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

Nicole Rodriguez Cavero, Lorenzo Marra+108 more|arXiv (Cornell University)|May 18, 2023

Astrophysical Phenomena and Observations5 citations

TL;DR

This paper presents the first X-ray polarization measurement of the black hole X-ray binary 4U 1630-47 in the steep power-law state, revealing a polarization degree (PD) of 10.3% ± 1.8% at 8 keV, increasing with energy. The results suggest a vertically extended or cone-shaped corona with a thickened accretion disk, favoring Thomson scattering in a geometrically thick corona over wind scattering or synchrotron emission as the dominant polarization mechanism.

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°.3 +/- 0°.9 East of North (all errors at 1σ 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.

Motivation & Objective

• To measure X-ray polarization in the steep power-law state of the black hole X-ray binary 4U 1630-47 using the IXPE mission.
• To determine the origin of high X-ray polarization in the source by comparing observational signatures with theoretical models.
• To investigate whether scattering in a thickened corona or disk geometry, rather than wind or jet emission, explains the observed polarization.
• To constrain the geometry and evolution of the corona and accretion disk during state transitions, particularly from hard to steep power-law states.

Proposed method

• Utilized data from the Imaging X-ray Polarimetry Explorer (IXPE) to measure the degree and angle of X-ray polarization in 4U 1630-47 during the steep power-law state.
• Performed spectral fitting using a multicolor disk blackbody model combined with a power-law component to model the continuum emission.
• Applied polarization modeling based on Thomson scattering in a geometrically thick corona, incorporating varying scattering optical depths and corona height.
• Compared observed polarization degree (PD) and energy dependence with predictions from alternative models, including wind scattering, synchrotron emission, and magnetized compact spots near the ISCO.
• Used reverberation lag modeling and spectral timing to infer corona size and height evolution during state transitions.
• Constrained the neutral hydrogen column density and compared it with previous hard state observations to assess absorption effects on polarization.

Experimental results

Research questions

• RQ1 What is the degree and energy dependence of X-ray polarization in 4U 1630-47 during the steep power-law state?
• RQ2 Is the high polarization primarily due to Thomson scattering in a thickened corona or disk, or does it arise from scattering in a wind or jet?
• RQ3 How does the corona geometry evolve between the hard and steep power-law states, and does this affect polarization signatures?
• RQ4 Can the observed polarization be explained by synchrotron emission from a jet or magnetic field-aligned electrons near the ISCO?
• RQ5 Does the change in disk temperature and column density between states support a thickened disk model in the steep power-law state?

Key findings

• The observed X-ray polarization degree (PD) is 10.3% ± 1.8% at 8 keV, with a significant increase in PD toward higher energies, indicating energy-dependent polarization.
• The spectral fitting shows an increase in disk temperature $kT_{\textrm{bb}}$ in the steep power-law state compared to the hard state, consistent with a thicker accretion disk.
• The neutral hydrogen column density is significantly lower in the steep power-law state than in the hard state, reducing absorption effects on polarization.
• The observed polarization behavior is inconsistent with dominant scattering in a wind or distant reflector, as such mechanisms would produce energy-independent PD.
• The energy-dependent PD is best explained by Thomson scattering in a vertically extended or cone-shaped corona, where higher-energy photons originate closer to the black hole and experience more scattering.
• The results support a model in which the corona expands and becomes geometrically thick during the hard-to-steep power-law state transition, leading to increased polarization with energy.