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Georgios Vasilopoulos

Publications and source records attributed to Georgios Vasilopoulos.

At least 19 recordsLinked to original sources
arXiv2026

Rapid Orbital Decay in the Ultracompact Double-degenerate Binary eRASSU J060839.5$-$704014

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 $\dot{P}= -4.7\,(1) \times 10^{-11} \mathrm{~s~s^{-1}}$. 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 $\sim0.43\, M_{\odot}$, 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.

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arXiv2026

Time-resolved leptonic modeling of the prompt emission of GRB 211211A

GRB 211211A is a long duration gamma-ray burst with a compact object merger origin. In this work, we model the time-resolved prompt-emission spectra of GRB 211211A within a leptonic radiation framework. Our goal is to infer the physical properties of the emitting region, study the temporal evolution of the radiating particle distribution, and make predictions for prompt emission at TeV energies. We perform Markov Chain Monte Carlo fitting of the time-resolved numerical spectral energy distribution (SED) models computed with the time-dependent non-thermal radiation code LeHaMoC. Our calculations include synchrotron emission and self-absorption, inverse Compton scattering including cooling in the Klein-Nishina regime, and photon-photon pair production. We find that the prompt emission of GRB 211211A between 10 keV and 10 MeV can be successfully reproduced by synchrotron radiation from a population of relativistic electrons. The spectral evolution during the first minute of the burst reflects different physical conditions in the emitting region. Our best-fit models favor fast-cooling solutions for the first 8 s, followed by a transition to slow-cooling solutions at later times. The accompanying synchrotron self-Compton emission extends to TeV energies, with predicted fluxes that would be detectable by CTAO for a burst similar to GRB 211211A, provided a sufficiently rapid response to a Fermi-GBM trigger or if the burst occurs within the CTAO field of view. The observed short variability of this burst requires very high Doppler factors ($\sim1000-2500$) throughout the burst evolution. Such extreme Doppler factors are difficult to reconcile with the jet Lorentz factor inferred from afterglow modeling unless the prompt-emitting regions are themselves moving relativistically with respect to the jet plasma.

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arXiv2026

A homogeneous view of asymptotic giant branch carbon stars as seen by Gaia

Carbon stars on the asymptotic giant branch are major contributors to galactic dust enrichment, with gas mass-loss rates up to 1e-4 Msun/yr. We present a homogeneous spectral energy distribution analysis of the Gaia DR3 Golden Sample of carbon stars in the Milky Way and Magellanic Clouds. Our dataset includes 14,747 sources with multi-band photometry from Gaia, 2MASS, and WISE, combined with recent distance and extinction estimates. For a subsample of 2,494 Mira variables, we model multi-band light curves to derive accurate mean magnitudes. Stellar and circumstellar parameters are obtained by fitting observations with a large grid of synthetic spectra computed with the DUSTY radiative transfer code using COMARCS atmospheres. We derive effective temperature, optical depth, and gas mass-loss rate for each source. The distributions peak around Teff = 3150 K, with mass-loss rates spanning 1e-11 to 1e-4 Msun/yr and inner dust temperatures near 1000 K. We find a correlation between variability amplitude and mass-loss rate. This framework provides a statistically robust view of carbon stars across environments with different metallicities. Apparent environmental dependencies are influenced by luminosity distributions and selection effects rather than purely intrinsic metallicity differences. The combined Gaia and WISE selection limits the detection of both highly obscured and faint Magellanic Cloud sources, but the observed trends remain significant within the sampled populations.

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arXiv2026

Broadband study of the Be/X-ray binary pulsar eRASSU J012422.9-724248 in the Magellanic Bridge, near the Eastern Wing of the Small Magellanic Cloud

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 broadband 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 J012422.9-724248 as 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 days from the 15 year 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 J012422.9-724248 is a new member of the class of persistent BeXRBs.

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arXiv2025

Accelerating SED Modeling of Astrophysical Objects Using Neural Networks

Interpreting the spectral energy distributions (SEDs) of astrophysical objects with physically motivated models is computationally expensive. These models require solving coupled differential equations in high-dimensional parameter spaces, making traditional fitting techniques such as Markov Chain Monte Carlo or nested sampling prohibitive. A key example is modeling non-thermal emission from blazar jets - relativistic outflows from supermassive black holes in Active Galactic Nuclei that are among the most powerful emitters in the Universe. To address this challenge, we employ machine learning to accelerate SED evaluations, enabling efficient Bayesian inference. We generate a large sample of lepto-hadronic blazar emission models and train a neural network (NN) to predict the photon spectrum with strongly reduced run time while preserving accuracy. As a proof of concept, we present an NN-based tool for blazar SED modeling, laying the groundwork for future extensions and for providing an open-access resource for the astrophysics community.

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arXiv2025

Detection of the optical counterpart of the transient ULX NGC300 ULX-1: a nascent black hole - neutron star binary?

The end points of massive star evolution are poorly known, especially those in interacting binary systems containing compact objects, such as neutron stars or black holes. Such systems are bright in X-rays, and the most luminous among them are called ultra-luminous X-ray sources (ULXs). In this paper, we address the enigmatic NGC 300 ULX-1. It's X-ray activity started in 2010 with the supernova impostor-like event SN 2010da. In the following few years the ULX was powered by persistent super-Eddington accretion but then it dimmed in X-rays. We present the most recent X-ray and optical observations. The Chandra and Swift telescopes confirm that SN 2010da/NGC 300 ULX-1 is not accreting at super-Eddington level anymore. We attribute this switch in accretion regime to the donor star variability and its fast evolution. In order to gain a better understanding of the donor star's nature, we consider its optical light curve on a decade-long time scale and show that the optical counterpart of SN 2010da/NGC 300 ULX-1 dimmed significantly over recent years. The most recent detection in optical by the Gemini telescope reveals that the source is now > 2.5 mag fainter in the r' band compared to the epoch when it was spectroscopically classified as a red supergiant. We discuss the nature of the abrupt changes in the donor star properties, and consider among other possibilities the silent collapse of the donor star into a black hole.

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arXiv2025

Two decades of optical variability of Small Magellanic Cloud high-mass X-ray binaries

We present an analysis of the long-term optical/IR behavior of 111 high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud based on data from the OGLE collaboration. Most systems exhibit variability on a range of time scales. This variability regulates the mass transfer to the compact object, while the compact object can, in turn, affect the donor star's behavior. To better understand this complex interaction and the resulting X-ray properties in these systems, we define a new taxonomy for the observed super-orbital variability. This taxonomy connects to the color changes, orbital periods, and X-ray behavior of the sources. In most cases, these properties can be explained by differences between the flux of the disk around the Be star and the flux from the star itself. We also refine and present new potential orbital periods and sub-orbital variability in the sources.

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arXiv2024

Particle acceleration and multi-messenger radiation from Ultra-Luminous X-ray Sources: A new class of Galactic PeVatrons

Super-Eddington accretion onto stellar-mass compact objects powers fast outflows in ultra-luminous X-ray sources (ULXs). Such outflows, which can reach mildly relativistic velocities, are often observed forming bubble structures. Wind bubbles are expected to develop strong wind termination shocks, which are sites of great interest for diffusive shock acceleration. We developed a model of diffusive shock acceleration in the wind bubbles powered by ULXs. We find that the maximum energy in these objects can easily reach the PeV range, promoting winds from ULXs as a new class of PeVatrons. We specialized our model in the context of the Galactic source SS433 and show that high-energy protons in the bubble might explain the highest energy photons (>100 TeV) and their morphology recently observed by LHAASO. In this paper, we discuss the detectability of such a source in neutrinos, and we analyze the possible radio counterpart of ULXs focusing on the case of W50, the nebula surrounding SS433. Finally, we discuss the possible contribution of Galactic ULXs to the cosmic-ray flux at the knee, concluding that their role could be significant only if one of these sources, currently undetected, were sufficiently close.

astro-ph.HE↗
arXiv2024

Theoretical Analysis of the RX J0209.6-7427 X-ray Spectrum During Giant Outburst

We model the spectral formation occurring in the binary X-ray pulsar RX~J0209.6-7427 during the 2019 super-Eddington outburst. Using a theoretical model previously developed by the authors, we are able to produce spectra that closely resemble the phase-averaged X-ray spectra observed using NuSTAR and Insight-HXMT during low and high luminosity states of the outburst, respectively. The theoretical model simulates the accretion of fully ionized gas in a dipole magnetic field, and includes a complete description of the radiation hydrodynamics, matter distribution, and spectral formation. Type II X-ray outbursts provide an opportunity to study accretion over a large range of luminosities for the same neutron star. The analysis performed here represents the first time both the outburst low and high states of an accretion-powered X-ray pulsar are modeled using a physics-based model rather than standard phenomenological fitting with arbitrary mathematical functions. We find the outer polar cap radius remains constant and the column is more fully-filled with increasing luminosity, Comptonized bremsstrahlung dominates the formation of the phase-averaged X-ray spectrum, and a negative correlation exists between cyclotron centroid energy and luminosity, as expected. The super-Eddington nature of the outburst is rendered possible due to the low scattering cross section for photons propagating parallel to the magnetic field. We also find emission through the column top dominates in both the low and high states, implying the pulse profiles should have a roughly sinusoidal shape, which agrees with observed properties of ultra-luminous X-ray pulsars.

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arXiv2023

The High Energy X-ray Probe (HEX-P): Studying Extreme Accretion with Ultraluminous X-ray Sources

Ultraluminous X-ray sources (ULXs) represent an extreme class of accreting compact objects: from the identification of some of the accretors as neutron stars to the detection of powerful winds travelling at 0.1-0.2 c, the increasing evidence points towards ULXs harbouring stellar-mass compact objects undergoing highly super-Eddington accretion. Measuring their intrinsic properties, such as the accretion rate onto the compact object, the outflow rate, the masses of accretor/companion -- hence their progenitors, lifetimes, and future evolution -- is challenging due to ULXs being mostly extragalactic and in crowded fields. Yet ULXs represent our best opportunity to understand super-Eddington accretion physics and the paths through binary evolution to eventual double compact object binaries and gravitational wave sources. Through a combination of end-to-end and single-source simulations, we investigate the ability of HEX-P to study ULXs in the context of their host galaxies and compare it to XMM-Newton and NuSTAR, the current instruments with the most similar capabilities. HEX-P's higher sensitivity, which is driven by its narrow point-spread function and low background, allows it to detect pulsations and broad spectral features from ULXs better than XMM-Newton and NuSTAR. We describe the value of HEX-P in understanding ULXs and their associated key physics, through a combination of broadband sensitivity, timing resolution, and angular resolution, which make the mission ideal for pulsation detection and low-background, broadband spectral studies.

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arXiv2023

The High Energy X-ray Probe (HEX-P): the most powerful jets through the lens of a superb X-ray eye

A fraction of the active supermassive black holes at the centers of galaxies in our Universe are capable of launching extreme kiloparsec-long relativistic jets. These jets are known multiband (radio to $γ$-ray) and multimessenger (neutrino) emitters, and some of them have been monitored over several decades at all accessible wavelengths. However, many open questions remain unanswered about the processes powering these highly energetic phenomena. These jets intrinsically produce soft-to-hard X-ray emission that extends from $E\sim0.1\,\rm keV$ up to $E>100\,\rm keV$. Simultaneous broadband X-ray coverage, combined with excellent timing and imaging capabilities, is required to uncover the physics of jets. Indeed, truly simultaneous soft-to-hard X-ray coverage, in synergy with current and upcoming high-energy facilities (such as IXPE, COSI, CTAO, etc.) and neutrino detectors (e.g., IceCube), would enable us to disentangle the particle population responsible for the high-energy radiation from these jets. A sensitive hard X-ray survey ($F_{8-24\,\rm keV}<10^{-15}\,\rm erg~cm^{-2}~s^{-1}$) could unveil the bulk of their population in the early Universe. Acceleration and radiative processes responsible for the majority of their X-ray emission would be pinned down by microsecond timing capabilities at both soft and hard X-rays. Furthermore, imaging jet structures for the first time in the hard X-ray regime could unravel the origin of their high-energy emission. The proposed Probe-class mission concept High Energy X-ray Probe (HEX-P) combines all these required capabilities, making it the crucial next-generation X-ray telescope in the multi-messenger, time-domain era. HEX-P will be the ideal mission to unravel the science behind the most powerful accelerators in the universe.

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arXiv2023

The Luminosity Phase Space of Galactic and Extragalactic X-ray Transients Out to Intermediate Redshifts

We present a detailed compilation and analysis of the X-ray phase space of low- to intermediate-redshift ($ 0\le z \le 1$) transients that consolidates observed light curves (and theory where necessary) for a large variety of classes of transient/variable phenomena in the 0.3--10 keV energy band. We include gamma-ray burst afterglows, supernovae, supernova shock breakouts and shocks interacting with the environment, tidal disruption events and active galactic nuclei, fast blue optical transients, cataclysmic variables, magnetar flares/outbursts and fast radio bursts, cool stellar flares, X-ray binary outbursts, and ultraluminous X-ray sources. Our overarching goal is to offer a comprehensive resource for the examination of these ephemeral events, extending the X-ray duration-luminosity phase space (DLPS) to show luminosity evolution. We use existing observations (both targeted and serendipitous) to characterize the behavior of various transient/variable populations. Contextualizing transient signals in the larger DLPS serves two primary purposes: to identify areas of interest (i.e., regions in the parameter space where one would expect detections, but in which observations have historically been lacking) and to provide initial qualitative guidance in classifying newly discovered transient signals. We find that while the most luminous (largely extragalactic) and least luminous (largely Galactic) part of the phase space is well-populated at $t > 0.1$ days, intermediate luminosity phenomena (L$_x = 10^{34} - 10^{42}$ erg s$^{-1}$) represent a gap in the phase space. We thus identify L$_x = 10^{34} - 10^{42}$ erg s$^{-1}$ and $t = 10^{-4} - 0.1$ days as a key discovery phase space in transient X-ray astronomy.

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arXiv2023

Constraining the evolution of the unstable accretion disk in SMC X-1 with NICER

Neutron star high mass X-ray binaries with superorbital modulations in luminosity host warped inner accretion disks that occult the neutron star during precession. In SMC X-1, the instability in the warped disk geometry causes superorbital period "excursions:" times of instability when the superorbital period decreases from its typical value of 55 days to $\sim$40 days. Disk instability makes SMC X-1 an ideal system in which to investigate the effects of variable disk geometry on the inner accretion flow. Using the high resolution spectral and timing capabilities of the Neutron Star Interior Composition Explorer (NICER) we examined the high state of four different superorbital cycles of SMC X-1 to search forchanges in spectral shape and connections to the unstable disk geometry. We performed pulse phase-averaged and phase-resolved spectroscopy to closely compare the changes in spectral shape and any cycle-to-cycle variations. While some parameters including the photon index and absorbing column density show slight variations with superorbital phase, these changes are most evident during the intermediate state of the supeorbital cycle. Few spectral changes are observed within the high state of the superorbital cycle, possibly indicating the disk instability does not significantly change SMC X-1's accretion process.

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arXiv2023

Dust-scattering rings of GRB 221009A as seen by the Neil Gehrels Swift satellite: can we count them all?

We present the first results for the dust-scattering rings of GRB 221009A, coined as the GRB of the century, as observed by the Neil Gehrels Swift satellite. We perform analysis of both time resolved observations and stacked data. The former approach enable us to study the expansion of the most prominent rings, associate their origin with the prompt X-ray emission of the GRB and determine the location of the dust layers. The stacked radial profiles increase the signal-to-noise ratio of the data and allows detection of fainter and overlapping peaks in the angular profile. We find a total of 16 dust concentrations (with hints of even more) that span about 15 kpc in depth and could be responsible for the highly structured X-ray angular profiles. By comparing the relative scattered fluxes of the five most prominent rings we show that the layer with the largest amount of dust is located at about 0.44 kpc away from us. We finally compare the location of the dust layers with results from experiments that study the 3D structure of our Galaxy via extinction or CO radio observations, and highlight the complementarity of dust X-ray tomography to these approaches.

astro-ph.HE↗
arXiv2022

Comparing the Super-Eddington accretion of SMC X-3 and RX J0209.6-7427 with Swift J0243.6+6124

We study the giant outbursts of SMC X-3 and RX J0209.6-7427 to compare their super-Eddington accretion regime with that of Swift J0243.6+6124. The high double-peak profile of SMC X-3 is found to be 0.25 phase offset from that below $2.3\times10^{38}$erg\,s$^{-1}$, which is similar to Swift J0243 (happened around $0.9\times10^{38}$erg\,s$^{-1}$). The profile of RX J0209 shows a similar 0.25 phase offset between high double-peak and low double-peak around $1.25\times10^{38}$erg\,s$^{-1}$. The 0.25 phase offset corresponds to a 90 degree angle change of the emission beam and strongly supports for a transition from a fan beam to a pencil beam. Their critical luminosities imply a surface magnetic field $\sim4\times10^{13}$ G and $2\times10^{13}$ G for SMC X-3 and RX J0209, respectively, based on the recently measured cyclotron line of Swift J0243. The spin-up rate and luminosity of SMC X-3 follows a relation of $\dotν\propto L^{0.94\pm0.03}$, while that of RX J0209 follows $\dotν\propto L^{1.00\pm0.03}$, which are similar to Swift J0243 and consistent with the prediction of a radiation-pressure-dominated (RPD) disk. These results indicate that accretion columns are indeed formed above Eddington luminosity, and the population of ULXPs likely corresponds to X-ray pulsars of highest magnetic field.

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arXiv2022

Timing six energetic rotation-powered X-ray pulsars, including the fast-spinning young PSR J0058-7218 and Big Glitcher PSR J0537-6910

Measuring a pulsar's rotational evolution is crucial to understanding the nature of the pulsar. Here we provide updated timing models for the rotational evolution of six pulsars, five of which are rotation phase-connected using primarily NICER X-ray data. For the newly-discovered fast energetic young pulsar, PSR J0058-7218, we increase the baseline of its timing model from 1.4 days to 8 months and not only measure more precisely its spin-down rate nudot = (-6.2324+/-0.0001)x10^-11 Hz s^-1 but also for the first time the second time derivative of spin rate nuddot = (4.2+/-0.2)x10^-21 Hz s^-2. For the fastest and most energetic young pulsar, PSR J0537-6910 (with 16 ms spin period), we detect 4 more glitches, for a total of 15 glitches over 4.5 years of NICER monitoring, and show that its spin-down behavior continues to set this pulsar apart from all others, including a long-term braking index n = -1.234+/-0.009 and interglitch braking indices that asymptote to <~ 7 for long times after a glitch. For PSR J1101-6101, we measure a much more accurate spin-down rate that agrees with a previous value measured without phase-connection. For PSR J1412+7922 (also known as Calvera), we extend the baseline of its timing model from our previous 1-year model to 4.4 years, and for PSR J1849-0001, we extend the baseline from 1.5 years to 4.7 years. We also present a long-term timing model of the energetic pulsar, PSR J1813-1749, by fitting previous radio and X-ray spin frequencies from 2009-2019 and new ones measured here using 2018 NuSTAR and 2021 Chandra data.

astro-ph.HE↗
arXiv2021

Multi-messenger emission from the parsec-scale jet of the flat-spectrum radio quasar PKS 1502+106 coincident with high-energy neutrino IceCube-190730A

On July 30th, 2019 IceCube detected a high-energy astrophysical muon neutrino candidate, IC-190730A, with a $67\%$ probability of astrophysical origin. The flat spectrum radio quasar (FSRQ) PKS 1502+106 is in the error circle of the neutrino. Motivated by this observation, we study PKS 1502+106 as a possible source of IC-190730A. PKS 1502+106 was in a quiet state in terms of UV/optical/X-ray/gamma-ray flux at the time of the neutrino alert, we therefore model the expected neutrino emission from the source during its average long-term state, and investigate whether the emission of IC-190730A as a result of the quiet long-term emission of PKS 1502+106 is plausible. We analyse UV/optical and X-ray data and collect additional observations from the literature to construct the multi-wavelength spectral energy distribution of PKS 1502+106. We perform leptohadronic modelling of the multi-wavelength emission of the source and determine the most plausible emission scenarios and the maximum expected accompanying neutrino flux. A model in which the multi-wavelength emission of PKS 1502+106 originates beyond the broad-line region and inside the dust torus is most consistent with the observations. In this scenario, PKS 1502+106 can have produced up to of order one muon neutrino with energy exceeding 100 TeV in the lifetime of IceCube. An appealing feature of this model is that the required proton luminosity is consistent with the average required proton luminosity if blazars power the observed ultra-high-energy-cosmic-ray flux and well below the source's Eddington luminosity. If such a model is ubiquitous among FSRQs, additional neutrinos can be expected from other bright sources with energy $\gtrsim 10$ PeV.

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arXiv2021

Radio emission from colliding outflows in high-mass X-ray binaries with strongly magnetized neutron stars

We present a toy model for radio emission in HMXBs with strongly magnetized neutron stars (NS) where a wind-collision region is formed by the NS outflow and the stellar wind of the massive companion. Radio emission is expected from the synchrotron radiation of shock-accelerated electrons and the free-free emission of the stellar wind. We found that the predicted relation between the GHz luminosity ($L_R$) and the accretion X-ray luminosity ($L_X$) can be written as $L_R \propto L_X^b$ for most parameters. No correlation with X-rays is expected ($b=0$) when the thermal emission of the stellar wind dominates in radio. We typically find a steep correlation ($b=12/7$) for sub-Eddington X-ray luminosities and a more shallow one ($b=2(p-1)/7$) for super-Eddington X-ray luminosities, where $p$ is the power-law index of accelerated electrons. The maximum predicted radio luminosity is independent of the NS properties, while it depends on the stellar wind momentum, binary separation distance, and the minimum electron Lorentz factor. Using a Bayesian approach we modelled the radio observations of \sj that cover a wide range of mass accretion rates. Our results support a shock origin for the radio detections at sub-Eddington X-ray luminosities. However, no physically meaningful parameters could be found for the super-Eddington phase of the outburst, suggesting a different origin. Future observations with more sensitive instruments might reveal a large number of HMXBs with strongly magnetized NSs in radio, allowing determination of the slope in the $L_R-L_X$ relation, and putting the wind-collision scenario into test.

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