Related papers: Untangling the Complex Nature of AGN Variability w…
Interband lags in the optical continua of active galactic nuclei (AGN) have been observed over years of monitoring, yet their physical origins remain unclear. While variable interband lags have been found in a few individual AGN…
Active galactic nuclei are known to exhibit flux variations across the entire electromagnetic spectrum. Among these, correlations between UV/optical and X-ray flux variations serve as a key diagnostics for understanding the physical…
Emission from Active Galactic Nuclei is known to vary strongly over time over a wide energy band, but the origin of the variability and especially of the inter-band correlations is still not well established. Here we present the results of…
We perform long-term ($\approx 15$ yr, observed-frame) X-ray variability analyses of the 68 brightest radio-quiet active galactic nuclei (AGNs) in the 6 Ms $Chandra$ Deep Field-South (CDF-S) survey; the majority are in the redshift range of…
I present new comparisons of AGN optical, UV, and X-ray variations. These reveal complex relationships between the different passbands that can change with time in a given object. While there is evidence from several objects that X-ray and…
Active Galactic Nuclei (AGN) are generally considered as the scaled-up counterparts of X-ray binaries (XRBs). It is known that the power spectral density (PSD) of the X-ray emission of XRBs shows significant evolution with spectral states.…
Aims: We used data from the 58 month long, continuous Swift/Burst Alert Telescope (BAT) observations of the five brightest Seyfert galaxies at hard X-rays, to study their flux and spectral variability in the 20-100 keV energy band. The…
Active galactic nuclei (AGN) emit radiation over a wide range of wavelengths, with a peak of emission in the far-UV region of the electromagnetic spectrum, a spectral region that is historically difficult to observe. Using optical, GALEX UV…
Strongly magnetized, accreting neutron stars show periodic and aperiodic variability over a wide range of time scales. By obtaining spectral and timing information on these different time scales, we can have a closer look into the physics…
We analyse the spectra of the archival XMM-Newton data of the Seyfert 1 AGN Zw 229.015 in the energy range $0.3 - 10.0$ keV. When fitted with a simple power-law, the spectrum shows signatures of weak soft excess below 1.0 keV. We find that…
We explore the connections between the infrared and X-ray properties of AGNs. Using the well constrained infrared SEDs of a sample of local (i.e., z < 0.1) sample of X-ray AGNs, we develop new diagnostics that exploit 24um and 70um flux…
We report the discovery of large amplitude X-ray variability in the low luminosity AGN (LLAGN) MGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor…
We investigate the spectral variability of the Seyfert galaxy Fairall 9 using almost 6 years of monitoring with the Rossi X-ray Timing Explorer (RXTE) with an approximate time resolution of 4 days. We discover the existence of pronounced…
The origin of the optical flux variability in active galactic nuclei (AGN) is largely unknown. Previous studies have correlated features of the variability structure function (SF) with AGN properties, though they mostly involved…
Variable continuum emission from AGN can be used to probe the structure of their accretion disks via reverberation mapping. Assuming a variable, hot inner light source irradiates the surrounding accretion disk, time delays between different…
We present the first results from the ongoing, intensive, multi-wavelength monitoring program of the luminous Seyfert 1 galaxy Mrk 817. While this AGN was, in part, selected for its historically unobscured nature, we discovered that the…
We study the relationship between the UV and X-ray variability of the narrow-line Seyfert 1 galaxy 1H~0707-495. Using a year long {\it Swift} monitoring and four long {\it XMM-Newton} observations, we perform cross-correlation analyses of…
Short-term variability of X-ray spectra has been reported for several Active Galactic Nuclei. Significant X-ray flux variations are observed within time scales down to 10^4 -- 10^5 seconds. We discuss short variability time scales in the…
Long-time-scale X-ray and optical variability is a key characteristic of AGN. Here, we summarise our current understanding of the X-ray and optical continuum variability of radio-quiet AGN and the relation between the two bands. We…
X-ray obscuration of active galactic nuclei (AGNs) is considered in the context of ionized winds of stratified structure launched from accretion disks. We argue that a Compton-thick layer of a large-scale disk wind can obscure continuum…