REANALYSIS OF DATA TAKEN BY THE CANGAROO 3.8 METER IMAGING ATMOSPHERIC CHERENKOV TELESCOPE: PSR B1706–44, SN 1006, AND VELA

We have reanalyzed data from observations of PSR B1706–44, SN 1006, and the Vela pulsar region made with the CANGAROO 3.8 m Imaging Atmospheric Cherenkov Telescope between 1993 and 1998 in response to the results reported for these sources by the H.E.S.S. Collaboration. Although detections of TeV gamma-ray emission from these sources were claimed by CANGAROO more than 10 years ago, upper limits to the TeV gamma-ray signals from PSR B1706–44 and SN 1006 derived by H.E.S.S. are about an order of magnitude lower. The H.E.S.S. group detected strong diffuse TeV gamma-ray emission from Vela but with a morphology differing from the CANGAROO result. In our reanalysis, in which gamma-ray selection criteria have been determined exclusively using gamma-ray simulations and off-source data as background samples, no significant TeV gamma-ray signals have been detected from compact regions around PSR B1706–44 or within the northeast rim of SN 1006. The upper limits to the integral gamma-ray fluxes at the 95% confidence level have been estimated for the 1993 data of PSR B1706–44 to be F(>3.2 ± 1.6 TeV) < 8.03 × 10−13 photons cm−2 s−1, for the 1996 and 1997 data of SN 1006 to be F(>3.0 ± 1.5 TeV) < 1.20 × 10−12 photons cm−2 s−1 and F(>1.8 ± 0.9 TeV) < 1.96 × 10−12 photons cm−2 s−1, respectively. We discuss reasons why the original analyses gave the source detections. The reanalysis did result in a TeV gamma-ray signal from the Vela pulsar region at the 4.5σ level using 1993, 1994, and 1995 data. The excess was located at the same position, 0.°13 to the southeast of the Vela pulsar, as that reported in the original analysis. We have investigated the effect of the acceptance distribution in the field of view of the 3.8 m telescope, which rapidly decreases toward the edge of the field of the camera, on the detected gamma-ray morphology. The expected excess distribution for the 3.8 m telescope has been obtained by reweighting the distribution of HESS J0835–455 measured by H.E.S.S. with the acceptance of the 3.8 m telescope. The result is morphologically comparable to the CANGAROO excess distribution, although the profile of the acceptance-reweighted H.E.S.S. distribution is more diffuse than that of CANGAROO. The integral gamma-ray flux from HESS J0835–455 has been estimated for the same region as defined by H.E.S.S. from the 1993–1995 data of CANGAROO to be F(>4.0 ± 1.6 TeV) = (3.28 ± 0.92) × 10−12 photons cm−2 s−1, which is statistically consistent with the integral flux obtained by H.E.S.S.