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Satoko Osone, Kazuo Makishima, Keiichi Matsuzaki, Yoshitaka Ishisaki, Yasushi Fukazawa, Search for Hot Gas in the Local Group with ASCA, Publications of the Astronomical Society of Japan, Volume 54, Issue 3, 25 June 2002, Pages 387–392, https://doi.org/10.1093/pasj/54.3.387
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Abstract
An X-ray study was made to examine whether some part of the soft X-ray background is coming from hot gas in the Local Group. For this purpose, four consecutive pointings were made with ASCA toward a sky region between M31 and M33, which is close to the direction of the center of the Local Group. By comparing the X-ray surface brightness in this sky direction with that in another blank sky region near the north equatorial pole, an upper limit on any soft excess X-ray background was determined to be |$2.8 \times 10^{-9} \,\mathrm{erg} \,\mathrm{cm}^{-2} \,\mathrm{s}^{-1} \,\mathrm{sr}^{-1}$| with a 90% confidence level statistical error. Assuming an optically-thin thermal bremsstrahlung energy spectrum (Raymond–Smith model) for a temperature of 1 keV and a |$\beta$|-model electron density distribution for a core radius of 100 kpc for the X-ray halo, the upper limit of the central plasma density was obtained to be |$1.3 \times 10^{-4} \,\mathrm{cm}^{-3}$|. The plasma column density is too low to contribute significantly to the observed quadrupole anisotropy in the cosmic microwave background.
