Discovery of X-Ray Polarization from the Black Hole Transient Swift J1727.8−1613

Abstract

We report the first detection of the X-ray polarization of the bright transient Swift J1727.8−1613 with the Imaging X-ray Polarimetry Explorer. The observation was performed at the beginning of the 2023 discovery outburst, when the source resided in the bright hard state. We find a time- and energy-averaged polarization degree of 4.1% ± 0.2% and a polarization angle of 2.°2 ± 1.°3 (errors at 68% confidence level; this translates to ∼20σ significance of the polarization detection). This finding suggests that the hot corona emitting the bulk of the detected X-rays is elongated, rather than spherical. The X-ray polarization angle is consistent with that found in submillimeter wavelengths. Since the submillimeter polarization was found to be aligned with the jet direction in other X-ray binaries, this indicates that the corona is elongated orthogonal to the jet.

Dramatic Drop in the X-Ray Polarization of Swift J1727.8–1613 in the Soft Spectral State

“…The polarization degree (PD) grows with the system inclination and energy and can achieve ∼15%-20% at high energies if the medium is viewed nearly edge-on. The first IXPE data on the hard-state systems revealed that the polarization signatures are largely consistent with these expectations (Krawczynski et al 2022;Ingram et al 2023;Veledina et al 2023).…”

“…The MAXI monitoring of Swift J1727.8-1613 up to the new IXPE observation is summarized in Figure 2. The plot serves as an updated version of Figure 1 in Ingram et al (2023) and Veledina et al (2023) with slight variations due to the enhanced data selection, subtracting the region around GX 9+9.…”

“…IXPE first observed this source on 2023 September 7, recording a PD of ≈4% (Veledina et al 2023). The X-ray polarization was found to be roughly in the north-south direction with a polarization angle (PA) ≈ 2°east of north, which aligns with the submillimeter polarization (Vrtilek et al 2023), as well as with the optical polarization (Kravtsov et al 2023).…”

Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1

“…As reported in the IXPEOBSSIM notes 57 and Veledina et al (2023), the polarization obtained by the PCUBE algorithm, when using the gray filter, does not properly account for the telescope response matrices at low energy. In particular, an overestimate of the polarization below 3 keV is observed with respect to the one obtained by the more solid XSPEC (Arnaud 1996) spectropolarimetric analysis, which properly considers the response matrices of the instrument at low energy.…”

X-Ray Polarization of the Black Hole X-Ray Binary 4U 1630–47 Challenges the Standard Thin Accretion Disk Scenario

“…In particular, the 4% polarization observed for the archetypal BHXRB Cyg X-1 (Krawczynski et al 2022), parallel to the radio jet, constrained the geometry of the hot corona responsible for the power-law emission in the hard state, to be extended perpendicular to the jet direction (see Done et al 2007, for detailed descriptions of the different X-ray states). IXPE monitored the polarization properties of the recently discovered BH binary candidate, Swift J1727.8−1613, throughout its state transition from a hard to a soft state, suggesting that the Comptonizing corona extends over the disk, similar to the case of Cyg X-1 (Veledina et al 2023a;Ingram et al 2023). In the disk-dominated spectral states of LMC X-1, LMC X-3, and 4U 1957+115, IXPE observations indicate that the polarization measurements are consistent with the standard geometrically thin, optically thick accretion disks (Marra et al 2023;Podgorný et al 2023;Svoboda et al 2024).…”