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

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 LombScargle 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.

Gamma-ray binaries and related systems

“…In HE gamma rays, Malyshev et al (2013) and Bodaghee et al (2013) report a 3 to 4σ detection of steady gamma-ray emission from Cyg X-1 using Fermi/LAT observations. The source is detected only during the hard X-ray state, with a luminosity L(≥ 100 MeV) ≈ 2 × 10 33 erg s −1 for a 2 kpc distance, consistent with the upper limits derived using AGILE data (Del Monte et al 2010;Sabatini et al 2013). This is surprising because the non-thermal power-law tail detected at MeV energies is weaker during the hard state than in the soft state ( §5.1.1), so a direct extrapolation would have predicted a gamma-ray detection in the soft state.…”

“…The presence of a radio jet in the hard state, absent in the soft state, could be the key ingredient. Del Monte et al (2010) excluded the possibility of HE flares analogous to the MAGIC flare in the AGILE data, based on the extrapolation of the soft MAGIC spectrum (Γ VHE ≈ 3) to the AGILE range. However, episodic detections of Cyg X-1 are also reported by AGILE: over a timescale ≤ 1 day during a hard X-ray state (Sabatini et al 2010) and over a timescale ≤ 2 days during a hard-to-soft transition (Sabatini et al 2013).…”

Rolling and tumbling: status of the SuperAGILE experiment

“…With its wide FoV, SuperAGILE is simultaneously monitoring many Compact Galactic Sources and studying their flux variation over time. In particular, we observed the whole orbital period and three more pre-periastron passages of the high mass X-ray binary GX 301-2 17 for a total net observation time of about 3.7 Ms. We also studied the temporal properties of Cyg X-1 18 , applying for the first time the analysis of the first order structure function to a Galactic Source, and we found antipersistence in the source emission, when a flux increase in the past is probably followed by a decrease in the future, with a negative feedback mechanism. The techniques used in the study of GRBs allowed the localization of the much softer X-ray bursts, thermonuclear explosions on the surface of a neutron star.…”

GAMMA-RAY OBSERVATIONS OF CYGNUS X-1 ABOVE 100 MeV IN THE HARD AND SOFT STATES

“…The measurements of the mass for the compact object range from 4.8 to 14.8 M ⊙ (Herrero et al, 1995;Shaposhnikov & Titarchuk, 2007;Orosz et al, 2011), suggesting identification with a black hole. Being one of the brightest sources in the X-ray sky and having a persistent emission, the literature on the system is extremely rich and extensive monitoring in radio, IR, UV and X-rays has been carried out (Mirabel et al, 1996;Pooley et al, 1999;Fender et al, 2000;McConnell et al, 2002;Gallo et al, 2003;Pandey et al, 2006;Del Monte et al, 2010;Rahoui et al, 2011;Jourdain et al, 2012), leading to interesting correlations and being of great importance for understanding the process of accretion onto black holes in general.…”