Astronomy and Astrophysics, volume 596A, 55-55 (2016/12-1)
Gamma rays detected from Cygnus X-1 with likely jet origin.
ZANIN R., FERNANDEZ-BARRAL A., DE ONA WILHELMI E., AHARONIAN F., BLANCH O., BOSCH-RAMON V. and GALINDO D.
Abstract (from CDS):
Aims. We probe the high-energy (>60 MeV) emission from the black hole X-ray binary system, Cygnus X-1, and investigate its origin. Methods. We analyzed 7.5yr of data by Fermi-LAT with the latest Pass 8 software version. Results. We report the detection of a signal at ∼8σ statistical significance that is spatially coincident with Cygnus X-1 and has a luminosity of 5.5x1033erg/s, above 60MeV. The signal is correlated with the hard X-ray flux: the source is observed at high energies only during the hard X-ray spectral state, when the source is known to display persistent, relativistic radio-emitting jets. The energy spectrum, extending up to ∼20GeV without any sign of spectral break, is well fit by a power-law function with a photon index of 2.3±0.2. There is a hint of orbital flux variability, with high-energy emission mostly coming around the superior conjunction. Conclusions. We detected GeV emission from Cygnus X-1 and probed that the emission is most likely associated with the relativistic jets. The evidence of flux orbital variability indicates the anisotropic inverse-Compton on stellar photons as the mechanism at work, thus constraining the emission region to a distance 1011-1013cm from the black hole.