Related papers: The relation between X-ray and UV emission in quas…
We present our long term Chandra X-ray monitoring data for the gravitationally lensed quasar Q2237+0305 with 20 epochs spanning 10 years. We easily detect microlensing variability between the images in the full (0.2--8 keV), soft (0.2--2…
The UV/optical variation, likely driven by accretion disc turbulence, is a defining characteristic of type 1 active galactic nuclei (AGNs) and quasars. In this work we investigate an interesting consequence of such turbulence using quasars…
We study the spectral energy distributions and evolution of a large sample of optically selected quasars from the Sloan Digital Sky Survey (SDSS) that were observed in 323 Chandra images analyzed by the Chandra Multiwavelength Project…
Quasars are bright active galactic nuclei powered by the accretion of matter around supermassive black holes at the center of galaxies. Their stochastic brightness variability depends on the physical properties of the accretion disk and…
We study a model of an accretion disc surrounded by an extended corona of the temperature 10^7 - 10^8 K. This corona modifies the disk spectrum since it redirects a significant fraction of the emission from the central parts towards the…
We examine unresolved nuclear X-ray sources in 57 brightest cluster galaxies to study the relationship between nuclear X-ray emission and accretion onto supermassive black holes (SMBHs). The majority of the clusters in our sample have…
The non-linear relation between X-ray and UV luminosity in quasars can be used to estimate their distance. Recently, we have shown that despite the large dispersion of the relation, a Hubble Diagram made of large samples of quasars can…
The main purpose of this work is to improve the existing knowledge about the most powerful engines in the Universe - quasars. Although a lot is already known, we still have only a vague idea how these engines work exactly, why they behave…
We have recently found fundamental differences in the radio properties of red quasars when compared to typical blue quasars. In this paper we use X-shooter data, providing spectral coverage from $\sim 3000-25000$ Ang, of a sample of 40 red…
We analyze the optical, UV, and X-ray microlensing variability of the lensed quasar SDSS J0924+0219 using six epochs of Chandra data in two energy bands (spanning 0.4-8.0 keV, or 1-20 keV in the quasar rest frame), 10 epochs of F275W…
This work studies the relationship between accretion-disk size and quasar properties, using a sample of 95 quasars from the SDSS-RM project with measured lags between the $g$ and $i$ photometric bands. Our sample includes disk lags that are…
Using Chandra and HST archival data, we have studied the individual Spectral Energy Distribution (SED) of 11 quasars. All UV spectra show a spectral break around 1100A. 5 X-ray spectra show the presence of a ``soft excess'' and 7 spectra…
Radio-loud quasars (RLQs) are more X-ray luminous than predicted by the X-ray-optical/UV relation (i.e. $L_\mathrm{x}\propto L_\mathrm{uv}^\gamma$) for radio-quiet quasars (RQQs). The excess X-ray emission depends on the radio-loudness…
The UV/optical and X-ray variability of active galactic nuclei (AGN) have long been expected to be well correlated as a result of the X-ray illumination of the accretion disk. Recent monitoring campaigns of nearby AGN, however, found that…
We discuss a model for the central region of radio-quiet AGN in which coronal accretion is the source of energy for X-ray emission. We start from the Haardt & Maraschi model but supplement it with pressure balance condition, two…
Current estimates of the normalized accretion rates of quasars (L/L_Edd), rely on measuring the velocity widths of broad optical-UV emission lines (e.g., H$\beta$ and Mg II $\lambda2800$). However, such lines tend to be weak or inaccessible…
The well-known anti-correlation between the optical/ultraviolet (UV) emission line equivalent widths of active galactic nuclei and the continuum luminosity (the so-called Baldwin effect) is a long-standing puzzle. One common hypothesis is…
The observed relation between the X-ray radiation from AGNs, originating in the corona, and the optical/UV radiation from the disk is usually described by the anticorrelation between the UV to X-ray slope alpha_ox and the UV luminosity.…
Quiescent emission from the neutron star low-mass X-ray binary Cen X-4 is seen to be variable on timescales from hundreds of seconds to years, suggesting that at least in this object, low-level accretion is important during quiescence. Here…
We use the ROSAT All Sky Survey (RASS) to study the soft X-ray properties of a homogeneous sample of 46,420 quasars selected from the third data release of the Sloan Digital Sky Survey (SDSS). Optical luminosities, both at rest-frame…