arXiv subjects

G. Vasilopoulos

Publications and source records attributed to G. Vasilopoulos.

At least 19 recordsLinked to original sources

arXiv2026

Delayed Radio Flares in Neutrino-associated Blazars: The Case of TXS 0506+056

Radio flares have been postulated to be associated with the production of astrophysical neutrinos. For example, TXS 0506+056 exhibits a 2-3 yr delay between the 2017 IceCube-170922A/$\gamma$-ray flare and a GHz radio maximum. We quantitatively test if the delayed radio flare originates from the same compact region where neutrinos and $\gamma$-rays are produced as it expands downstream and synchrotron self-absorption (SSA) is reduced. Starting from the 2017 flare blob parameters, we model the expanding production region and its evolving radio emission with LeHaMoC in a fully time-dependent framework, and compare our 1.2-22 GHz light curves to RATAN-600 data. We study different scenarios with increasing levels of sophistication, including continuous injection and energy re-dissipation on pc scales. While a simple expanding blob scenario fails to reproduce the radio data, a downstream dissipation episode of particles in the optically thin regime, followed by jet deceleration, successfully describes the radio evolution. Within our one-zone time-dependent framework, the delayed radio flare is unlikely to come from an expanding neutrino production zone becoming transparent to radio emission. Additional ingredients are needed, such as re-dissipation downstream with a subsequent Doppler-factor decline. The radio flare is powered by leptonic synchrotron emission and is largely insensitive to the proton population relevant for neutrino production, implying that the delayed radio flare mainly probes downstream dissipation and beaming in certain jet configurations rather than being a genuine feature associated with the neutrino production.

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arXiv2026

A comprehensive catalogue of high-mass X-ray binaries in the Large Magellanic Cloud detected during the first eROSITA all-sky survey

The Magellanic Clouds, the closest star-forming galaxies to the Milky Way, offer an excellent environment to study high-mass X-ray binaries. While the Small Magellanic Cloud has been thoroughly investigated with over 120 systems identified, the Large Magellanic Cloud has lacked a complete survey due to its large angular size. Most prior studies targeted central or high-star-formation regions. The SRG/eROSITA all-sky surveys now enable a comprehensive coverage of the LMC, particularly due to its close vicinity to the south ecliptic pole. This work aims to improve our understanding of the HMXB population in the LMC by building a flux-limited catalogue. This allows us to compare sample properties with those of HMXB populations in other nearby galaxies. Using detections during the first eROSITA all-sky survey, we cross-matched X-ray positions with optical and infrared catalogues to identify candidate HMXBs. We assigned flags based on multi-wavelength follow-up observations and archival data, using properties of known LMC HMXBs. These flags defined confidence classes for our candidates. We detect sources down to X-ray luminosities of a few $10^{34}$ erg s$^{-1}$, resulting in a catalogue of 53 objects, including 28 confirmed HMXBs and 21 new eROSITA detections. We identify several likely supergiant systems, including a candidate supergiant fast X-ray transient with phase-dependent flares. We find three Be stars with likely white dwarf companions. Two of the Be/WD candidates show steady luminosities across four eROSITA scans, unlike the post-nova states seen in the majority of previous Be/WD reports. Our catalogue is the first to cover the entire LMC since the ROSAT era, providing a basis for statistical population studies. Using the HMXB population, we estimate the LMC star-formation rate to be $(0.22^{+0.06}_{-0.07})$ M$_{\odot}$yr$^{-1}$, which is in agreement with other tracers.

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arXiv2025

Black-hole mass estimation through accretion disk spectral fitting for high-redshift blazars

High-redshift ($z>2$) blazars, with relativistic jets aligned toward us, probe the most powerful end of the active galactic nuclei (AGN) population. We aim at determining the black hole masses and mass accretion rates of high-$z$ blazars in a common framework that utilizes a Markov Chain Monte Carlo (MCMC) fitting method and the Shakura-Sunayev multi-temperature accretion disk model, accounting also for attenuation due to neutral hydrogen gas in the intergalactic medium (IGM). We compiled a sample of 23 high-redshift blazars from the literature with publicly available infrared-to-ultraviolet photometric data. We performed a Bayesian fit to the spectral energy distribution (SED) of the accretion disk, accounting for upper limits, and determined the black hole masses and mass accretion rates with their uncertainties. We also examined the impact of optical-ultraviolet attenuation due to gas in the IGM. We find that neglecting IGM attenuation in SED fits leads to systematically larger black-hole mass estimates and correspondingly lower Eddington ratios, with the bias becoming more severe at higher redshift. Our MCMC fits yield median black-hole masses in the range $\sim (10^{8}-10^{10})\,M_{\odot}$ and a broad distribution of median Eddington ratios ($\lambda_{\rm Edd}\sim0.04$ up to $\sim1$). Comparison with previous literature shows no clear method-dependent systematic offsets, although individual mass estimates can differ by up to a factor of a few. We also demonstrate that assumptions about black-hole spin introduce a systematic degeneracy. This work is to our knowledge the first systematic study to model the accretion-disk emission of a large sample of high-$z$ blazars within a single, consistent statistical framework. Our results emphasize the importance of accounting for IGM attenuation and of using uniform fitting methods when comparing disk-based black hole estimates across samples.

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arXiv2025

Discovery of 146.8 s pulsations from EP J005146.9-730930, a new transient Be/X-ray binary in the SMC

Recent observations of the Small Magellanic Cloud (SMC) with Einstein Probe (EP) revealed a new transient X-ray source, most likely identified as Be/X-ray binary. To characterise the X-ray properties of EP J005146.9-730930 and in particular to look for pulsations in the X-ray flux, we triggered an XMM-Newton anticipated target of opportunity observation. To follow the flux evolution during the outburst we monitored the source for about three months with the Follow-up X-ray Telescope of EP. The XMM-Newton observation was performed on 2024 September 15 and we used the data from the European Photon Imaging Camera (EPIC) for detailed spectral and timing analyses. The EPIC X-ray spectrum is well described by an absorbed power law with photon index of 1.25 +/- 0.04 and the timing analysis revealed pulsations with 146.79 +\- 0.03 s. The source flux had decreased by a factor of about 10 since the observed maximum about one month before the XMM-Newton observation. EP J005146.9-730930 was never detected significantly during serendipitous observations before September 2024. The characteristics of the X-ray brightening suggest the source was discovered during a type II outburst reaching an X-ray peak luminosity of ~2 x 10^37 erg/s. The trend of spectral hardening towards higher luminosities observed by EP suggests that the source is accreting below the critical luminosity, yielding an estimated lower limit for the pulsar magnetic field strength of 3.3 x 10^12 G. The improved X-ray position confirms the candidate Be star OGLE J005147.58-730924.7 as optical counterpart. We conclude that EP J005146.9-730930 = SXP 146.8 is a new Be/X-ray binary pulsar in the SMC.

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arXiv2025

Don't torque like that. Measuring compact object magnetic fields with analytic torque models

Context. Changes of the rotational period observed in various magnetized accreting sources are generally attributed to the interaction between the in-falling plasma and the large-scale magnetic field of the accretor. A number of models have been proposed to link these changes to the mass accretion rate, based on different assumptions on the relevant physical processes and system parameters. For X-ray binaries with neutron stars, with the help of precise measurements of the spin periods provided by current instrumentation, these models render a way to infer such parameters as the strength of the dipolar field and a distance to the system. Often, the obtained magnetic field strength values contradict those from other methods used to obtain magnetic field estimates. Aims. We want to compare the results of several of the proposed accretion models. To this end an example application of these models to data is performed. Methods. We reformulate the set of disk accretion torque models in a way that their parametrization are directly comparable. The application of the reformulated models is discussed and demonstrated using Fermi/GBM and Swift/BAT monitoring data covering several X-ray outbursts of the accreting pulsar 4U 0115+63. Results. We find that most of the models under consideration are able to describe the observations to a high degree of accuracy and with little indication for one model being preferred over the others. Yet, derived parameters from those models show a large spread. Specifically the magnetic field strength ranges over one order of magnitude for the different models. This indicates that the results are heavily influenced by systematic uncertainties.

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arXiv2025

Optical flares due to X-ray irradiation during BeXRB major outbursts

Be X-ray binaries (BeXRBs) may show strong X-ray and optical variability, and can exhibit some of the brightest outbursts that break through the Eddington limit. Major X-ray outbursts are often accompanied by strong optical flares that evolve parallel to the X-ray outburst. Our goal is to provide a simple quantitative explanation for the optical flares with an application to a sample of the brightest outbursts of BeXRBs in the Magellanic clouds and the Galactic Ultra-luminous X-ray (ULX) pulsar Swift J0243.6+6124. We constructed a numerical model to study X-ray irradiation in a BeXRB system. We then conducted a parametric investigation of the model parameters and made predictions for the intensity of the optical flares based on geometric and energetic constraints. From our modeling we found that the optical emission during major outbursts is consistent with being the result of X-ray irradiation of the Be disk. For individual systems, if this method is combined with independent constraints of the geometry of the Be disk, the binary orbital plane, and the plane of the observer, it can provide estimates of the Be disk size during major outbursts. Moreover, we computed a semi-analytical relation between optical flare luminosity and X-ray luminosity that is consistent with both model predictions and observed properties of flares.

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arXiv2024

Broadband study of the Be X-ray binary RX J0520.5-6932 during its outburst in 2024

A new giant outburst of the Be X-ray binary RX J0520.5-6932 was detected and subsequently observed with several space-borne and ground-based instruments. This study presents a comprehensive analysis of the optical and X-ray data, focusing on the spectral and timing characteristics of selected X-ray observations. A joint fit of spectra from simultaneous observations performed by the X-ray telescope (XRT) on the Neil Gehrels Swift Observatory (Swift) and Nuclear Spectroscopic Telescope ARray (NuSTAR) provides broadband parameter constraints, including a cyclotron resonant scattering feature (CRSF) at 32.2(+0.8/-0.7) keV with no significant energy change since 2014, and a weaker Fe line. Independent spectral analyses of observations by the Lobster Eye Imager for Astronomy (LEIA), Einstein Probe (EP), Swift-XRT, and NuSTAR demonstrate the consistency of parameters across different bands. Luminosity variations during the current outburst were tracked. The light curve of the Optical Gravitational Lensing Experiment (OGLE) aligns with the X-ray data in both 2014 and 2024. Spin evolution over 10 years is studied after adding Fermi Gamma-ray Burst Monitor (GBM) data, improving the orbital parameters, with an estimated orbital period of 24.39 days, slightly differing from OGLE data. Despite intrinsic spin-up during outbursts, a spin-down of ~0.04s over 10.3 years is suggested. For the new outburst, the pulse profiles indicate a complicated energy-dependent shape, with decreases around 15 keV and 25 keV in the pulsed fraction, a first for an extragalactic source. Phase-resolved NuSTAR data indicate variations in parameters such as flux, photon index, and CRSF energy with rotation phase.

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arXiv2024

LeHaMoC: a versatile time-dependent lepto-hadronic modeling code for high-energy astrophysical sources

Recent associations of high-energy neutrinos with active galactic nuclei (AGN) have revived the interest in leptohadronic models of radiation from astrophysical sources. The rapid increase in the amount of acquired multi-messenger data will require soon fast numerical models that may be applied to large source samples. We develop a time-dependent leptohadronic code, LeHaMoC, that offers several notable benefits compared to other existing codes, such as versatility and speed. LeHaMoC solves the Fokker-Planck equations of photons and relativistic particles (i.e. electrons, positrons, protons, and neutrinos) produced in a homogeneous magnetized source that may also be expanding. The code utilizes a fully implicit difference scheme that allows fast computation of steady-state and dynamically evolving physical problems. We first present test cases where we compare the numerical results obtained with LeHaMoC against exact analytical solutions and numerical results computed with ATHE$ν$A, a well-tested code of similar philosophy but different numerical implementation. We find a good agreement (within 10-30%) with the numerical results obtained with ATHE$ν$A without evidence of systematic differences. We then demonstrate the capabilities of the code through illustrative examples. First, we fit the spectral energy distribution from a jetted AGN in the context of a synchrotron-self Compton model and a proton-synchrotron model using Bayesian inference. Second, we compute the high-energy neutrino signal and the electromagnetic cascade induced by hadronic interactions in the corona of NGC 1068. LeHaMoC is easily customized to model a variety of high-energy astrophysical sources and has the potential to become a widely utilized tool in multi-messenger astrophysics.

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arXiv2024

TeV pion bumps in the gamma-ray spectra of flaring blazars

(Abridged) Very high-energy (VHE, $E>100$ GeV) observations of the blazar Mrk 501 with MAGIC in 2014 provided evidence for an unusual narrow spectral feature at about 3 TeV during an extreme X-ray flaring activity. The one-zone synchrotron-self Compton scenario, widely used in blazar broadband spectral modeling, fails to explain the narrow TeV component. Motivated by this rare observation, we propose an alternative model for the production of narrow features in the VHE spectra of flaring blazars. These spectral features may result from the decay of neutral pions ($π^0$ bumps) that are in turn produced via interactions of protons (of tens of TeV energy) with energetic photons, whose density increases during hard X-ray flares. We explored the conditions needed for the emergence of narrow $π^0$ bumps in VHE blazar spectra during X-ray flares reaching synchrotron energies $\sim 100$ keV using time-dependent radiative transfer calculations. We focused on high-synchrotron peaked (HSP) blazars, which comprise the majority of VHE-detected extragalactic sources. We find that synchrotron-dominated flares with peak energies $\gtrsim100$ keV can be ideal periods for the search of $π^0$ bumps in the VHE spectra of HSP blazars. The flaring region is optically thin to photopion production, its energy content is dominated by the relativistic proton population, and the inferred jet power is highly super-Eddington. Application of the model to the spectral energy distribution of Mrk 501 on MJD 56857.98 shows that the VHE spectrum of the flare is described well by the sum of a synchrotron self-Compton (SSC) component and a distinct $π^0$ bump centered at 3 TeV. Spectral fitting of simulated SSC+$π^0$ spectra for the Cherenkov Telescope Array (CTA) show that a $π^0$ bump could be detected at a 5$σ$ significance level with a 30-min exposure.

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arXiv2024

eRASSUJ060839.5-704014: A double degenerate ultra-compact binary in the direction of the LMC

Context. During four all-sky surveys (eRASS1--4), eROSITA, the soft X-ray instrument aboard Spektrum-Roentgen-Gamma (SRG) detected a new supersoft X-ray source, eRASSU J060839.5-704014, in the direction of the Large Magellanic Cloud (LMC). Methods. We arranged follow-up observations in the X-ray and optical wavelengths and further searched in archival observations to reveal the nature of the object. Results. We discover pulsations at ~374 s with a pulse profile consistent with 100% modulation. We identify two other periodicities in the eROSITA data, which we establish as aliases due to the sampling of the eROSITA light curve. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The timing and spectral characteristics of the source are consistent with a double degenerate ultra-compact binary system in the foreground of the LMC. eRASSU J060839.5-704014 belongs to a rare class of AM CVns, which are important to study in the context of progenitors of SN Ia and for persistent gravitational wave detection. Conclusions. We identify eRASSU J060839.5-704014 as a new double degenerate ultra-compact binary located in the foreground of the LMC.

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arXiv2023

From Stellar Death to Cosmic Revelations: Zooming in on Compact Objects, Relativistic Outflows and Supernova Remnants with AXIS

Compact objects and supernova remnants provide nearby laboratories to probe the fate of stars after they die, and the way they impact, and are impacted by, their surrounding medium. The past five decades have significantly advanced our understanding of these objects, and showed that they are most relevant to our understanding of some of the most mysterious energetic events in the distant Universe, including Fast Radio Bursts and Gravitational Wave sources. However, many questions remain to be answered. These include: What powers the diversity of explosive phenomena across the electromagnetic spectrum? What are the mass and spin distributions of neutron stars and stellar mass black holes? How do interacting compact binaries with white dwarfs - the electromagnetic counterparts to gravitational wave LISA sources - form and behave? Which objects inhabit the faint end of the X-ray luminosity function? How do relativistic winds impact their surroundings? What do neutron star kicks reveal about fundamental physics and supernova explosions? How do supernova remnant shocks impact cosmic magnetism? This plethora of questions will be addressed with AXIS - the Advanced X-ray Imaging Satellite - a NASA Probe Mission Concept designed to be the premier high-angular resolution X-ray mission for the next decade. AXIS, thanks to its combined (a) unprecedented imaging resolution over its full field of view, (b) unprecedented sensitivity to faint objects due to its large effective area and low background, and (c) rapid response capability, will provide a giant leap in discovering and identifying populations of compact objects (isolated and binaries), particularly in crowded regions such as globular clusters and the Galactic Center, while addressing science questions and priorities of the US Decadal Survey for Astronomy and Astrophysics (Astro2020).

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arXiv2023

Application of neural networks to synchro-Compton blazar emission models

Jets from supermassive black holes in the centers of active galaxies are the most powerful persistent sources of electromagnetic radiation in the Universe. To infer the physical conditions in the otherwise out-of-reach regions of extragalactic jets we usually rely on fitting of their spectral energy distribution (SED). The calculation of radiative models for the jet non-thermal emission usually relies on numerical solvers of coupled partial differential equations. In this work machine learning is used to tackle the problem of high computational complexity in order to significantly reduce the SED model evaluation time, which is needed for SED fitting with Bayesian inference methods. We compute SEDs based on the synchrotron self-Compton model for blazar emission using the radiation code ATHE$ν$A, and use them to train Neural Networks exploring whether these can replace the original computational expensive code. We find that a Neural Network with Gated Recurrent Unit neurons can effectively replace the ATHE$ν$A leptonic code for this application, while it can be efficiently coupled with MCMC and nested sampling algorithms for fitting purposes. We demonstrate this through an application to simulated data sets and with an application to observational data. We offer this tool in the community through a public repository. We present a proof-of-concept application of neural networks to blazar science. This is the first step in a list of future applications involving hadronic processes and even larger parameter spaces.

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arXiv2023

The High Energy X-ray Probe (HEX-P): A New Window into Neutron Star Accretion

Accreting neutron stars (NSs) represent a unique laboratory for probing the physics of accretion in the presence of strong magnetic fields ($B\gtrsim 10^8$ G). Additionally, the matter inside the NS itself exists in an ultra-dense, cold state that cannot be reproduced in Earth-based laboratories. Hence, observational studies of these objects are a way to probe the most extreme physical regimes. Here we present an overview of the field and discuss the most important outstanding problems related to NS accretion. We show how these open questions regarding accreting NSs in both low-mass and high-mass X-ray binary systems can be addressed with the High-Energy X-ray Probe (HEX-P) via simulated data. In particular, with the broad X-ray passband and improved sensitivity afforded by a low X-ray background, HEX-P will be able to 1) distinguish between competing continuum emission models; 2) provide tighter upper limits on NS radii via reflection modeling techniques that are independent and complementary to other existing methods; 3) constrain magnetic field geometry, plasma parameters, and accretion column emission patterns by characterizing fundamental and harmonic cyclotron lines and exploring their behavior with pulse phase; 4) directly measure the surface magnetic field strength of highly magnetized NSs at the lowest accretion luminosities; as well as 5) detect cyclotron line features in extragalactic sources and probe their dependence on luminosity in the super-Eddington regime in order to distinguish between geometrical evolution and accretion-induced decay of the magnetic field. In these ways HEX-P will provide an essential new tool for exploring the physics of NSs, their magnetic fields, and the physics of extreme accretion.

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arXiv2023

On the cyclotron absorption line and evidence of the spectral transition in SMC X-2 during 2022 giant outburst

We report comprehensive spectral and temporal properties of the Be/X-ray binary pulsar SMC X-2 using X-ray observations during the 2015 and 2022 outbursts. The pulse profile of the pulsar is unique and strongly luminosity dependent. It evolves from a broad-humped into a double-peaked profile above luminosity 3$\times$10$^{38}$ ergs s$^{-1}$. The pulse fraction of the pulsar is found to be a linear function of luminosity as well as energy. We also studied the spectral evolution of the source during the latest 2022 outburst with NICER. The observed photon index shows a negative and positive correlation below and above the critical luminosity, respectively, suggesting evidence of spectral transition from the sub-critical to super-critical regime. The broadband spectroscopy of four sets of NuSTAR and XRT/NICER data from both outbursts can be described using a cutoff power-law model with a blackbody component. In addition to the 6.4 keV iron fluorescence line, an absorption-like feature is clearly detected in the spectra. The cyclotron line energy observed during the 2015 outburst is below 29.5 keV, however latest estimates in the 2022 outburst suggest a value of 31.5 keV. Moreover, an increase of 3.4 keV is detected in the cyclotron line energy at equal levels of luminosity observed in 2022 with respect to 2015. The observed cyclotron line energy variation is explored in terms of accretion induced screening mechanism or geometrical variation in line forming region.

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arXiv2023

A shared accretion instability for black holes and neutron stars

Accretion disks around compact objects are expected to enter an unstable phase at high luminosity. One instability may occur when the radiation pressure generated by accretion modifies the disk viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole. Although there are hints of these cycles in a few isolated cases, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a lingering puzzle. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum-from radio to X-ray-of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects.

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arXiv2023

SRG/eROSITA-triggered XMM-Newton observations of three Be/X-ray binaries in the LMC: Discovery of X-ray pulsations

Using data from eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma (SRG), we report the discovery of two new hard transients, eRASSU J050810.4-660653 and eRASSt J044811.1-691318, in the Large Magellanic Cloud. We also report the detection of the Be/X-ray binary RX J0501.6-7034 in a bright state. We initiated follow-up observations to investigate the nature of the new transients and to search for X-ray pulsations coming from RX J0501.6-7034. We analysed the X-ray spectra and light curves from our XMM-Newton observations, obtained optical spectra using the South African Large Telescope to look for Balmer emission lines and utilised the archival data from the Optical Gravitational Lensing Experiment (OGLE) for the long-term monitoring of the optical counterparts. We find X-ray pulsations for eRASSU J050810.4-660653, RX J0501.6-7034, and eRASSt J044811.1-691318 of 40.6 s, 17.3 s, and 784 s, respectively. The Halpha emission lines with equivalent widths of -10.4 A (eRASSU J050810.4-660653) and -43.9 A (eRASSt J044811.1-691318) were measured, characteristic for a circumstellar disc around Be stars. The OGLE I- and V-band light curves of all three systems exhibit strong variability. A regular pattern of deep dips in the light curves of RX J0501.6-7034 suggests an orbital period of ~451 days. We identify the two new hard eROSITA transients eRASSU J050810.4-660653 and eRASSt J044811.1-691318 and the known Be/X-ray binary RX J0501.6-7034 as Be/X-ray binary pulsars.

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arXiv2022

A Bayesian approach for torque modelling of BeXRB pulsars with application to super-Eddington accretors

In this study we present a method to estimate posterior distributions for standard accretion torque model parameters and binary orbital parameters for X-ray binaries using a nested sampling algorithm for Bayesian Parameter Estimation. We study the spin evolution of two Be X-ray binary systems in the Magellanic Clouds, RX J0520.5-6932 and RX J0209-7427, during major outbursts, in which they surpassed the Eddington-limit. Moreover, we apply our method to the recently discovered Swift J0243.6+6124; the only known Galactic pulsating ultra-luminous X-ray source. This is an excellent candidate for studying the disc evolution at super-Eddington accretion rates, for its luminosity span several orders of magnitude during its outburst, with a maximum $L_{\rm X}$ that exceeded the Eddington limit by a factor of $\sim 10$. Our method, when applied to RX J0520.5-6932 and RX J0209-7427, is able to identify the more favourable torque model for each system, while yielding meaningful ranges for the NS and orbital parameters. Our analysis for Swift J0243.6+6124 illustrates that, contrary to the standard torque model predictions, the magnetospheric radius and the Alfvén radius are not proportional to each other when surpassing the Eddington limit. Reported distance estimates of this source range between 5 and 7 kpc. Smaller distances require non-typical neutron star properties (i.e. mass and radius) and possibly lower radiative efficiency of the accretion column.

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arXiv2022

eRASSt J040515.6-745202, an X-ray burster in the Magellanic Bridge

During the third all-sky survey (eRASS3), eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma, detected a new hard X-ray transient, eRASSt J040515.6-745202, in the direction of the Magellanic Bridge. We arranged follow-up observations and searched for archival data to reveal the nature of the transient. Using X-ray observations with XMM-Newton, NICER, and Swift, we investigated the temporal and spectral behaviour of the source for over about 10 days. The X-ray light curve obtained from the XMM-Newton observation with an 28 ks exposure revealed a type-I X-ray burst with a peak bolometric luminosity of at least 1.4e37 erg/s. The burst energetics are consistent with a location of the burster at the distance of the Magellanic Bridge. The relatively long exponential decay time of the burst of 70 s indicates that it ignited in a H-rich environment. The non-detection of the source during the other eROSITA surveys, twelve and six months before and six months after eRASS3, suggests that the burst was discovered during a moderate outburst which reached 2.6e36 erg/s in persistent emission. During the NICER observations, the source showed alternating flux states with the high level at a similar brightness as during the XMM-Newton observation. This behaviour is likely caused by dips as also seen during the last hour of the XMM-Newton observation. Evidence for a recurrence of the dips with a period of 21.8 hr suggests eRASSt J040515.6-745202 is a low-mass X-ray binary (LMXB) system with an accretion disk seen nearly edge on. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. (abbreviated)

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