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Issue A&A
Volume 479, Number 1, February III 2008
Page(s) 35 - 40
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20078401



A&A 479, 35-40 (2008)
DOI: 10.1051/0004-6361:20078401

The 26 year-long X-ray light curve and the X-ray spectrum of the BL Lacertae object 1E 1207.9+3945 in its brightest state

A. Maselli1, P. Giommi2, 3, M. Perri3, R. Nesci1, A. Tramacere1, F. Massaro4, and M. Capalbi3

1  Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
    e-mail: alessandro.maselli@uniroma1.it
2  Agenzia Spaziale Italiana, Unità Osservazione dell'Universo, Viale Liegi 26, 00198 Roma, Italy
3  ASI Science Data Center, ESRIN, via G. Galilei, 00044 Frascati, Italy
4  Dipartimento di Fisica, Università Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy

(Received 2 August 2007 / Accepted 24 September 2007)

Abstract
Aims.We studied the temporal and spectral evolution of the synchrotron emission from the high energy peaked BL Lac object 1E 1207.9+3945.
Methods.Two recent observations have been performed by the XMM-Newton and Swift satellites; we carried out X-ray spectral analysis for both of them, and photometry in optical-ultraviolet filters for the Swift one. Combining the results thus obtained with archival data we built the long-term X-ray light curve, spanning a time interval of 26 years, and the Spectral Energy Distribution (SED) of this source.
Results.The light curve shows a large flux increasing, by about a factor of six, in a time interval of a few years. After reaching its maximum in coincidence with the XMM-Newton pointing in December 2000 the flux decreased in later years, as revealed by Swift. The very good statistics available in the 0.5-10 keV XMM-Newton X-ray spectrum reveals a highly significant deviation from a single power law. A log-parabolic model with a best fit curvature parameter of 0.25 and a peak energy at ~1 keV describes well the spectral shape of the synchrotron emission. The simultaneous fit of Swift UVOT and XRT data provides a milder curvature ($b\sim0.1$) and a peak at higher energies (~15 keV), suggesting a different state of source activity. In both cases UVOT data support the scenario of a single synchrotron emission component extending from the optical/UV to the X-ray band.
Conclusions.New X-ray observations are important to monitor the temporal and spectral evolution of the source; new generation $\gamma$-ray telescopes like AGILE and GLAST may for the first time detect its inverse Compton emission.


Key words: radiation mechanisms: non-thermal -- galaxies: active -- X-rays: individuals: 1E 1207.9+3945 -- X-rays: galaxies



© ESO 2008


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