Notes for reducing XIS0 data with area discrimination after 2009-06-27.

2010-05-31 by the XIS team
After the XIS0 anomaly in June 2009, we apply an area discrimination (AD) to all the XIS0 data starting from June 27, 2009 16:27 (UT). The discriminated area is in the segment A, covering about 1/8 of the entire chip. The geometry is shown in the XIS quick reference.

In reducing the data taken before and after the date, users need to pay attention to the following items.
  1. Selection of events and construction of responses.

    For all data taken after the start of the AD, users need to exclude the ADed region explicitly in extracting events using a region file. By so doing, the response generators (xisrmfgen and xisarfgen) consider the effects of the AD masking appropriately.

    A shell script can be used to generate the region file to mask the ADed area. The region should be multiplied with your own source and background extraction regions.

    Example. 
    ) # Display the image.
) ds9 xis0.evt -zoom to fit &

) # Print the region file.
) xis_fov.sh xis0.evt
global color=magenta
fk5
box(263.7798,-32.6083,1067.2128",1067.2128",90.0013)
-box(263.779803,-32.491921,83.376",1067.2128",90.0013)

) # Plot the region file over the image.
) xis_fov.sh xis0.evt | xpaset ds9 regions
  2. Construction of background using xisnxbgen

    Users constructing the NXB spectrum using the xisnxbgen tool should consider the following.

    xisnxbgen retrieves the NXB data within 150 days before and after the observation start by default. If (a) the duration covers across the XIS0 anomaly event and the subsequent AD operation and (b) the source extraction region includes the ADed region, the resultant NXB spectrum is inappropriate.

    • If (a) the data taken within 150 days before the XIS0 anomaly event (between January 24, 2009 and June 27, 2009) and (b) the source extraction region includes the ADed region, users are advised to limit the duration to retrieve the NXB database using a hidden parameter "time_max". This reduces the statistics in the resultant NXB spectrum. Fortunately, the NXB spectrum above 0.5 keV is fairly stable throughout the mission, so the reduced statistics can be recovered by extending the NXB retrieval duration backward in time beyond 150 days using another hidden parameter "time_min".

      Example. 
      ) xisnxbgen output.pi input.pha (other parameters) time_min="2008-08-01T00:00:00"  time_max="2009-06-23T00:00:00"

    • If the data taken after the XIS0 anomaly event, no problem is anticipated in practice; all users are supposed to discard the ADed area, anyway, following the item 1 above.
  3. Spurious events at the edge of the XIS0

    After the anomaly, spurious events sometimes appear close to the boundary of the ADed area. See figure 2 in the Suzaku memo 2010-01. These events only appear in the XIS0, thus can be easily recognized by comparing with images with other sensors.