arXiv subjects

Hannah C. I. Wichern

Publications and source records attributed to Hannah C. I. Wichern.

2 recordsLinked to original sources

arXiv2026

Reawakening of GS 1843+00: A Multiwavelength Study of its 2026 Giant X-ray Outburst

GS 1843+00 (Ginga 1843+009) is a poorly studied Be/X-ray binary system that underwent a major X-ray outburst in 2026 February-May after nearly sixteen years of quiescence. We present an X-ray, optical, and radio study of GS 1843+00 during this outburst, focusing on the evolution of the neutron star emission geometry, Be-disk connection, and possible outflow activity. We analyse three NuSTAR observations obtained at different stages of the outburst, complemented by archival RXTE and Chandra data, optical spectroscopy and imaging with the Nordic Optical Telescope, and a MeerKAT radio observation. NuSTAR observations reveal coherent X-ray pulsations around 29.7 s with a spin-up rate of $(-5.1\pm0.1)\times10^{-8}$ s s$^{-1}$ during the outburst. The pulse profiles show strong evolution with energy and outburst phase, with distinct morphologies even at comparable luminosities. The pulsed fraction varies between 1 and 16% and exhibits a complex reversal in energy dependence above 7 keV, increasing with energy during the first two observations and decreasing during the third. The 3-79 keV broadband spectra are described by thermal Comptonization with a blackbody and an Fe K$α$ emission line, with no evidence for a cyclotron absorption feature. Phase-resolved spectroscopy reveals variations in the Comptonization and blackbody parameters, indicating changes in the accretion-column structure. Optical spectroscopy confirms the presence of a Be-circumstellar disk through H$α$ and Paschen-series emission lines. No radio counterpart is detected, with a $3σ$ upper limit of 36 $μ$Jy beam$^{-1}$ at 1.28 GHz. Weaker H$α$ emission compared with previous epochs indicates that the Be-circumstellar disk was in a different state during the current episode, while the radio non-detection suggests that any associated outflow activity was faint or below the detection limit.

astro-ph.HE↗

arXiv2026

Low-hard to high-soft spectral state transitions in the faintest early-X-ray-detected optical tidal disruption event TDE 2025aarm

We report the X-ray and optical spectroscopic properties of TDE 2025aarm, the second closest tidal disruption event (TDE) discovered to date. The proximity of this source, combined with a deep and intense X-ray monitoring campaign spanning six months, allowed us to probe the source down to an unprecedented 0.2-10 keV luminosity of $\sim7\times10^{39}$ erg s$^{-1}$ close to the optical peak. This renders TDE 2025aarm the faintest early-X-ray-detected TDE to date. After the first X-ray detection, the source brightened by nearly two orders of magnitude, reaching a peak luminosity of $\sim5\times10^{41}$ erg s$^{-1}$ about four months after the optical peak. Through time-resolved X-ray spectral analysis, we find that TDE 2025aarm evolved from an initially hard, power-law-dominated X-ray state into a softer, disk-dominated state as the luminosity increased, before hardening again at later times. Such low-hard-to-high-soft state transitions are commonly observed in black hole X-ray binaries (XRBs) but have not previously been reported in thermal TDEs. We show that the spectral evolution can be described by variations in the relative contributions of an accretion disk and a Comptonizing component, qualitatively resembling the disk--corona evolution observed in XRBs. We also present the results of our optical spectroscopic follow-up campaign with HET/LRS2, confirming the TDE classification and revealing NIII Bowen fluorescence features. The extremely faint early-time X-ray emission of TDE 2025aarm further supports the idea that the historical dichotomy between X-ray-bright and X-ray-undetected TDEs is largely driven by selection effects related to the depth, cadence, and duration of X-ray follow-up observations. TDE 2025aarm therefore provides new insight into both the accretion physics of TDEs and the possible universality of accretion across several orders of magnitude in black hole mass.

astro-ph.HE↗