Abstract
An X-ray pulsar has been detected for the first time as a very high-energy (VHE) γ-ray pulsar. X-ray emission from Hercules X-1 is multiply periodic with a pulsar period of 1.24 s, a binary orbital period of 1.7 day and a 35-day amplitude modulation of unknown origin characterized by a sharp turn-on followed by a slow decline over 11 days. During a drift-scan we have detected a significant 3-min outburst of VHE γ rays, which had a 1.24-s periodicity. This outburst occurred 35 days before an observed X-ray turn-on. The γ-ray luminosity during the outburst was of the same order as the X-ray luminosity. Together with the coincidence in time, this suggests a connection between the mechanisms giving rise to the γ-ray outburst and the X-ray turn-on. Subsequent monitoring of the source during a period when the X-ray flux was decreasing yielded some evidence of much weaker pulsed γ-ray activity at the pulsar period. We note a possible similarity between this system and Cygnus X-3.
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