2023ApJ...949L..43R

We detect a high level of polarization in the X-ray emission of the black hole binary 4U 1630-47 in an observation with the Imaging X-ray Polarimetry Explorer. The 2-8 keV polarization degree is 8% at a position angle of 18°, with the polarization degree increasing significantly with energy, from ∼6% at ∼2 keV to ∼11% at ∼8 keV. The continuum emission in the spectrum of simultaneous observations with the Neutron Star Interior Composition Explorer (NICER) is well described with only a thermal disk spectrum, with stringent upper limits to any Comptonized emission from the corona. Together with the lack of significant variability in the Fourier power spectrum, this suggests that the source was in the high soft state at the time of these observations. The NICER spectrum reveals the presence of several absorption lines in the 6-9 keV band that we fit with two ionized absorbers, providing evidence of the presence of a strong disk wind, which supports the idea that the source was in the soft state. Previous measurements of X-ray polarization in other sources in harder states were associated with the corona or the jet in those systems. Given that the corona is significantly absent in this observation of 4U 1630-47, and that the jet in black hole binaries is quenched in the high soft state, we speculate that in this observation of 4U 1630-47, the polarization likely arises from the direct and reflected radiation of the accretion disk in this source.