ROSAT X-ray sources in the field of the LMC

III. The log – log of background AGN and the LMC gas

¹ Sternwarte, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Corresponding author: P. Kahabka, pkahabka@astro.uni-bonn.de

Received: 3 September 2001
Accepted: 22 March 2002

Abstract

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 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   map derived from observations with the Parkes radio telescope). We compare the observed relation with a theoretical relation of the soft extragalactic X-ray background (SXRB) which comprises an AGN and a cluster of galaxy contribution. We find that the observed has a deficiency with respect to the theoretical . 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 in the LMC field from the derived from a large sample of AGN in several fields in the sky, (3) the existence of gas additional to the  represented in the Parkes  map of the LMC field, restricted to the high column ≥ 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 in the field of the LMC from a mean theoretical , 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   represented in the Parkes  map would be responsible for the deficiency and if this additional gas is restricted to the high column ≥ 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 at 90% confidence of additional gas would be required which, if purely molecular, would be equal to a molecular mass fraction of 63%. Such a value would be larger than but within the uncertainties consistent with a molecular mass fraction of ~ 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  for the high column regime of the LMC gas is likely to be required to explain the observed . 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 of the SXRB in the field of the LMC.