Search for hot gas in the local group with ASCA Original Research Article

An X-ray study was made to examine whether some part of the soft X-ray background is coming from hot gas halo that might be associated with the Local Group. For this purpose, four consecutive pointings were made with ASCA onto a general sky region in between M31 and M33, which is thought to be close to the direction of the Local Group Center. After removing point sources with 2–10 keV fluxes exceeding 2 × 10−13 erg s−1 cm−2, the X-ray background brightness in this direction was quantified in terms of two (steep and flat) power-law components. By comparing the soft-component brightness with that in another blank sky region near the equatorial pole, any excess soft X-ray background coming from this M31/M33 region was determined to be ≤ 1.27 × 10−8 erg s−1 cm−2 str−1. Assuming a Raymond-Smith type spectrum of temperature 0.3-1.2 keV and a β-model density distribution for the X-ray halo, the central plasma density N0 and core radius Rc for the β model was constrained as [ N010−3cm−3]2 [Rc100kpc] ≤ 5.80×10−3. The implied plasma column density is too low to cause any noticeable quardrupole anisotropy in the cosmic microwave backbround.