A model of the TeV flare of Cygnus X-1: electron acceleration and extended pair cascades

TL;DR

This paper models TeV photon emission from Cygnus X-1 during a flare, exploring electron acceleration near the black hole and the development of extended pair cascades to explain observed spectra.

Contribution

It introduces a comprehensive model combining electron acceleration and pair cascades to explain TeV emission from Cygnus X-1 during flares.

Findings

Emission site can be close to the black hole with low optical depth.

Extended pair cascades occur at energies >3 TeV, explaining the spectrum.

Optical depth varies with photon energy, affecting photon propagation.

Abstract

We consider theoretical models of emission of TeV photons by Cyg X-1 during a flare discovered by the MAGIC detector. We study acceleration of electrons to energies sufficient for TeV emission, and find the emission site is allowed to be close to the black hole. We then consider pair absorption in the photon field of the central X-ray source and a surrounding accretion disc, and find its optical depth is <1, allowing emission close to the black hole. On the other hand, the optical depth in the stellar field is about 10 at 1 TeV. However, the optical depth drops with increasing energy, allowing a model with the initial energy of >3 TeV, in which photons travel far away from the star, initiating a spatially extended pair cascade. This qualitatively explains the observed TeV spectrum, though still not its exact shape.