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

¹ Max-Planck-Institut fur Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
e-mail: Roberta.Zanin@mpi-hd.mpg.de
² Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), Campus UAB, 08193 Bellaterra (Barcelona), Spain
³ Institute of Space Sciences (CSIC-IEEC), 08193 Barcelona, Spain
⁴ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁵ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain

Received: 12 May 2016
Accepted: 16 September 2016

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.