384 research outputs found
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 2007–2015, characterized by several extreme flares. The ratio between the optical,
X-ray and γ -ray fluxes is very variable. The γ -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 ∼50 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
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
Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift during the 2006--2007 outburst
The blazar AO 0235+164 was claimed to show a quasi-periodic behaviour in the
radio and optical bands. Moreover, an extra emission component contributing to
the UV and soft X-ray flux was detected, whose nature is not yet clear. A
predicted optical outburst was observed in late 2006/early 2007. We here
present the radio-to-optical WEBT light curves during the outburst, together
with UV data acquired by Swift in the same period. We found the optical
outburst to be as strong as the big outbursts of the past: starting from late
September 2006, a brightness increase of 5 mag led to the outburst peak in
February 19-21, 2007. We also observed an outburst at mm and then at cm
wavelengths, with an increasing time delay going toward lower frequencies
during the rising phase. Cross-correlation analysis indicates that the 1 mm and
37 GHz flux variations lagged behind the R-band ones by about 3 weeks and 2
months, respectively. These short time delays suggest that the corresponding
jet emitting regions are only slightly separated and/or misaligned. In
contrast, during the outburst decreasing phase the flux faded contemporaneously
at all cm wavelengths. This abrupt change in the emission behaviour may suggest
the presence of some shutdown mechanism of intrinsic or geometric nature. The
behaviour of the UV flux closely follows the optical and near-IR one. By
separating the synchrotron and extra component contributions to the UV flux, we
found that they correlate, which suggests that the two emissions have a common
origin.Comment: 9 pages, 7 figures, in press for Astronomy and Astrophysic
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&
WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase. Detection of the little and big blue bumps
The blazar 3C 454.3 underwent an unprecedented optical outburst in spring
2005. This was first followed by a mm and then by a cm radio outburst, which
peaked in February 2006. We report on follow-up observations by the WEBT to
study the multiwavelength emission in the post-outburst phase. XMM-Newton
observations on July and December 2006 added information on the X-ray and UV
fluxes. The source was in a faint state. The radio flux at the higher
frequencies showed a fast decreasing trend, which represents the tail of the
big radio outburst. It was followed by a quiescent state, common at all radio
frequencies. In contrast, moderate activity characterized the NIR and optical
light curves, with a progressive increase of the variability amplitude with
increasing wavelength. We ascribe this redder-when-brighter behaviour to the
presence of a "little blue bump" due to line emission from the broad line
region, which is clearly visible in the source SED during faint states.
Moreover, the data from the XMM-Newton OM reveal a rise of the SED in the UV,
suggesting the existence of a "big blue bump" due to thermal emission from the
accretion disc. The X-ray spectra are well fitted with a power-law model with
photoelectric absorption, possibly larger than the Galactic one. However, the
comparison with previous X-ray observations would imply that the amount of
absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum
presents a curvature, which may depend on the X-ray brightness. In this case,
two scenarios are possible.Comment: 9 pages, 7 figures, accepted for publication in A&
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&
The structure and emission model of the relativistic jet in the quasar 3C 279 inferred from radio to high-energy gamma-ray observations in 2008-2010
We present time-resolved broad-band observations of the quasar 3C 279
obtained from multi-wavelength campaigns conducted during the first two years
of the Fermi Gamma-ray Space Telescope mission. While investigating the
previously reported gamma-ray/optical flare accompanied by a change in optical
polarization, we found that the optical emission appears delayed with respect
to the gamma-ray emission by about 10 days. X-ray observations reveal a pair of
`isolated' flares separated by ~90 days, with only weak gamma-ray/optical
counterparts. The spectral structure measured by Spitzer reveals a synchrotron
component peaking in the mid-infrared band with a sharp break at the
far-infrared band during the gamma-ray flare, while the peak appears in the
mm/sub-mm band in the low state. Selected spectral energy distributions are
fitted with leptonic models including Comptonization of external radiation
produced in a dusty torus or the broad-line region. Adopting the interpretation
of the polarization swing involving propagation of the emitting region along a
curved trajectory, we can explain the evolution of the broad-band spectra
during the gamma-ray flaring event by a shift of its location from ~ 1 pc to ~
4 pc from the central black hole. On the other hand, if the gamma-ray flare is
generated instead at sub-pc distance from the central black hole, the
far-infrared break can be explained by synchrotron self-absorption. We also
model the low spectral state, dominated by the mm/sub-mm peaking synchrotron
component, and suggest that the corresponding inverse-Compton component
explains the steady X-ray emission.Comment: 23 pages, 18 figures 5 tables, Accepted for publication in The
Astrophysical Journa
AGILE detection of extreme gamma-ray activity from the blazar PKS 1510-089 during March 2009. Multifrequency analysis
We report on the extreme gamma-ray activity from the FSRQ PKS 1510-089
observed by AGILE in March 2009. In the same period a radio-to-optical
monitoring of the source was provided by the GASP-WEBT and REM. Moreover,
several Swift ToO observations were triggered, adding important information on
the source behaviour from optical/UV to hard X-rays. We paid particular
attention to the calibration of the Swift/UVOT data to make it suitable to the
blazars spectra. Simultaneous observations from radio to gamma rays allowed us
to study in detail the correlation among the emission variability at different
frequencies and to investigate the mechanisms at work. In the period 9-30 March
2009, AGILE detected an average gamma-ray flux of (311+/-21)x10^-8 ph cm^-2
s^-1 for E>100 MeV, and a peak level of (702+/-131)x10^-8 ph cm^-2 s^-1 on
daily integration. The gamma-ray activity occurred during a period of
increasing activity from near-IR to UV, with a flaring episode detected on
26-27 March 2009, suggesting that a single mechanism is responsible for the
flux enhancement observed from near-IR to UV. By contrast, Swift/XRT
observations seem to show no clear correlation of the X-ray fluxes with the
optical and gamma-ray ones. However, the X-ray observations show a harder
photon index (1.3-1.6) with respect to most FSRQs and a hint of
harder-when-brighter behaviour, indicating the possible presence of a second
emission component at soft X-ray energies. Moreover, the broad band spectrum
from radio-to-UV confirmed the evidence of thermal features in the optical/UV
spectrum of PKS 1510-089 also during high gamma-ray state. On the other hand,
during 25-26 March 2009 a flat spectrum in the optical/UV energy band was
observed, suggesting an important contribution of the synchrotron emission in
this part of the spectrum during the brightest gamma-ray flare, therefore a
significant shift of the synchrotron peak.Comment: 13 pages, 7 figures, 3 tables. Accepted for publication in Astronomy
and Astrophysic
Multiwavelength observations of 3C 454.3. III. Eighteen months of AGILE monitoring of the "Crazy Diamond"
We report on 18 months of multiwavelength observations of the blazar 3C 454.3
(Crazy Diamond) carried out in July 2007-January 2009. We show the results of
the AGILE campaigns which took place on May-June 2008, July-August 2008, and
October 2008-January 2009. During the May 2008-January 2009 period, the source
average flux was highly variable, from an average gamma-ray flux F(E>100MeV) >
200E-8 ph/cm2/s in May-June 2008, to F(E>100MeV)~80E-8 ph/cm2/s in October
2008-January 2009. The average gamma-ray spectrum between 100 MeV and 1 GeV can
be fit by a simple power law (Gamma_GRID ~ 2.0 to 2.2). Only 3-sigma upper
limits can be derived in the 20-60 keV energy band with Super-AGILE. During
July-August 2007 and May-June 2008, RXTE measured a flux of F(3-20 keV)=
8.4E-11 erg/cm2/s, and F(3-20 keV)=4.5E-11 erg/cm2/s, respectively and a
constant photon index Gamma_PCA=1.65. Swift/XRT observations were carried out
during all AGILE campaigns, obtaining a F(2-10 keV)=(0.9-7.5)E-11 erg/cm2/s and
a photon index Gamma_XRT=1.33-2.04. BAT measured an average flux of ~5 mCrab.
GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period from the radio
to the optical. A correlation analysis between the optical and the gamma-ray
fluxes shows a time lag of tau=-0.4 days. An analysis of 15 GHz and 43 GHz VLBI
core radio flux observations shows an increasing trend of the core radio flux,
anti- correlated with the higher frequency data. The modeling SEDs, and the
behavior of the long-term light curves in different energy bands, allow us to
compare the jet properties during different emission states, and to study the
geometrical properties of the jet on a time-span longer than one year.Comment: Accepted for publication in ApJ. Adapted Abstract. 17 pages, 19
Figures, 5 Table
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