Detection of a thermonuclear X-ray burst from XTE J1810-189 with NuSTAR during its October 2026 outburst

ATel #18115; Manoj Mandal (Physical Research Laboratory), Sachindra Naik (Physical Research Laboratory), Jaya Chowdhury (Physical Research Laboratory), Souvik Manik (Indian Institute of Technology Kanpur), Sabyasachi Pal (Midnapore City College), Aman Kaushik (Tata Institute of Fundamental Research), Sudip Bhattacharyya (Tata Institute of Fundamental Research), Andrea Sanna (Università degli Studi di Cagliari), Gaurava K. Jaisawal (DTU Space)
on 21 Oct 2026; 19:29 UT
Credential Certification: Manoj Mandal (manojmandal@prl.res.in)

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We report preliminary timing and spectral results from NuSTAR and Neil Gehrels Swift Observatory/XRT observations of the neutron star low-mass X-ray binary XTE J1810−189 during its ongoing October 2026 outburst. The source was discovered during its 2008 outburst (ATel #1424), and a subsequent detection of a thermonuclear (Type-I) X-ray burst established its neutron star nature (ATel #1443). Following the recent detection of renewed X-ray activity by MAXI/GSC (Negoro et al., ATel #18101), we initiated coordinated NuSTAR and Swift/XRT Target-of-Opportunity (ToO) observations to investigate its temporal and broadband spectral properties.

NuSTAR observed XTE J1810−189 on 2026 October 19 for approximately 23 ks, while Swift/XRT provided a complementary observation of approximately 1.5 ks. The NuSTAR light curves reveal a bright X-ray burst detected independently in both FPMA and FPMB, with a rapid rise of about 3–4 s, followed by an approximately exponential decay lasting about 20 s. The FPMA light curve, constructed with 0.5 s time bins, peaks at 923 ± 90 counts s⁻¹, compared with a pre-burst persistent count rate of ~7 counts s⁻¹. The burst morphology is consistent with a thermonuclear (Type-I) X-ray burst. The Swift/XRT observation, obtained in photon-counting (PC) mode, shows an average count rate of ~1.7 counts s⁻¹, with no evidence of X-ray bursts during the observation. A simultaneous 1-hour MeerKAT L-band (856–1712 MHz) observation was conducted alongside the NuSTAR observation. Additionally, a 6-hour uGMRT observation in Bands 4 (550–850 MHz) and 5 (1050–1450 MHz) has been approved and is awaiting scheduling.

We performed a joint spectral analysis of the persistent emission using Swift/XRT (0.8–9 keV) and NuSTAR FPMA/FPMB (3–30 keV) data. Our preliminary spectral analysis indicates that the broadband spectrum can be described by an absorbed cutoff power-law and blackbody continuum, supplemented by a broad Gaussian emission component (χ²/dof = 763.34/741). The best-fitting parameters are N_H = 5.92^+0.55
_−0.49 × 10²² cm⁻², photon index Γ = 3.14^+0.31
_−0.27, and blackbody temperature kT_bb = 1.47^+0.13
_−0.07 keV. The spectrum exhibits a broad emission excess around 5–7 keV, potentially associated with X-ray reflection from the accretion disc. The presence and possible origin of this feature will be further examined using alternative continuum and reflection models. The absorbed 0.8–30 keV flux is approximately 2.87 × 10⁻¹⁰ erg cm⁻² s⁻¹. All quoted spectral parameter uncertainties correspond to the 90% confidence level.

We thank the NuSTAR and Swift teams for approving and promptly scheduling our ToO observations. Further investigations, including time-resolved burst spectroscopy and a detailed timing study, are underway and will be presented in a forthcoming publication.