SIMBAD references

We use a sample of 50 background X-ray sources (AGN) and candidate AGN in the field of the LMC observed with more than 50 counts in archival ROSAT PSPC observations to derive the observed logN-logS relation. We correct for the inhomogenous ROSAT PSPC exposure and the varying absorption due to the galactic and the LMC gas (for which we used an HI map derived from observations with the Parkes radio telescope). We compare the observed logN-logS relation with a theoretical logN-logS relation of the soft extragalactic X-ray background (SXRB) which comprises an AGN and a cluster of galaxy contribution. We find that the observed logN-logS has a deficiency with respect to the theoretical logN-logS. There are several factors which can account for such a deficiency: (1) incompleteness of the selected AGN and cluster of galaxies sample, (2) deviation of the theoretical logN-logS in the LMC field from the logN-logS derived from a large sample of AGN in several fields in the sky, (3) the existence of gas additional to the HI represented in the Parkes HI map of the LMC field, restricted to the high column ≥10²¹cm^–2 regime. We investigate the likely contribution of these effects and find that (1) a fraction (of at most ∼30%) of the AGN and clusters of galaxies in the LMC field may not have been found in our analysis and may contribute to the observed deficiency. The existence of extended regions with hot diffuse gas and source crowding makes the detection of all AGN and clusters of galaxies very difficult. (2) We cannot exclude a deviation of the logN-logS in the field of the LMC from a mean theoretical logN-logS, especially the cluster of galaxy contribution which is of importance in the flux range we are comprising may show variations across the sky. (3) If LMC gas in addition to the HI represented in the Parkes HI map would be responsible for the deficiency and if this additional gas is restricted to the high column ≥10²¹cm^–2 regime, and assuming that the metallicity of the ISM of the LMC is -0.3 dex lower than the metallicity of the galactic ISM, then a factor of 1.9±^3.3_1.6 at 90% confidence of additional gas would be required which, if purely molecular, would be equal to a molecular mass fraction of 63±²⁰₄₂%. Such a value would be larger than but within the uncertainties consistent with a molecular mass fraction of ∼30% derived from CO observations for the high column regime of the LMC gas. From this analysis, it follows that some gas additional to the measured HI for the high column regime of the LMC gas is likely to be required to explain the observed logN-logS. But the amount of such additional gas is dependent on the completeness of our selected AGN and clusters of galaxies sample and on the assumptions made about the description of the logN-logS of the SXRB in the field of the LMC.