ROSAT X-ray sources in the field of the LMC

I. Total LMC gas from the background AGN spectral fits

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: 17 November 2000
Accepted: 14 March 2001

Abstract

We analyzed a sample of 26 background X-ray sources in a ~60 square degree field of the Large Magellanic Cloud observed with the ROSAT PSPC. The sample has been selected from previously classified and optically identified X-ray sources. In addition, pointlike and spectrally hard sources with at least 100 to 200 observed counts have been used for the analysis. We performed X-ray spectral fitting and derived total hydrogen absorbing column densities due to LMC gas in the range -. We compared these columns with the H i columns derived from a 21-cm Parkes survey of the LMC. For 7 optically identified sources we find, within the uncertainties derived from the X-ray spectral fit, agreement for both columns. For a further 19 sources, we constrain the LMC columns from the X-ray spectral fit assuming that the power law photon index is that of AGN type spectra. We derive for 20 sources gas columns which are within the uncertainties, in agreement with the H i columns. We derive for two background sources (RX J0536.9-6913 and RX J0547.0-7040) hydrogen absorbing column densities due to LMC gas, which are in excess compared to the H i columns. These sources -located in regions of large (~) LMC H i column densities -could be seen through additional gas which may be warm and diffuse, cold or molecular. For 10 sources we derive upper limits for the gas columns additional to H i and constrain the molecular mass fraction to <(30-140)% .