A year-long AGILE observation of Cygnus X-1 in hard spectral state

¹ INAF IASF Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: ettore.delmonte@iasf-roma.inaf.it
² Dip. di Fisica, Università degli Studi di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy
³ ENEA, via E. Fermi 45, 00044 Frascati (Rm), Italy
⁴ INFN Trieste, Padriciano 99, 34012 Trieste, Italy
⁵ Dip. di Fisica, Università di Trieste, via Valerio 2, 34127 Trieste, Italy
⁶ INFN Pavia, via Bassi, 6 27100 Pavia, Italy
⁷ INAF IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁸ INAF IASF Milano, via E. Bassini 15, 20133 Milano, Italy
⁹ Dip. di Fisica, Università degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁰ ENEA C.R. “E. Clementel”, via Martiri di Monte Sole 4, 40129 Bologna, Italy
¹¹ INAF Osservatorio Astronomico di Cagliari, loc. Poggio dei Pini, strada 54, 09012 Capoterra (Ca), Italy
¹² Dip. di Fisica e Matematica, Università dell'Insubria, via Valleggio 11, 20100 Como, Italy
¹³ INAF IASF Palermo, via U. La Malfa 153, 90146 Palermo, Italy
¹⁴ Consorzio Interuniversitario per la Fisica Spaziale, Viale Settimio Severo 63, 10133 Torino, Italy
¹⁵ ASI Science Data Center, via G. Galilei, 00044 Frascati (Rm), Italy
¹⁶ INAF Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone (Rm), Italy
¹⁷ INAF staff resident at ASI Science Data Center
¹⁸ Agenzia Spaziale Italiana, Unità Osservazione dell'Universo, Viale Liegi 26, 00198 Roma, Italy

Received: 11 August 2009
Accepted: 30 March 2010

Abstract

Context. Cygnus X-1 (Cyg X-1) is a high mass X-ray binary system, known to be a black hole candidate and one of the brightest sources in the X-ray sky, which shows both variability on all timescales and frequent flares. The source spends most of the time in a hard spectral state, dominated by a power-law emission, with occasional transitions to the soft and intermediate states, where a strong blackbody component emerges.

Aims. We present the observation of Cyg X-1 in a hard spectral state performed during the AGILE science verification phase and observing cycle 1 in hard X-rays (with SuperAGILE) and gamma rays (with the gamma ray imaging detector) and lasting for about 160 days with a live time of ~6 Ms.

Methods. We investigated the variability of Cyg X-1 in hard X-rays on different timescales, from ~300 s up to one day, and we applied different tools of timing analysis, such as the autocorrelation function, the first-order structure function, and the Lomb-Scargle periodogram, to our data (from SuperAGILE) and to the simultaneous data in soft X-rays (from RXTE/ASM). We concluded our investigation with a search for emission in the energy range above 100 MeV with the maximum likelihood technique.

Results. In the hard X-ray band, the flux of Cyg X-1 shows its typical erratic fluctuations on all timescales with variations of about a factor of two that do not significantly affect the shape of the energy spectrum. From the first-order structure function, we find that the X-ray emission of Cyg X-1 is characterized by antipersistence (anticorrelation in the time series, with an increase in the emission likely followed by a decrease), indicative of a negative feedback mechanism at work. In the gamma ray data a statistically significant point-like source at the position of Cyg X-1 is not found, and the upper limit on the flux is 5 × 10^-8 ph cm^-2   s^-1 over the whole observation (160 days). Finally we compared our upper limit in gamma rays with the expectation of various models of the Cyg X-1 emission, both of hadronic and leptonic origin, in the GeV–TeV band.

Conclusions. The time history of Cyg X-1 in the hard X-ray band over 13 months (not continuous) is shown. Different analysis tools do not provide fully converging results of the characteristic timescales in the system, suggesting that the timescales found in the structure function are not intrinsic to the physics of the source. While Cyg X-1 is not detected in gamma rays, our upper limit is a factor of two lower than the EGRET one and is compatible with the extrapolation of the flux measured by COMPTEL in the same spectral state.