A possible very high energy gamma-ray burst from Hercules X-1

Abstract

We observed a large increase in the trigger rate in the direction of Hercules X-1 in the Atmospheric Cerenkov array at Pachmarhi, India. The burst lasted from 2147 UT to 2201 UT on 1986 April 11. The accidental coincidence rate did not show any increase during the burst. Barring any electronic noise or celestial or terrestrial optical phenomenon with time structure similar to that of atmospheric Cerenkov phenomenon, we ascribe the increase to TeV gamma rays from Her X-1. The number of gamma-ray events during the burst amounted to ∼54% of the cosmic-ray flux, resulting in a 42σ effect. This is the largest TeV gamma-ray signal seen from any source till now. The time-averaged flux for the burst period is 1.8 × 10^-8 photons cm^-2 s^-1 above a threshold energy of 0.4 TeV, which results in a luminosity of 1.8 × 10³⁷ ergs s^-1. The burst took place at the end of the "high on" state in the 35 day cycle of the Her X-1 binary system indicating accretion disk as the possible production site.