Search for Hot Gas in the Local Group with ASCA

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 M 31 and M 33, 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$\times10^{-9}$ erg cm$^{-2}$ s$^{-1}$ 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$\times10^{-4}$ cm$^{-3}$. The plasma column density is too low to contribute significantly to the observed quadrupole anisotropy in the cosmic microwave background.Comment: 21pages, 6 figures, paper will be published to PASJ Vol.54, No.

The Arrival Direction Distribution of Extremely High Energy Cosmic Rays Observed by AGASA

Study of the arrival direction distribution plays a key role in finding sources of the Extremely High Energy Cosmic Rays (EHECR). Greisen-Zatsepin-Kuz\u27min (GZK) mechanism limits the propagation of EHE cosmic rays significantly and only nearby sources can contribute to the local EHECR flux. Furthermore EHECRs travel intergalactic spaces almost linearly. Hence, we expect they can be traced back to their sources. Here we report a small-sacle anisotropy of the arrival direction distribution of EHECRs observed by AGASA above 1019eV . This result strongly suggests the existence of point sources of EHECRs