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Abstract: We applied the Maximum Likelihood method, as an image reconstructionalgorithm, to the BAT X-ray Survey BXS. This method was specifically designedto preserve the full statistical information in the data and to avoidmosaicking of many exposures with different pointing directions, thus reducingsystematic errors when co-adding images. We reconstructed, in the 14-170 keVenergy band, the image of a 90x90 deg$^2$ sky region, centered onRA,DEC=105$^{\circ}$,-25$^{\circ}$, which BAT surveyed with an exposure timeof $\sim1$ Ms in Nov. 2005. The best sensitivity in our image is $\sim0.85$mCrab or $2.0\times 10^{-11}$ erg cm$^{-2}$. We detect 49 hard X-ray sourcesabove the 4.5 $\sigma$ level; of these, only 12 were previously known as hardX-ray sources $>$15 keV. Swift-XRT observations allowed us to firmly identifythe counterparts for 15 objects, while 2 objects have Einstein IPC counterparts\citep{harris90}; in addition to those, we found a likely counterpart for 13objects by correlating our sample with the ROSAT All-Sky Survey Bright SourceCatalog \citep{voges99}. 7 objects remain unidentified. Analysis of the noiseproperties of our image shows that $\sim75$% of the area is surveyed to a fluxlimit of $\sim$1 mCrab. This study shows that the coupling of the MaximumLikelihood method to the most sensitive, all-sky surveying, hard X-rayinstrument, BAT, is able to probe for the first time the hard X-ray sky to themCrab flux level. The successful application of this method to BAT demonstratesthat it could also be applied with advantage to similar instruments likeINTEGRAL-IBIS.

Autor: M. Ajello, J. Greiner, G. Kanbach, A. Rau, A. W. Strong, J. A. Kennea


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