Tracking the X-Ray Polarization of the Black Hole Transient Swift J1727.8–1613 during a State Transition

Abstract

We report on an observational campaign on the bright black hole (BH) X-ray binary Swift J1727.8–1613 centered around five observations by the Imaging X-ray Polarimetry Explorer. These observations track for the first time the evolution of the X-ray polarization of a BH X-ray binary across a hard to soft state transition. The 2–8 keV polarization degree decreased from ∼4% to ∼3% across the five observations, but the polarization angle remained oriented in the north–south direction throughout. Based on observations with the Australia Telescope Compact Array, we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher-spatial-resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long (≳10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8–1613 is a member of a hitherto undersampled subpopulation.

A Timing View of the Additional High-energy Spectral Component Discovered in the Black Hole Candidate Swift J1727.8-1613

“…Combined with the submillimeter band observation, these results indicate that the corona is elongated orthogonal to the jet. Similar accretion geometry was also proposed in the hard state by Ingram et al (2024) and Podgorný et al (2024). As the outburst evolves, Svoboda et al (2024) found that the polarization degree decreases dramatically to 1% in the soft state, which suggested that the polarization is mainly contributed by the upscattered radiation in the hot corona.…”

Swift J1727.8–1613 Has the Largest Resolved Continuous Jet Ever Seen in an X-Ray Binary

“…Radio observations from the Submillimetre Array (SMA), the Karl G. Jansky Very Large Array, the enhanced Multi Element Remotely Linked Interferometer (eMERLIN), and the Allen Telescope Array (ATA) showed a bright, flat-spectrum, unresolved source (Baglio et al 2023;Miller-Jones et al 2023;Vrtilek et al 2023;Williams-Baldwin et al 2023). Polarization observations at X-ray wavelengths with the Imaging X-ray Polarimetry Explorer (IXPE), at optical wavelengths with the 60 cm Tohoku telescope, at 230 GHz with the SMA, and at 5.5 and 9 GHz with the Australian Telescope Compact Array (ATCA) revealed J1727 to be polarized across the electromagnetic spectrum, with a position angle consistent with being aligned in the north-south direction (Dovciak et al 2023;Ingram et al 2023;Kravtsov et al 2023;Veledina et al 2023;Vrtilek et al 2023). These observations suggested that J1727 had a bright, compact, hard-state jet aligned in the north-south direction.…”