Swift/XRT follow-up observations of SAX J1808.4-3658
ATel #17326; Caterina Ballocco (INAF-OAR), Alessandro Papitto (INAF-OAR), Arianna Miraval Zanon (ASI), Giulia Illiano (INAF-OAB), Filippo Ambrosino (INAF-OAR)
on 10 Aug 2025; 18:54 UT
Credential Certification: Alessandro Papitto (alessandro.papitto@inaf.it)
Subjects: X-ray, Binary, Transient, Pulsar
On 2025 August 6, XB-NEWS reported a new outburst from the accreting millisecond X-ray pulsar SAX J1808.4-3658 from a brightening observed in the optical band (ATel #17323). Shortly after, the EP/FXT telescope confirmed the event in X-rays, reporting a flux of 4.30 (-0.35, +0.43)✕ 10-12erg cm-2 s-1 in the 0.5-10 keV band (ATel #17324).
On 2025 August 9 at 15:04:57 UTC, we performed a Swift/XRT target of opportunity observation in Photon Counting (PC) mode with an exposure time of 0.82 ks. The average XRT count rate during the observation was 39.4+/-0.7 counts/s.
The 0.3-10 keV X-ray spectrum could be fitted with an absorbed power-law model with an hydrogen column density of 7.5 (+2.5, -2.3) ✕ 1020 cm-2 and a photon index of 1.50+/-0.08. The corresponding unabsorbed flux in the 0.3-10 keV band is 1.93 (+0.09, -0.08) ✕ 10-9erg cm-2 s-1, which is equivalent to a luminosity of ~ 2.8✕ 1036erg s-1 assuming a distance of 3.5 kpc ( Galloway et al., 2006). This luminosity is comparable to the peak values observed during previous outbursts of the source (~1036erg s-1; see e.g. Illiano et al. 2023). This ToO observation is the first of an approved 10-days Swift monitoring program (1 ks/day) and we will continue to monitor the outburst evolution.
We would like to thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible.
