Very Deep X-ray Observations of Three Globular Clusters

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Session 9 -- Galactic X-Ray/Gamma-Ray Sources
Display presentation, Monday, 30, 1994, 9:20-6:30

[9.06] Very Deep X-ray Observations of Three Globular Clusters

B. Margon, E. Deutsch, A. Silber (Univ. Wash.), W. H. G. Lewin (MIT), J. Van Paradijs, M. Van der Klis (Univ. Amsterdam)

The ROSAT PSPC has been used for long soft X-ray exposures of three globular clusters, in an effort to clarify the nature of the low luminosity cluster X-ray sources. The clusters 47 Tuc, NGC 6205 (M13), and NGC 6341 (M92) were observed for 61 ks, 46 ks, and 46 ks, respectively. A primary goal was to determine if a large population of cluster sources, possibly associated with cataclysmic variable stars, lurks just beneath the flux threshold of previous Einstein observations.

All three images show several dozen sources, but in all cases this number is consistent with that expected from from AGN and stellar corona sources in any random ROSAT field of this exposure depth. In the region more than a few core radii from the center of each cluster, we can rule out the existence of any significant population (more than a few) of cluster sources of luminosity in excess of $10^{32}$~erg~s$^{-1}$ in the 0.1 -- 2.4 keV band.

Within the inner one core radius, the situation is more complex. In 47 Tuc we detect the well-known central source first seen by Einstein, and resolved by the ROSAT HRI into multiple objects. Our observations suggest complex structure for the central region of this cluster, perhaps implying the existence of even more faint sources than yet resolved. Both NGC 6205 and NGC 6341 have a single faint ($10^{33}$~erg~s$^{-1}$) but well-detected source at a position consistent with the cluster center, although the modest spatial resolution of the PSPC implies that offsets of $10-30''$ are also possible. Of course these two sources may also prove to be multiple when observed with higher angular resolution. The steadily growing number of globular clusters with low luminosity central sources implies that a few of these objects may be common in most cluster cores.

Spectral information is presented for the central sources. There is strong evidence that many or most of the spectra are considerably softer than the values of $\sim$5~keV previously arbitrarily assumed.

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