arXiv subjects

S. Sheth

Publications and source records attributed to S. Sheth.

4 recordsLinked to original sources

arXiv2026

The SRG/eROSITA All-Sky Survey DR2: Cumulative X-ray catalogues from the first three surveys and multi-wavelength counterparts in the western Galactic hemisphere

The eROSITA telescope array on board the Spektrum-Roentgen-Gamma (SRG) mission began its all-sky survey program in December 2019, scanning the sky at an approximately six-month cadence. Here, we present new catalogues of point-like and extended X-ray sources derived from the first three completed eROSITA all-sky surveys (eRASS:3), covering the western Galactic hemisphere, which Germany's eROSITA consortium holds proprietary rights for. We describe the observational strategy, data processing, and analysis pipelines. We also characterise the resulting X-ray source populations. The eRASS:3 main catalogue contains nearly two million sources, including 1,911,744 point-like and 63,796 extended sources, detected in the 0.2-2.3 keV energy band, eROSITA's most sensitive energy window. This volume has doubled the X-ray source content of eRASS1 and provides a comprehensive census of X-ray-emitting objects across diverse astrophysical classes. We also released a secondary hard catalogue of nearly 15,000 sources detected in the harder 2.3-5.0 keV energy band. In addition, we released six catalogues, three for the main sample and three for the hard sample, in which we identify and classify the optical and infrared counterparts of eRASS:3 point-like sources. This approach delivers a homogeneous, high-quality identification of mostly extragalactic X-ray emitters; for example, we estimate that approximately 88% of the 1.4 million counterparts identified within the footprints of the Legacy Survey Imaging for DESI are extragalactic sources. This work has enabled the generation of samples optimised for completeness and purity, while expanding the discovery space for rare populations. The second data release (DR2) of the SRG/eROSITA all-sky survey is a catalogue-only release, comprising the catalogues presented in this work together with an updated version of the eROSITA upper flux limit server.

astro-ph.HE↗

arXiv2026

A phase-coherent timing solution for the X-ray dim isolated neutron star eRASSU J131716.9-402647

Based on its predominantly thermal X-ray emission and long spin period, the isolated neutron star eRASSU J131716.9-402647 is one of the most promising candidates for membership in the still small class of X-ray dim isolated neutron stars (XDINSs). Confirmation of this classification, however, requires a more detailed characterisation of the source's timing and spectral properties. In this work, we present new NICER observations which, together with previous X-ray follow-up, allow us to constrain the timing properties and long-term evolution of eRASSU J131716.9-402647. We obtain a coherent timing solution with a spin period of $P\sim12.8$ s and a period derivative of $\dot{P}\sim9\times 10^{-14}$ s s$^{-1}$, which best-describes the spin evolution of the source. These parameters imply a dipolar magnetic field strength of $3\times10^{13}$ G and a spin-down luminosity of order $10^{30}$ erg s$^{-1}$. Spectral modelling reveals no significant change in the spectral state over the 15 months of observational monitoring and indicates a thermal luminosity that likely exceeds the rotational energy loss. This suggests a thermal evolution that has been significantly influenced by past reheating. The energy dependence of the double-humped pulse profile closely resembles that observed in the XDINS RX J1308.6+2127, with the pulsed fraction increasing towards higher energies. Taken together, these results unambiguously confirm the XDINS nature of eRASSU J131716.9-402647, making it the first newly confirmed XDINS in more than two decades.

astro-ph.HE↗

arXiv2025

X-ray, optical, and radio follow-up of five thermally emitting isolated neutron star candidates

We report on follow-up observations with XMM-Newton, the FORS2 instrument at the ESO-VLT, and FAST, aiming to characterise the nature of five thermally emitting isolated neutron star (INS) candidates recently discovered from searches in the footprint of the Spectrum Roentgen Gamma (SRG)/eROSITA All-sky Survey. We find that the X-ray spectra are predominantly thermal and can be described by low-absorbed blackbody models with effective temperatures ranging from 50 to 210 eV. In two sources, the spectra also show narrow absorption features at $300 - 400$ eV. Additional non-thermal emission components are not detected in any of the five candidates. The soft X-ray emission, the absence of optical counterparts in four sources, and the consequent large X-ray-to-optical flux ratios $>3000 - 5400$ confirm their INS nature. For the remaining source, eRASSU J144516.0-374428, the available data do not allow a confident exclusion of an active galactic nucleus nature. However, if the source is Galactic, the small inferred X-ray emitting region is reminiscent of a heated pulsar polar cap, possibly pointing to a binary pulsar nature. X-ray timing searches do not detect significant modulations in all candidates, implying pulsed fraction upper limits of 13 - 19% ($0.001-13.5$ Hz). The absence of pulsations in the FAST observations targeting eRASSU J081952.1-131930 and eRASSU J084046.2-115222 excludes periodic magnetospheric emission at 1 - 1.5 GHz with an $8\sigma$ significance down to 4.08 $\mu$Jy and 2.72 $\mu$Jy, respectively. The long-term X-ray emission of all sources does not imply significant variability. Additional observations are warranted to establish exact neutron star types. At the same time, the confirmation of the predominantly thermal neutron star nature in four additional sources highlights the power of SRG/eROSITA to complement the Galactic INS population.

astro-ph.HE↗

arXiv2025

Massive runaway star HD 254577: the pre-supernova binary companion to the progenitor of the supernova remnant IC 443

The secondaries of the massive binary systems can be found as runaway stars after being ejected due to the supernova (SN) of the more massive component. We search for such stars inside the supernova remnants (SNRs), where a recent SN is guaranteed to have happened. In this paper, we present the massive runaway star HD~254577 as the pre-supernova binary companion to the progenitor of the supernova remnant IC~443 and the neutron star (NS) CXOU~J61705.3$+$222127. We performed spectroscopic observations of the runaway star and specified its atmospheric parameters. Together with the precise \textit{Gaia} DR3 astrometry and photometry, we identified the possible birth origin of the runaway star, and by isochrone fitting, we determined the progenitor mass. From the \textit{Gaia} DR3 proper motions, we specified the possible explosion sites and calculated the neutron star (NS) velocity. HD~254577 is a hot and evolved star with an effective temperature of $24000\pm1000$ K (B0.5II) and a surface gravity of $\log({g~\rm[cm/s^2]})=2.75\pm0.25$. It is probably a single star with a peculiar 3-D velocity of $31.3^{+1.2}_{-0.9}$ km~s$^{-1}$, lying at a heliocentric distance of $1701^{+55}_{-54}$ pc. The cometary tail of the NS implies that it is moving away from the same site as the runaway star. From the flight trajectories, we calculated that the NS has typical pulsar velocities such as $254-539$ km~s$^{-1}$ at a distance of $1.7$ kpc for $10-20$ kyr ages. Together with the blue-only shifted interstellar medium lines on its spectrum, HD~254577 must be the pre-supernova binary companion to the progenitor of IC~443. By identifying the pre-supernova companion and the possible parent cluster, we showed that the progenitor zero-age main sequence mass is high: $31-64$ M$_\odot$. We also discuss the expansion dynamics of the SNR due to the highly off-centered explosion site.

astro-ph.SR↗