The First X-ray Polarization Observation of the Black Hole X-ray Binary 4U 1630-47 in the Steep Power Law State

TL;DR

This paper reports the first X-ray polarization measurement of the black hole binary 4U 1630-47 in a specific accretion state, revealing consistent polarization properties across different states and providing insights into the emission mechanisms.

Contribution

It presents the first polarization observation of 4U 1630-47 in the steep power law state and compares it with previous measurements, offering new constraints on emission models.

Findings

Polarization degree of 6.8% at 2-8 keV energy range.

Polarization angle remains constant across energies.

Similar polarization signatures in different accretion states.

Abstract

The Imaging X-ray Polarimetry Explorer (IXPE) observed the black hole X-ray binary 4U 1630-47 in the steep power law (or very high) state. The observations reveal a linear polarization degree of the 2-8 keV X-rays of 6.8 +/- 0.2 % at a position angle of 21{\deg}.3 +/- 0{\deg}.9 East of North (all errors at 1{\sigma} confidence level). Whereas the polarization degree increases with energy, the polarization angle stays constant within the accuracy of our measurements. We compare the polarization of the source in the steep power-law state with the previous IXPE measurement of the source in the high soft state. We find that even though the source flux and spectral shape are significantly different between the high soft state and the steep power-law state, their polarization signatures are similar. Assuming that the polarization of both the thermal and power-law emission components are constant over time, we estimate the power-law component polarization to be 6.8-7.0% and note that the polarization angle of the thermal and power-law components must be approximately aligned. We discuss the implications for the origin of the power-law component and the properties of the emitting plasma.