X-ray study of star-forming region NGC 6357

Inside the brightest constituent of this large HII region complex is the massive open cluster Pismis 24. It contains two of the brightest and bluest stars known, yet remains poorly studied; only a handful of optically bright stellar members have been identified.

We investigate the cluster extent and initial mass function and detect ~800 X-ray sources with a limiting sensitivity of ~10^30^ergs/s; this provides the first reliable probe of the rich intermediate-mass and low-mass population of this massive cluster, increasing the number of known members from optical study by a factor of ~50.