The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa
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The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

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4.3. CODED APERTURE ANALYSIS FOR X-RAY ASTRONOMERS 21 Figure 4.3: Side view of the BAT mechanical structures. Photons enter from the top and pass downwards. Figure 4.4: Comparison of BAT detector space (left) and sky space (right). A point source casts the complicated shadow shown on the left. After reconstruction a sky image using batfftimage, the point source is revealed. This is a small subimage of the entire BAT sky image for this particular observation. 4.3 Coded Aperture Analysis for X-ray Astronomers For an observer with experience in X-ray astronomy, coded aperture analysis with BAT may appear to be similar to their experiences, but they are not. This section gives a description of some of the major differences. BAT has no focussing optics. This means that most of the familiar analysis operations of X-ray and optical astronomy do not apply. The BAT has two distinct spaces: detector space and sky space. The BAT software is used to convert from detected counts to sky fluxes. Figure 4.4 compares the two kinds of images. Another example, Figure 4.5 shows the mask modulation pattern of a bright source, Sco X-1, as detected by BAT. Note however that for all but the brightest sources the mask modulation pattern will not be visible to the naked eye.

22 CHAPTER 4. INTRODUCTION TO BAT ANALYSIS Figure 4.5: BAT Detector plane image (DPI) with the bright source Sco X-1 in the field of view. The mottled texture of the image is the mask shadow pattern. The small triangular patch in the lower right hand corner is a portion of the array not illuminated by Sco X-1. The black squares correspond to disabled detectors. Each detector can be illuminated by many points on the sky; and each point on the sky illuminates many detectors. This entanglement means that one must use the special BAT software to do the disentangling. You cannot “select” photons with spatial regions. The BAT software does not reconstruct where each individual photon came from. Neighboring sky image pixels are highly correlated, so it does not make sense to “select photons” within a region. For images, it is preferred to use the PSF fitting method of the batcelldetect task. For light curves and spectra, batmaskwtevt is used to select a point of interest on the sky, and batbinevt is used to extract the fluxes. The noise is gaussian, not poissonian. BAT software provides a background-subtracted flux for a source. The deconvolution technique produces errors which are gaussian in nature, not poissonian. This means that any downstream software that you use must be able to cope with gaussian errors. It also means that you do not need to use “grouping” strategies commonly used with XSPEC in X-ray astronomy (such as GRPPHA or “setplot group”). The BAT has a very large field of view. The complete field of view is approximately 120 × 60 square degrees, although sensitivity is lower at the edges. The solid angle is approximately 2 steradians. Since this is a significant fraction of the sky, it is likely that BAT will observe a given source even when Swift is not pointed at the source. The BAT typically covers ∼75-85% of the sky on any given day. The BAT is background dominated. The typical full array background count rate is about 10,000 to 12,000 counts per second. Even the Crab is background dominated (1 Crab ∼ 10% of background). Thus, the most sensitive analysis will involve removing as much background contamination as possible.

4.3. CODED APERTURE ANALYSIS FOR X-RAY ASTRONOMERS 21<br />

Figure 4.3: Side view of the <strong>BAT</strong> mechanical structures. Photons enter from the top and pass<br />

downwards.<br />

Figure 4.4: Comparison of <strong>BAT</strong> detector space (left) and sky space (right). A point source casts<br />

the complicated shadow shown on the left. After reconstruction a sky image using batfftimage, the<br />

point source is revealed. This is a small subimage of the entire <strong>BAT</strong> sky image for this particular<br />

observation.<br />

4.3 Coded Aperture Analysis for X-ray Astronomers<br />

For an observer with experience in X-ray astronomy, coded aperture analysis with <strong>BAT</strong> may appear<br />

to be similar to their experiences, but they are not. This section gives a description of some of the<br />

major differences.<br />

<strong>BAT</strong> has no focussing optics. This means that most of the familiar analysis operations of<br />

X-ray and optical astronomy do not apply.<br />

<strong>The</strong> <strong>BAT</strong> has two distinct spaces: detector space and sky space. <strong>The</strong> <strong>BAT</strong> software is<br />

used to convert from detected counts to sky fluxes. Figure 4.4 compares the two kinds of images.<br />

Another example, Figure 4.5 shows the mask modulation pattern of a bright source, Sco X-1, as<br />

detected by <strong>BAT</strong>. Note however that for all but the brightest sources the mask modulation pattern<br />

will not be visible to the naked eye.

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