195 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
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&
Multicolour CCD Measurements of Visual Double and Multiple Stars. III
Recent CCD observations were performed in the period 1998-2004 for a large
sample of visual double and multiple stars selected from the Hipparcos
Catalogue and/or from the Gliese Catalogue of Nearby Stars. Accurate
astrometric and photometric data allowing to characterize the individual
components are provided. These data are confronted to Hipparcos data or to data
from an older epoch in order to assess the nature of the observed systems. We
simultaneously apply a Moffat-Lorentz profile with a similar shape to all
detected components and adjust the profile parameters from which we obtain the
relative astrometric position (epoch, position angle, angular separation) as
well as differential multi-colour photometry (filters (B)VRI). We thus acquired
recent data for 71 visual systems of which 6 are orbital binaries, 27 are
nearby and 30 are multiple systems. In three cases, the systems remained
unresolved. 23 new components were detected and measured. Two new visual double
stars of intermediate separation were also found. The estimated accuracies in
relative position are 0.04 deg and 0.01" respectively, while those in
differential photometry are of the order of 0.01-0.02 mag in general. The
nature of the association of 55 systems is evaluated. New basic binary
properties are derived for 20 bound systems. Component colours and masses are
provided for two orbital binaries.Comment: 7 pages, 2 Postscript figures, 1 appendix, 1 annex containing 3
tables. Appendix A and Tables~4-6 are only available in electronic form.
Astronomy and Astrophysics, in press (2006
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
Multi-band optical variability of three TeV Blazars on Diverse Timescales
We present our optical photometric observations of three TeV blazars, PKS
1510-089, PG 1553+113 and Mrk 501 taken using two telescopes in India, one in
Bulgaria, one in Greece and one in Serbia during 2012 - 2014. These
observations covered a total of 95 nights with a total of 202 B filter frames,
247 images in V band, 817 in R band while 229 images were taken in the I
filter. This work is focused on multi-band flux and colour variability studies
of these blazars on diverse timescales which are useful in understanding the
emission mechanisms. We studied the variability characteristics of above three
blazars and found all to be active over our entire observational campaigns. We
also searched for any correlation between the brightness of the sources and
their colour indices. During the times of variability, no significant evidence
for the sources to display spectral changes correlated with magnitude was found
on timescales of a few months. We briefly discuss the possible physical
mechanisms most likely responsible for the observed flux variability.Comment: 11 pages, 7 figures, 9 tables. Accepted for publication in MNRA
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