MAXI observations of Swift J1727.8-1613 indicate transition of the source to the low/hard state

ATel #16541; Jakub Podgorny (AI CAS, Prague), Jiri Svoboda (AI CAS, Prague), Michal Dovciak (AI CAS, Prague)
on 20 Mar 2024; 06:18 UT
Credential Certification: Jakub Podgorny (jakub.podgorny@asu.cas.cz)

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We report a change in the X-ray spectral state of the black hole X-ray binary Swift J1727.8-1613 (originally GRB 230824A: GCN #34536, #34537). This low-mass X-ray binary (LMXB) was first discovered in August 2023, when it went to a prominently bright outburst (ATel #16205, #16206, #16207,#16210, #16215). The MAXI monitoring (Matsuoka et al., 2009) allowed to trace its evolution in the hardness-intensity diagram (see Figure 1 in Svoboda et al., arXiv:2403.04689). The source evolved from the hard state to the soft state with continuously decaying flux over past months. The X-ray flux dropped by more than two orders of magnitude from the outburst, with current X-ray flux approximately 1e-9 erg cm-2 s-1 in 2-10 keV.

Using the MAXI on-demand service that extracts the X-ray contamination by a nearby source GX 9+9, we report a new significant change in the time averaged data from the past 10 days. Between MJD 60379.031157 to 60381.743843 and MJD 60388.051887 to 60388.957894, the 4-10 keV/2-4 keV hardness went from (0.08 +- 0.03) to (0.43 +- 0.12). The 2-20 keV flux in the same period evolved from (0.12 +- 0.01) cts/cm^2/s to (0.17 +- 0.03) cts/cm^2/s. It indicates that the source is likely beginning a transition towards the low/hard spectral state. We provide the MAXI light curves with 1-day time bin across the past 60 days and the corresponding hardness-intensity diagram (since the outburst beginning) here.

Multiwavelength observations are encouraged to catch the soft-to-hard transition of Swift J1727.8-1613.

This ATel has made use of the MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images (MAXI; Matsuoka et al., PASJ, 61, 999, 2009) data provided by RIKEN, JAXA and the MAXI team.