300 research outputs found
Results from the Blazar Monitoring Campaign at the Whipple 10m Gamma-ray Telescope
In September 2005, the observing program of the Whipple 10 m gamma-ray
telescope was redefined to be dedicated almost exclusively to AGN monitoring.
Since then the five Northern Hemisphere blazars that had already been detected
at Whipple are monitored routinely each night that they are visible. Thanks to
the efforts of a large number of multiwavelength collaborators, the first year
of this program has been very successful. We report here on the analysis of
Markarian 421 observations taken from November, 2005 to May, 2006 in the
gamma-ray, X-ray, optical and radio bands.Comment: 4 pages; contribution to the 30th International Cosmic Ray
Conference, Merida, Mexico, July 200
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&
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
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
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
Another look at the BL Lacertae flux and spectral variability
The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope
(WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical
frequencies. During this period, high-energy observations were performed by
XMM-Newton, Swift, and Fermi. We analyse these data with particular attention
to the calibration of Swift UV data, and apply a helical jet model to interpret
the source broad-band variability. The GASP-WEBT observations show an optical
flare in 2008 February-March, and oscillations of several tenths of mag on a
few-day time scale afterwards. The radio flux is only mildly variable. The UV
data from both XMM-Newton and Swift seem to confirm a UV excess that is likely
caused by thermal emission from the accretion disc. The X-ray data from
XMM-Newton indicate a strongly concave spectrum, as well as moderate flux
variability on an hour time scale. The Swift X-ray data reveal fast (interday)
flux changes, not correlated with those observed at lower energies. We compare
the spectral energy distribution (SED) corresponding to the 2008 low-brightness
state, which was characterised by a synchrotron dominance, to the 1997 outburst
state, where the inverse-Compton emission was prevailing. A fit with an
inhomogeneous helical jet model suggests that two synchrotron components are at
work with their self inverse-Compton emission. Most likely, they represent the
radiation from two distinct emitting regions in the jet. We show that the
difference between the source SEDs in 2008 and 1997 can be explained in terms
of pure geometrical variations. The outburst state occurred when the
jet-emitting regions were better aligned with the line of sight, producing an
increase of the Doppler beaming factor. Our analysis demonstrates that the jet
geometry can play an extremely important role in the BL Lacertae flux and
spectral variability.Comment: 12 pages, 10 figures, accepted for publication in A&
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