Search for very high energy gamma ray emission from Hercules X-1

Abstract

Data from the Haleakala Gamma-ray Observatory taken during 1987 and 1989 is searched for the presence of TeV Gamma-rays from r he direction of Hercules X-1. These two data sets were compiled on two different instruments: the first in operation from 1985 through the end of 1987 and the second, upgraded instrument in operation from early 1989 to the present. After the initial data selection and calibration, the two data segments were analyzed for periodicity near the interpolated pulse period, a counting rate (or DC) excess during short intervals, and an integrated DC excess over longer periods of time. The 1987 data set revealed one interval of significant periodicity. The detected period, 1.23578 ±0.00020, s, is not in agreement with the expected X-ray pulse period, but it is in agreement with periodicity observed from Her X-1 by several groups during 1986 in the TeV - PeV energy range. No significant rate excess was observed during this interval. A integrated DC excess calculation was not possible on these data because of the mode in which the data were taken. The 1989 data set contained no periodicity; however, an integrated excess corresponding to two of the three high on X-ray intervals during which data was taken was observed. The overall flux observed during this year was (5.9 ± 2.6) X 10^-11 cm^-2s^-1 above 0.15 TeV. No significant short (15 minute) bursts of VHE gamma-rays were detected in this data set.