272 research outputs found
Short-term optical variability of high-redshift QSO's
This paper presents results of a search for short-term variability in the
optical band of selected high-luminosity, high-redshift radio-quiet quasars.
Each quasar has been monitored typically for 2 - 4 hours with a time resolution
of 2 - 5 minutes and a photometric accuracy of about 0.01 - 0.02 mag. Due to
the significant redshift (z>2), the covered wavelength range falls into the UV
region (typically 1500 - 2500A). We found no statistical evidence for any
continuum variations larger than 0.01 - 0.02 for any of the monitored objects.
Our results suggest that the presence of a short-term variability in
radio-quiet quasars is unlikely even in the UV region, contrary to reports by
other authors. This conclusion holds true at least for high-luminosity (large
black hole mass and accretion rate?) objects. The results are consistent with
the idea that significant short-term (less than 1 hour) variations in AGN,
where observed, should be attributed primarily to processes in a relativistic
jet.Comment: 7 pages, accepted for publication in MNRA
Quasar optical variability: searching for interband time delays
Aims. The main purpose of this paper is to study time delays between the
light variations in different wavebands for a sample of quasars. Measuring a
reliable time delay for a large number of quasars may help constraint the
models of their central engines. The standard accretion disk irradiation model
predicts a delay of the longer wavelengths behind the shorter ones, a delay
that depends on the fundamental quasar parameters. Since the black hole masses
and the accretion rates are approximately known for the sample we use, one can
compare the observed time delays with the expected ones.
Methods. We applied the interpolation cross-correlation function (ICCF)
method to the Giveon et al. sample of 42 quasars, monitored in two (B and R)
colors, to find the time lags represented by the ICCF peaks. Different tests
were performed to assess the influence of photometric errors, sampling, etc.,
on the final result.
Results. We found that most of the objects show a delay in the red light
curve behind the blue one (a positive lag), which on average for the sample is
about +4 days (+3 for the median), although the scatter is significant. These
results are broadly consistent with the reprocessing model, especially for the
well-sampled objects. The normalized time-lag deviations do not seem to
correlate significantly with other quasar properties, including optical, radio,
or X-ray measurables. On the other hand, many objects show a clear negative
lag, which, if real, may have important consequences for the variability
models.Comment: 5 pages, 4 figures, accepted in A&
Average UV Quasar Spectra in the Context of Eigenvector 1: A Baldwin Effect Governed by Eddington Ratio?
We present composite UV spectra for low redshift Type 1 AGN binned to exploit
the information content of the Eigenvector 1 (E1) parameter space. Composite
spectra allow a decomposition of the CIV1549 line profile - one of the
strongest high-ionization lines. The simplest CIV decomposition into narrow
(NLR), broad (BLR) and very broad (VBLR) components suggests that different
components have an analog in Hb with two major exceptions. VBLR emission is
seen only in population B (FWHM(Hb)>4000 km/s) sources. A blue
shifted/asymmetric BLR component is seen only in pop. A (FWHM(Hb)<4000 km/s)
HIL such as CIV. The blueshifted component is thought to arise in a wind or
outflow. Our analysis suggests that such a wind can only be produced in pop. A
(almost all radio-quiet) sources where the accretion rate is relatively high.
Comparison between broad UV lines in radio-loud (RL) and radio-quiet (RQ)
sources shows few significant differences. Clear evidence is found for a narrow
CIV component in most radio-loud sources. We find also some indirect
indications that the black hole (BH) spin, rather than BH mass or accretion
rate is a key trigger in determining whether an object will be RL or RQ. We
find a ten-fold decrease in EW CIV with Eddington ratio (decreasing from ~1 to
\~0.01) while NV shows no change. These trends suggest a luminosity-independent
"Baldwin effect" where the physical driver may be the Eddington ratio.Comment: 39 pages, 6 figures. To appear in Ap
Multi-Band Intra-Night Optical Variability of BL Lacertae
We monitored BL Lacertae frequently during 2014 - 2016 when it was generally
in a high state. We searched for intra-day variability for 43 nights using
quasi-simultaneous measurements in the B, V, R, and I bands (totaling 143 light
curves); the typical sampling interval was about eight minutes. On hour-like
timescales, BL Lac exhibited significant variations during 13 nights in various
optical bands. Significant spectral variations are seen during most of these
nights such that the optical spectrum becomes bluer when brighter. The
amplitude of variability is usually greater for longer observations but is
lower when BL Lac is brighter. No evidence for periodicities or characteristic
variability time-scales in the light curves was found. The color variations are
mildly chromatic on long timescales.Comment: 15 pages, 5 Figures, 3 Tables; Accepted for publication in Galaxies;
a special issue on Microvariability of Blazar
Intranight variability of 3C 454.3 during its 2010 November outburst
Context. 3C 454.3 is a very active flat spectrum radio quasar (blazar) that
has undergone a recent outburst in all observed bands, including the optical.
Aims. In this work we explore the short-term optical variability of 3C 454.3
during its outburst by searching for time delays between different optical
bands. Finding one would be important for understanding the evolution of the
spectrum of the relativistic electrons, which generate the synchrotron jet
emission.
Methods. We performed photometric monitoring of the object by repeating
exposures in different optical bands (BVRI). Occasionally, different telescopes
were used to monitor the object in the same band to verify the reliability of
the smallest variations we observed.
Results. Except on one occasion, where we found indications of a lag of the
blue wavelengths behind the red ones, the results are inconclusive for most of
the other cases. There were either no structures in the light curves to be able
to search for patterns, or else different approaches led to different
conclusions.Comment: 4 pages, 4 figures. Accepted for publication in A&
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