The IXPE-detected X-ray polarization in Swift J1727.8-1613 in the low/hard state remarkably similar to the bright hard-intermediate state

ATel #16596; Jiri Svoboda (Astronomical Institute of the Czech Academy of Sciences, ASU), Michal Dovciak (ASU), Elise Egron (INAF Osservatorio Astronomico di Cagliari), Giorgio Matt (Universita degli Studi Roma Tre), Alexandra Veledina (University of Turku), Jakub Podgorny (ASU), Henric Krawczynski (Washington University in St. Louis), Phil Kaaret (NASA Marshall Space Center) on behalf of a larger collaboration
on 19 Apr 2024; 15:45 UT
Credential Certification: Jiri Svoboda (jiri.svoboda@asu.cas.cz)

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We report the new X-ray polarization measurements of a black-hole X-ray binary Swift J1727.8-1613 in the low/hard state with the Imaging X-ray Polarimetry Explorer (IXPE). The source (R.A. 17:27:43.31, DEC -16:12:19.23) was discovered in August 2023 when it underwent an exceptionally bright outburst, peaking at around 7 Crab in the 2-20 keV energy range (GCN #34540, GCN #34544). The first X-ray polarization measurements, performed a few weeks after the discovery, when the source was in the hard-intermediate state, revealed the polarization degree PD ≈ 3-4% and the polarization angle PA ≈ 2° east of north (Veledina et al. (2023), Ingram et al. (2024)). In the soft state, with the spectrum dominated by the thermal emission of the accretion disk, the polarization fraction significantly decreased to the level below 1% Svoboda et al. (2024). Following the recent soft-to-hard transition (ATEL #16541), we triggered a new IXPE observation in the low/hard state (performed on Apr 3 - 8, MJD 60403.6576-60408.1005) and obtained PD = (3.5 ± 0.6)% and PA = 5° ± 5°, i.e. identical to the bright hard state. These measurements suggest remarkable similarity of accretion geometries despite the flux being currently two orders of magnitude lower (about 0.04 Crab in 2-20 keV), as compared to the observations taken during the bright hard-to-soft transition. The radio observations performed with the RATAN-600 and SRT simultaneously with the IXPE observation indicate a weak flux density between 1 and 8 mJy at 4.7 GHz, and show a significant decrease of the radio flux compared to the transition phase (ATEL #16552). Other multi-wavelength observations are strongly encouraged.
We thank the IXPE operation team for their efforts in scheduling the observation.