Georgios Vasilopoulos

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.

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 P ̇ = − 4.7 ( 1 ) × 1 0 − 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.

Radio flares have been postulated as being associated with the production of astrophysical neutrinos. For example, TXS 0506+056 exhibits a 2–3 yr delay between the 2017 IceCube-170922A/ γ -ray flare and a GHz radio maximum. We quantitatively test if the delayed radio flare originates from the same compact region where neutrinos and γ rays are produced as it expands downstream and synchrotron self-absorption 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 redissipation on parsec 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. 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, instead of being a direct radiative counterpart of the compact neutrino-production zone.

Context. High-redshift ( z > 2) blazars, with relativistic jets aligned toward us, probe the most powerful end of the active galactic nuclei (AGNs) population. Aims. We aimed to determine 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). Methods. 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. Results. 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 ∼(10 ⁸ − 10 ¹⁰ ) M _⊙ and a broad distribution of median Eddington ratios ( λ _Edd ∼ 0.04 up to ∼1), with several objects accreting at λ _Edd ≳ 0.4 both at z ∼ 2 − 3 and at z ≳ 5. 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, changing the inferred M _BH and Ṁ by factors up to five. Conclusions. 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.

Context. Carbon stars on the asymptotic giant branch are major contributors to the dust enrichment of galaxies, with gas mass-loss rate values up to Ṁ ≈ 10 ⁻⁴ M _⊙ yr ⁻¹ . They represent the final evolutionary stage of low- and intermediate-mass stars, during which recurrent dredge-up episodes enrich their atmospheres with carbon and trigger the formation of dust. Through their intense winds, they inject large amounts of newly formed carbonaceous dust into the interstellar medium, playing a central role in the chemical evolution of galaxies. Their stellar and dust properties have been studied for decades, with a particular focus on the carbon stars in the Magellanic Clouds (MCs). Aims. Our aim is to homogeneously analyse the Gaia DR3 Golden Sample of Carbon Stars through the spectral energy distribution (SED) fitting. Our focus is on sources belonging to the Milky Way (MW) and the MCs. Methods. Our dataset consists of 14 747 stars with complete multi-band photometry from Gaia , 2MASS, and WISE, combined with recent distance and extinction estimates. For a subsample of Mira variables made of 2494 stars, we also modelled multi-band light curves to obtain accurate mean magnitudes. Stellar and circumstellar properties were derived by fitting the observations with a large grid of synthetic models computed with the DUSTY radiative transfer code, using COMARCS model atmospheres as input. For each target, we determined stellar and dust parameters such as the effective temperature, optical depth, and gas mass-loss rate. Results. The resulting distributions reveal typical effective temperatures of around 3150 K. Mass-loss rates range from 10 ⁻¹¹ to 10 ⁻⁴ M _⊙ yr ⁻¹ . The average dust temperature at the inner bound of the dust shell is about T _d = 1000 K. We also observe a correlation between photometric variability amplitude and mass-loss rate. Conclusions. This homogeneous framework provides a unified view of carbon stars across environments spanning a wide range of metallicities, supported by strong statistical coverage. Our results show that some of the physical properties of carbon stars exhibit a dependence on the galactic environment. However, these dependences do not necessarily reflect intrinsic metallicity effects, but are influenced by differences in luminosity distributions and by the selection biases affecting the available samples. The use of Gaia and WISE introduces combined selection effects that are significant, limiting the detection of both the most dust-enshrouded objects and the less luminous sources in the Magellanic Clouds. While this limits the completeness of the comparison, the observed trends remain statistically robust within the selected samples.

Context . The Magellanic Clouds, the closest star-forming galaxies to the Milky Way, offer an excellent environment to study high-mass X-ray binaries (HMXBs). While the Small Magellanic Cloud (SMC) has been thoroughly investigated with over 120 systems identified, the Large Magellanic Cloud (LMC) 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. Aims . 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. Methods . Using detections during the first eROSITA all-sky survey (eRASS1), 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. Results . 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. Compared to the SMC, the LMC hosts fewer HMXBs and more systems with supergiant companions. We identify several likely supergiant systems, including a candidate supergiant fast X-ray transient with phase-dependent flares. We also find three Be stars with likely white dwarf companions. Two of the candidate Be/WD binaries show steady luminosities across four eROSITA scans, unlike the post-nova states seen in the majority of previous Be/WD reports. Conclusions . 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 _−0.07 ^+0.06 ) M _⊙ yr ⁻¹ , which is in agreement with results using other tracers.

The first four all-sky surveys with eROSITAthe 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, NuSTARand 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 NuSTARdata 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 NuSTARspectra with ∼1.8σ. 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.

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.

Context. Changes in the rotational period observed in various magnetized accreting sources are generally attributed to the interaction between the infalling 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 provide 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 was performed. Methods. We reformulated the set of disk accretion torque models in a way that their parameterizations 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 that one model is preferred over the others. Even so, the derived parameters from these 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. Conclusions. The application of torque models provides a generic way to access system parameters of the accreting object. The values obtained via these models must be treated with caution, since the systematics of the models must be taken into account. Our example suggests that the current state of analytic torque models does not allow for quantitative measurements of the magnetic field of an accreting object. Systematic application to a sample of sources with known magnetic fields and distances will provide a selection criterion between models in the future.