Reawakening of GS 1843+00: A multiwavelength study of its 2026 giant X-ray outburst
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 analyzed three νstar observations obtained at different stages of the outburst, complemented with archival and data, optical spectroscopy and imaging with the Nordic Optical Telescope, and a MeerKAT radio observation. νstar observations reveal coherent X-ray pulsations around 29.7 s with a spin-up rate of (-5.1 $ 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 Kα emission line, with no evidence of a cyclotron absorption feature. Phase-resolved spectroscopy reveals variations in the Comptonization and blackbody parameters, indicating changes in the accretion-column structure. Optical spectroscopy confirmed the presence of a Be-circumstellar disk through Hα and Paschen-series emission lines. No radio counterpart was detected, with a 3σ upper limit of 36 μJy beam$^ s s -1 -1 at 1.28 GHz. The observed pulse profile evolution, spectral variability, and reversal of the pulsed-fraction energy dependence suggest substantial changes in the accretion-column geometry during the outburst. Weaker Hα 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. Together, these results reveal a complex interplay between Be-disk evolution, outflows, and neutron-star accretion geometry in GS 1843+00.