Publications (6)19.78 Total impact
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Article: The Whole Earth Blazar Telescope Campaign on the Intermediate BL Lac Object 3C 66A in 2007-2008
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ABSTRACT: Prompted by a high optical state in 2007 September, the Whole Earth Blazar Telescope consortium organized an intensive optical, near-IR (JHK) and radio observing campaign on the intermediate BL Lac object 3C 66A throughout the fall and winter of 2007-2008. In this paper, we present data from 28 observatories in 12 countries, covering the observing season from late 2007 July through 2008 February. The source remained in a high optical state throughout the observing period and exhibited several bright flares on timescales of ~10 days. This included an exceptional outburst around 2007 September 15-20, reaching a peak brightness at R~ 13.4. Our campaign revealed microvariability with flux changes up to |dR/dt|~ 0.02 mag hr–1. Our observations do not reveal evidence for systematic spectral variability in the overall high state covered by our campaign, in agreement with previous results. In particular, we do not find evidence for spectral hysteresis in 3C 66A for which hints were found in an earlier campaign in a somewhat lower flux state. We also did not find any evidence for spectral lags in the discrete correlation functions between different optical bands. We infer a value of the magnetic field in the emission region of B~ 19 e 2/7 B τ–6/7 h D 13/7 1 G, where eB is the magnetic field equipartition fraction, τ h is the shortest observed variability timescale in units of hours, and D 1 is the Doppler factor in units of 10. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D ≤ 28 τ–1/8 h e 3/16 B . This is in perfect agreement with superluminal motion measurements with the VLBI/VLBA of βapp ≤ 27 and argues against models with very high Lorentz factors of Γ 50, required for a one-zone synchrotron-self-Compton interpretation of some high-frequency-peaked BL Lac objects detected at TeV γ-ray energies.The Astrophysical Journal 03/2009; 694(1):174. · 6.02 Impact Factor -
Article: Multiwavelength Observations of Markarian 421 in 2005 - 2006
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ABSTRACT: Since September 2005, the Whipple 10m Gamma-ray Telescope has been operated primarily as a blazar monitor. The five Northern Hemisphere blazars that have already been detected at the Whipple Observatory, Markarian 421, H1426+428, Markarian 501, 1ES 1959+650 and 1ES 2344+514, are monitored routinely each night that they are visible. We report on the Markarian 421 observations taken from November 2005 to June 2006 in the gamma-ray, X-ray, optical and radio bands. During this time, Markarian 421 was found to be variable at all wavelengths probed. Both the variability and the correlations among different energy regimes are studied in detail here. A tentative correlation, with large spread, was measured between the X-ray and gamma-ray bands, while no clear correlation was evident among the other energy bands. In addition to this, the well-sampled spectral energy distribution of Markarian 421 (1101+384) is presented for three different activity levels. The observations of the other blazar targets will be reported separately.01/2009; -
Article: The high activity of 3C 454.3 in autumn 2007 Monitoring by the WEBT during the AGILE detection
Astronomy and Astrophysics 01/2008; 485(2):17. · 4.59 Impact Factor -
Article: A new activity phase of the blazar 3C 454.3 Multifrequency observations by the WEBT and XMM-Newton in 2007—2008
Astronomy and Astrophysics 01/2008; 491(3):755. · 4.59 Impact Factor -
Article: WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase. Detection of the little and big blue bumps
Astronomy and Astrophysics 10/2007; 473(3):819. · 4.59 Impact Factor -
Article: WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase
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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.http://dx.doi.org/10.1051/0004-6361:20078289.
