NuSTAR confirms ongoing X-ray outburst from Be/X-ray binary pulsar GS 1843+00
ATel #17722; G. K. Jaisawal (DTU Space) and G. Vasilopoulos (NKUA)
on 13 Mar 2026; 09:48 UT
Credential Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
GS 1843+00 (Ginga 1843+009) is a Be/X-ray binary hosting a ~29.5 s accreting X-ray pulsar (Koyama et al. 1990; Israel et al. 2001). Since 2010, the source has remained inactive. On 2026 February 27 (MJD 61098), MAXI/GSC reported renewed X-ray activity from the direction of GS 1843+00 (ATel #17706). Public MAXI data suggest that the brightening began around 2026 February 19 (MJD 61090), with the daily-averaged 2-20 keV flux reaching ~50 mCrab on 2026 March 11.
We observed GS 1843+00 with NuSTAR between 2026 March 10 22:32:49 and March 11 20:06:20 UTC for a total effective exposure of ~36 ks. The source was clearly detected at the known position of GS 1843+00, confirming the ongoing outburst from the system. The NuSTAR 3-79 keV light curve shows an average count rate of ~56 counts per second per focal plane module. Timing analysis of the barycenter-corrected light curve reveals coherent X-ray pulsations with a best-fit spin period of 29.8346(1) s. The pulse profile is complex and shows significant energy dependence, evolving from softer to harder X-ray bands. The pulsed fraction increases from ~3% in the soft band to <20% toward higher energies in the 3-80 keV range.
The broadband 3-79 keV NuSTAR spectrum can be described by an absorbed Comptonization model (comptt) with an additional blackbody component and an iron emission line near 6.4 keV. Preliminary best-fit parameters are: blackbody kT = 1.23 ± 0.02 keV, blackbody norm = 13.4 ± 0.5; Comptonization seed photon temperature T0 = 3.35 ± 0.03 keV, electron temperature kTe = 10.9 ± 0.2 keV, optical depth = 3.77 ± 0.02, and reduced χ2 = 1.13 for 2613 degrees of freedom. Uncertainties are quoted at the 90% confidence level.
The unabsorbed 3-80 keV flux is (5.84 ± 0.02) × 10-9 erg cm-2 s-1. Assuming a distance of 10 kpc (Piraino et al. 2000), this corresponds to an X-ray luminosity of 7 × 1037 erg s-1, indicating that the source is currently accreting near the critical luminosity regime typical of bright Be/X-ray binary outbursts. These results confirm that GS 1843+00 has entered a renewed active accretion phase after a long period of quiescence. Detailed spectral and timing analyses of the NuSTAR observation are ongoing.
We thank the NuSTAR PI, Fiona Harrison, for approving the DDT observation and the NuSTAR SOC for promptly scheduling and executing the observation.
