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

E. Del Monte, M. Feroci, Y. Evangelista, E. Costa, I. Donnarumma, I. Lapshov, F. Lazzarotto, L. Pacciani, M. Rapisarda, P. Soffitta, A. Argan, G. Barbiellini, F. Boffelli, A. Bulgarelli, P. Caraveo, P.W. Cattaneo, A. Chen, F. D'Ammando, G. Di Cocco, F. FuschinoM. Galli, F. Gianotti, A. Giuliani, C. Labanti, P. Lipari, F. Longo, M. Marisaldi, S. Mereghetti, E. Moretti, A. Morselli, A. Pellizzoni, F. Perotti, G. Piano, P. Picozza, M. Pilia, M. Prest, G. Pucella, A. Rappoldi, S. Sabatini, E. Striani, M. Tavani, M. Trifoglio, A. Trois, E. Vallazza, S. Vercellone, V. Vittorini, A. Zambra, L.A. Antonelli, S. Cutini, C. Pittori, B. Preger, P. Santolamazza, F. Verrecchia, P. Giommi, L. Salotti

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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 × 10-8ph cm-2s-1over 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. © 2010 ESO.
Original languageEnglish
Article numberA67
Pages (from-to)-
JournalAstronomy and Astrophysics
Volume520
Issue number10
DOIs
Publication statusPublished - 2010
Externally publishedYes

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All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Del Monte, E., Feroci, M., Evangelista, Y., Costa, E., Donnarumma, I., Lapshov, I., ... Salotti, L. (2010). A year-long AGILE observation of Cygnus X-1 in hard spectral state. Astronomy and Astrophysics, 520(10), -. [A67]. https://doi.org/10.1051/0004-6361/200913104