BAT X‐Ray Survey. I. Methodology and X‐Ray Identification

Abstract

We applied the maximum likelihood ( ML) method, as an image reconstruction algorithm, to the BAT X-Ray Survey (BXS). This method was specifically designed to preserve the full statistical information in the data and to avoid mosaicking of many exposures with different pointing directions, thus reducing systematic errors when co-adding images. We reconstructed, in the 14Y170 keV energy band, the image of a 90 ; 90 deg 2 sky region, centered on (R:A:; decl:) ¼ (105 ; À25 ), which BAT surveyed with an exposure time of $1 Ms (in 2005 November). The best sensitivity in our image is $0.85 mcrab or 2:0 ; 10 À11 ergs cm À2 . We detect 49 hard X-ray sources above the 4.5 level; of these, only 12 were previously known as hard X-ray sources (>15 keV ). Swift XRT observations allowed us to firmly identify the counterparts for 15 objects, while 2 objects have Einstein IPC counterparts (Harris et al. 1990); in addition to those, we found a likely counterpart for 13 objects by correlating our sample with the ROSAT All-Sky Survey Bright Source Catalog (Voges et al. 1999). Seven objects remain unidentified. Analysis of the noise properties of our image shows that $75% of the area is surveyed to a flux limit of $1 mcrab. This study shows that the coupling of the ML method to the most sensitive, all-sky surveying, hard X-ray instrument, BAT, is able to probe for the first time the hard X-ray sky to the millicrab flux level. The successful application of this method to BAT demonstrates that it could also be applied with advantage to similar instruments such as INTEGRAL IBIS.

Intermediate polars in the Swift/BAT survey: spectra and white dwarf masses

“…BAT continuously monitors the hard X-ray sky surveying gamma-ray bursts. Results of the BAT survey (Markwardt et al 2005;Ajello et al 2008a) show that BAT reaches a sensitivity of ≈1 mCrab in a 1 Ms exposure. With its sensitivity and the large amount of exposure already accumulated over the whole sky, BAT is a very good instrument for studying IPs above 20 keV.…”

“…The upper limit is taken to avoid the presence of strong background lines which could worsen the overall sensitivity. The data screening was performed analogously to the method presented in Ajello et al (2008a). The final all-sky image is obtained as the weighted average of all the shorter observations.…”

“…All the candidates are then fit with the BAT point spread function (using the standard BAT tool batcelldetect) to derive the best source position. The BAT position error is between 3−7 arcmin, depending on the source intensity (see Ajello et al 2008a). The identification of IPs from this detection list was done using the online catalog maintained by Mukai 1 and allowing for each source's position error.…”

GALAXY CLUSTERS IN THESWIFT/BURST ALERT TELESCOPE ERA: HARD X-RAYS IN THE INTRACLUSTER MEDIUM

“…The upper limit was chosen so as to avoid the presence of strong background lines, which could worsen the overall sensitivity. The data screening was performed according to Ajello et al (2008a). The all-sky image is obtained as the weighted average of all the shorter observations.…”

The blazar content in theSwift-BAT hard X-ray sky

“…At present, the Burst Alert Telescope (BAT, Barthelmy et al 2005) aboard the Swift satellite (Gehrels et al 2004) is performing the most sensitive and complete survey of the sky in the hard X rays. The analysis of progressively longer periods provided with BAT survey data has produced in recent years catalogues and lists of Galactic and extragalactic sources (Markwardt et al 2005;Ajello et al 2008;Tueller et al 2008;Ajello et al 2009b;Tueller et al 2010). The largest (to date) hard X-ray catalogue, obtained from the reduction of 39 months of BAT survey data accumulated since its A&A 520, A47 (2010) launch, is the First Palermo BAT Catalogue (hereafter PBC39, Cusumano et al 2010).…”