Detection of gamma rays of likely jet origin in Cygnus X-1

TL;DR

This study reports the detection of high-energy gamma-ray emission from Cygnus X-1, likely originating from its relativistic jets, with emission correlated to the hard X-ray state and showing orbital variability.

Contribution

First detection of GeV gamma-ray emission from Cygnus X-1, linking it to jet activity and constraining the emission region based on orbital variability.

Findings

Gamma-ray emission detected at 8 sigma significance

Emission correlates with hard X-ray spectral state

Orbital flux variability suggests anisotropic inverse Compton process

Abstract

Aims: Probe the high-energy ($>$60 MeV) emission from the black hole X-ray binary system, Cygnus X-1, and investigate its origin. Methods: We analysed 7.5 yr of data by Fermi/LAT with the latest PASS8 software version. Results: We report the detection of a signal at $\sim$8 $\sigma$ statistical significance spatially coincident with Cygnus X-1 and a luminosity above 60 MeV of 5.5$\times$10$^{33}$ erg s$^{-1}$. 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 $\sim$20 GeV without any sign of spectral break, is well fitted by a power-law function with a photon index of 2.3$\pm$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 points to the anisotropic inverse Compton on stellar photons as the mechanism at work, thus constraining the emission region to a distance $10^{11}-10^{13}$ cm from the black hole.