Article

The long-lasting activity of 3C 454.3. GASP-WEBT and satellite observations in 2008-2010

Astronomy & Astrophysics - ASTRON ASTROPHYS 07/2011; 534. DOI: 10.1051/0004-6361/201117026
Source: arXiv

ABSTRACT We present multiwavelength observations of 3C 454.3 from April 2008 to March
2010. The radio to optical data are mostly from the GASP-WEBT, UV and X-ray
data from Swift, and gamma-ray data from the AGILE and Fermi satellites. We
improved the calibration of optical-UV data from the UVOT and OM instruments
and estimated the Lyalpha flux to disentangle the contributions from different
components in this spectral region. The observations reveal prominent
variability above 8 GHz. In the optical-UV band, the variability amplitude
decreases with increasing frequency due to a steadier radiation from both a
broad line region and an accretion disc. The optical flux reaches nearly the
same levels in the 2008-2009 and 2009-2010 observing seasons; the mm one shows
similar behaviour, whereas the gamma and X-ray flux levels rise in the second
period. Two prominent gamma-ray flares in mid 2008 and late 2009 show a
double-peaked structure, with a variable gamma/optical flux ratio. The X-ray
flux variations seem to follow the gamma-ray and optical ones by about 0.5 and
1 d, respectively. We interpret the multifrequency behaviour in terms of an
inhomogeneous curved jet, where synchrotron radiation of increasing wavelength
is produced in progressively outer and wider jet regions, which can change
their orientation in time. In particular, we assume that the long-term
variability is due to this geometrical effect. By combining the optical and mm
light curves to fit the gamma and X-ray ones, we find that the gamma (X-ray)
emission may be explained by inverse-Comptonisation of synchrotron optical (IR)
photons by their parent relativistic electrons (SSC process). A slight,
variable misalignment between the synchrotron and Comptonisation zones would
explain the increased gamma and X-ray flux levels in 2009-2010, as well as the
change in the gamma/optical flux ratio during the outbursts peaks.

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    ABSTRACT: This is the first report of Fermi Gamma-ray Space Telescope observations of the quasar 3C 454.3, which has been undergoing pronounced long-term outbursts since 2000. The data from the Large Area Telescope (LAT), covering 2008 July 7 - October 6, indicate strong, highly variable gamma-ray emission with an average flux of ~3 x 10^{-6} photons cm^{-2} s^{-1}, for energies above 100 MeV. The gamma-ray flux is variable, with strong, distinct, symmetrically-shaped flares for which the flux increases by a factor of several on a time scale of about three days. This variability indicates a compact emission region, and the requirement that the source is optically thin to pair-production implies relativistic beaming with Doppler factor delta > 8, consistent with the values inferred from VLBI observations of superluminal expansion (delta ~ 25). The observed gamma-ray spectrum is not consistent with a simple power-law, but instead steepens strongly above ~2 GeV, and is well described by a broken power-law with photon indices of ~2.3 and ~3.5 below and above the break, respectively. This is the first direct observation of a break in the spectrum of a high luminosity blazar above 100 MeV, and it is likely direct evidence for an intrinsic break in the energy distribution of the radiating particles. Alternatively, the spectral softening above 2 GeV could be due to gamma-ray absorption via photon-photon pair production on the soft X-ray photon field of the host AGN, but such an interpretation would require the dissipation region to be located very close (less than 100 gravitational radii) to the black hole, which would be inconsistent with the X-ray spectrum of the source. Comment: Accepted by the Astrophysical Journal; corresponding authors: Greg Madejski (madejski@slac.stanford.edu) and Benoit Lott (lott@cenbg.in2p3.fr)
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    ABSTRACT: We present images from a long-term program (MOJAVE: Monitoring of Jets in active galactic nuclei (AGNs) with VLBA Experiments) to survey the structure and evolution of parsec-scale jet phenomena associated with bright radio-loud active galaxies in the northern sky. The observations consist of 2424 15 GHz Very Long Baseline Array (VLBA) images of a complete flux-density-limited sample of 135 AGNs above declination -20°, spanning the period 1994 August to 2007 September. These data were acquired as part of the MOJAVE and 2 cm Survey programs, and from the VLBA archive. The sample-selection criteria are based on multi-epoch parsec-scale (VLBA) flux density, and heavily favor highly variable and compact blazars. The sample includes nearly all the most prominent blazars in the northern sky, and is well suited for statistical analysis and comparison with studies at other wavelengths. Our multi-epoch and stacked-epoch images show 94% of the sample to have apparent one-sided jet morphologies, most likely due to the effects of relativistic beaming. Of the remaining sources, five have two-sided parsec-scale jets, and three are effectively unresolved by the VLBA at 15 GHz, with essentially all of the flux density contained within a few tenths of a milliarcsecond.
    The Astronomical Journal 01/2009; · 4.97 Impact Factor
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    ABSTRACT: Context: The quasar-type blazar 3C 454.3 was observed to undergo an unprecedented optical outburst in spring 2005, affecting the source brightness from the near-IR to the X-ray frequencies. This was first followed by a millimetric and then by a radio outburst, which peaked in February 2006. Aims: In this paper we report on follow-up observations to study the multiwavelength emission in the post-outburst phase. Methods: Radio, near-infrared, and optical monitoring was performed by the Whole Earth Blazar Telescope (WEBT) collaboration in the 2006-2007 observing season. XMM-Newton observations on July 2-3 and December 18-19, 2006 added information on the X-ray and UV states of the source. Results: 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 near-IR 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 spectral energy distribution (SED) during faint states. Moreover, the data from the XMM-Newton Optical Monitor reveal a rise of the SED in the ultraviolet, 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. i) There is no extra absorption, and the X-ray spectrum hardens at low energies, the hardening being more evident in bright states; ii) there is a constant amount of extra absorption, likely in the quasar environment, and the X-ray spectrum softens at low energies, at least in faint X-ray states. This softening might be the result of a flux contribution by the high-frequency tail of the big blue bump.
    Astronomy and Astrophysics 10/2007; 473(3):819. · 5.08 Impact Factor

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