Very high energy gamma-ray radiation from the stellar mass black hole binary Cygnus X-1

We report on the results from the observations in very high energy band (VHE, E_gamma > 100 GeV) of the black hole X-ray binary (BHXB) Cygnus X-1. The observations were performed with the MAGIC telescope, for a total of 40 hours during 26 nights, spanning the period between June and November 2006. Searches for steady gamma-ray signals yielded no positive result and upper limits to the integral flux ranging between 1 and 2% of the Crab nebula flux, depending on the energy, have been established. We also analyzed each observation night independently, obtaining evidence of gamma-ray signals at the 4.0 standard deviations (sigma) significance level (3.2 sigma after trial correction) for 154 minutes effective on-time (EOT) on September 24 between 20h58 and 23h41 UTC, coinciding with an X-ray flare seen by RXTE, Swift and INTEGRAL. A search for faster-varying signals within a night resulted in an excess with a significance of 4.9 sigma (4.1 sigma after trial correction) for 79 minutes EOT between 22h17 and 23h41 UTC. The measured excess is compatible with a point-like source at the position of Cygnus X-1, and excludes the nearby radio nebula powered by its relativistic jet. The differential energy spectrum is well fitted by an unbroken power-law described by dN/(dA dt dE) = (2.3+- 0.6) x 10^{-12} (E/1 TeV)^{-3.2 +- 0.6}. This is the first experimental evidence of VHE emission from a stellar-mass black hole, and therefore from a confirmed accreting X-ray binary.
4 ApJL-formatted pages, 4 figures. Revised version accepted for publication at ApJL (26-06-2007)