M. Villata
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Publication
Featured researches published by M. Villata.
The Astrophysical Journal | 1998
Ann E. Wehrle; E. Pian; Claudia M. Urry; L. Maraschi; I. M. McHardy; A. J. Lawson; G. Ghisellini; R. C. Hartman; Greg M. Madejski; F. Makino; Alan P. Marscher; S. J. Wagner; J. R. Webb; G. S. Aldering; Margo F. Aller; Hugh D. Aller; Dana E. Backman; T. J. Balonek; P. Boltwood; Jerry T. Bonnell; J. Caplinger; A. Celotti; W. Collmar; J. Dalton; A. Drucker; R. Falomo; C. E. Fichtel; Wolfram Freudling; Walter Kieran Gear; N. Gonzales
The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.
The Astrophysical Journal | 2001
R. C. Hartman; M. Böttcher; G. Aldering; Hugh D. Aller; Margo F. Aller; Dana E. Backman; Thomas J. Balonek; D. L. Bertsch; S. D. Bloom; H. Bock; Paul Boltwood; Michael T. Carini; W. Collmar; G. de Francesco; Elizabeth Colleen Ferrara; Wolfram Freudling; Walter Kieran Gear; Patrick B. Hall; J. Heidt; Philip A. Hughes; Stanley D. Hunter; Shardha Jogee; W. N. Johnson; G. Kanbach; S. Katajainen; M. Kidger; Tsuneo Kii; M. Koskimies; A. Kraus; H. Kubo
Of the blazars detected by EGRET in GeV γ-rays, 3C 279 is not only the best observed by EGRET but also one of the best monitored at lower frequencies. We have assembled 11 spectra, from GHz radio through GeV γ-rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial γ-ray contribution to the total luminosity of the object; in a high state, the γ-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and γ-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton plus external Compton γ-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low γ-ray intensity.
Astronomy and Astrophysics | 2001
C. M. Raiteri; M. Villata; Hugh D. Aller; Margo F. Aller; J. Heidt; Omar M. Kurtanidze; L. Lanteri; M. Maesano; E. Massaro; Franco Montagni; R. Nesci; Kim K. Nilsson; Maria G. Nikolashvili; P Nurmi; L Ostorero; Tapio Pursimo; R Rekola; A. Sillanpää; L. Takalo; H. Teräsranta; G. Tosti; T. J. Balonek; Markus Feldt; A Heines; C Heisler; J. Hu; M. Kidger; J. R Mattox; E. J McGrath; A. Pati
The BL Lacertae object AO 0235+16 is well known for its extreme optical and radio variability. New optical and radio data have been collected in the last four years by a wide international collaboration, which conrm the intense activity of this source: on the long term, overall variations of 5 mag in the R band and up to a factor 18 in the radio fluxes were detected, while short-term variability up to 0:5 mag in a few hours and 1: 3m ag in one day was observed in the optical band. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) rst-light campaign organized in November 1997, involving a dozen optical observatories. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection and Discrete Correlation Function (DCF) analysis. On the long term, the autocorrelation function of the optical data shows a double-peaked maximum at 4100{4200 days (11:2{11:5 years), while a double-peaked maximum at 3900{4200 days (10:7{11:5 years) is visible in the radio autocorrelation functions. The existence of this similar characteristic time scale of variability in the two bands is by itself an indication of optical-radio correlation. A further analysis by means of Discrete Fourier Transform (DFT) technique and folded light curves reveals that the major radio outbursts repeat quasi-regularly with a periodicity of5:7 years, i.e. half the above time scale. This period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them. Visual inspection and DCF analysis of the optical and radio light curves then reveal that in some cases optical outbursts seem to be simultaneous with radio ones, but in other cases they lead the radio events. Moreover, a deep inspection of the radio light curves suggests that in at least two occasions (the 1992{1993 and 1998 outbursts) flux variations at the higher frequencies may have led those at the lower ones.
Astronomy and Astrophysics | 2002
M. Villata; C. M. Raiteri; Maria G. Nikolashvili; I. E. Papadakis; K. Tsinganos; Kozo Sadakane; S. Ciprini; A. Frasca; E. Marilli; R. Nesci; M. Maesano; Julian N. Basler; C. Pullen; J. A. de Diego; Marko Moilanen; Arto Oksanen; Dolores Rodriguez; M. A. Kelly; H. R. Miller; Santo Catalano; D. Dultzin-Hacyan; Hannu Karttunen; N. A. Kudryavtseva; Katsura Matsumoto; F. Montagni; G. Nucciarelli; Luisa Ostorero; J. Papamastorakis; M. Pasanen; G. Sobrito
We present UBVRI light curves of BL Lacertae from May 2000 to January 2001, obtained by 24 telescopes in 11 countries. More than 15 000 observations were performed in that period, which was the extension of the Whole Earth Blazar Telescope (WEBT) campaign originally planned for July–August 2000. The exceptional sampling reached allows one to follow the flux behaviour in fine detail. Two different phases can be distinguished in the light curves: a first, relatively low-brightness phase is followed by an outburst phase, after a more than
The Astrophysical Journal | 2001
F. Tavecchio; L. Maraschi; E. Pian; L. Chiappetti; A. Celotti; Giovanni Fossati; G. Ghisellini; Eliana Palazzi; C. M. Raiteri; Rita M. Sambruna; A. Treves; Claudia M. Urry; M. Villata; A. Djannati-Ataï
1\rm\,mag
Astronomy and Astrophysics | 2008
M. Villata; Claudia Maria Raiteri; V. M. Larionov; Omar M. Kurtanidze; K. Nilsson; M. F. Aller; M. Tornikoski; A. Volvach; Hugh D. Aller; A. A. Arkharov; U. Bach; P. Beltrame; G. Bhatta; C. S. Buemi; M. Böttcher; P. Calcidese; D. Carosati; A. J. Castro-Tirado; D. Da Rio; A. Di Paola; M. Dolci; E. Forné; A. Frasca; V. A. Hagen-Thorn; J. Heidt; D. Hiriart; Martin Jelinek; G. N. Kimeridze; T. S. Konstantinova; E. N. Kopatskaya
brightening in a few weeks. Both the time duration (about
Astronomy and Astrophysics | 2008
V. M. Larionov; S. G. Jorstad; Alan P. Marscher; Claudia Maria Raiteri; M. Villata; I. Agudo; M. F. Aller; A. A. Arkharov; I. M. Asfandiyarov; U. Bach; A. Berdyugin; C. S. Buemi; Alok C. Gupta; M. A. Gurwell; V. A. Hagen-Thorn; M. A. Ibrahimov; B. Jordan; M. Kamada; T. S. Konstantinova; E. N. Kopatskaya; Y. Y. Kovalev; Yu. A. Kovalev; Omar M. Kurtanidze; L. Lanteri; L. V. Larionova; P. Leto; E. Lindfors; E. Marilli; I. M. McHardy; M. G. Mingaliev
100\rm\,d
Astronomy and Astrophysics | 2004
M. Villata; C. M. Raiteri; Hugh D. Aller; M. F. Aller; H. Teräsranta; P. Koivula; S. Wiren; Omar M. Kurtanidze; M. G. Nikolashvili; M. A. Ibrahimov; I. E. Papadakis; G. Tosti; Filip Hroch; L. Takalo; A. Sillanpää; V. A. Hagen-Thorn; V. M. Larionov; R. D. Schwartz; Julian N. Basler; L. F. Brown; T. J. Balonek
) and the variation amplitude (roughly
The Astrophysical Journal | 2000
F. Tavecchio; L. Maraschi; G. Ghisellini; A. Celotti; L. Chiappetti; A. Comastri; Giovanni Fossati; P. Grandi; E. Pian; G. Tagliaferri; A. Treves; C. M. Raiteri; Rita M. Sambruna; M. Villata
0.9\rm\,mag
Astronomy and Astrophysics | 2008
Claudia Maria Raiteri; M. Villata; V. M. Larionov; M. A. Gurwell; W. P. Chen; P. Charlot
) are similar in the two phases. Rapid flux oscillations are present all the time, involving variations up to a few tenths of mag on hour time scales, and witnessing an intense intraday activity of this source. In particular, a half-mag brightness decrease in about