Related papers: A search for rapid optical variability in radio-qu…
The potential of the Sloan Digital Sky Survey for wide-field variability studies is illustrated using multi-epoch observations for 3,000,000 point sources observed in 700 deg2 of sky, with time spans ranging from 3 hours to 3 years. These…
We present the results of a multi-cycle Chandra program to systematically monitor the X-ray variability of 10 weak-line quasars (WLQs) that previously had limited multi-epoch X-ray observations. Three new Chandra 2.8 to 8.2 ks observations…
We present results from a mini-survey of relatively high redshift (1.7 < z < 4) gravitationally lensed radio-quiet quasars observed with the Chandra X-ray Observatory and with XMM-Newton. The lensing magnification effect allows us to search…
Photometric measurements are prone to systematic errors presenting a challenge to low-amplitude variability detection. In search for a general-purpose variability detection technique able to recover a broad range of variability types…
We present near-infrared imaging with ESO VLT+ISAAC of the host galaxies of low luminosity quasars in the redshift range 1 < z < 2, aimed at investigating the relationship between the nuclear and host galaxy luminosities at high redshift.…
We present a new measurement of the space density of high redshift (3.0<z<4.5), X-ray selected QSOs obtained by exploiting the deep and uniform multiwavelength coverage of the COSMOS survey. We have assembled a statistically large (40…
We investigate C IV broad absorption line (BAL) variability within a sample of 46 radio-loud quasars (RLQs), selected from SDSS/FIRST data to include both core-dominated (39) and lobe-dominated (7) objects. The sample consists primarily of…
For some samples, it has been shown that spectra of QSOs with low redshift are bluer during their brighter phases. For the FIRST bright QSO sample, we assemble their spectra from SDSS DR7 to investigate variability between the spectra from…
We present deep HST/WFPC2, rest-frame U images of 17 ~L* quasars at z=1 and z=2 (V and I bands respectively), designed to explore the host galaxies. We fit the images with simple axisymmetric galaxy models, including a point-source, in…
We report results from an 18-day ROSAT HRI monitoring campaign on the ultrasoft Narrow-Line Seyfert 1 (NLS1) class quasar PHL 1092. This luminous, radio-quiet quasar showed strong X-ray variability in a short ROSAT PSPC observation, and…
The first seven years of RXTE monitoring of Seyfert 1 active galactic nuclei have been systematically analyzed to yield five homogeneous samples of 2-12 keV light curves, probing hard X-ray variability on successively longer durations from…
In this paper, we used the database of the university of Michigan Radio Astronomy Observatory (UMRAO) at three (4.8 GHz, 8.0 GHZ, and 14.5 GHz) radio frequency to analyze the radio light curves by the power spectral analysis method in…
In the near future, new surveys promise a significant increase in the number of quasars (QSO) at large redshifts. This will help to constrain the dark energy models using quasars. The Large Synoptic Survey Telescope (LSST) will cover over…
In the two parts of this contribution we describe two related XMM-Newton programs. The first part summarizes our study of the X-ray spectral properties and variability of z>4 quasars (Shemmer et al. 2005). The second part presents…
We report on two programs to address differential evolution between the radio-loud and radio-quiet quasar populations at high (z>4) redshift. Both programs entail studying the radio properties of optically-selected quasars. First, we have…
Changing-look quasars exhibit dramatic variability in broad emission-line fluxes on short timescales. This behavior is challenging to many models of the quasar broad line region, due in large part to the short transition times between high…
Observations of quasars (QSOs) shining through or close to galaxies offer a way to probe the properties of the foreground matter through dust extinction and gravitational lensing. In this paper the feasibility of measuring the dust…
Different properties of quasars may be observed and analysed through the many ranges of the electromagnetic spectrum. Pioneering studies showed that an "H-R diagram" for quasars was needed to organize these data, and that more than two…
We obtained optical spectra of four objects identified with variable radio sources. Three objects (0029+0554, 0400+0550, 2245+0500) were found to be quasars with redshifts of 1.314, 0.761, and 1.091. One object (2349+0534) has a continuum…
The variability of 30 PG quasars has been observed in the red band during three years. A rest-frame structure function analysis shows an increase of variability with the time interval up to 0.2 mag r.m.s. after two years. A comparison with…