Issue A&A Volume 489, Number 2, October II 2008 Page(s) L17 - L20 Section Letters DOI http://dx.doi.org/10.1051/0004-6361:200810711 Published online 25 August 2008

A&A 489, L17-L20 (2008)
DOI: 10.1051/0004-6361:200810711

Letter

Long-term AGILE monitoring of the puzzling gamma-ray source 3EG J1835+5918

A. Bulgarelli¹, M. Tavani^{2, 3}, P. Caraveo⁴, A. W. Chen^{5, 4}, F. Gianotti¹, M. Trifoglio¹, M. Marelli⁴, A. Argan², G. Barbiellini⁶, F. Boffelli^{7, 8}, P. W. Cattaneo⁷, V. Cocco², E. Costa¹, F. D'Ammando^{2, 3}, E. Del Monte², G. De Paris², G. Di Cocco¹, I. Donnarumma², Y. Evangelista², M. Feroci², M. Fiorini⁴, T. Froysland^{2, 3}, F. Fuschino⁵, M. Galli⁹, A. Giuliani⁴, C. Labanti¹, I. Lapshov², F. Lazzarotto², P. Lipari¹⁰, F. Longo⁶, M. Marisaldi¹, S. Mereghetti⁴, A. Morselli¹¹, L. Pacciani², A. Pellizzoni⁴, F. Perotti⁴, G. Piano^{2, 3}, P. Picozza^{3, 11}, M. Prest¹², G. Pucella², M. Rapisarda¹³, A. Rappoldi⁷, P. Soffitta², A. Trois², E. Vallazza⁶, S. Vercellone⁴, V. Vittorini^{2, 5}, A. Zambra⁴, D. Zanello¹⁰, P. Giommi¹⁴, C. Pittori¹⁴, F. Verrecchia¹⁴, P. Santolamazza¹⁴, D. Gasparrini¹⁴, S. Cutini¹⁴, S. Colafrancesco¹⁴, and L. Salotti<sup>12

1</sup>  INAF/IASF–Bologna, via Gobetti 101, 40129 Bologna, Italy
    e-mail: bulgarelli@iasfbo.inaf.it
²  INAF/IASF–Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy
³  Dip. di Fisica, Univ. “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴  INAF/IASF–Milano, via E. Bassini 15, 20133 Milano, Italy
⁵  CIFS–Torino, Viale Settimio Severo 3, 10133 Torino, Italy
⁶  Dip. di Fisica and INFN Trieste, via Valerio 2, 34127 Trieste, Italy
⁷  INFN–Pavia, via Bassi 6, 27100 Pavia, Italy
⁸  Dipartimento di Fisica Nucleare e Teorica, Universitá di Pavia, via Bassi 6, 27100 Pavia, Italy
⁹  ENEA–Bologna, via Biancafarina 2521, 40059 Medicica (BO), Italy
¹⁰  INFN–Roma “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy
¹¹  INFN–Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
¹²  Dip. di Fisica, Univ. dell'Insubria, via Valleggio 11, 22100 Como, Italy
¹³  ENEA–Roma, via E. Fermi 45, 00044 Frascati (Roma), Italy
¹⁴  ASI–ASDC, via G. Galilei, 00044 Frascati (Roma), Italy

Received 30 July 2008 / Accepted 13 August 2008

Abstract
Context. We present the AGILE gamma-ray observations of the field containing the puzzling gamma-ray source 3EG J1835+5918. This source is one of the most remarkable unidentified EGRET sources.
Aims. An unprecedentedly long AGILE monitoring of this source yields important information on the positional error box, flux evolution, and spectrum.
Methods. 3EG J1835+5918 has been in the AGILE field of view several times in 2007 and 2008 for a total observing time of 138 days from 2007 Sep. 04 to 2008 June 30 encompassing several weeks of continuous coverage.
Results. With an exposure time approximately twice that of EGRET, AGILE confirms the existence of a prominent gamma-ray source (AGL J1836+5926) at a position consistent with that of EGRET, although with a remarkably lower average flux value for photon energies greater than 100 MeV. A 5-day bin temporal analysis of the whole data set of AGL J1836+5926 shows some evidence for variability of the gamma-ray flux. The source spectrum between 100 MeV and 1 GeV can be fitted with a power law with photon index in the range 1.6–1.7, fully consistent with the EGRET value.
Conclusions. The faint X-ray source RX J1836.2+5925 that has been proposed as a possible counterpart of 3EG J1835+5918 is well within the AGILE error box. Future continuous monitoring (both by AGILE and GLAST) is needed to confirm the gamma-ray flux variability and to unveil the source origin, a subject that is currently being pursued through a multiwavelength search for counterparts.

Key words: gamma rays: observations

© ESO 2008

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