Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1972
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1972saosr.341.....e&link_type=abstract
SAO Special Report #341 (1972)
Astronomy and Astrophysics
Astronomy
6
Scientific paper
The feasibility of using atmospheric fluorescence as an observational technique for X-ray astronomy has been investigated. X-rays are totally absorbed by the Earth's atmosphere, causing fluorescence emission, proportional to the total amount of X-ray energy absorbed, in the 1N (0,0) band (3914 Å) of N+2. The process of fluorescence emission occurs in less than 100 ns, and because quenching of the 1N bands occurs at altitudes below 80 km, X-rays in the 0.7-15 KeV energy range are the most efficient in producing atmospheric fluorescence. The fluorescent light can be detected from a ground-based station, providing a cheap method of observing X-ray sources. Here we have investigated the possibility of detecting only X-ray sources. whose flux varies on a time scale of one second or shorter. The sensitivity of the fluorescence technique is limited by the shot noise of the night-sky background light. A station to detect atmospheric fluorescence caused by cosmic X-rays was set up at Agassiz Station, Massachusetts, and then moved to Mt. Hopkins, Arizona. A thorough study of light pulses that could be confused with fluorescence was made. The sources of background pulses that were studied are meteor trails, stellar scintillations, man-made light, lightning, the xenon flashlamp, and Čerenkov light from air showers. New types of 100 μ sec light pulses, previously reported by Ögelman et al. (1971), have been observed. Ögelman et al. have suggested a geophysical origin for one type of these pulses. Arguments against this suggestion are presented and evidence is given for a man-made origin. The present fluorescence station lacks the sensitivity to detect X-rays from any presently known source of X-rays, such as the Crab pulsar, but pilot searches have been made for energetic bursts of X-rays, perhaps originating in a supernova explosion, on time scales of 100 μsec and 1 sec. The lack of definite fluorescence events in the search for 1 sec pulses allows us to place an upper limit of 5 X 10-4 sec-1 on the flux, at the Earth, of 0.7-15 keV X-ray pulses with a total energy greater than 630 MeV cm-2. That no definite fluorescence pulses were detected in the 100 μsec search allow us to place an upper limit of 3 X 1047 ergs, in the form of 0.7-15 keV X-rays, released by a supernova explosion on a time scale of 100 μsec or less. This limit is uncertain because of the 100 μsec background pulses. For the above energy parameters, the present station was sensitive (in the 100μsec pulse search) to supernova explosions occurring at distances less than 65 Mpc. It is concluded that the fluorescence technique is well suited for searching for energetic, but infrequent, pulses of X-rays, and improved techniques are suggested for future searches of this type.
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