arXiv subjects

A. Marino

Publications and source records attributed to A. Marino.

At least 19 recordsLinked to original sources

arXiv2026

A FAST search for radio pulsations during the dormant state of the AMSPs IGR J00291+5934 and MAXI J1957+032

Accreting millisecond pulsars (AMSPs) and transitional millisecond pulsars (tMSPs) are neutron star low-mass X-ray binaries which can evolve into "recycled" radio millisecond pulsars. In both types of systems, X-ray pulsations have been detected during phases of X-ray activity when matter accretion through a disc is turned on. On the other hand, when accretion stops, and these systems enter the quiescent, low-luminosity X-ray state, only tMSPs become visible as radio pulsars. Despite several attempts, radio pulsations have never been detected in quiescent AMSPs, except for IGR J18245$-$2452. In this manuscript, we present the results of two observational campaigns performed on the AMSPs IGR J00291+5934 and MAXI J1957+032 with the Five-hundred-meter Aperture Spherical Telescope ($\it{FAST}$) in L-band (1-1.5 GHz). Both sources have most likely been observed in quiescence, as suggested by the upper limits on their X-ray and optical flux obtained with Swift and the Las Cumbres Observatory, respectively. We have performed a deep search for coherent periodicities in radio but found no significant candidate signal, either at the known spin frequency of those sources or at other frequencies. Assuming a pulse duty cycle of 10%, we derive upper limits on the pulsed radio flux density of 3.3 $\mu$Jy and 5.6 $\mu$Jy for IGR J00291+5934 and MAXI J1957+032, respectively, which are the most stringent limits so far for any known persistent AMSP.

astro-ph.HE↗

arXiv2026

Discovery of EP J175257.3-351923 as a Candidate Black Hole Low-Mass X-ray Binary

We report the discovery of a new X-ray transient, EP J175257.3-351923 (EP250916a), by the Einstein Probe (EP) near the Galactic plane. The outburst lasted for at least 250\,days, reached a peak 2--10 keV flux of $\sim 4 \times 10^{-10}$~erg~cm$^{-2}$~s$^{-1}$, and exhibited a fast-rise exponential-decay (FRED) profile typical of X-ray binary outbursts. The source remained in the hard state throughout the outburst, with only modest variations in the photon index ($\sim 1.6$--$2.2$) and no evidence for a spectral state transition. Broadband spectral modeling suggests a truncated disk, a weak reflection component, and a high-energy cutoff at $\sim 217$~keV, consistent with hard-state accretion in black-hole systems. No reliable optical or radio counterpart is detected within the Swift/XRT error circle. The inferred X-ray-to-optical and X-ray-to-radio flux ratios are consistent with a low-mass X-ray binary classification. Neither pulsations nor significant aperiodic variability are detected. Although the compact object cannot yet be firmly identified, the timing, spectral, and multiwavelength evidence favors EP~J175257.3--351923 as a black-hole low-mass X-ray binary candidate, highlighting EP's potential to uncover a faint, previously hidden population of X-ray binaries.

astro-ph.HE↗

arXiv2026

The first IXPE view of the eclipsing ADC source 4U 1822-37

Accretion-disc corona sources are high-inclination low-mass X-ray binaries in which the innermost regions are hidden and the observed X-ray emission is dominated by radiation scattered above the disc. 4U 1822-37 is a key binary system of this class, but its geometry is still debated. X-ray polarimetry offers a direct probe of the scattering structure. We present the first X-ray spectro-polarimetric study of 4U 1822-37 and test whether its spectrum and polarisation can be explained within the accretion-disc-corona scenario. We analysed a coordinated campaign with IXPE, XMM-Newton, NuSTAR, and Swift. We performed broadband spectral modelling and model-independent, energy-resolved, and orbital-phase-resolved polarimetric analyses. The broadband spectrum requires a soft thermal component, a Comptonised continuum, a hard power-law tail, and relativistically blurred reflection. The observed 0.1-100 keV luminosity, $L_{\rm obs}\simeq6.1\times10^{36}\ {\rm erg\,s^{-1}}$, is far below the intrinsic luminosity expected from the orbital evolution, supporting a geometry in which only a small fraction of the intrinsic emission is scattered into the line of sight by an extended, optically thin corona. In the 2-8 keV band, IXPE measures ${\rm PD}=7.9\pm0.6\%$ and ${\rm PA}=-24^\circ\pm2^\circ$. The PD increases with energy, while the PA remains approximately constant. During eclipse, the PD decreases to ${\rm PD}=5.5\pm1.7\%$, with no significant PA variation. This behaviour is consistent with the companion occulting the most polarimetrically efficient part of the extended corona. The high PD, stable PA, energy-dependent polarisation, and eclipse behaviour support a picture in which 4U 1822-37 is observed in an extreme high-inclination, scattering-dominated regime. The extended corona is the main structure shaping both the observed X-ray emission and its polarisation.

astro-ph.HE↗

arXiv2026

Multi-wavelength outburst activity from EP J174942.2-384834: a very faint X-ray transient discovered by Einstein Probe

We report the discovery and multi-wavelength characterization of the Galactic transient EP J174942.2$-$384834, first detected by the Einstein Probe during a faint X-ray outburst in March 2025. Coordinated follow-up observations revealed two major outbursts and a rebrightening over a seven-month period. Broadband X-ray spectral modeling shows that the outburst emission was dominated by thermal Comptonization of very soft seed photons. The absence of a detected thermal disk component, together with the low inferred seed-photon temperature, is consistent with a cool and possibly truncated accretion disk. The X-ray spectrum remained consistently hard throughout the outburst activity, with a power-law photon index of $\Gamma \approx 1$-2, gradually softening as the flux declined. The optical/UV counterpart brightened in tandem with the X-ray emission and exhibited a blue continuum with broad Balmer absorption features. Together with the optical/UV - X-ray luminosity correlation, this supports a disk-dominated origin of the optical/UV outburst emission, with viscous heating likely playing a major role and irradiation possibly contributing, especially in the UV. No radio counterpart was detected, implying at most very faint jet activity. Taken together, the observed properties support the classification of EP J174942.2$-$384834 as a very faint X-ray transient black hole candidate. This study demonstrates the ability of Einstein Probe to uncover and characterize the faintest accreting compact objects in the Galaxy.

astro-ph.HE↗

arXiv2026

Einstein Probe discovery of EP J171159.4-333253: an eclipsing neutron star low-mass X-ray binary with clocked bursts

EP J171159.4-333253 is a new neutron-star low-mass X-ray binary discovered in outburst by the Einstein Probe (EP) on 2025 June 23, exhibiting clocked type-I X-ray bursts, eclipses and dips. In this paper, we report on the results of the X-ray spectral and timing analyses for EP J171159.4-333253 using data collected by EP and NuSTAR during the first 21 days of the outburst. The X-ray burst recurrence time can be characterized over a subset of nine bursts spanning 1.6 days around the NuSTAR observation, and the result is $t_{\rm rec}=8196 \pm 177\,$s with indications of a possible decreasing trend. From the X-ray eclipse events, the binary orbital period and the eclipse duration are estimated to be $P_{\rm orb}=6.48301 \pm 0.00003\,$hr and $D_{\star,X} = 1245.5^{+6.9}_{-6.5}\,$s, respectively. These enable an estimate of the mass and radius of the companion star and the binary inclination, which are $M_2\approx0.6-0.8\,M_\odot$, $R_2\approx0.7-0.8\,R_\odot$ and $i\approx73-75^\circ$, respectively. We also report on joint ULTRACAM and EP observations on 2025 July 21--22, detecting the source optical counterpart and covering an eclipse in both X-ray and optical bands. The optical eclipse is wavelength-dependent and broader than in X-rays, indicating that part of the optical emission arises from an extended region in the accretion flow. Despite a moderate variation in the source flux, the properties of the persistent X-ray emission are typical of a hard spectral state. We further evaluated the ratio of the accretion energy to the thermonuclear energy to be 120--130, implying helium bursts with the accreted hydrogen being depleted in-between bursts.

astro-ph.HE↗

arXiv2026

A high-resolution X-ray view of the ultra-fast outflow in MAXI J1810-222

In previous work, it was reported that the Galactic black hole candidate MAXI J1810-222 exhibited a notable absorption spectral feature at around 1 keV in low-resolution X-ray spectra of CCD-like detectors. The feature was correlated with the spectral state of the source, being stronger in the soft states, as it occurs in the typical Fe K winds of X-ray binaries (XRBs). However, the results hinted towards rather extreme wind velocities of up to ~0.1 c. We therefore requested and obtained an observation with XMM-Newton to take advantage of the 10-fold higher spectral resolution (R ~200-400) provided by the RGS detector in order to resolve the lines and break the degeneracy between different models and interpretations. We applied state-of-the-art models of plasma in photoionisation equilibrium and multiphase interstellar medium. Further comparisons are performed with a re-analysis of NICER and NuSTAR data. The XMM-Newton/RGS spectrum is consistent with the presence of a mildly relativistic wind, confirming the earlier indications obtained with NICER, but places tighter constraints on the outflow properties, with the lines being intrinsically broad. The data would then favour magnetically driven winds, although thermal effects may still contribute to mass loading. NuSTAR and XMM-Newton (EPIC) show a further hotter component indicating a stratified or multiphase outflow. Fe K spectra taken with calorimetric detectors (e.g., Resolve on XRISM) will enable a high-resolution view of the complex extreme outflow in this source and shed new light on outflow processes in XRBs.

astro-ph.HE↗

arXiv2025

XMM-Newton multi-year campaign on NGC 55 ULX-1: Resolving the wind and its variability with RGS

Winds are an important ingredient in the evolution of X-ray binary (XRB) systems, particularly those at high accretion rates such as ultra-luminous X-ray sources (ULXs), because they may regulate the accretion of matter onto the compact object. We aim at understanding the properties of ULX winds and their link with the source spectral and temporal behavior. We performed high-resolution X-ray spectroscopy of the variable source NGC 55 ULX-1 to resolve emission and absorption lines as observed with XMM-Newton at different epochs. Optically-thin plasma models are used to characterise the wind. We confirmed and thoroughly strengthened previous evidence of outflows in NGC 55 ULX-1. The presence of radiative recombination signatures and the ratios between the fluxes of the emission lines favours photoionisation balance and low-to-moderate densities, which confirm that the lines originate from classical XRB disc winds. An in-depth parameter space exploration shows line emission from a slowly moving, cool, and variable plasma perhaps associated with a thermal wind. Mildly-relativistic Doppler shifts (about -0.15c) associated with the absorption lines confirm, at higher confidence, the presence of powerful, radiatively-driven, winds. The comparison between results obtained at different epochs revealed that the wind responds to the variability of the underlying continuum and these variations may be used to understand the actual accretion regime and the nature of the source.

astro-ph.HE↗

arXiv2025

Accretion and Ejection Physics at High Time Resolution

Accretion onto compact objects is one of the most fundamental phenomena in the astrophysics, powering some of the most luminous objects in the sky. Along with this, accretion has also a key impact on the evolution of the Universe, through the launch of powerful outflows that affect the surrounding medium. In the last years sub-second optical-infrared observations of accreting X-ray binaries have opened a new window in the study of inflow-outflow connection, discovering a wide range of previously unkown phenomena. Here we review the key open questions in accretion and ejection physics and discuss how a dedicated facility, equipped with photon-counting detectors and high spectral resolution from the UV to the mid-infrared, can enable transformative advances in our understanding of accretion processes.

astro-ph.IM↗

arXiv2025

Flashing fast: characterising the 2025 outburst of MAXI J1957+032

MAXI J1957+032 is an accreting millisecond X-ray pulsar that shows brief, recurrent outbursts in an ultra-compact ~1 h orbit. We characterise the 2025 outburst using X-ray timing and spectroscopy from XMM-Newton and Swift (and a late-time NuSTAR observation), together with contemporaneous optical photometry from LCO, and compare the spin frequency with the 2022 outburst. Timing searches detect coherent pulsations at ~313.6 Hz with no measurable frequency derivative during the XMM-Newton exposure. Relative to its 2022 outburst, we measure a long-term spin-down of ~-2x10^-14 Hz s^-1, consistent with magnetic-dipole braking in quiescence. The pulse profile is nearly sinusoidal, with significant power at the fundamental, second, and fifth harmonics; the fractional amplitude decreases with increasing flux and shows soft lags up to a few keV. The 0.5-10 keV spectrum is well described by absorbed thermal Comptonisation (photon index ~2.4) plus a cool blackbody (kT ~0.23 keV) consistent with emission from a surface hotspot; no reflection or Fe-line features are detected. Requiring R_m \leq R_co implies B_s ~(0.5-3)x10^8 G for d=(5 \pm 2) kpc and ξ=0.3-0.5, below the upper limit from the secular spin-down (B_p \leq 10^9 G), possibly indicating a mildly leaky propeller. The optical emission lies on the neutron-star branch of the L_OIR-L_X relation, consistent with reprocessing in a compact disc. The optical SEDs are broadly flat, while an early red excess suggests a transient jet contribution during the initial hard X-ray phase; an optical peak delayed relative to the X-rays may trace an outward-propagating heating front and rapid disc evolution in these short-lived outbursts.

astro-ph.HE↗

arXiv2025

The multiple facets of millisecond pulsar binaries

Millisecond pulsar (MSP) binaries are unique laboratories for studying matter and radiation under extreme conditions that are unattainable on Earth. Recent detections of optical millisecond pulsations from three systems in distinct evolutionary stages have opened an entirely new observational window to investigate particle acceleration, pulsar-disk interplay, and intrabinary wind interactions. These discoveries reveal unexpectedly diverse emission mechanisms across accretion regimes, with optical efficiencies in some systems far exceeding those predicted by rotation-powered models. Despite decades of research, key questions remain unresolved: are optical pulsations a universal property of MSPs? How does the presence of an accretion disk boost the conversion of spin-down power into coherent optical emission? What physical processes drive the observed fast variability, and how do pulsar and companion winds regulate mass transfer and binary evolution? Addressing these fundamental problems requires high-time-resolution optical observations, rapid-response observing capabilities, and time-resolved spectroscopy at moderate spectral resolution to map disk and intrabinary-shock variability. Future facilities dedicated to time domain astronomy present a unique opportunity to perform a systematic exploration of optical pulsars across all MSP evolutionary stages for the first time and answer the above-mentioned fundamental questions.

astro-ph.IM↗

arXiv2025

Aql X-1 from dawn 'til dusk: the early rise, fast state transition and decay of its 2024 outburst

Transient Low-Mass X-ray Binaries (LMXBs) are usually first detected by all-sky X-ray monitors when they enter new outbursts, typically at X-ray luminosities above $\sim$10$^{36}$ erg/s. Observations of these sources during the early rise of the outbursts have so far been very limited. However, the launch of the Einstein Probe (EP) has greatly improved our ability to detect fainter X-ray activity, unlocking access to the outburst early rise. In September 2024, EP detected the early onset of a new outburst from the neutron star LMXB Aql X-1, catching the source at a luminosity below 10$^{35}$ erg/s. In this paper we present results from a comprehensive, multi-wavelength campaign of this event, combining data from EP, NICER, NuSTAR, Swift and Las Cumbres Observatory covering the full outburst from its early rise through its decay. By comparing X-ray and optical light curves obtained with Las Cumbres Observatory during the initial rise, we show that the start of the X-ray emission lagged the optical rise by, at most, 13 days. Time-resolved X-ray spectroscopy revealed how the geometry and the physical properties of the accretion flow evolve during this early stage of the outburst, as well as at higher luminosities as the source transitioned through the canonical X-ray spectral states - hard, intermediate and soft. These data show that the source underwent a very rapid, about 12-h long, transition from the hard to the soft state about two weeks after the optical onset of the outburst. The evolution of the temperature and physical sizes of both the inner region of the disk and a black body near the NS surface suggest that at the state transition, a boundary and spreading layer likely formed. We discuss these results in the context of time-scales for outburst evolution and state transitions in accreting neutron stars and black holes.

astro-ph.HE↗

arXiv2025

Einstein Probe Discovery of EP J182730.0-095633: A New Black Hole X-ray Binary Candidate in Faint Outburst?

Black hole X-ray binaries (candidates) currently identified in our galaxy are mainly transient sources, with the majority discovered through the detection of their X-ray outbursts. Among these, only four were found during faint outbursts exhibiting peak X-ray luminosities $L_{\rm X}\lesssim10^{36}~{\rm erg~s^{-1}}$, likely due to the previous lack of sensitive, wide-field monitoring instruments in the X-ray band. In this Letter, we present the discovery of an intriguing X-ray transient, EP J182730.0-095633, via the Einstein Probe (EP) and subsequent multi-wavelength follow-up studies. This transient, located on the Galactic plane, experienced a faint and brief X-ray outburst lasting about 20 days. Its X-ray spectrum is non-thermal and consistent with a power-law model with a nearly constant photon index of $Γ\sim2$ throughout the outburst. A long-lasting millihertz quasi-periodic oscillation (QPO) signal was detected in its X-ray light curve, centered around a frequency of $\sim0.04$ Hz. A transient near-infrared source was identified as its counterpart, although no optical emission was detectable, likely due to significant extinction. A radio counterpart was also observed, displaying an inverted radio spectrum with $α\sim0.45$. The X-ray spectral and temporal characteristics, along with the multi-wavelength properties, indicate that the source is a faint low-mass X-ray binary, with the compact object likely being a black hole. This work demonstrates the potential of the EP in discovering new X-ray binaries by capturing faint-level X-ray outbursts.

astro-ph.HE↗

arXiv2025

Einstein Probe discovery of the short period intermediate polar EP J115415.8-501810

The X-ray transient source EP240309a/EP\,J115415.8$-$501810 was first detected by the Wide-Field X-ray Telescope (WXT) on board Einstein Probe (EP) during the commissioning phase. Subsequent optical observations confirmed it as a Cataclysmic Variable of the intermediate polar type with a 238.2\,s spinning white dwarf in a $\sim$3.76\,hr orbit. We report on the source discovery and follow-up studies made with the Follow-up X-ray Telescope (FXT) of EP. A periodic variation of 231\,s is detected in the 0.3$-$2\,keV band, while no obvious pulsation appears in the 2$-$10\,keV band. The spectral analysis shows that the X-ray emission could be described by an absorbed bremsstrahlung model with $kT$\textgreater\,11\,keV. The partial covering absorption, with an hydrogen column density $N_H$ = 2.0$\times 10^{22}\,\rm cm^{-2}$ and covering fraction around 0.9, is much larger than the interstellar absorption along the line of sight. According to the distance $d = 309.5$\,pc obtained from Gaia parallax, we estimate that the luminosity of this source in the 0.3$-$10\,keV range is $\sim 2\times10^{32}$\,erg\,s$^{-1}$. In addition, phase-resolved spectral analysis reveals that the detected periodic variation is mainly caused by the change in the absorption column density. In this scenario the spin modulation arises due to absorption from the pre-shock accretion flow of the X-ray emitting pole, while the optical radiation is modulated at the orbital side band ($ω_{\rm spin} - Ω_{\rm orbit}$) due to reprocessing in regions within the binary system. Due to its unusual transient behaviour for an intermediate polar, we have also searched for radio signals similar to those observed in the new class of long period transients. We derived upper limits with ASKAP (200--300\,$μ$Jy\,beam$^{-1}$ between 800--1500 MHz) and MWA (40--90\,mJy\,beam$^{-1}$ between 80--300 MHz).

astro-ph.HE↗

arXiv2025

On the nature of the X-ray binary transient MAXI J1834-021: clues from its first observed outburst

MAXI J1834-021 is a new X-ray transient that was discovered in February 2023. We analysed the spectral and timing properties of MAXI J1834-021 using NICER, NuStar and Swift data collected between March and October 2023. The light curve showed a main peak followed by a second activity phase. The majority of the spectra extracted from the individual NICER observations could be adequately fitted with a Comptonisation component alone, while a few of them required an additional thermal component. The spectral evolution is consistent with a softening trend as the source gets brighter in X-rays. We also analysed the broadband spectrum combining data from simultaneous NICER and NuStar observations on 2023 March 10. This spectrum can be fitted with a disc component with a temperature at the inner radius of $kT_{\rm in} \sim 0.4$ keV and a Comptonisation component with a power-law photon index of $Γ\sim 1.8$. By including a reflection component in the modelling, we obtained a 3$σ$ upper limit for the inner disc radius of 11.4 gravitational radii. We also detected a quasi-periodic oscillation (QPO), whose central frequency varies with time (from 2 Hz to $\sim$0.9 Hz) and anti-correlates with the hardness ratio. Based on the observed spectral-timing properties, MAXI J1834-021, can be classified as a low-mass X-ray binary in outburst. However, we are not able to draw a definitive conclusion on the nature of the accreting compact object, which at the moment could as well be a black hole or a neutron star.

astro-ph.HE↗

arXiv2025

Unveiling the reflection spectrum in the ultracompact LMXB 4U 1820-30

4U 1820-30 is a ultracompact X-ray binary located in the globular cluster NGC 6624, consisting of a neutron star accreting material from a helium white dwarf companion characterized by the shortest known orbital period for this type of star (11.4 minutes). Despite extensive studies, the detection of the relativistic Fe K emission line, has been inconsistently reported and no measurement of the system inclination has been achieved. In this work, we investigate the broadband spectral and polarimetric properties of 4U 1820-30, exploring the presence of a reflection component and its role in shaping the polarization signal. We analyzed simultaneous X-ray observations from NICER, NuSTAR, and IXPE. The spectral continuum was modeled with a disk blackbody, a power law, and a Comptonization component with seed photons originating from a boundary layer. We detected a strong reflection component, described, for the first time, with two self-consistent models ({\tt Relxillns} and {\tt Rfxconv}), allowing us to provide a measurement of the system inclination angle (about 31 degrees), supporting the low-inclination hypothesis. Subsolar iron abundances were detected in the accretion disk and interstellar medium, probably related to the source location in a metal-poor globular cluster. The polarization increases from an upper limit of $1.2\%$ in the 2--4 keV band up to about $8\%$ in the 7--8 keV range. The disk is expected to be orthogonally polarized to these components, which may help to explain the decreasing of the observed polarization at low energies. However, the high polarization degree we found challenges the current models, also taking into consideration the relatively low inclination angle derived from the spectral analysis.

astro-ph.HE↗

arXiv2025

Identification and characterisation of the gamma-ray counterpart of the transitional pulsar candidate CXOU J110926.4-650224

Transitional millisecond pulsars (tMSPs) represent a crucial link between the rotation-powered and accretion-powered states of binary pulsars. During their active X-ray state, tMSPs are the only low-mass X-ray binary systems detected up to GeV energies by the Fermi Large Area Telescope (LAT). CXOU J110926.4-650224 is a newly discovered tMSP candidate in an active X-ray state, potentially spatially compatible with a faint gamma-ray source listed in the latest Fermi-LAT point-source catalogue as 4FGL J1110.3-6501. Confirming the association between CXOU J110926.4-650224 and the Fermi source is a key step toward validating its classification as a tMSP. In this study, we analyse Fermi-LAT data collected from August 2008 to June 2023 to achieve a more accurate localisation of the gamma-ray source, characterise its spectral properties, and investigate potential time variability. By thoroughly reconstructing the gamma-ray background around the source using a weighted likelihood model, we obtain a new localisation that aligns with the position of the X-ray source at the 95% confidence level, with a Test Statistic value of $\sim 42$. This establishes a spatial association between the gamma-ray source and CXOU J110926.4-650224. The gamma-ray emission is adequately described by a power-law model with a photon index of $Γ= 2.5 \pm 0.1$ and a corresponding flux of $(3.7\pm0.9) \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 0.1-300 GeV range.

astro-ph.HE↗

arXiv2024

High-Precision Excited-State Nuclear Recoil Spectroscopy with Superconducting Sensors

Superconducting sensors doped with rare isotopes have recently demonstrated powerful sensing performance for sub-keV radiation from nuclear decay. Here, we report the first high-resolution recoil spectroscopy of a single, selected nuclear state using superconducting tunnel junction (STJ) sensors. The STJ sensors were used to measure the eV-scale nuclear recoils produced in $^7$Be electron capture decay in coincidence with a 478 keV $γ$-ray emitted in decays to the lowest-lying excited nuclear state in $^7$Li. Details of the Doppler broadened recoil spectrum depend on the slow-down dynamics of the recoil ion. The measured spectral broadening is compared to empirical stopping power models as well as modern molecular dynamics simulations at low energy. The results have implications in several areas from nuclear structure and stopping powers at eV-scale energies to direct searches for dark matter, neutrino mass measurements, and other physics beyond the standard model.

physics.ins-det↗

arXiv2024

Einstein Probe discovery of EP J005245.1-722843: a rare BeWD binary in the Small Magellanic Cloud?

On May 27 2024, the Wide-field X-ray Telescope onboard the Einstein Probe (EP) mission detected enhanced X-ray emission from a new transient source in the Small Magellanic Cloud (SMC) 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$\sim$0.1 keV at the outburst peak) with an X-ray luminosity of $L\sim4\times10^{38}$ erg s$^{-1}$, labelled EP J005245.1-722843. This super-soft outburst faded very quickly in a week 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 SSS 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 days periodicity in the optical band. The discovery of this super-soft 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.

astro-ph.HE↗