Cygnus X-1 (Cyg X-1) is the archetypal black hole binary system in our Galaxy. We report the main results of an extensive search for transient gamma-ray emission from Cygnus X-1 carried out in the energy range 100 MeV-3 GeV by the AGILE satellite, during the period 2007 July-2009 October. The total exposure time is about 300 days, during which the source was in the "hard" X-ray spectral state. We divided the observing intervals in 2-4 week periods, and searched for transient and persistent emission. We report an episode of significant transient gamma-ray emission detected on 2009 October 16 in a position compatible with Cyg X-1 optical position. This episode, which occurred during a hard spectral state of Cyg X-1, shows that a 1-2 day time variable emission above 100 MeV can be produced during hard spectral states, having important theoretical implications for current Comptonization models for Cyg X-1 and other microquasars. Except for this one short timescale episode, no significant gamma-ray emission was detected by AGILE. By integrating all available data, we obtain a 2 sigma upper limit for the total integrated flux of F(gamma,U.L.) = 3 x 10(-8) ph cm(-2) s(-1) in the energy range 100 MeV-3 GeV. We then clearly establish the existence of a spectral cutoff in the energy range 1-100 MeV that applies to the typical hard state outside the flaring period and that confirms the historically known spectral cutoff above 1 MeV.
EPISODIC TRANSIENT GAMMA-RAY EMISSION FROM THE MICROQUASAR CYGNUS X-1 / Sabatini S; Tavani M; Striani E; Bulgarelli A; Vittorini V; Piano G; Del Monte E; Feroci M; de Pasquale F; Trifoglio M; Gianotti F; Argan A; Barbiellini G; Caraveo P; Cattaneo PW; Chen AW; DAmmando F; Costa E; De Paris G; Di Cocco G; Donnarumma I; Evangelista Y; Ferrari A; Fiorini M; Fuschino F; Galli M; Giuliani A; Giusti M; Labanti C; Lazzarotto F; Lipari P; Longo F; Marisaldi M; Mereghetti S; Morelli E; Moretti E; Morselli A; Pacciani L; Pellizzoni A; Perotti F; Picozza P; Pilia M; Pucella G; Prest M; Rapisarda M; Rappoldi A; Rubini A; Scalise E; Soffitta P; Trois A; Vallazza E; Vercellone S; Zambra A; Zanello D; Pittori C; Verrecchia F; Santolamazza P; Giommi P; Colafrancesco S; Antonelli LA; Salotti L. - In: THE ASTROPHYSICAL JOURNAL LETTERS. - ISSN 2041-8205. - STAMPA. - 712:1(2010), pp. 10-15. [10.1088/2041-8205/712/1/L10]
EPISODIC TRANSIENT GAMMA-RAY EMISSION FROM THE MICROQUASAR CYGNUS X-1
D'AMMANDO, FILIPPO;
2010
Abstract
Cygnus X-1 (Cyg X-1) is the archetypal black hole binary system in our Galaxy. We report the main results of an extensive search for transient gamma-ray emission from Cygnus X-1 carried out in the energy range 100 MeV-3 GeV by the AGILE satellite, during the period 2007 July-2009 October. The total exposure time is about 300 days, during which the source was in the "hard" X-ray spectral state. We divided the observing intervals in 2-4 week periods, and searched for transient and persistent emission. We report an episode of significant transient gamma-ray emission detected on 2009 October 16 in a position compatible with Cyg X-1 optical position. This episode, which occurred during a hard spectral state of Cyg X-1, shows that a 1-2 day time variable emission above 100 MeV can be produced during hard spectral states, having important theoretical implications for current Comptonization models for Cyg X-1 and other microquasars. Except for this one short timescale episode, no significant gamma-ray emission was detected by AGILE. By integrating all available data, we obtain a 2 sigma upper limit for the total integrated flux of F(gamma,U.L.) = 3 x 10(-8) ph cm(-2) s(-1) in the energy range 100 MeV-3 GeV. We then clearly establish the existence of a spectral cutoff in the energy range 1-100 MeV that applies to the typical hard state outside the flaring period and that confirms the historically known spectral cutoff above 1 MeV.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
