Temporal and spectral characteristics of active galactic nuclei in X-rays using NuSTAR

Abstract

Flux variability is one of the defining characteristics of active galactic nuclei

(AGN). This has been known over the past six decades ever since the discov- ery of quasars in 1963. Since then AGN have been observed for flux variability

over all accessible wavelengths on a range of timescales from hours to days and months. Studying flux variability of AGN is important as it is a effective tool to probe the central regions of AGN that are not accessible by any current imaging techniques set by Rs < c×tvar, where tvar is the time scale of variability and Rs is the size of the emitting region. Of all the wavelengths that are used to probe AGN, hard X-ray is very important, because firstly it is known to originate in the immediate vicinity of the central supermassive black hole (SMBH) and secondly, it is less affected by absorption. However, available studies on the hard X-ray variability characteristics of different classes of AGN on hour like timescale is very limited.

The primary X-ray emission in AGN is believed to originate in a compact region called the corona situated very close to the SMBH and the accretion disk. The knowledge of the cut-off energy (Ecut) of the primary X-ray continuum in AGN is very important as it carries information on the physical characteristics of the hot X-ray emitting corona. Though Ecut has been measured in AGN in the past, the existing measurements have large error bars largely attributed to the sensitivity of the instruments used to carry out the observations. The availability of a new

hard X-ray focussing instrument Nuclear Spectroscopic Telescope Array (NuS- TAR) which is about 100 times more sensitive than earlier hard X-ray missions

has opened up the possibility to explore both the hard X-ray variability charac- teristics of AGN as well as to obtain precise Ecut values on a large sample of AGN

to infer their coronal properties. Exploiting the high sensitivity of NuSTAR to observations in the 3−79 keV band, we in this thesis work aimed to address two problems (a) to carry out a comparative analysis of the hard X-ray flux variability characteristics of different classes of AGN to look for similarities and/or differences in the hard X-ray variability characteristics of radio-loud vis-a-vis radio-loud AGN on hour like time scales and (b) determine new Ecut values for a sample of AGN to infer the coronal properties and look for correlations if any between Ecut and other physical properties of AGN.

The first part of the thesis objective is addressed in Chapter 3, wherein we have analysed 557 sets of observations pertaining to 335 AGN that comprises of 24 BL Lac objects, 24 flat spectrum quasars (FSRQs), 20 Narrow Line Seyfert 1 galaxies, 121 Seyfert 1 galaxies and 146 Seyfert 2 galaxies. Our analysis indicates that on hour like time scales, blazars (that includes FSRQs and BL Lac objects) are more variable that their radio-quiet counterparts namely the Seyfert galaxies, which could be attributed to the contribution of relativistic jets to the observed X-ray emission in blazars. We also found brighter AGN to be less variable as well as AGN powered by more massive black holes to be less variable.

The second part of the thesis objective is addressed in two chapters, namely Chap- ter 4 and Chapter 5. In Chapter 4, we report the first time measurement of Ecut

value for the radio-loud AGN, namely 3C 120, whereas in Chapter 5, we report first time measurement of Ecut values for nine AGN and an upper limit of one AGN. Combining our new Ecut measurements with those available in literature,

totalling 30 AGN, we found that the correlation between Ecut and the photon in- dex of AGN is complicated, thereby requiring more Ecut measurements on a large

number of AGN in the future to understand the complicated behaviour between Ecut and photon index.