Abstract
We reviewed X-ray flux and spectral variability properties studied to date by various X-ray satellites for Mrk 421 and PKS 2155-304, which are TeV emitting blazars. Mrk 421 and PKS 2155-304 are the most X-ray luminous blazars in the northern and southern hemispheres, respectively. Blazars show flux and spectral variabilities in the complete electromagnetic spectrum on diverse timescales ranging from a few minutes to hours, days, weeks, months and even several years. The flux and spectral variability on different timescales can be used to constrain the size of the emitting region, estimate the super massive black hole mass, find the dominant emission mechanism in the close vicinity of the super massive black hole, search for quasi-periodic oscillations in time series data and several other physical parameters of blazars. Flux and spectral variability is also a dominant tool to explain jet as well as disk emission from blazars at different epochs of observations.
Highlights
Blazar is a subclass of radio-loud (RL) active galactic nuclei (AGN) which includes BL Lacertae (BL Lacs) objects and flat spectrum radio quasars (FSRQs)
From photon spectral index (α), flux, synchrotron radiation peak energy (E p ), electron spectral index (p) and hardness ratio (HR), it was found that when considering TeV blazars as a whole, α and X-ray luminosity are positively correlated, E p is negatively correlated with p and α and E p is positively correlated with HR [61]
Extensive study of XMM-Newton provided the following results: (i) the excess variance and the fractional rms variability amplitude show linear correlation with source flux, (ii) using the normalized excess variance, the black hole mass of PKS 2155-304 was estimated to be 1.45 × 108 M, (iii) the hardness ratio versus flux plots showed that the spectral changes were mainly significant during flares, (iv) the cross-correlation of the light curves in different energies were well correlated with different time lags, (v) the source has shown large amplitude X-ray intra-day variability (IDV) [95,96,97]
Summary
Blazar is a subclass of radio-loud (RL) active galactic nuclei (AGN) which includes BL Lacertae (BL Lacs) objects and flat spectrum radio quasars (FSRQs). Based on the location of the first peak of SED, blazars are classified into low-energy peaked blazars (LBLs) and high-energy peaked blazars (HBLs) [6]. Telescope Array System (VERITAS) e.g., [18,19,20], etc These facilities have a very strong impact and made a complete revolution in TeV γ-ray astronomy. A few blazars including Mrk 421 and PKS 2155-304 were predicted to be potential neutrino emitting objects on the basis of protons accelerated in the cores can produce neutrinos if the soft radiation background in the core is sufficiently high [21]. We discuss in detail the flux and spectral variability properties studied to date by various X-ray satellites for Mrk 421 and PKS 2155-304, which are TeV emitting HBL/HSP blazars.
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