Energy-independent X-Ray Polarization in 4U 1630-47 in the Intermediate State from Spectroscopy and Polarimetry

Abstract

The IXPE mission enabled measurements of X-ray polarization in accreting binaries. The dependence of the X-ray polarization degree on the binary inclination and the optical depth of the emitting medium $τ_0$ are the new important tools in X-ray Astronomy. We analyzed the black hole X-ray binary 4U~1630--47 as an example of one of the enigmatic sources in our Galaxy. We consider spectral and polarimetric observations of outburst episodes from 4U~1630--47 observed with the IXPE and the NICER. We carefully investigated a time interval of the IXPE observations during which 4U~1630--47 was in a single spectral state (the intermediate state) and found that the polarization degree did not change significantly during this time interval with the photon energy in the 2--8 keV range ($P\sim$ 6\%). We interpreted this energy independence behavior in $P(E)$ as a result of the Comptonization (up-scattering) of soft photons in the flat Compton cloud. We argued that the X-ray source broadband energy spectra can be reproduced by a physical model composed of the Comptonized component and iron-line ({\it Gaussian}) component. Spectral analysis using this model shows that during the intermediate state the 4U~1630--47 spectrum was characterized by a photon index $Γ\sim 2.5$ and the electron temperature $kT_e\sim ~9 ~{\rm keV}$. In fact, this approach allows one to easily find the half-optical depth $τ_0\sim 1.8$ of the Compton cloud in 4U~1630--47 applying the inclination $69\pm1^\circ$ and using the relation $(P,\cos(i),τ_0)$ when solving the full radiative transfer equation for a flat Compton cloud by an iterative method. This approach is also accompanied by the fact that the polarization degree is independent of the photon energy, which we demonstrate observationally, at least for 4U~1630--47 in the 2--8 keV range.