X-ray Polarization from Black Holes in the Thermal State

📝 Abstract

We present new calculations of X-ray polarization from black hole accretion disks in the thermally-dominated state, using a Monte-Carlo ray-tracing code in full general relativity. In contrast to many previously published studies, our approach allows us to include returning radiation that is deflected by the strong-field gravity of the BH and scatters off of the disk before reaching a distant observer. Although carrying a relatively small fraction of the total observed flux, the scattered radiation tends to be highly polarized and in a direction perpendicular to the direct radiation. We show how these new features of the polarization spectra may be developed into a powerful tool for measuring black hole spin and probing the gas flow in the innermost disk.

💡 Analysis

We present new calculations of X-ray polarization from black hole accretion disks in the thermally-dominated state, using a Monte-Carlo ray-tracing code in full general relativity. In contrast to many previously published studies, our approach allows us to include returning radiation that is deflected by the strong-field gravity of the BH and scatters off of the disk before reaching a distant observer. Although carrying a relatively small fraction of the total observed flux, the scattered radiation tends to be highly polarized and in a direction perpendicular to the direct radiation. We show how these new features of the polarization spectra may be developed into a powerful tool for measuring black hole spin and probing the gas flow in the innermost disk.

📄 Content

arXiv:0906.0559v1 [astro-ph.HE] 2 Jun 2009 1 X-ray Polarization from Black Holes in the Thermal State Jeremy D. Schnittman (Johns Hopkins University) Julian H. Krolik (Johns Hopkins University) Abstract We present new calculations of X-ray polarization from black hole accretion disks in the thermally-dominated state, using a Monte-Carlo ray-tracing code in full general relativity. In contrast to many previously published studies, our approach allows us to include returning radiation that is de- flected by the strong-field gravity of the BH and scatters offof the disk before reaching a distant observer. Although carrying a relatively small fraction of the total observed flux, the scattered radiation tends to be highly polarized and in a direction perpendicular to the direct radiation. We show how these new features of the polarization spectra may be developed into a powerful tool for measuring black hole spin and probing the gas flow in the innermost disk. 1.1 Introduction A recent flurry of new mission proposals has renewed interest in X-ray po- larization from a variety of astrophysical sources, hopefully marking the “Coming of Age of X-ray Polarimetry” in the very near future. The Gravity and Extreme Magnetism SMEX (GEMS) mission†, for example, should be able to detect a degree of polarization δ < 1% for a flux of a few mCrab (e.g. (13) and these proceedings). A similar detector for the International X-ray Observatory (IXO) could achieve sensitivity roughly 10× greater (δ < 0.1%; (8; 5), Alessandro Brez in these proceedings). In this talk, based on our recent paper (11), we focus on the polarization signal from accreting stellar- mass black holes (BHs) in the thermal state, which are characterized by a broad-band spectrum peaking around 1 keV. The typical level of polariza- † heasarc.gsfc.nasa.gov/docs/gems 1 2 Schnittman & Krolik tion from these sources should be a few percent in the 1 −10 keV range, depending on BH spin and the inclination angle of the accretion disk. Symmetry arguments demand that in the flat-space (Newtonian) limit, the observed polarization from the disk must be either parallel or perpendicular to the BH/disk rotation axis. However, the effects of relativistic beaming, gravitational lensing, and gravito-magnetic frame-dragging can combine to give a non-trivial net rotation to the integrated polarization vector. Early work exploring these effects (12; 3; 4) showed that they create changes in the angle and degree of polarization that are strongest for higher photon energy. Quite recently, Dovciak et al. (7) investigated the effect of atmospheric op- tical depth on the polarization signal, and Li et al. (9) applied the original calculations of thermal X-ray polarization to the problem of measuring the inclination of the inner accretion disk. Nearly all previous work has modeled the relativistic effects by calculating the transfer function along geodesics between the observer and emitter. By its very nature, this method precludes the possibility of including the ef- fects of returning radiation—photons emitted from one part of the disk and bent by gravitational lensing so that they are absorbed or reflected from another part of the disk (6). As described in greater detail in (11), the most important feature of our approach is that the photons are traced from the emitting region in all directions, either returning to the disk, scatter- ing through a corona, getting captured by the BH, or eventually reaching a distant observer. Using this method, we study an important new polarization feature, namely the transition between horizontal- and vertical-oriented polarization as the photon energy increases, an effect first discussed in (1). At low energies we reproduce the “Newtonian” result of a semi-infinite scattering atmosphere emitting radiation weakly polarized in a direction parallel to the emission surface, an orientation we call horizontal polarization (2). At higher energy, corresponding to the higher temperature of the inner disk, a greater frac- tion of the emitted photons returns to the disk and is then scattered to the observer. These scattered photons have a high degree of polarization and are aligned parallel to the disk rotation axis (vertical), as projected onto the image plane. At the transition point between horizontal and vertical polar- ization, the relative contributions of direct and reflected photons are nearly equal, and little net polarization is observed. Since the effects of returning radiation are greatest for photons coming from the innermost regions of the disk, the predicted polarization signature is strongly dependent on the be- havior of gas near and inside the inner-most stable circular orbit (ISCO). X-ray Polarization from Black Holes in the Thermal State 3 D = 40M

10-5 10-4 10-3 10-2 10-1 1 I/Imax deg=5% D = 40M i=75o Fig. 1.1. Ray-traced image of a thermal disk, including (left) only the direct, as well as (right) both direct and returning radiation. The observe

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