Title: Impact of reflection Comptonization on X-ray reflection spectroscopy: the case of EXO 1846-031
Authors: Songcheng Li, Honghui Liu, Cosimo Bambi, James F. Steiner, Zuobin Zhang
First Author’s Institution: Center for Astronomy and Astrophysics, Center for Field Theory and Particle Physics, and Department of Physics, Fudan University
Status: Published in PRD [open access]
This bite was written and published as part of Astrobites’s new partnership with the American Physical Society (APS). As part of this partnership, we cover selected articles from the Physical Review Journals, APS’s premier publications covering all aspects of physics. For more coverage as part of this partnership, see our other PRJ posts.
Black holes are rather simple objects, as they are believed to be fully characterized by just 2 fundamental parameters, mass and spin; this is the infamous No-Hair Theorem. We can probe black hole masses using the dynamics of the system (i.e. studying how fast things are moving around the black hole; see various Astrobites on the topic of measuring black hole masses!). Measuring spin is a bit more complicated, but these measurements are crucial to understanding how black holes came to be! For supermassive black holes, the average spin has important cosmological implications, since it tells us whether those black holes grew primarily from mergers of smaller black holes (where the random spin directions will add up to a net lower spin) or from accretion of gas (which can “spin up” the black holes to higher spin). For stellar mass black holes, the spin encodes some information about the stellar explosion that created the black hole!
Measuring spin, in practice
How do we actually measure the spin of black holes in practice? Today we will focus on the “reflection spectroscopy” technique, which uses information from the accretion disk around black holes that are gobbling down material from their surroundings. Measuring the inner edge of the accretion disk is a proxy for the black hole spin because more rapidly spinning black holes can support stable orbits closer to the event horizon! To measure the inner edge of the disk, we can use the X-ray spectrum, which consists of three main parts: (1) the accretion disk that emits thermal radiation, (2) the corona (a hot, ionized plasma) that scatters the thermal radiation to higher energies via a process known as inverse Compton scattering, and (3) the reflected emission that arises from the interaction between the Comptonized photons and the disk (see Figure 1). Despite its name, the reflected emission is not quite a mirror image of the incoming light because this light is scattered and absorbed by the material in the disk. This results in a complex reflection spectrum with emission lines from various atoms in the disk, which hold the key to unlocking the spin! These emission lines, like the iron Kα line near 6.4 keV, are relativistically blurred by the strong gravity of the black hole. As the inner disk gets closer to the black hole (i.e. the higher the black hole spin), the reflected photons have to climb out of a deeper gravitational potential well. In this process, they lose energy through a process called gravitational redshifting, leaving a highly skewed emission line like we see in green in Figure 1! So if we model the low energy wing of the iron Kα line, then we can essentially measure the spin of the black hole!
Add a little bit of complexity
Now, this is great in theory, but is this simplified picture really enough to capture the complex physics of accretion? One of the major unknowns and active areas of research in accretion physics is determining the geometry of the corona. If the corona is extended radially, it could actually scatter not only the thermal photons from the disk, but also the reflected photons. This additional scattering will effectively blur out the reflection features and could therefore influence what we infer about the spin and other accretion parameters by modeling the reflection spectrum. It’s important to rule this out as a bias in current spin measurements, and this additional scattering can also help us understand the geometry of the corona better.
Today’s authors investigate this exact problem by studying a well-known stellar mass black hole, EXO 1846-031, that is accreting material from a star that is in orbit around it. They use X-ray spectroscopy data from NuSTAR, a hard X-ray telescope. With this data, they model the X-ray spectra with two complementary models. Model 1 is a more traditional approach to fitting the X-ray spectra, with only scattering of the disk photons (i.e. the reflected photons are not scattered). Model 2, on the other hand, includes this additional scattering of the reflected photons from an extended corona.
Finding consistency!
With detailed statistical tests, the authors assess which model provides a better fit and compare the fit values across these two models. They found that both models can produce very similar fits to the data, and that the important fit parameters, including both the black hole spin and the disk inclination, are not affected by the inclusion of scattering of reflected photons. Figure 3 highlights the good agreement between Models 1 and 2, showing that the best fit black hole spin value and uncertainties are very similar between the two models. In addition the authors also check whether the inclusion of data from the NICER, an X-ray telescope that covers a lower energy range, impacts their results. They find that the inclusion of NICER data does not impact the spin or inclination measurements, but it helps place tighter and more physical constraints on other key accretion properties, like the total luminosity and the radial dependence of the reflected emission (i.e. the emissivity profile of the disk). Ultimately, this is an important step in validating the spin measurements from stellar mass black holes, but further testing with different systems in different spectral states will be important for verifying these results!
Astrobite edited by Diana Solano-Oropeza
Featured image credit: Gabriel Pérez Díaz, SMM (IAC)
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