Separating the BL Lac and cluster X-ray emissions in Abell 689 withChandra

We present the results of aChandraobservation of the galaxy cluster Abell 689 (z= 0.279). Abell 689 is one of the most luminous clusters detected in theROSATAll Sky Survey (RASS), but was flagged as possibly including significant point source contamination. The small point spread function of theChandratelescope allows us to confirm this and separate the point source from the extended cluster X-ray emission. For the cluster, we determine a bolometric luminosity ofLbol= (3.3 +/- 0.3) 1044ergs-1 and a temperature ofkT= 5.1+2.2- 1.3keV when including a physically motivated background model. We compare our measured luminosity for A689 to that quoted in the RASS, and findL0.1-2.4keV= 2.8 1044ergs-1, a value similar to 10 times lower than theROSATmeasurement. Our analysis of the point source shows evidence for significant pile-up, with a pile-up fraction of [sime]60 per cent. Sloan Digital Sky Survey spectra andHubble Space Telescope(HST) images lead us to the conclusion that the point source within Abell 689 is a BL Lac object. Using radio and optical observations from the Very Large Array andHSTarchives, we determine alpha ro= 0.50, alpha ox= 0.77 and alpha rx= 0.58 for the BL Lac, which would classify it as being of 'high-energy peak BL Lac' type. Spectra extracted of A689 show a hard X-ray excess at energies above 6keV that we interpret as inverse-Compton emission from aged electrons that may have been transported into the cluster from the BL Lac.