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We present results from our analysis of a short Chandra observation of the unidentified X-ray source 1RXSJ114720.0-125253 detected in the ROSAT All-Sky Survey. Our 10 ks Chandra observation established unambiguously that the bulk of the X-ray emission (85%) originates from the 6dF QSO g1147207-125310 (z = 0.496), although diffuse emission from a suspected alternative optical counterpart, the galaxy cluster LCDCS S031 (z = 0.580), is also detected. We measure X-ray luminosities of (8.0 ± 0.8) × 10 44 erg s −1 and (2.7 ± 0.4) × 10 44 for the QSO and the cluster, respectively (0.3−8 keV). Both objects are thus found to be typical representatives of their respective population at the quoted redshifts. Thanks to over 1200 net photons having been detected from the QSO, we were able to characterize its spectrum further, finding a photon index of 1.4 ± 0.1 and no sign of significant self absorption. For the cluster, photon statistics (250 net photons) were too poor to constrain the intra-cluster gas temperature to better than (6.7± 4.4) keV, consistent with the estimate of 3.8 keV from the LX − kT relation. The cluster emission was also found to be significantly elongated and poorly described by a single β model, suggesting an unevolved system or a recent merger event, a hypothesis supported also by the presence of a diffuse radio source about 200 kpc from the cluster core. Prompted by this rare chance alignment of two X-ray luminous extragalactic sources, we discuss the probabilities of QSOs or clusters dominating the X-ray emission from RASS sources with ambiguous optical counterparts and apply the resulting probabilistic argument to the case of H1821+643.

Cluster or QSO? Identifying 1RXSJ114720.0-125253