Black hole-jet systems: From blazars to microquasars

Yongquan Xue, Purdue University

Abstract

Understanding black holes is one of the most intriguing and important topics in high energy astrophysics. Many astronomical black hole systems are known to contain three basic components: a black hole, an accretion disk, and a pair of collimated jets that are likely coupled physically to the accretion disk. For this thesis work, the focus is on two classes of black hole-jet systems, known as blazars and microquasars. There is growing evidence that the central engines in both types of systems are qualitatively similar. Collectively, they may, therefore, provide an excellent laboratory for studying common physical processes over a vast range of physical scales. Although both types of systems have been studied extensively, there are still many outstanding issues. The goal of this thesis work is to cast light on some of these important issues. First, to understand the energetics of the flaring phenomenon in blazars, it is necessary to get a handle on the size of the emitting region. An effective way to do so is to quantify timescales over which a source varies. I systematically studied X-ray flaring activities of the TeV blazar Mrk 501 and found flares over a wide range of timescale, with the most rapid one lasting for only about 800 s, which is the shortest ever seen in this system. The latter sets an upper limit of ∼ 2.4 × 1014 cm (i.e., 800 light seconds) on the size of the region that produces the flare, which is already comparable to the characteristic size of the black hole (of ∼ 109 [special characters omitted]). Second, a related question is what causes the observed flares in blazars. To this end, I studied X-ray spectral evolution of TeV blazars Mrk 421 and Mrk 501 during individual flares that last for a few days. Such a study has become possible only recently, with high-quality X-ray data taken during very active periods of the sources. I fitted the time-resolved X-ray spectra with a synchrotron model and found that, in order to account for the observed spectral variability, multiple parameters (that characterize the electron distribution and magnetic field) must vary, in contrast to earlier studies that invoke a change in, e.g., only the normalization of the electron distribution, to fit data of poorer quality. Third, to examine the roles of the jets and accretion flows to the spectral energy distribution (SED) of microquasars, I picked two sources (XTE J1550-564 and H 1743-322) whose jets are seen both in X-ray and radio bands, which allows a precise assessment of the contribution from the jets to the overall SED. I applied a synchrotron plus inverse Compton model to fit the broadband SED. The main conclusion is that the synchrotron radiation from the jets may account for 100% of the observed radio fluxes but it contributes little to the observed X-ray emission, which is at odds with some of the recent works by others (which attribute emission in all bands to the jets alone). The X-ray emission, according to my work, comes almost entirely from the accretion flows. Finally, to facilitate understanding of the coupling between the jets and accretion flows in microquasars, I critically examined possible correlation between radio and X-ray variabilities in a sample of microquasars. I found that the diverse behaviors of the radio/X-ray correlation cannot possibly be characterized by a single, universal function, as others have claimed. On the other hand, radio and X-ray variabilities are, to varying degrees, positively correlated in most cases, perhaps indicating the presence of a coupling between the jets and accretion flows. I should note that the claimed universal correlation would have serious implications on theoretical models, which is why it has generated a lot of excitement. However, as shown here, the phenomenology is not that simple.

Degree

Ph.D.

Advisors

Cui, Purdue University.

Subject Area

Astronomy

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