Swift/XRT detects an X-ray decline in 1A 0538-66
ATel #18051; G. K. Jaisawal (DTU Space)
on 17 Sep 2026; 11:47 UT
Credential Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
We report on a Swift/XRT observation of the Be/X-ray binary pulsar 1A 0538-66 following the trigger of a major X-ray outburst detected by MAXI/GSC on 2026 August 28 (ATel #18020).
We observed the pulsar with the Swift/XRT for a net exposure of approximately 1.7 ks in Windowed Timing mode on 2026 September 9. The 0.5-10 keV spectrum is fitted with an absorbed power-law model using Cash statistics, yielding a photon index of 1.8 ± 0.4. The equivalent hydrogen column density is poorly constrained, with lower and upper limits of 1 × 1020 cm−2 and 2.5 × 1021 cm−2, respectively. The best-fit model gives a Cash statistic of 30.5 for 27 degrees of freedom. The quoted uncertainties correspond to a 90% confidence level. We estimate an unabsorbed flux of (7.9 ± 1.6) × 10−12 erg cm−2 s−1 in the 0.5-10 keV band and (3.1 ± 1.4) × 10−12 erg cm−2 s−1 in the 4-10 keV band. Assuming a distance of 50 kpc, the latter corresponds to an X-ray luminosity of (9 ± 4) × 1035 erg s−1, about three orders of magnitude below the peak value of 1.1 × 1039 erg s−1 reported by MAXI in the same band (ATel #18020).
The source remained X-ray active roughly 12 days after the MAXI/GSC detection, though far fainter than at peak. This places the outburst well into its decline phase.
