One stone for two birds: a promising solution to the puzzling steep power-law problem in black hole X-ray binaries

(News & views on the paper by Yan & Wang 2011, vol.11, 631-636)

Shuangnan Zhang (Institute of High Energy Physics)

A black hole X-ray binary (BHXB) consists of a stellar mass black hole with a mass from several solar masses to tens of solar masses, and a companion star that transfers its gas to the black hole through an accretion disk. The so-called standard model of an accretion disk can account for the commonly observed soft thermal-like X-ray spectra from BHXBs (Shakura & Sunyaev 1973). However Cygnus X-1, the very first BHXB found, produces a hard power-law-like X-ray spectrum most of the time and only occasionally makes a transition to the state exhibiting a soft thermal-like spectrum (e.g. Zhang et al. 1997a). It is generally believed that the thermal Comptonization process is mostly responsible for producing the observed hard power-law-like X-ray spectrum (e.g., Sunyaev & Titarchuk 1980). However, GRO J1655-40 (Zhang et al. 1994) showed a broadband spectrum with a prominent soft thermal-like component and a strong power-law component much steeper than the hard power-law observed in Cygnus X-1 and many other BHXBs (Zhang et al. 1997b). More observations revealed that GRO J1655-40 sometimes also produces the canonical soft thermal-like or hard power-law-like spectra. McClintock & Remillard (2006) thus classified all major spectral states of BHXBs into three classes: the canonically known thermal or hard state, or the recently discovered steep power-law (SPL) state, which is the one initially observed in GRO J1655-40 (Zhang et al. 1997b).

However, the nature and origin of the SPL state have been debated and remain unresolved so far, although several models have been proposed. Most of these models simply extend the original model used to produce the hard power-law spectrum, i.e., the Comptonization process; the energy distribution of electrons, responsible for up-scattering the soft photons from the accretion disk, is tuned to match the observed steep power-law X-ray spectrum. However, the X-ray spectrum is not the only characteristic of the SPL; it was already found that the power density spectrum (PDS) of GRO J1655-40 in the SPL is also quite different from that of the hard state (Zhang et al. 1997b). It is not clear how a very different PDS can be produced by only changing the energy distribution of electrons in the same models. A yet more serious problem for Comptonization models is that stronger variability and quasi-periodic oscillations (QSOs) have been found to be more pronounced at higher energies; however higher energy photons are scattered more by electrons and thus should exhibit less variability and weaker quasi-periodic oscillations in the Comptonization process. Similarly, another model involving bulk motion Comptonization (Titarchuk & Shrader 2002) also has the above difficulty in explaining the timing properties of the SPL state.

Yan & Wang recently proposed a model to solve this long standing problem. The novelty of the work of Yan & Wang (2011) is that their model naturally explains both the observed spectral and timing properties simultaneously. In their model, the SPL is produced in the hot-spots near the inner stable circular orbit (ISCO) of the black hole. The hot-spots are assumed to have high energy electrons that produce high energy photons via synchrotron radiation; some of the high energy photons are then down-scattered to lower energies by low energy electrons in the same hot-spots, producing the observed SPL. Their simple, yet elegant calculations show that the observed spectral properties of the SPL state can be reproduced with physically plausible parameters of the assumed hot-spots. In this new model, the lower energy photons have less variability and weaker QSOs, because they are scattered more, thus naturally accounting for the observed timing properties of the SPL state. Therefore in both quantitative and qualitative arguments, their model is superior to other competing models for the SPL state and appears very promising for solving this long-standing puzzle in astrophysics.

A major unresolved issue, which may be of considerable scientific value and interest, is the origin of the hot-spots. Previously, it has been found that the atmospheres of the Sun and BHXBs share many common characteristics, which have been used to argue that magnetic energy release, most likely through magnetic reconnections, dominates the physical processes both in the Sun and the accretion disks around BHXBs (Zhang et al. 2000). Comparisons of many similar phenomena at very different scales suggest that magnetic energy release is actually very common in astrophysics (Zhang 2007). Through studies of the Sun, our best and nearest astrophysical laboratory in nature, it has been understood that efficient particle accelerations can happen in magnetic reconnections. It is therefore possible that magnetic reconnections in accretion disks around black holes produce the hot-spots required in the work of Yan & Wang (2011).

A scientific theory must be verifiable, and making testable predictions for comparison to data is the most common practice of verifying or proving a theory. Unfortunately, the present work of Yan & Wang (2011) falls short in making testable predictions. This is not satisfying to me. Being able to explain all existing data and to continuously accommodate new data by incorporating more complexity does not prove the correctness and validness of a model, in my opinion. I thus urge the authors and interested readers to make testable predictions of this new model. Here I suggest two possible tests.

One possible test involves calculations of the expected timing properties in the SPL state, in order to quantitatively compare with the observed amplitudes of variability and QSOs as a function of photon energy. In addition to testing their model, such comparison may also allow researchers to determine the physical properties of the hot-spots and to measure the radius of ISCO, thus providing another method of measuring the spin of black holes in BHXBs, complimentary to the commonly used X-ray continuum fitting method first proposed by Zhang et al. (1997c).

Another possible test could be a more detailed comparison between synchrotron radiation and inverse Compton scattering processes in the hot-spots. It is known that the relative importance between these two processes depends on the ratio of the energy densities of the magnetic field and photon field. The “equi-partition” argument is normally used to estimate the magnetic fields in the accretion disks around BHXBs, in absence of direct measurement of the magnetic fields there (e.g., Zhang et al. 2000). This would imply that the inverse Compton scattering of the photons from the accretion disk should be equally as important as the synchrotron radiation, in terms of the total power output of the electrons in the hot-spots; this process should produce another spectral component. Actually, the observed photons from SPL can also be the seed photons for further inverse Compton scattering, a process very similar to the so-called Synchrotron-Self Comptonization process, applied to explain the high energy gamma-ray emissions from the relativistic jets of active galactic nuclei. Therefore, it is also natural that BHXBs in the SPL state can produce high energy gamma-ray emissions. Future observations of BHXBs in the SPL state with higher quality and in broader energy bands may allow researchers to distinguish between the model of Yan & Wang (2011) and other previously proposed models, as well as to determine the energy distribution of electrons and the strength of magnetic fields in the hot-spots.

It is therefore clear that, based upon the promising model of Yan & Wang (2011), more theoretical and observational investigations are required to further understand the physics and astrophysics of accreting black holes in the Milky Way and beyond.


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