The Chandra High Resolution Camera is not equally-sensitive at all positions. At off-axis positions, the sensitivity drops quite a bit, due to inherent difficulties in building X-ray mirrors. Making this more challenging, the sensitivity changes are also energy-dependent - at high energies, the loss is larger than at low (soft) ones. Therefore, to get a true measure of the brightness of the halo independent of these changes, I need to calculate the actual response of the camera to the specific source, in this case GX5-1, at all positions of interest. Fortunately, this is a common problem, and tools exist to
determine the 'exposure map' semi-automatically using the CIAO software package. The tricky part is determining the spectrum, but since I've already done this part, I can reuse my
previous result. The calculation is done in two parts - first, getting the 'instrument map' and then the full 'exposure map.' The instrument map calculates the effects of the detector and the X-ray mirror on the final sensitivity. The exposure map is the overall impact on the observed sensitivity. The difference between the two is because Chandra doesn't sit and stare at one point, but rather 'dithers' around the focus point. This is done for a
number of reasons, but in this case it simply means we have to move the instrument map around and co-add it to get the final impact. These steps are easy, but take a significant amount of time and memory (hours on a 2 GHz machine and hundreds of MB, respectively).
The image in blue shows the instrument map, with a color scale at bottom. This is in units of square centimeters, and measures the 'effective area' of the detector at each point to the source. If the source is emitting 1 photon/sq. cm/s, then if we have 60 sq. cm of effective area, we'll get 60 counts/s from the source. Note the clever conversion from 'photons' to 'counts' - 'photons' are independent of any real-life detector, while 'counts' are real measured blips with all of the calibration and other issues that are associated with reality. Also, the maximum sensitivity is in the corner of the detector because this particular observation was done with the detector offset to put the source in the corner. Normally, the maximum sensitivity (ie, the white spot) would be in the center. As a side note, this figure can be immediately identified as an instrument map and not a detector map because it's square and aligned on the figure. By custom, images in sky coordinates have north 'up' and east pointing 'left'. The exposure map will be in sky coordinates, and will thus show a rotated detector - as shown in the second image. I've done these in blue and orange, using the
SAO ds9 program, but could have used any colors. Now that I have these, I'm ready to calculate the radial profile of the source - or, well, almost.
The Chandra High Resolution Camera is not equally-sensitive at all positions. At off-axis positions, the sensitivity drops quite a bit, due to inherent difficulties in building X-ray mirrors. Making this more challenging, the sensitivity changes are also energy-dependent - at high energies, the loss is larger than at low (soft) ones. Therefore, to get a true measure of the brightness of the halo independent of these changes, I need to calculate the actual response of the camera to the specific source, in this case GX5-1, at all positions of interest. Fortunately, this is a common problem, and tools exist to determine the 'exposure map' semi-automatically using the CIAO software package. The tricky part is determining the spectrum, but since I've already done this part, I can reuse my previous result. The calculation is done in two parts - first, getting the 'instrument map' and then the full 'exposure map.' The instrument map calculates the effects of the detector and the X-ray mirror on the final sensitivity. The exposure map is the overall impact on the observed sensitivity. The difference between the two is because Chandra doesn't sit and stare at one point, but rather 'dithers' around the focus point. This is done for a number of reasons, but in this case it simply means we have to move the instrument map around and co-add it to get the final impact. These steps are easy, but take a significant amount of time and memory (hours on a 2 GHz machine and hundreds of MB, respectively).
The image in blue shows the instrument map, with a color scale at bottom. This is in units of square centimeters, and measures the 'effective area' of the detector at each point to the source. If the source is emitting 1 photon/sq. cm/s, then if we have 60 sq. cm of effective area, we'll get 60 counts/s from the source. Note the clever conversion from 'photons' to 'counts' - 'photons' are independent of any real-life detector, while 'counts' are real measured blips with all of the calibration and other issues that are associated with reality. Also, the maximum sensitivity is in the corner of the detector because this particular observation was done with the detector offset to put the source in the corner. Normally, the maximum sensitivity (ie, the white spot) would be in the center. As a side note, this figure can be immediately identified as an instrument map and not a detector map because it's square and aligned on the figure. By custom, images in sky coordinates have north 'up' and east pointing 'left'. The exposure map will be in sky coordinates, and will thus show a rotated detector - as shown in the second image. I've done these in blue and orange, using the SAO ds9 program, but could have used any colors. Now that I have these, I'm ready to calculate the radial profile of the source - or, well, almost.
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