Appendix A: X-ray sources in a deep merged observation of the northern field of the supergiant shell LMC 4

In the previous analysis in this paper I have made use of the X-ray sources given in the catalog of HP99. Part of the LMC region has been observed during several observations and a large integrated exposure exists for these fields. In particular I investigated the field of the northern area of the supergiant shell LMC 4 to find out how many X-ray sources can be detected in deep merged observations of this specific field.

LMC 4 is the northern and largest (with a diameter of $\sim$1200 pc) of five supergiant shells in the LMC which are characterized by circular regions of filamentary H  II emission (Meaburn 1980). McGee & Milton (1966) noted the existence of a deep minimum in the column density of neutral hydrogen in LMC 4 in their 21-cm H  I observations. Inside this hole is the stellar association Shapley III (Nail & Shapley 1953).

I merged the observations existing for the field in the northern area of the supergiant shell LMC 4 making use of source detection routines (local, map, and maximum likelihood) which are available in EXSAS. I applied the local, map, and the maximum likelihood source detection task to one energy band (0.5-2.4 keV) to derive the catalog of sources. 97 X-ray sources were detected in the 1.8 square degree field. I accepted only sources with a likelihood ratio of existence >8 and checked the reality of the sources on a displayed image. The catalog of these sources is given in Table 3. Column 1 gives the source index, Col. 2 the ROSAT source name, Cols. 3 and 4 the source position, the right ascension (RA) and the declination (Dec) for the epoch J2000 with the 90% confidence positional uncertainty (Col. 5), the count rate (0.1-2.4 keV, Col. 6), the hardness ratios $H\!R1$ and $H\!R2$ (Col. 7 and 8), the source extent in arcsec in case the extent likelihood ratio is >10 (Col. 9), the likelihood ratio of existence $L_{\rm exi} = -\ln(P)$, with P the probability that the detected source is due to excess counts measured above a mean local background (Col. 10), the distance of the source from the center of the field of the merged observation (Col. 11), the source index from the catalog of HP99 (Col. 12), the distance to that source in arcsec (Col. 13), the classification of the source (with A = AGN, B = X-ray binary, R = supernova remnant, F = foreground star) in Col. 14 and remarks in Col. 15.

The exposure time of the merged observation varies over a large range and has a region of high exposure ($\sim$50 to 70 ksec) in one merged PSPC pointing. The limiting flux for the sources detected in this merged observation and given in the catalog of Table 3 is $8\times 10^{-15}\ {\rm erg}\ {\rm cm^{-2}}\ {\rm s^{-1}}$ assuming a powerlaw spectrum with a photon index of $-\Gamma=2$ and a galactic absorbing column density of $5\times 10^{20}\ {\rm cm}^{-2}$.

If one considers the location of the sources detected in this field with accurate values for the hardness ratios $\delta H\!R1 \le 0.20$ and $\delta H\!R2\ \le 0.20$ in the hardness ratio $H\!R1$ - $H\!R2$ plane then one finds that most of the sources are located in the region bounded by the powerlaw tracks $-\Gamma=1.0$, 2.0 and 3.0 and are consistent with AGN and X-ray binaries (cf. Sect. 4). A few bright sources which are located outside this region are (consistent with) supernova remnants.

A fraction of the detected sources is contained in the sample of sources investigated in the previous sections and is also given in Table 1. 60 sources correlate within a search radius of 20$\arcsec$ with a source in the catalog of Haberl & Pietsch (1999). I made a tentative classification of the 97 X-ray sources detected in the field of the supergiant shell LMC 4 from the location of these sources in the $H\!R1$ - $H\!R2$ plane (cf. Sect. 4.1). I classified 35 of the sources with more than 30 observed counts as AGN (or likely AGN). This low threshold in counts has been chosen as the X-ray survey in the field of the Supergiant Shell LMC 4 was considerably deeper than the X-ray survey of the general LMC field. In Paper III it will be made use of this AGN sample to derive the $\log N - \log S$ of background X-ray sources in the field of the Supergiant Shell LMC 4.

Acknowledgements

The ROSAT project is supported by the Max-Planck-Gesellschaft and the Bundesministerium für Forschung und Technologie (BMFT). This research has made use of the SIMBAD data base operated at CDS, Strasbourg, France. I thank C. Brüns for making available the Parkes 21-cm map of the galactic and LMC H  I in the field of the LMC. I thank J. Kerp for comments on an earlier version of the manuscript. I thank K. S. de Boer for suggestions to improve the article. I thank an anonymous referee for the suggestions to improve the manuscript. PK is supported by the Graduiertenkolleg on the "Magellanic Clouds and other Dwarf galaxies'' (DFG GRK 118).