294 research outputs found
Long-Term Multiwavelength Studies of High-Redshift Blazar 0836+710
Aims. The observation of gamma -ray flares from blazar 0836+710 in 2011,
following a period of quiescence, offered an opportunity to study correlated
activity at different wavelengths for a high-redshift (z=2.218) active galactic
nucleus. Methods. Optical and radio monitoring, plus Fermi-LAT gamma-ray
monitoring provided 2008-2012 coverage, while Swift offered auxiliary optical,
ultraviolet, and X-ray information. Other contemporaneous observations were
used to construct a broad-band spectral energy distribution. Results. There is
evidence of correlation but not a measurable lag between the optical and
gamma-ray flaring emission. On the contrary, there is no clear correlation
between radio and gamma-ray activity, indicating radio emission regions that
are unrelated to the parts of the jet that produce the gamma-rays. The
gamma-ray energy spectrum is unusual in showing a change of shape from a power
law to a curved spectrum when going from the quiescent state to the active
state.Comment: 11 pages, 10 figures, Accepted for publication in A&
Optical and Radio Variability of BL Lacertae
We observed the prototype blazar, BL Lacertae, extensively in optical and
radio bands during an active phase in the period 2010--2013 when the source
showed several prominent outbursts. We searched for possible correlations and
time lags between the optical and radio band flux variations using
multifrequency data to learn about the mechanisms producing variability. During
an active phase of BL Lacertae, we searched for possible correlations and time
lags between multifrequency light curves of several optical and radio bands. We
tried to estimate any possible variability timescales and inter-band lags in
these bands. We performed optical observations in B, V, R and I bands from
seven telescopes in Bulgaria, Georgia, Greece and India and obtained radio data
at 36.8, 22.2, 14.5, 8 and 4.8 GHz frequencies from three telescopes in
Ukraine, Finland and USA. Significant cross-correlations between optical and
radio bands are found in our observations with a delay of cm-fluxes with
respect to optical ones of ~250 days. The optical and radio light curves do not
show any significant timescales of variability. BL Lacertae showed many optical
'mini-flares' on short time-scales. Variations on longer term timescales are
mildly chromatic with superposition of many strong optical outbursts. In radio
bands, the amplitude of variability is frequency dependent. Flux variations at
higher radio frequencies lead the lower frequencies by days or weeks.
The optical variations are consistent with being dominated by a geometric
scenario where a region of emitting plasma moves along a helical path in a
relativistic jet. The frequency dependence of the variability amplitude
supports an origin of the observed variations intrinsic to the source.Comment: 10 pages, 9 figures, Accepted for publication in A&
Long-Term Multiwavelength Studies of High-Redshift Blazar 0836+710
Following gamma-ray flaring activity of high-redshift (z=2.218) blazar 0836+710 in 2011, we have assembled a long-term multiwavelength study of this object. Although this source is monitored regularly by radio telescopes and the Fermi Large Area Telescope, its coverage at other wavelengths is limited. The optical flux appears generally correlated with the gamma-ray flux, while little variability has been seen at X-ray energies. The gamma-ray/radio correlation is complex compared to some other blazars. As for many blazars, the largest variability is seen at gamma-ray wavelengths
Flaring radio lanterns along the ridge line: Long-term oscillatory motion in the jet of S5 1803 + 784
© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. We present a detailed analysis of 30 very long baseline interferometric (VLBI) observations of the BL Lac object S5 1803+784 (z= 0.679), obtained between mean observational time 1994.67 and 2012.91 at observational frequency 15 GHz. The long-term behaviour of the jet ridge line reveals the jet experiences an oscillatory motion superposed on its helical jet kinematics on a time-scale of about 6 yr. The excess variance of the positional variability indicates the jet components being farther from the VLBI core have larger amplitude in their position variations. The fractional variability amplitude shows slight changes in 3 yrbins of the component's position. The temporal variability in the Doppler boosting of the ridge line results in jet regions behaving as flaring 'radio lanterns'. We offer a qualitative scenario leading to the oscillation of the jet ridge line that utilizes the orbital motion of the jet emitter black hole due to a binary black hole companion. A correlation analysis implies composite origin of the flux variability of the jet components, emerging due to possibly both the evolving jet structure and its intrinsic variability
Multiband optical variability of the blazar OJ 287 during its outbursts in 2015 -- 2016
We present recent optical photometric observations of the blazar OJ 287 taken
during September 2015 -- May 2016. Our intense observations of the blazar
started in November 2015 and continued until May 2016 and included detection of
the large optical outburst in December 2016 that was predicted using the binary
black hole model for OJ 287. For our observing campaign, we used a total of 9
ground based optical telescopes of which one is in Japan, one is in India,
three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the
USA. These observations were carried out in 102 nights with a total of ~ 1000
image frames in BVRI bands, though the majority were in the R band. We detected
a second comparably strong flare in March 2016. In addition, we investigated
multi-band flux variations, colour variations, and spectral changes in the
blazar on diverse timescales as they are useful in understanding the emission
mechanisms. We briefly discuss the possible physical mechanisms most likely
responsible for the observed flux, colour and spectral variability.Comment: 11 pages, 6 figures, 4 tables; Accepted for publication in MNRA
Characterizing optical variability of OJ 287 in 2016 - 2017
We report on a recent multi-band optical photometric and polarimetric
observational campaign of the blazar OJ 287 which was carried out during
September 2016 -- December 2017. We employed nine telescopes in Bulgaria,
China, Georgia, Japan, Serbia, Spain and the United States. We collected over
1800 photometric image frames in BVRI bands and over 100 polarimetric
measurements over ~175 nights. In 11 nights with many quasi-simultaneous
multi-band (V, R, I) observations, we did not detect any genuine intraday
variability in flux or color. On longer timescales, multiple flaring events
were seen. Large changes in color with respect to time and in a
color--magnitude diagram were seen, and while only a weak systematic
variability trend was noticed in color with respect to time, the
color--magnitude diagram shows a bluer-when-brighter trend. Large changes in
the degree of polarization, and substantial swings in the polarization angle
were detected. The fractional Stokes parameters of the polarization showed a
systematic trend with time in the beginning of these observations, followed by
chaotic changes and then an apparently systematic variation at the end. These
polarization changes coincide with the detection and duration of the source at
very high energies as seen by VERITAS. The spectral index shows a systematic
variation with time and V-band magnitude. We briefly discuss possible physical
mechanisms that could explain the observed flux, color, polarization, and
spectral variability.Comment: 16 pages, 8 figures, 7 tables; Accepted for Publication to A
The optical behaviour of BL Lacertae at its maximum brightness levels: a blend of geometry and energetics
In 2021 BL Lacertae underwent an extraordinary activity phase, which was
intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration.
We present the WEBT optical data in the BVRI bands acquired at 36 observatories
around the world. In mid 2021 the source showed its historical maximum, with R
= 11.14. The light curves display many episodes of intraday variability, whose
amplitude increases with source brightness, in agreement with a geometrical
interpretation of the long-term flux behaviour. This is also supported by the
long-term spectral variability, with an almost achromatic trend with
brightness. In contrast, short-term variations are found to be strongly
chromatic and are ascribed to energetic processes in the jet. We also analyse
the optical polarimetric behaviour, finding evidence of a strong correlation
between the intrinsic fast variations in flux density and those in polarisation
degree, with a time delay of about 13 h. This suggests a common physical
origin. The overall behaviour of the source can be interpreted as the result of
two mechanisms: variability on time scales greater than several days is likely
produced by orientation effects, while either shock waves propagating in the
jet, or magnetic reconnection, possibly induced by kink instabilities in the
jet, can explain variability on shorter time scales. The latter scenario could
also account for the appearance of quasi-periodic oscillations, with periods
from a few days to a few hours, during outbursts, when the jet is more closely
aligned with our line of sight and the time scales are shortened by
relativistic effects.Comment: 15 pages, 16 figures, submitted to MNRA
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