Reanalysis of Data Taken by the CANGAROO 3.8 Meter Imaging Atmospheric Cherenkov Telescope: PSR B1706-44, SN 1006, and Vela

T. Yoshikoshi, M. Mori, P. G. Edwards, S. Gunji, S. Hara, T. Hara, A. Kawachi, Y. Mizumoto, T. Naito, K. Nishijima, T. Tanimori, G. J. Thornton, T. Yoshida

(Astrophys. J. in press)
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.