High energy variability of 3C 273 during the AGILE multiwavelength campaign of December 2007–January 2008

¹ 
INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: luigi.pacciani@iasf-roma.inaf.it
² 
INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via E. Bassini 15, 20133 Milano, Italy
³ 
ASI Science Data Center, via G. Galilei, 00044 Frascati (Roma), Italy
⁴ 
Consorzio Inter-universitario Fisica Spaziale, Viale Settimio Severo 3, 10133 Torino, Italy
⁵ 
Dip. di Fisica, Univ. Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁶ 
Osservatorio Astronomico, Univ. di Perugia, via B. Bonfigli, 06126 Perugia, Italy
⁷ 
INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁸ 
Dip. di Fisica and INFN-Trieste, via Valerio 2, 34127 Trieste, Italy
⁹ 
INFN-Pavia, via Bassi 6, 27100 Pavia, Italy
¹⁰ 
ENEA-Bologna, via Martiri di Monte Sole 4, 40129 Bologna, Italy
¹¹ 
INFN-Roma , Piazzale A. Moro 2, 00185 Roma, Italy
¹² 
INFN-Roma , via della Ricerca Scientifica 1, 00133 Roma, Italy
¹³ 
Dip. di Fisica, Univ. della Insubria, via Valleggio 11, 22100 Como, Italy
¹⁴ 
ENEA-Frascati, via E. Fermi 45, 00044 Frascati (Roma), Italy
¹⁵ 
ASI, Viale Liegi 26, 00198 Roma, Italy


Received: 16 August 2008
Accepted: 26 October 2008

Abstract

Context. 
We report the results of a 3-week multi-wavelength campaign targeting the flat spectrum radio quasar 3C 273 carried out with the AGILE gamma-ray mission, covering the 30 MeV–50 GeV and 18–60 keV, the REM observatory (covering the near-IR and optical), Swift (near-UV/Optical, 0.2–10 keV and 15–50 keV), INTEGRAL (3–200 keV) and Rossi XTE (2–12 keV). This is the first observational campaign including gamma-ray data, after the last EGRET observations, more than 8 years ago.

Aims. This campaign has been organized by the AGILE team with the aim of observing, studying and modelling the broad band energy spectrum of the source, and its variability on a week timescale, testing the emission models describing the spectral energy distribution of this source.

Methods. Our study was carried out using simultaneous light curves of the source flux from all the involved instruments, in the different energy ranges, to search for correlated variability. Then a time-resolved spectral energy distribution was used for a detailed physical modelling of the emission mechanisms.

Results. The source was detected in gamma-rays only in the second week of our campaign, with a flux comparable to the level detected by EGRET in June 1991. We found an indication of a possible anti-correlation between the emission at gamma-rays and at soft and hard X-rays, supported by the complete set of instruments. Instead, optical data do not show short term variability, as expected for this source. Only in two preceding EGRET observations (in 1993 and 1997) 3C 273 showed intra-observation variability in gamma-rays. In the 1997 observation, flux variation in gamma-rays was associated with a synchrotron flare.
The energy-density spectrum with almost simultaneous data partially covers the regions of synchrotron emission, the big blue bump, and the inverse-Compton. We adopted a leptonic model to explain the hard X/gamma-ray emissions, although from our analysis hadronic models cannot be ruled out.
In the adopted model, the soft X-ray emission is consistent with combined synchrotron-self Compton and external Compton mechanisms, while hard X and gamma-ray emissions are compatible with external Compton from thermal photons of the disk. Under this model, the time evolution of the spectral energy distribution is well interpreted and modelled in terms of an acceleration episode of the electron population, leading to a shift in the inverse Compton peak towards higher energies.