AstroSat/LAXPC and NICER light curves reveal pronounced short-duration flaring activity lasting ∼400–700 s with enhancements in intensity by about a factor of two. The pulse profile exhibits a strong dependence on both energy and intensity, evolving from a simple single-peaked structure at low energies to complex multi-peaked shapes at intermediate energies and reverting to simpler morphologies at higher energies. Pulse profiles during the flares differ significantly from those in the persistent state, indicating changes in the pulsed beam pattern with a change in the intensity on a short timescale. Broadband spectral analysis revealed a soft excess and an emission feature at ∼1 keV likely arising from reprocessed emission in the accretion disk and fluorescence from Ne K and Fe L ions. Continuous NICER monitoring over nearly one orbital cycle enabled us to track spin evolution with accretion-driven spin-up and spectral variability in the soft X-ray band. Additionally, we observed a declining spin-up rate during the outburst, likely due to a gradual reduction in mass accretion rate. Conclusions. Our results provide a comprehensive view of the complex accretion dynamics in RX J0520.5–6932 during its 2024 outburst. The strong variability in pulse shape and spin behavior highlights rapid changes in the accretion geometry and torque as a function of accretion rate.

The first four all-sky surveys with eROSITA the soft X-ray instrument on board the Spektrum-Roentgen-Gamma (SRG) satellite revealed a new X-ray source, eRASSU J012422.9-724248, in the Magellanic Bridge, near the Eastern Wing of the Small Magellanic Cloud (SMC). We performed a broad-band timing and spectral analysis using the optical and X-ray data of eRASSU J012422.9-724248. Using the X-ray observations with eROSITA, Swift, NuSTAR and optical data from the optical Gravitational Lensing Experiment (OGLE) and the Las Cumbres Observatory (LCO), we confirm the nature of eRASSU J012422.9-724248as a Be/X-ray binary (BeXRB) pulsar in the Magellanic Bridge. The position is coincident with that of an early-type star (OGLE ID SMC732.10.7). We detect the spin period at 341.71 s in NuSTAR data and infer a period of 63.65 d from the 15 yr monitoring with OGLE, that we interpret as the orbital period of the system. A tentative CRSF at 12.3 keV is identified in NuSTAR spectra with 1.8 sigma. The source appears to show a persistent X-ray luminosity and an optical magnitude transition on the long timescale. We propose eRASSU J012422.9-724248is a new member of the class of persistent BeXRBs.

We present timing and spectral analysis of the recently identified ultracompact double-degenerate (DD) white dwarf binary eRASSU J060839.5-704014 using observations from NICER and Einstein Probe (EP), together with archival XMM-Newton data. By phase-connecting the long-term XMM-Newton, NICER, and EP observations, we obtain a coherent quadratic timing solution, yielding an orbital period of 374.15013 (2) s and an orbital decay rate of Ṗ= -4.7 (1) × 10^-11 s s^-1. This orbital decay exceeds that measured in the prototypical DD binaries HM Cnc and V407 Vul. Assuming that the observed orbital evolution is primarily driven by gravitational-wave (GW) angular momentum loss, the inferred chirp mass is ∼0.43 M_⊙, placing the source among the most massive known systems of this class. The phase-averaged spectra of NICER and EP-Follow-up X-ray Telescope (FXT) are described by a soft thermal component with temperatures of 126 and 144 eV, respectively, confirming the supersoft nature of the source. Phase-resolved spectroscopy reveals a clear decrease in temperature across the bright phase in both instruments, indicating a structured emission region with significant temperature gradients. These results establish eRASSU J060839.5-704014 as one of the most rapidly evolving ultracompact DD binaries presently known, belonging to the rare class of direct-impact ultracompact binaries, and a promising verification source for future low-frequency GW studies.

The Magellanic Bridge stellar population is a relic of the tidal interaction between the Large and Small Magellanic Clouds. A comprehensive view of the evolution of the Bridge stellar population requires probing the compact remnants of stellar evolution, otherwise hidden at optical wavelengths. The all-sky survey conducted by the eROSITA instrument on-board the Spectrum Roentgen Gamma observatory has discovered a significant population of compact-object-powered systems in the Bridge using X-ray. The candidate super-soft source eRASSU J043115.8-711730 (hereafter J0431-71) was discovered as a part of this campaign, and we present here a deeper study of this source using XMM-Newton and SALT spectroscopy, long-term optical-infrared photometry using OGLE, ATLAS, ASAS-SN, WISE, and GAIA data. J0431-71 is a highly variable super-soft X-ray source, classified as a red giant with the GAIA color-magnitude diagram. The source exhibits: (a) a thermal X-ray spectrum with a temperature of kT∼30 eV and a bright state luminosity of 3.2×10^37 erg s^-1 in the 0.15-1 keV band, (b) Balmer emission lines, [Fe X] coronal line, HeII emission, and the Bowen fluorescence blend, (c) an optical and infrared periodicity of ∼500-560 days in phase with the X-ray-UV emission, (d) a 'redder-when-brighter' trend in the stellar emission with a ∼520 day period indicating a pulsating donor star. We argue that the observed spectral and temporal properties in J0431-71 are consistent with a high-accretion rate onto a white-dwarf via Roche-lobe overflow, making J0431-71 the first symbiotic source discovered in the Bridge.

Context. The study of astronomical transients at high energies provides insights into some of the most extreme physical events in the universe; however, carrying out their detection and fast follow-up studies are limited by instrumental constraints. Search for Transient Object in New observations using Known Sources (STONKS) is a near-real-time transient detection system for XMM-Newton offering the capability to detect transients in XMM-Newton observations at fainter fluxes than can be achieved with wide survey instruments. Aims. We present the transients detected with the STONKS pipeline found in an XMM-Newton multi-year heritage survey of the Galactic plane to identify and classify highly variable X-ray sources that have recently been reported in this region. Methods. We examined the alerts created by the STONKS pipeline from over 200 XMM observations of the Galactic plane, screening for instrumental effects. The 78 alerts associated with 70 real astrophysical sources were then subjected to further temporal and spectral analysis. Results. From the 70 sources we identified, we were able to classify 32 with a high degree of confidence, including 7 X-ray binaries, 1 γ-Cas analogue, and 1 magnetar candidate. Of the 70 sources, 23 were detected for the first time in X-rays. Conclusions. This systematic analysis of publicly available data has shown the value and potential of STONKS in the application to XMM-Newton observations. It will enable the community to detect transient and highly variable sources at fainter fluxes than with any other X-ray transient detection systems.

Aims. Supernova remnants (SNRs) are responsible for the injection of energy and chemical elements into the interstellar medium (ISM). The emission from SNRs can be studied to infer information about the supernova (SN) explosion itself as well as about the properties of the surrounding ISM. Studying a sample of SNRs in a galaxy provides an opportunity to better understand stellar feedback, and the best laboratory for such an investigation is the Large Magellanic Cloud (LMC). The LMC is the nearest star-forming galaxy, it lies outside the Galactic plane line of sight, and therefore its foreground absorption is low. Methods. The eROSITA telescopes are the best instruments currently available to perform a survey of the SNRs in the entire LMC due to their large field of view and their high sensitivity towards soft X-rays. We used the sample of SNRs reported in the previous paper and performed a spectral analysis on a part of the sample. We estimated the flux and the luminosity of the fainter sources using the energy conversion factor obtained assuming a non-equilibrium ionisation plasma model. Results. The X-ray luminosity function (XLF) of SNRs in the LMC shows a relatively large number of SNRs at high luminosities. We fitted the distribution with two Gaussian components, which yielded best-fit maxima for the L-X[0.3-8.0keV] distribution at m(1) = 10(34.7 +/- 0.2) erg s(-1) and m(2) = 10(36.5 +/- 0.4) erg s(-1). We compared the XLF of the LMC with the XLFs of the Small Magellanic Cloud (SMC), M31, and M33 using a power-law fit and an Anderson-Darling (DA) test. The power indices of the XLFs of the LMC and SMC appear consistent with each other, while those of M33 and M31 are larger. Thus, the latter have steeper power laws, indicating a lower number of X-ray luminous SNRs with respect to the Magellanic Clouds. The DA test showed that the luminosity distributions of SNRs in the SMC and LMC are compatible with being extracted from the same underlying distribution. They are also compatible for different galaxies if we consider the same lower limit for L-X [0.3-8.0 keV] for the entire distribution. Finally, we compared the luminosity and the X-ray sizes (diameter) of the SNRs in our sample. We observed a general trend of anti-correlation between size and X-ray luminosity that can be interpreted as a result of fading with time.

Aims. Supernova remnants (SNRs) are responsible for the injection of energy and chemical elements into the interstellar medium (ISM). The emission from SNRs can be studied to infer information about the supernova (SN) explosion itself as well as about the properties of the surrounding ISM. Studying a sample of SNRs in a galaxy provides an opportunity to better understand stellar feedback, and the best laboratory for such an investigation is the Large Magellanic Cloud (LMC). The LMC is the nearest star-forming galaxy, it lies outside the Galactic plane line of sight, and therefore its foreground absorption is low. Methods. The eROSITA telescopes are the best instruments currently available to perform a survey of the SNRs in the entire LMC due to their large field of view and their high sensitivity towards soft X-rays. We used the sample of SNRs reported in the previous paper and performed a spectral analysis on a part of the sample. We estimated the flux and the luminosity of the fainter sources using the energy conversion factor obtained assuming a non-equilibrium ionisation plasma model. Results. The X-ray luminosity function (XLF) of SNRs in the LMC shows a relatively large number of SNRs at high luminosities. We fitted the distribution with two Gaussian components, which yielded best-fit maxima for the LX[0.3-8.0keV] distribution at m1 = 1034.7±0.2 erg s−1 and m2 = 1036.5±0.4 erg s−1. We compared the XLF of the LMC with the XLFs of the Small Magellanic Cloud (SMC), M31, and M33 using a power-law fit and an Anderson-Darling (DA) test. The power indices of the XLFs of the LMC and SMC appear consistent with each other, while those of M33 and M31 are larger. Thus, the latter have steeper power laws, indicating a lower number of X-ray luminous SNRs with respect to the Magellanic Clouds. The DA test showed that the luminosity distributions of SNRs in the SMC and LMC are compatible with being extracted from the same underlying distribution. They are also compatible for different galaxies if we consider the same lower limit for LX [0.3-8.0 keV] for the entire distribution. Finally, we compared the luminosity and the X-ray sizes (diameter) of the SNRs in our sample. We observed a general trend of anti-correlation between size and X-ray luminosity that can be interpreted as a result of fading with time.

The Einstein Probe (EP) is an interdisciplinary mission of time-domain and X-ray astronomy. Equipped with a wide-field lobster-eye X-ray focusing imager, EP will discover cosmic X-ray transients and monitor the X-ray variability of known sources in 0.5–4 keV, at a combination of detecting sensitivity and cadence that is not accessible to the previous and current wide-field monitoring missions. EP can perform quick characterisation of transients or outbursts with a Wolter-I X-ray telescope onboard. In this paper, the science objectives of the EP mission are presented. EP is expected to enlarge the sample of previously known or predicted but rare types of transients with a wide range of timescales. Among them, fast extragalactic transients will be surveyed systematically in soft X-rays, which include γ-ray bursts and their variants, supernova shock breakouts, and the predicted X-ray transients associated with binary neutron star mergers. EP will detect X-ray tidal disruption events and outbursts from active galactic nuclei, possibly at an early phase of the flares for some. EP will monitor the variability and outbursts of X-rays from white dwarfs, neutron stars and black holes in our and neighbouring galaxies at flux levels fainter than those detectable by the current instruments, and is expected to discover new objects. A large sample of stellar X-ray flares will also be detected and characterised. In the era of multi-messenger astronomy, EP has the potential of detecting the possible X-ray counterparts of gravitational wave events, neutrino sources, and ultra-high energy γ-ray and cosmic ray sources. EP is expected to help advance the studies of extreme objects and phenomena revealed in the dynamic X-ray universe, and their underlying physical processes. Besides EP’s strength in time-domain science, its follow-up telescope, with excellent performance, will also enable advances in many areas of X-ray astronomy.

Context. The Large Magellanic Cloud (LMC), being a nearby and actively star-forming satellite galaxy of the Milky Way, is an ideal site to observe the multiphase interstellar medium (ISM) of a galaxy across the electromagnetic spectrum. Aims. We aimed to exploit the available SRG/eROSITA all-sky survey data to study the distribution, composition and properties of the diffuse X-ray emitting hot gas in the LMC. Methods. We constructed multiband X-ray images of the LMC, reflecting the morphology and temperatures of the diffuse hot gas. By performing spatially resolved X-ray spectroscopy of 175 independent regions, we constrained the distribution, temperature, mass, energetics and composition of the hot ISM phase throughout the galaxy, while also testing for the presence of X-ray synchrotron emission. We combined our constraints with multiwavelength data to obtain a comprehensive view of the different ISM phases. Results. We measure a total X-ray luminosity of the hot ISM phase of 1.9 x 10(38) erg s(-1) (0.2-5.0 keV band), and constrain its thermal energy to around 9 x 10(54) erg. The typical density and temperature of the X-ray emitting plasma are around 5 x 10(-3) cm(-3) and 0.25 keV, respectively, with both exhibiting broad peaks in the southeast of the LMC. The observed degree of X-ray absorption correlates strongly with the distribution of foreground H I gas, whereas a spatial anticorrelation between the hot and cold ISM phases is visible on sub-kpc scales within the disk. The abundances of light metals show a strong gradient throughout the LMC, with the north and east exhibiting a strong alpha-enhancement, as expected from observed massive stellar populations there. In contrast, the enigmatic "X-ray spur" exhibits a local deficit in alpha-elements, and a peak in hot-gas pressure at P/k similar to 10(5) K cm(-3), consistent with a dominant energy input through tidally driven gas collisions. Finally, we tentatively identify spectroscopic signatures of nonthermal X-ray emission from the supergiant shell LMC 2, although contamination by straylight cannot be excluded.

On 2024 May 27, the Wide-field X-ray Telescope on board the Space Sciences, University of Chinese Academy of Einstein Probe (EP) mission detected enhanced X-ray emission from a new transient source in the Small Magellanic Cloud during its commissioning phase. Prompt follow-up with the EP Follow-up X-ray Telescope, the Swift X-ray Telescope. and NICER have revealed a very soft, thermally emitting source (kT ~ 0.1 keV at the outburst peak) with an X-ray luminosity of L ~ 4 × 1038 erg s−1, labeled EP J005245.1−722843. This supersoft outburst faded very quickly in a week's time. Several emission lines and absorption edges were present in the X-ray spectrum, including deep nitrogen (0.67 keV) and oxygen (0.87 keV) absorption edges. The X-ray emission resembles the supersoft source phase of typical nova outbursts from an accreting white dwarf (WD) in a binary system, despite the X-ray source being historically associated with an O9-B0e massive star exhibiting a 17.55 day periodicity in the optical band. The discovery of this supersoft outburst suggests that EP J005245.1−722843 is a BeWD X-ray binary: an elusive evolutionary stage where two main-sequence massive stars have undergone a common envelope phase and experienced at least two episodes of mass transfer. In addition, the very short duration of the outburst and the presence of Ne features hint at a rather massive, i.e., close to the Chandrasekhar limit, Ne–O WD in the system.

When a star dies, it can explode in a supernova, causing a strong shock wave and forming an interstellar object called a supernova remnant (SNR). Observational studies of SNRs allow us to learn about the different types of progenitors, the explosion mechanisms, the physics of interstellar shocks, and the matter cycle in galaxies. We report on the first detection of SNRs located on the outskirts of a galaxy; namely, the Large Magellanic Cloud (LMC), the largest satellite galaxy of our Galaxy. The sources were discovered similar to 3 degrees outside the main stellar and gas distribution of the LMC in the recent surveys in radio with the Australian Square Kilometre Array Pathfinder (ASKAP) and in X-rays with the extended Roentgen Survey with an Imaging Telescope Array (eROSITA). We studied them in follow-up observations with the X-ray Multi-Mirror Mission-Newton telescope and MeerKAT and confirmed them to be SNRs. Their progenitors are most likely stars that had left the LMC due to tidal interaction between the Magellanic Clouds and the Milky Way. SNR J0614-7251 is located in an environment with a similar density to those of the other known SNRs in the LMC, and has similar X-ray properties. SNR J0624-6948, on the other hand, is located in a region with a lower density, n(0) < 0.01 cm(-3). Its radio shell shows a spectral index and polarisation typical of an SNR.

ABSTRACTeROSITA is the soft X‐ray instrument aboard the Spectrum Roentgen Gamma (SRG) satellite that is most sensitive in the energy range between 0.2 and 2.3 keV. Between December 2019 and December 2021, eROSITA completed four all‐sky surveys, producing all‐sky X‐ray source lists and sky maps of unprecedented depth. In the energy range between 0.2 keV and 1 keV, we detected about 38,000 sources with a hardness ratio below −0.94, covering a small sample of known white dwarfs found with eROSITA in the dataset to which the German eROSITA consortium has rights (half sky). Two hundred and sixty four of these soft sources have a probability of more than 90% to be a white dwarf. This is more than the 175 white dwarfs ROSAT found in the whole sky. Here we present the results of a pilot study to increase the sensitivity of eROSITA for soft sources by extending the detection threshold down to 0.1 keV. First tests with dedicated sky regions are promising.

With its first All-Sky Survey (eRASS1), the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) mission has offered an unprecedented, comprehensive view of the variable X-ray sky. Featuring enhanced sensitivity, broader energy coverage, and improved resolution compared to prior surveys, the eRASS1 Data Release 1 (DR1) catalogue underwent a variability analysis, and in this paper, we performed an advanced variability analysis focusing on a substantial subset of 128 669 sources, all exhibiting a net count exceeding ten. We performed multiple variability tests, utilising conventional normalised excess variance (NEV), maximum amplitude variability (AMP), and Bayesian excess variance methods (bexvar). The analysis focused on binned light curves; specifically, employing one eroday (a great circle scan with a duration of 4 hours) binning of the German part of the first eROSITA all-sky survey (eROSITA-DE) data, i.e., the source sample covers only half of the sky. Within the 128 669 DR1 sources with light curves, our research pinpointed 808 light curves that show hints of variability according to the AMP test, and 298 according to the NEV test. However, after applying suitable thresholds, 90 (123) sources were found to be significantly variable according to the AMP (NEV) tests. In addition, 1342 sources are considered variable according to the Bayesian test bexvar. The total number of unique sources is 1709, and they form the catalogue of variable sources released with this paper. We cross-matched with existing X-ray catalogues and identified 258, 318, 598, and 120 sources in 4XMM DR13, 2SXPS, 2RXS, and CSC2.1, respectively. Only 27 sources overlap across all catalogues, while 882 are new X-ray detections from eROSITA DR1. About 70% are coronal stars, 5% are Quasi-Stellar Objects, and 1.6% are normal galaxies. We further subclassified 18 sources as LMXBs, 11 as HMXBs, and 14 as bright stars. In this paper, we analyse the variability of eRASS1 sources on a timescale of only a few days. To study the physics of variable sources, we need more deeply pointed observations with other X-ray missions or at least the final depth of the eRASS: 8 observations. The timescale of the eRASS1 observations is not representative of the timescales of the expected upcoming eRASS catalogues. A substantial 52% of the eRASS1 variable sources were first discovered with eROSITA. The DR1 variability catalogue is excellent for follow-up observations with telescopes such as XMM-Newton , Chandra , or Swift .

We present a study of X-ray normal galaxies using data from the first all-sky scan of the eROSITA X-ray survey. eRASS1 provides the first unbiased X-ray census of normal galaxies allowing us to study the X-ray emission from XRBs and the hot ISM in the full range of stellar population parameters present in the local Universe. By combining the HECATE value-added galaxy catalogue with the eRASS1, we study the X-ray emission from normal galaxies as a function of their SFR, M$_{*}$, Metallicity, and stellar population age. After applying optical and mid-IR activity classification criteria, we constructed a sample of 18790 star-forming galaxies with measurements of their L$_{X}$. By stacking the X-ray data in SFR-M$_{*}$-distance bins we study the correlation between the average L$_{X}$ and stellar population parameters. We also present updated L$_{\rm{X}}$-SFR and L$_{\rm{X}}$/SFR-Metallicity scaling relations accounting for the scatter dependence on the SFR. We find that the integrated L$_{X}$ of the HEC-eR1 star-forming galaxies is significantly elevated with respect to that expected from the current scaling relations. The observed scatter is also significantly larger. This excess persists even when we measure the average L$_{X}$ of galaxies in SFR-M$_{*}$-distance and metallicity bins and it is stronger in lower SFRs. The excess is not the result of hot gas, LMXBs, background AGN, LLAGN (including TDEs), or stochastic sampling of the XRB XLF. We find that while the excess correlates with lower metallicity, its primary driver is the age of the stellar populations. Our analysis reveals a sub-population of X-ray luminous starbursts with high sSFRs, low metallicities, and young stellar populations. This population drives upwards the X-ray scaling relations for star-forming galaxies, and has important implications for understanding the population of XRBs in the local and high-z Universe.

Detecting a pulsar associated with a supernova remnant (SNR) and/or pulsar wind nebula (PWN) is crucial for unraveling its formation history and pulsar wind dynamics, yet the association with a radio pulsar is observed only in a small fraction of known SNRs and PWNe. In this paper, we report the discovery of a young pulsar J1631-4722, associated with the Galactic SNR G336.7+0.5 using Murriyang, Commonwealth Scientific and Industrial Research Organisation (CSIRO)'s Parkes radio telescope. It is also potentially associated with a PWN revealed by the Rapid ASKAP (Australian Square Kilometre Array Pathfinder) Continuum Survey (RACS). This 118 ms pulsar has a high dispersion measure of similar to 873 pc cm(-3) and a rotation measure of -1004 rad m(-2). Because of such a high DM, at frequencies below 2 GHz, the pulse profile is significantly scattered, making it effectively undetectable in previous pulsar surveys at similar to 1.4 GHz. Follow-up observations yield a period derivative of P-center dot=5.5x10(-14), implying a characteristic age, tau(c)=33.9 kyr, and spin-down luminosity, E-center dot=1.3x10(36 )erg s(-1). PSR J1631-4722, with its high spin-down luminosity and potential link to a PWN, stands out as a promising source of the high-energy gamma-ray emission observed in the region.

We used a supervised machine learning algorithm (probabilistic random forest) to classify similar to 130 million sources in the VISTA Survey of the Magellanic Clouds (VMC). We used multiwavelength photometry from optical to far-infrared as features to be trained on, and spectra of active galactic nuclei (AGNs), galaxies and a range of stellar classes including from new observations with the Southern African Large Telescope (SALT) and South African Astronomical Observatory (SAAO) 1.9-m telescope. We also retain a label for sources that remain unknown. This yielded average classifier accuracies of similar to 79 per cent [Small Magellanic Cloud (SMC)] and similar to 87 per cent [Large Magellanic Cloud (LMC)]. Restricting to the 56 696 719 sources with class probabilities (P-class) > 80 per cent yields accuracies of similar to 90 per cent (SMC) and similar to 98 per cent (LMC). After removing sources classed as 'Unknown', we classify a total of 707 939 (SMC) and 397 899 (LMC) sources, including >77 600 extragalactic sources behind the Magellanic Clouds. The extragalactic sources are distributed evenly across the field, whereas the Magellanic sources concentrate at the centres of the Clouds, and both concentrate in optical/IR colour-colour/magnitude diagrams as expected. We also test these classifications using independent data sets, finding that, as expected, the majority of X-ray sources are classified as AGN (554/883) and the majority of radio sources are classed as AGN (1756/2694) or galaxies (659/2694), where the relative AGN-galaxy proportions vary substantially with radio flux density. We have found >49 500 hitherto unknown AGN candidates, likely including more AGN dust dominated sources which are in a critical phase of their evolution; >26 500 new galaxy candidates and >2800 new young stellar object (YSO) candidates.

The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with 1.5” imaging over a wide 24' field of view and an order of magnitude greater collecting area than Chandra in the 0.3-12 keV band. Combining sharp imaging, high throughput, and rapid response capabilities, AXIS will open new windows on virtually every aspect of modern astrophysics, exploring the birth and growth of supermassive black holes, the feedback processes that shape galaxies, the life cycles of stars and exoplanet environments, and the nature of compact stellar remnants, supernova remnants, and explosive transients. This book compiles 138 community-contributed science cases developed by five Science Working Groups focused on AGN and supermassive black holes, galaxy evolution and feedback, compact objects and supernova remnants, stellar physics and exoplanets, and time-domain and multi-messenger astrophysics. Together, these studies establish the scientific foundation for next-generation X-ray exploration in the 2030s and highlight strong synergies with facilities of the 2030s, such as JWST, Roman, Rubin/LSST, SKA, ALMA, ngVLA, and next-generation gravitational-wave and neutrino networks.

Context. Nova explosions occur on accreting white dwarfs. A thermonuclear runaway in the H-rich accreted envelope causes its ejection without destroying the white dwarf, and an increase in the luminosity by several magnitudes. Accretion is re-established some time after the explosion. The explosion of the nova itself is expected to affect the mass-transfer rate from the secondary and the accretion rate, but these effects have been little explored observationally. Most novae are observed only in outburst, and the properties of the host systems are unknown. Aims. X-ray observations of novae happen mostly during outburst; only a few have been the target of dedicated X-ray observations years or decades after outburst. However, the X-ray emission long after the outburst provides a powerful diagnostic of the accretion rate and the possible magnetic nature of the white dwarf. Methods. We explored the first two years of the SRG/eROSITA All-Sky Survey (eRASS) for X-ray sources correlated with Galactic historical novae. We present the first population study of nova hosting systems in X-rays, focusing on the evolution of the accretion rate as a function of time since the last outburst, and look for new candidates for magnetic systems. Results. In total, 32 X-ray counterparts of novae are found in the western Galactic hemisphere. Combined with 53 nova detections published for the eastern hemisphere, the fraction of X-ray detected novae in quiescence is 18% of the Galactic novae. We have, for the first time, enough statistics to observationally determine the evolution of accretion rate as a function of time since the last nova outburst for a time span of 120 years. The results confirm that magnetic systems remain systematically at higher fluxes and that accretion is enhanced during the first years after the outburst, as theoretically predicted. We also identify new intermediate polar candidates in AT Cnc and RR Cha.

The eROSITA instrument on board Spectrum-Roentgen-Gamma (SRG) has completed four scans of the X-ray sky, leading to the detection of almost one million X-ray sources in eRASS1 alone, including multiple new X-ray binary candidates. We report on analysis of the X-ray binary 1eRASS J085039.9−421151, using a ∼55 ks long NuSTAR observation, following its detection in each eROSITA scan. An analysis of the eROSITA and NuSTAR X-ray spectra in combination with X-shooter data of the optical counterpart provide evidence of an X-ray binary with a red supergiant (RSG) companion, confirming previous results. However, we did determine a cooler spectral type for M2–3, owing to the presence of TiO bands in the optical and near-infrared spectra. The X-ray spectrum is well-described by an absorbed power law with a high-energy cutoff typically applied for accreting high mass X-ray binaries. In addition, we detected a strong fluorescent neutral iron line with an equivalent width of ∼700 eV and an absorption edge, the latter indicating strong absorption by a partial covering component. It is unclear whether the partial absorber is ionised. There is no significant evidence of a cyclotron resonant scattering feature. We did not detect any pulsations in the NuSTAR light curves, possibly on account of a large spin period that has gone undetected due to insufficient statistics at low frequencies or potentially large absorption that causes pulsations to be smeared out. Even so, the low persistent luminosity, the spectral parameters observed (photon index, Γ < 1.0), and the minuscule likelihood of detection of RSG-black hole systems suggest that the compact object is a neutron star.

Aims. During the Spectrum Roentgen Gamma (SRG)/eROSITA all-sky surveys, X-ray sources close to the South Ecliptic Pole (SEP) are observed almost every 4 h. We aim to identify the sources exhibiting the most significant long-term X-ray variability within 3 degrees of the SEP in the first three surveys, and investigate their properties. Methods. We determined the variability significance of similar to 10(4) sources observed by eROSITA within 3 degrees of the SEP by using thresholds on the Bayesian excess variance (SCATT_LO) and the maximum amplitude deviation (AMPL_SIG). Sources exhibiting a variability significance above 3 sigma were subdivided into likely Galactic and extragalactic sources, by using spectral and photometric information of their optical counterparts. We quantified the X-ray normalised excess variances of all variable sources, and also calculated the periodograms of the brightest ones. Results. Out of more than 10(4) X-ray sources detected by eROSITA within 3 degrees of the SEP, we identified 453 that exhibit significant X-ray variability. SCATT_LO is significantly more sensitive to detecting variable sources in this field, but AMPL_SIG helps to provide a more complete variability sample. Of those variable sources, 168 were classified as likely extragalactic, and 235 as likely Galactic. The periodograms of most bright and variable extragalactic sources are approximately described by an aliased power law (P infinity nu(-alpha)) with an index of alpha approximate to 1. We identified a potential tidal disruption event, and long-term transient sources. The stellar X-ray variability was predominantly caused by bright X-ray flares from coronally active stars.