Swift J1727.8-1613 has returned to the hard state as seen by MAXI

ATel #16276; Wenfei Yu (Shanghai Astronomical Observatory)
on 10 Oct 2023; 06:22 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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Swift J1727.8-1613, a new transient black hole X-ray binary, was found in outburst on August 24, 2023 initially by Swift (GRB 230824A: GCN #34536, #34537), followed with X-ray detections by other X-ray missions (ATel #16205, #16206, #16219, #16238, #16242) including the detection of black hole QPOs (Atel # 16215, #16219). It had stayed in the black hole hard state as suggested by X-ray monitorings by MAXI and Swift/BAT and radio observation of compact jets in the source (ATel #16211, #16218, #16231) until very recent detection of the quenching of the radio compact jet (ATel #16271) and evidence of X-ray state transition towards the soft state (or soft-intermediate state) with NICER X-ray timing observations (ATel #16273).

With the most updated on-line MAXI orbital data up to MJD 60226.5 (2023 Oct. 9 at UT 12.0 hour), we found the MAXI hardness ratio between 4-20 keV and 2-4 keV (in crab unit) is 0.93 +- 0.03, returned to hardness ratio of the source on MJD 60211.0 of 0.93 +- 0.01 and the hardest hardness ratio of the source before the transition on MJD 60221 (0.91 +- 0.02). With these detections, we conclude that Swift J1727.8-1613 has quickly returned to the hard spectral state with short transitions between the soft and the hard state. We strongly encourage X-ray observations as well as multi-wavelength observations to monitoring the rapid change (e.g., accretion flow, jets and wind configurations) in the source.

We have also noticed that MAXI also show a source flux drop in 2-4 keV below 5 crab and a flux rise in 10-20 keV to 2 crab accompanied by the current transition back to the hard state following a stable flux trend in both bands. Therefore the hysteresis effect of transitions usually seen in transient black hole binaries (Miyamoto et al. 1995) can not be significant. The recent X-ray spectral state transitions show great similarities to the hard-to-soft state transition as well as the soft-to-hard transition observed during the X-ray luminosity decline in an outburst of the neutron star LMXB transient Aquila X-1 around MJD 52095 (Yu & Dolence 2007); in both sources the back-and-forth spectral transitions between the two major spectral states were of short-duration (~ 5 days), showing no apparent hysteresis. The empirical relationship between the rate-of-increase of X-ray flux in the early rising phase and the observed hard-to-soft transition in LMXBs (Yu and Yan 2009) implies that Swift J1727.8-1613 could not transit to the black hole soft state at its hard X-ray peak level during the current outburst, which also suggests a decline in the hard state as observed was more or less expected.

We thank the MAXI team and the Swift team for making their X-ray monitoring data public.

References: (1) Miyamoto et al. 1995, ApJ, 442, L13; (2) Yu & Dolence, 2007, ApJ, 667, 1043; (3) Yu & Yan 2009, ApJ, 701,1940)