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arXiv · 2610.14494

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

Gaurava K. Jaisawal·Georgios Vasilopoulos·Francesco Carotenuto·Hannah C. I. Wichern
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Abstract

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.

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Gaurava K. Jaisawal, Georgios Vasilopoulos, Francesco Carotenuto, Hannah C. I. Wichern. 2026-10-10. Reawakening of GS 1843+00: A Multiwavelength Study of its 2026 Giant X-ray Outburst. https://arxiv.org/abs/2610.14494

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