INTEGRAL observations of the GeV blazar PKS 1502+106 and the hard X-ray bright Seyfert galaxy Mkn 841^⋆

¹ INAF, Trieste Astronomical Observatory, via G.B. Tiepolo 11, 34143 Trieste, Italy
e-mail: elena.pian@sns.it
² Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁴ INAF, IASF-Roma, via del Fosso del Cavaliere, 100, 00133 Roma, Italy
⁵ Centre François Arago, APC, Université Paris Diderot, CNRS/IN2P3, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
⁶ INAF/IASF Milano, via E. Bassini 15, 20133 Milano, Italy
⁷ INAF, Brera Astronomical Observatory, via E. Bianchi 46, 23807 Merate ( LC), Italy
⁸ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de la Palma, Spain
⁹ INTEGRAL Science Data Centre, University of Geneva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
¹⁰ Department of Physics, University “Roma Tre”, via della Vasca Navale 84, 00146 Roma, Italy
¹¹ INAF, IASF-Bolognai, via P. Gobett, 40129 Bologna, Italy
¹² Astronomical Institute, Academy of Sciences, Fricova 298, 25165 Ondrejov, Czech Republic
¹³ Czech Technical University in Prague, Faculty of Electrical Engineering, Czech Republic
¹⁴ INAF, Turin Astronomical Observatory, via Osservatorio 20, 10025 Pino Torinese ( TO), Italy
¹⁵ Laboratoire AIM - CNRS - CEA/DSM - Université Paris Diderot (UMR 7158), CEA Saclay, DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France
¹⁶ Department of Physics and Mathematics, University of Insubria, via Valleggio 11, 22100 Como, Italy

Received: 16 July 2010
Accepted: 7 October 2010

Abstract

Context. Extragalactic nuclear activity is most effectively explored with observations at high energies, where the most extreme flux and spectral variations are expected to occur, because of changes in either the accretion flow or the kinematics of the plasma. In active galactic nuclei of blazar type, these variations are the most dramatic.

Aims. By following blazar outbursts from their onset and correlating the observed variations at many different wavelengths, we can reconstruct the behavior of the plasma and map out the development of the flare within the jet.

Methods. The advent of the Fermi satellite has allowed the start of a systematic and intensive monitoring program of blazars. Blazar outbursts are very effectively detected by the LAT instrument in the MeV–GeV domain, and these can be promptly followed up with other facilities. Based on a Fermi LAT detection of a high MeV-GeV state, we observed the blazar PKS 1502+106 with the INTEGRAL satellite between 9 and 11 August 2008. Simultaneous Swift observations were also accomplished, as well as optical follow-up at the Nordic Optical Telescope.

Results. The IBIS instrument onboard INTEGRAL detected a source at a position inconsistent with the optical coordinates of PKS 1502+106, but consistent with those of the Seyfert 1 galaxy Mkn 841, located at 6.8 arcmin south-west of the blazar, which is therefore responsible for all the hard X-ray flux detected by IBIS. At the location of the blazar, IBIS sets an upper limit of  ~10^-11 erg s^-1 cm^-2 on the 15–60 keV flux, which is consistent with a model of inverse Compton scattering accounting for the soft X-ray and gamma-ray spectra measured by Swift XRT and Fermi LAT, respectively. The gamma-ray spectrum during the outburst indicates substantial variability in the characteristic energy of the inverse Compton component in this blazar. The hard X-ray state of the Seyfert appears to be nearly unchanged with respect to the past. On the other hand, its soft X-ray flux (0.3–10 keV) varies with a  ~50% amplitude on timescales from days to years. On long timescales, this is well correlated with the optical flux, with no measurable delay.

Conclusions. In PKS 1502+106, the critical parameters that control variability are the accelerating power transferred to the relativistic electrons, and the magnetic field in the emitting region. The spectrum of Mkn 841 in the 0.3–100 keV range is well described by a power-law with a cutoff at  ~150 keV and a Compton reflected continuum.