240 research outputs found
Multifrequency monitoring of the blazar 0716+714 during the GASP-WEBT-AGILE campaign of 2007
Since the CGRO operation in 1991-2000, one of the primary unresolved
questions about the blazar gamma-ray emission has been its possible correlation
with the low-energy (in particular optical) emission. To help answer this
problem, the Whole Earth Blazar Telescope (WEBT) consortium has organized the
GLAST-AGILE Support Program (GASP) to provide the optical-to-radio monitoring
data to be compared with the gamma-ray detections by the AGILE and GLAST
satellites. This new WEBT project started in early September 2007, just before
a strong gamma-ray detection of 0716+714 by AGILE. We present the GASP-WEBT
optical and radio light curves of this blazar obtained in July-November 2007,
about various AGILE pointings at the source. We construct NIR-to-UV spectral
energy distributions (SEDs), by assembling GASP-WEBT data together with UV data
from the Swift ToO observations of late October. We observe a contemporaneous
optical-radio outburst, which is a rare and interesting phenomenon in blazars.
The shape of the SEDs during the outburst appears peculiarly wavy because of an
optical excess and a UV drop-and-rise. The optical light curve is well sampled
during the AGILE pointings, showing prominent and sharp flares. A future
cross-correlation analysis of the optical and AGILE data will shed light on the
expected relationship between these flares and the gamma-ray events.Comment: 5 pages, 5 figures, to be published in A&A (Letters); revised to
match the final version (changes in Fig. 5 and related text
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&
Dissecting the long-term emission behaviour of the BL Lac object Mrk 421
We report on long-term multiwavelengthmonitoring of blazar Mrk 421 by the GLAST-AGILE
Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and
Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in
the period 2007â2015, characterized by several extreme flares. The ratio between the optical,
X-ray and Îł -ray fluxes is very variable. The Îł -ray flux variations show a fair correlation with
the optical ones starting from 2012.We analyse spectropolarimetric data and find wavelengthdependence
of the polarization degree (P), which is compatible with the presence of the
host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA).
Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of
the EVPA.We build broad-band spectral energy distributions with simultaneous near-infrared
and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite.
They show strong variability in both flux and X-ray spectral shape and suggest a shift of
the synchrotron peak up to a factor of âŒ50 in frequency. The interpretation of the flux and
spectral variability is compatible with jet models including at least two emitting regions that
can change their orientation with respect to the line of sight.http://10.0.4.69/mnras/stx2185Accepted manuscrip
The 72-Hour WEBT Microvariability Observation of Blazar S5 0716+714 in 2009
Context. The international whole earth blazar telescope (WEBT) consortium
planned and carried out three days of intensive micro-variability observations
of S5 0716+714 from February 22, 2009 to February 25, 2009. This object was
chosen due to its bright apparent magnitude range, its high declination, and
its very large duty cycle for micro-variations. Aims. We report here on the
long continuous optical micro-variability light curve of 0716+714 obtained
during the multi-site observing campaign during which the Blazar showed almost
constant variability over a 0.5 magnitude range. The resulting light curve is
presented here for the first time. Observations from participating
observatories were corrected for instrumental differences and combined to
construct the overall smoothed light curve. Methods. Thirty-six observatories
in sixteen countries participated in this continuous monitoring program and
twenty of them submitted data for compilation into a continuous light curve.
The light curve was analyzed using several techniques including Fourier
transform, Wavelet and noise analysis techniques. Those results led us to model
the light curve by attributing the variations to a series of synchrotron
pulses. Results. We have interpreted the observed microvariations in this
extended light curve in terms of a new model consisting of individual
stochastic pulses due to cells in a turbulent jet which are energized by a
passing shock and cool by means of synchrotron emission. We obtained an
excellent fit to the 72-hour light curve with the synchrotron pulse model
A new activity phase of the blazar 3C 454.3. Multifrequency observations by the WEBT and XMM-Newton in 2007-2008
We present and analyse the WEBT multifrequency observations of 3C 454.3 in
the 2007-2008 observing season, including XMM-Newton observations and near-IR
spectroscopic monitoring, and compare the recent emission behaviour with the
past one. In the optical band we observed a multi-peak outburst in July-August
2007, and other faster events in November 2007 - February 2008. During these
outburst phases, several episodes of intranight variability were detected. A mm
outburst was observed starting from mid 2007, whose rising phase was
contemporaneous to the optical brightening. A slower flux increase also
affected the higher radio frequencies, the flux enhancement disappearing below
8 GHz. The analysis of the optical-radio correlation and time delays, as well
as the behaviour of the mm light curve, confirm our previous predictions,
suggesting that changes in the jet orientation likely occurred in the last few
years. The historical multiwavelength behaviour indicates that a significant
variation in the viewing angle may have happened around year 2000. Colour
analysis reveals a complex spectral behaviour, which is due to the interplay of
different emission components. All the near-IR spectra show a prominent Halpha
emission line, whose flux appears nearly constant. The analysis of the
XMM-Newton data indicates a correlation between the UV excess and the
soft-X-ray excess, which may represent the head and the tail of the big blue
bump, respectively. The X-ray flux correlates with the optical flux, suggesting
that in the inverse-Compton process either the seed photons are synchrotron
photons at IR-optical frequencies or the relativistic electrons are those that
produce the optical synchrotron emission. The X-ray radiation would thus be
produced in the jet region from where the IR-optical emission comes.Comment: 10 pages, 12 figures (7 included in the text, 5 in GIF format),
accepted for publication in A&
Multifrequency Photo-polarimetric WEBT Observation Campaign on the Blazar S5 0716+714: Source Microvariability and Search for Characteristic Timescales
Here we report on the results of the WEBT photo-polarimetric campaign
targeting the blazar S5~0716+71, organized in March 2014 to monitor the source
simultaneously in BVRI and near IR filters. The campaign resulted in an
unprecedented dataset spanning \,h of nearly continuous, multi-band
observations, including two sets of densely sampled polarimetric data mainly in
R filter. During the campaign, the source displayed pronounced variability with
peak-to-peak variations of about and "bluer-when-brighter" spectral
evolution, consisting of a day-timescale modulation with superimposed hourlong
microflares characterized by \,mag flux changes. We performed an
in-depth search for quasi-periodicities in the source light curve; hints for
the presence of oscillations on timescales of \,h and \,h do
not represent highly significant departures from a pure red-noise power
spectrum. We observed that, at a certain configuration of the optical
polarization angle relative to the positional angle of the innermost radio jet
in the source, changes in the polarization degree led the total flux
variability by about 2\,h; meanwhile, when the relative configuration of the
polarization and jet angles altered, no such lag could be noted. The
microflaring events, when analyzed as separate pulse emission components, were
found to be characterized by a very high polarization degree () and
polarization angles which differed substantially from the polarization angle of
the underlying background component, or from the radio jet positional angle. We
discuss the results in the general context of blazar emission and energy
dissipation models.Comment: 16 pages, 17 Figures; ApJ accepte
The unprecedented optical outburst of the quasar 3C 454.3. The WEBT campaign of 2004-2005
The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting
more than 1 year and culminating in spring 2005. The maximum brightness
detected was R = 12.0, which represents the most luminous quasar state thus far
observed (M_B ~ -31.4). In order to follow the emission behaviour of the source
in detail, a large multiwavelength campaign was organized by the Whole Earth
Blazar Telescope (WEBT). Continuous optical, near-IR and radio monitoring was
performed in several bands. ToO pointings by the Chandra and INTEGRAL
satellites provided additional information at high energies in May 2005. The
historical radio and optical light curves show different behaviours. Until
about 2001.0 only moderate variability was present in the optical regime, while
prominent and long-lasting radio outbursts were visible at the various radio
frequencies, with higher-frequency variations preceding the lower-frequency
ones. After that date, the optical activity increased and the radio flux is
less variable. This suggests that the optical and radio emissions come from two
separate and misaligned jet regions, with the inner optical one acquiring a
smaller viewing angle during the 2004-2005 outburst. Moreover, the colour-index
behaviour (generally redder-when-brighter) during the outburst suggests the
presence of a luminous accretion disc. A huge mm outburst followed the optical
one, peaking in June-July 2005. The high-frequency (37-43 GHz) radio flux
started to increase in early 2005 and reached a maximum at the end of our
observing period (end of September 2005). VLBA observations at 43 GHz during
the summer confirm theComment: 7 pages, 4 figures, to be published in A&
The correlated optical and radio variability of BL Lacertae. WEBT data analysis 1994-2005
Since 1997, BL Lacertae has undergone a phase of high optical activity, with
the occurrence of several prominent outbursts. Starting from 1999, the Whole
Earth Blazar Telescope (WEBT) consortium has organized various multifrequency
campaigns on this blazar, collecting tens of thousands of data points. One of
the main issues in the study of this huge dataset has been the search for
correlations between the optical and radio flux variations, and for possible
periodicities in the light curves. The analysis of the data assembled during
the first four campaigns (comprising also archival data to cover the period
1968-2003) revealed a fair optical-radio correlation in 1994-2003, with a delay
of the hard radio events of ~100 days. Moreover, various statistical methods
suggested the existence of a radio periodicity of ~8 years. In 2004 the WEBT
started a new campaign to extend the dataset to the most recent observing
seasons, in order to possibly confirm and better understand the previous
results. In this campaign we have collected and assembled about 11000 new
optical observations from twenty telescopes, plus near-IR and radio data at
various frequencies. Here, we perform a correlation analysis on the long-term
R-band and radio light curves. In general, we confirm the ~100-day delay of the
hard radio events with respect to the optical ones, even if longer (~200-300
days) time lags are also found in particular periods. The radio
quasi-periodicity is confirmed too, but the "period" seems to progressively
lengthen from 7.4 to 9.3 years in the last three cycles. The optical and radio
behaviour in the last forty years suggests a scenario where geometric effects
play a major role. In particular, the alternation of enhanced and suppressed
optical activity (accompanied by hard and soft radio events, respectively) canComment: 6 pages, 4 figure
WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007-2008. Unveiling different emission components
In 2007-2008 we carried out a new multiwavelength campaign of the Whole Earth
Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the
XMM-Newton satellite, to study its emission properties. The source was
monitored in the optical-to-radio bands by 37 telescopes. The brightness level
was relatively low. Some episodes of very fast variability were detected in the
optical bands. The X-ray spectra are well fitted by a power law with photon
index of about 2 and photoelectric absorption exceeding the Galactic value.
However, when taking into account the presence of a molecular cloud on the line
of sight, the data are best fitted by a double power law, implying a concave
X-ray spectrum. The spectral energy distributions (SEDs) built with
simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations
suggest that the peak of the synchrotron emission lies in the near-IR band, and
show a prominent UV excess, besides a slight soft-X-ray excess. A comparison
with the SEDs corresponding to previous observations with X-ray satellites
shows that the X-ray spectrum is extremely variable. We ascribe the UV excess
to thermal emission from the accretion disc, and the other broad-band spectral
features to the presence of two synchrotron components, with their related SSC
emission. We fit the thermal emission with a black body law and the non-thermal
components by means of a helical jet model. The fit indicates a disc
temperature greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s.Comment: 11 pages, 7 figures, accepted for publication in A&
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