IXPE detection of a bright thermonuclear burst during the reflaring stage of SAX J1808.4-3658
ATel #17369; C. Ballocco (INAF-OAR), A. Trois (INAF-OAC), A. Papitto (INAF-OAR), E. Del Monte (INAF-IAPS), A. Di Marco (INAF-IAPS), F. La Monaca (INAF-IAPS), G. Illiano (INAF-OAB), F. Ambrosino (INAF-OAR), A. Miraval Zanon (ASI)
on 2 Sep 2025; 19:31 UT
Credential Certification: Alessandro Papitto (alessandro.papitto@inaf.it)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
During follow-up observations of the outburst of the accreting millisecond pulsar SAX J1808.4-3658 first reported on August 6, 2025 (ATel #17323, #17324, #17326, #17329), Imaging X-ray Polarimetry Explorer (IXPE) detected a bright type-I X-ray burst starting on August 30, 2025, at 15:02:29 UTC. The flux reached a peak count rate of ~ 750 c/s in raw data and then declined to ~ 30/45 c/s within ~ 2 minutes. Assuming an X-ray spectrum described by an absorbed black body with a temperature of 1.8 keV, we estimated the bolometric peak flux as 2.5E-07 erg/cm^2/s, comparable to the brightest ever observed from the source (see, e.g., Bult et al. 2019, ApJ, 885, L1). Together with observed deviations of the light curve from a simple exponential profile, the peak flux suggests the source reached the local Eddington limit at the neutron star envelope.
A few hours after the burst detection, Swift/XRT observations performed in Windowed Timing (WT) mode on August 31 at 03:55 UTC detected the source at a 0.3-10 keV observed flux of 1.67(8)E-10 erg/cm^2/s, with a spectrum described by an absorbed power law with a photon index of 2.2+/-0.1. At a distance of 3 kpc, the flux corresponds to a luminosity of ~1.8E35 erg/s. Variations in the X-ray flux by an order of magnitude during the approximately daily Swift XRT monitoring (ATel #17326) suggest that the source is currently in the reflaring stage, which characterises the final stages of its outbursts.
Multiwavelength follow-up of the outburst is encouraged. We thank the IXPE and Swift teams, Observatory Duty Scientists and the PIs for promptly scheduling and performing these observations.
