2003–2005 INTEGRAL and XMM-Newton observations of 3C 273

¹ INTEGRAL Science Data Center, Chemin d'Écogia 16, 1290 Versoix, Switzerland e-mail: Masha.Chernyakova@obs.unige.ch
² Geneva Observatory, University of Geneva, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
³ NASA Goddard Space Flight Center, Exploration of the Universe Division, Code 661, Greenbelt, MD 20771, USA
⁴ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warszawa, Poland
⁵ UK Swift Science Data Centre, Dept. of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁶ XMM-Newton Science Operations Centre, European Space Astronomy Centre (ESAC), Villafranca, PO Box 50727, 28080 Madrid, Spain
⁷ Institut für Astronomie und Astrophysik, Astronomie, 72076 Tübingen, Germany
⁸ School of Physics & Astronomy University of Southampton Highfield, Southampton SO17 1BJ, UK

Received: 22 August 2006
Accepted: 22 December 2006

Abstract

Aims.The aim of this paper is to study the evolution of the broadband spectrum of one of the brightest and nearest quasars 3C 273.

Methods.We analyze the data obtained during quasi-simultaneous INTEGRAL and XMM-Newton monitoring of the blazar 3C 273 in 2003–2005 in the UV, X-ray and soft γ-ray bands and study the results in the context of the long-term evolution of the source.

Results.The 0.2–100 keV spectrum of the source is well fitted by a combination of a soft cut-off power law and a hard power law. No improvement of the fit is achieved if one replaces the soft cut-off power law by either a blackbody, or a disk reflection model. During the observation period the source has reached the historically softest state in the hard X-ray domain with a photon index Γ = 1.82 ± 0.01. Comparing our data with available archived X-ray data from previous years, we find a secular evolution of the source toward softer X-ray emission (the photon index has increased by 0.3-0.4 over the last thirty years). We argue that existing theoretical models have to be significantly modified to account for the observed spectral evolution of the source.