Statistics
Scientific paper
Sep 1969
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1969phdt.........9s&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, SAN DIEGO, 1969.Source: Dissertation Abstracts International, Volume: 30-11, Section:
Statistics
20
Scientific paper
The Third Orbiting Solar Observatory (OSO-III) carried an X-ray detector collimated to a 23 deg. full width at half maximum aperture, and sensitive to photons between 7.7 and 210 keV. I have analyzed the data in five logarithmically spaced energy channels between 7.7 and 110 keV to study the spectrum and angular distribution of the diffuse component of cosmic X-rays. (I use the term "diffuse," although this experiment cannot eliminate the possibility that the X-rays actually emanate from numerous discrete sources.) Because this single instrument has scanned almost the entire sky, I can confidently look for relative angular variations even in the presence of systematic errors. I subtract the following sources of background from the raw count rates: phototube noise, counts due to cosmic ray induced gamma-rays, and counts due to gamma-rays associated with the beta-decay of Na24. At 10 keV about 98 percent of the counts represent diffuse X-rays; at 85 keV only about 25 percent remain after background correction. The programs eliminate a large portion of the data because of possible contamination by charged particles, albedo X-rays, solar X-rays, and X-rays from the discrete galactic sources. A program compiles counts within 30 deg. of the galactic poles to study the energy spectrum. Considering photon counting statistics and estimated errors in the background correction, the four lowest channels have standard deviations from 1.5 to 3 percent, and 14 percent for the 65-110 keV channel. A power law with a constant index cannot represent the data. Considering the detector efficiency and resolution, the lowest three channels fit a power law of index 1.7 +/- 0.1 with a normalization constant 16.7 photons (keV)0.7 (cm2 sec ster)-1. A discontinuity of 0.6 to 1.5 in the index between 32 and 46 keV can then fit the two highest channels. After avoiding all known galactic discrete sources, only 20 deg. strips around longitudes 150 deg. and 230 deg. remain on the galactic plane. These areas have an excess emission in the lowest channel of 0.012 +/- 0.004 photons (cm2 sec rad)-1 at 10 keV, if interpreted as a line source. The present experiment cannot distinguish between a point or a line source. No other anisotropies appear in the data. Whether the diffuse component arises from inverse Compton scattering in intergalactic space, or from the integrated effect of individual galaxies, sources at distances of the order of the Hubble radius must contribute significantly. Limits as low as five percent to inhomogeneities or to a 12-hour periodic component apply to much of the sky. The limit of one percent to the 24-hour period in any direction corresponds to an upper limit of 800 km/sec for the earth's resultant velocity relative to any rest frame in which the X-rays appear isotropic. The lack of lumpiness sets a lower limit of 100,000 to the number of necessary discrete sources in the sky, if discrete sources produce the measured diffuse X-rays.
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