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Description
This BAT Hard X-ray Survey data is the 70-month survey product of the BAT instrument on
the Swift observatory. Swift/BAT is a wide field-of-view (70x100
degrees) hard X-ray imager consisting of a coded mask and a large array
of CdZnTe detectors (with an effective area of ~ 5000 cm<sup>2</sup>).
<p>
BAT is sensitive in the energy range 14-195 keV. The data are divided
into 8 energy bands
<table border>
<tr><th>Band<th>Energy (keV)<th>Frequency (EHz) </tr>
<tr><td>1<td> 14-20 <td> 3.38-4.84</tr>
<tr><td>2<td> 20-24 <td> 4.84-5.80</tr>
<tr><td>3<td> 24-35 <td> 5.80-8.46</tr>
<tr><td>4<td> 35-50 <td> 8.46-12.1</tr>
<tr><td>5<td> 50-75 <td> 12.1-18.1</tr>
<tr><td>6<td> 75-100 <td> 18.1-24.2</tr>
<tr><td>7<td> 100-150<td> 24.2-36.3</tr>
<tr><td>8<td> 150-195<td> 36.3-47.2</tr>
<tr><td>Sum (SNR only)<td>14-195<td> 3.38-47.2</tr>
</table>
Each band is represented as as two separate surveys, a signal-to-noise (SNR) map and a flux map.
(e.g., BAT-snr-1 or BAT SNR 1 or BAT SNR 14-20, or BAT-Flux-1, ...).
An additional summed survey, BAT-SNR-SUM or BAT SNR SUM or BAT SNR 14-195, is also available, but there
is no summed flux survey. In our Web interface only the SNR surveys are
shown in the Web form. Users can get flux maps corresponding to a given SNR image
from the results pages. The batch interfaces may directly query any of the surveys since
the user chooses the names explicitly rather than from a selection box.
<p>
The values displayed in the significance
maps are the local signal to noise ratio in each pixel. The noise in
these coded-mask images follows a Gaussian distribution with center at
zero and a characteristic width (sigma) of 1.0. The noise is
calculated locally for each pixel by measuring the RMS value of all
pixel values in an annulus around each pixel and hence includs both
statistical and systematic components. Known sources are excluded from
the annuli.
<p>
The signal in each pixel is taken from the flux maps.
<p>
The flux values are in the native BAT survey units of
counts/sec/detector. The detector is an individual piece of CZT in
the BAT array with an area of 1.6 x 10<sup>-7</sup>m<sup>2</sub>.
<p>
While the Swift mission is primarily designed to follow gamma-ray bursts,
the random distribution of bursts in the sky means that these survey's sky coverage
is relatively uniform with the exposure at any point varying between about 6 to 16 megaseconds.
The survey limits for source detection are about 10<sup>-11</sup> ergs/s/cm<sup>2</sup>
over about half the sky and 1.3x10<sup>-11</sup> ergs/s/cm<sup>2</sup> over 90%.
<p>
These data replace the 9-month BAT datasets which we have retired. If you wish access to the older data please let us know.
Note that for the 9-month data we provided access through the web page to the flux data
and gave links to the signal-to-noise maps. Since the existence of sources is most
easily seen in the SNR maps, we decided to invert that for this release.
<p>
For the 8 band data, the source data were provided by the BAT team as 6 FITS files. Each of these contained the 8 bands in separate image
extensions for a region centered at l=0,b=+/-90 or l=0,90,180,270,b=0, the centers of 6 cubic facets. However these data are not the classical
cube-faced projections, e.g., as used in COBE data. The data on the facets overlap, so that this is just a convenient way to tile the sky.
<i>SkyView</i> separated each of the FITS image extensions into a separate file, but no other modifications were made to the data. The summed image was
provided as six separate files. Provenance: NASA BAT Team. This is a service of NASA HEASARC.
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