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

Abstract

We present the observation of Cyg X-1 in 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 GRID) and lasting for about 160 days with a live time of $\sim 6$ Ms. We investigate the variability of Cyg X-1 in hard X-rays at different timescales, from $\sim 300$ s up to one day, and we apply 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 conclude our investigation with a search for emission in the energy range above 100 MeV with the maximum likelihood technique. In the hard X-ray band the flux of Cyg X-1 shows its typical erratic fluctuations at all timescales with variations of about a factor of two that do not affect significantly 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 \textit{antipersistence}, indication 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 $5\times 10^{-8}\mathrm{ph}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, over the whole observation (160 days). Finally we compare our upper limit in gamma rays with the expectation of various models of the Cyg X-1 emission, of both hadronic and leptonic origin, in the GeV -- TeV band. The time history of Cyg X-1 in the hard X-ray band over 13 months (not continuous) is shown. Different tools of analysis 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.

15 pages and 11 figures (with subfigures). Accepted for publication by A&A.

References in corpus (10)

• SuperAGILE: the hard X-ray Imager for the AGILE space mission
• A transient relativistic radio jet from Cygnus X-1
• AGILE detection of a strong gamma-ray flare from the blazar 3C 454.3
• Episodic Transient Gamma-Ray Emission from the Microquasar Cygnus X-1
• The Ultraviolet Spectrum and Physical Properties of the Mass Donor Star in HD 226868 = Cygnus X-1
• High-energy emission from jet-clump interactions in microquasars
• Leptonic secondary emission in a hadronic microquasar model
• An intense state of hard X-ray emission of Cyg X-1 observed by INTEGRAL coincident with TeV measurements
• A model of the TeV flare of Cygnus X-1: electron acceleration and extended pair cascades
• Monitoring the hard X-ray sky with SuperAGILE

Cited by in corpus (4)

• Gamma-ray binaries and related systems
• Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states
• High energy gamma-ray emission from compact galactic sources in the context of observations with the next generation Cherenkov Telescope Arrays
• Expected gamma-ray emission from x-ray binaries