Other
Scientific paper
Jun 1971
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1971saosr.335.....h&link_type=abstract
SAO Special Report #335 (1971)
Other
2
Scientific paper
The gas-Čerenkov balloon-borne cosmic gamma ray detector described here has a variable energy threshold and is sensitive down to 15 MeV with a half-angle resolution of 5° to 7°. Gamma rays are detected by a coincidence between light pulses in a scintillator at the top of the detector and pulses of Čerenkov light collected by a mirror at the bottom of the detector. Gamma rays convert or Compton scatter in the scintillator, and the resulting electrons emit Čerenkov radiation as they move through the 2 m gas column between the scintillator and the mirror. The detector's efficiency and angular response were calculated by a Monte Carlo simulation and checked experimentally by a calibration with gamma rays and electrons. The detector was flown in a balloon from Palestine, Texas, on 29 September 1970 and observed M87, the Crab Nebula, and the atmospheric gamma ray background. The background measurements, combined with other measurements at higher energies, show a flattening of the atmospheric gamma ray spectrum below 50 MeV, caused by the superposition of a pion-produced gamma ray spectrum peaked at 70 MeV on a power law bremsstrahlung spectrum. No gamma ray flux was seen from M87 with a 95% confidence upper limit of Fγ (> 15 MeV) < 2.7 X 10-4 γ cm-2 sec-1. No statistically significant flux was detected from the direction of the Crab Nebula, although a 1σ excess of gamma r ray counts was observed. The corresponding 95% confidence upper limit to the total gamma ray flux from the Crab is Fγ (> 15 MeV) < 4.7 X 10-4γcm-2 sec-1. The gamma rays from the direction of the Crab were phase analyzed for the period of the pulsar NP 0532, but no pulsed structure was observed, setting a 95% confidence upper limit on the pulsed gamma ray flux from NP 0532 of Fγ (> 15 MeV) < 1.4 X 10-4 γcm-2 sec-1. Further balloon flights are planned, with improvements in the detector which should increase its sensitivity at leat threefold.
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