Gamma rays detected from Cygnus X-1 with likely jet origin

Abstract

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 7.5 yr 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 5.5 × 10³³ erg s^-1, above 60 MeV. 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 20 GeV 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 10¹¹-10¹³ cm from the black hole.