Detection of 205.7 s X-ray pulsations from the Be/X-ray binary pulsar LS V +44 17 during its September 2026 outburst

ATel #18089; Manoj Mandal (Physical Research Laboratory), Sachindra Naik (Physical Research Laboratory)
on 4 Oct 2026; 09:02 UT
Credential Certification: Manoj Mandal (manojmandal@prl.res.in)

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We report preliminary timing and spectral results from the XPoSat/XSPECT observations of the Be/X-ray binary pulsar LS V +44 17 (RX J0440.9+4431) during its ongoing September 2026 X-ray outburst. RX J0440.9+4431 is a Be/X-ray binary pulsar associated with the B0.2Ve optical companion LS V +44 17, located at a distance of approximately 2.44 kpc. The source hosts a slowly rotating neutron star with a spin period of approximately 205 s. The current X-ray activity of LS V +44 17 was reported by Einstein Probe (ATel #18063), Fermi/GBM (ATel #18067), and MAXI/GSC (ATel #18071), following a series of recurrent outbursts observed since March 2026 (ATels #17712, #17816, and #17931). Following these reports, we initiated XPoSat Target-of-Opportunity (ToO) observations on 2026 September 27 and performed preliminary timing and spectral analyses of the XSPECT data.

XPoSat, consisting of the co-aligned XSPECT and POLIX instruments, initiated ToO observations of LS V +44 17 on 2026 September 27 to monitor the source during the ongoing outburst. We performed timing and spectral analyses of the XSPECT data in the 1–10 keV energy range. During the September 27 observation, the barycenter-corrected and background-subtracted light curve shows strong X-ray pulsations. An epoch-folding analysis yields a pulse period of approximately 205.7 s, consistent with the pulsations detected by Fermi/GBM during the current outburst (ATel #18067). The folded pulse profile is nearly sinusoidal and single-peaked, with a pulse fraction of approximately 62%.

We simultaneously fitted the 1–10 keV XSPECT spectra from FOV2 and FOV3 with an absorbed power-law plus blackbody model. The best-fitting model yields an equivalent hydrogen column density of N_H = (1.67 ± 0.35) × 10²² cm⁻², a power-law photon index of Γ = 2.14 ± 0.52, and a blackbody temperature of kT = 1.81 ± 0.04 keV. The fit provides a reduced χ² of 1.01 for 1167 degrees of freedom. The corresponding unabsorbed 1–10 keV flux is (8.93 ± 0.07) × 10⁻¹⁰ erg cm⁻² s⁻¹. All quoted spectral uncertainties are at the 90% confidence level.

XSPECT (X-ray SPECtroscopy and Timing), onboard XPoSat, is a soft X-ray spectrometer operating in the 0.8–15 keV energy band. XSPECT employs Swept Charge Devices and has a net geometric area of 60 cm². It is designed to carry out long-term, pile-up-free observations of bright X-ray sources. The XPoSat project is managed and facilitated by the Indian Space Research Organisation (ISRO). We thank the XPoSat team for approving and promptly scheduling our Target-of-Opportunity observation of LS V +44 17. The XPoSat observations of LS V +44 17 are continuing, and further analysis of the spectral and timing evolution of the source during the current outburst is in progress.