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Suzaku Guest Observer Facility

HXD/PIN Background Files for V2.x Processed Data

Updated 2014 June 9


Two different types non X-ray background (NXB) models for HXD/PIN are available now.

The tuned background is available for observations since 2005-08-17, except for the period 2006-05-24 to 2006-05-29, when one of 64 PIN diodes showed an unusually high event rate probably caused by the radiation damage. For this period, users can use the quick background by selecting with a condition of "UNITID>3."

There are no usable "quick" background files for the period 2012 August through 2013 September.

Please note that the new tuend NXB files have been dead-time corrected, and hence the analysis procedure is exactly the same between "quick" and "tuned." (In this respect, the tuned NXB files are different from the V1.X bgd_d files that were previously available, which needed to be corrected for the dead time by users.)

The tuned background files cannot be produced until 1-2 months after the processing of the data. Once produced, they have estimated systematic uncertainties of about 1.3% (see suzakumemo-2008-03 for further details about the reproducibility of the tuned PIN background model). This systematic uncertainty level is about half of the quick model, so we recommend the use of tuned background for publication-quality analysis. Since the quick background files can be delivered a few weeks after processing, these can be used for quick-look purposes before the tuned background files become available.

Directoriese for both background files are divided into subdirectories by month. For example, background files for observations carried out in 2006 August can be found in the subdirectory 2006_08. Within these monthly directories, individual background files are listed alphabetically. Note these files are named using the sequence number, e.g., ae100005010hxd_pinnxb_cl.evt.gz (quick) or ae100005010hxd_pinbgd.evt.gz (tuned).

These files should only be used with version 2.x processed data, and vice versa.

One important change from version 1.x background files is that the new background files contain events from all units of PIN, regardless of whether the bias voltage for PIN is 500V or 400V.

Scripts for PIN Analysis

Two scripts, hxdpinxbpi and hxdpinxblc, allow users to create the source(+background) and background spectra/light curves in one step, following the step-by-step instructions below. These scripts have been released as part of Suzaku FTOOLS V14 in HEAsoft V6.8.

1. Usage for Spectral Analysis:

  1. The background event files have a GTI extension (extension #2). The background estimation is performed only within the GTIs listed. For further filtering, you should make a new GTI by ANDing the GTI from your filtering criteria with the GTI extension of the background files.

  2. It is necessary to correct for the dead time of the observed spectrum to apply the background file correctly. The dead time correction tool (hxddtcor, included in the latest release of the Suzaku ftools) updates the EXPOSURE keyword of the spectral file, by comparing the number of pseudo events injected by the analog electronics on-board with that found in the telemetry.

    A pseudo event file filtered with the same GTI as the cleaned event file can be found in the cleaned event file directory in data processed with version 2.x (event_cl/aeNNNNNNNNNhxd_0_pse_cl.evt.gz). This is the most convenient input to hxddtcor, if you are analyzing the cleaned event files. Otherwise, supply the unscreened event file(s) to hxddtcor. The syntax is:

    hxddtcor ae123456789hxd_0_pse_cl.evt ae123456789pin.pha
    
    if the spectral file is called ae123456789pin.pha. Note that the EXPOSURE keyword value will be rewritten.

    On the other hand, dead time correction is not necessary for the PIN background files, regardless of whether they are "quick" or "tuned."

  3. The event rate in the PIN background event file is 10 times higher than the real background to suppress the Poisson errors. Therefore, users should increase the exposure time of derived background spectra and light curves by a factor of 10 using, e.g., fmodhead.

  4. The background event file does not include the cosmic X-ray background (CXB). Since the CXB flux is about 5% of the background for PIN, you should take it into account after subtracting the background.
    • A recipe for estimating the CXB level in HXD/PIN data

  5. For the accuracy of the background model, users are refereed to Suzaku Memo 2007-09 and Suzaku Memo 2008-03 for quick and tuned ones, respectively.

    Guest observers are strongly recommended to verify the reliability of the background model

    • by comparing light curves of the observation and the background model.
    • by comparing the model spectrum with the "earth occultation spectrum," which can be obtained by screening with "ELV<-5." Note, however, that in this case you need to re-produce the pseudo event file from the unscreened event file, or by supplying the unscreened event file to hxddtcor.

With the background files, users can now proceed to spectral fitting. Please note that, due to changes in bias voltages and the low energy threshold, users must now choose PIN response matrices that are appropriate for the epoch of observation, available from the Suzaku CALDB.

2. Usage for Timing Analysis:

The users can also generate background-subtracted PIN light curves using these background files. In this process, users need to take the dead time into account, using the pseudo event files. Since pseudo events are generated by the HXD analog electronics every 4 seconds for each of 16 units, we expect 16/4 = 4.0 ct/s in the absence of dead time. Therefore, the live time is given by the measured pseudo event rate during the time bin divided by 4.

The following method for correcting for bin-by-bin dead time is recommended only for bins longer than 128 s, to ensure that the dead time estimate is statistically accurate enough.

  1. Merge the GTIs (see Usage for Spectral Analysis, Step 1).

  2. Extract pure pseudo event light curve (i.e., those pseudo events that have no coincidental trigger flags from the real detectors).
    fselect infile=ae123456789hxd_0_pse_cl.evt+1 outfile=pseudo_pure.evt \
                     expr ="GRADE_HITPAT<=1&&GRADE_QUALTY==0" histkw=yes
    
    Extract lightcurve from this "pure" pseudo event file, while applying the merged GTI file, and save it as pin_pseudo.lc, for example.

  3. Extract the source light curve using the merged GTI file. If this file is called pin_event.lc, the following steps will allow you to create a new RATE column which includes the dead time corrected RATE.
    fcalc pin_pseudo.lc+1 pin_pseudo_div4.lc DTCOR  "RATE/4"
    faddcol pin_event.lc+1 pin_pseudo_div4.lc+1 DTCOR
    fcalc pin_event.lc+1 pin_event_dtcor.lc RATE "RATE/DTCOR"
    fcalc pin_event_dtcor.lc+1 pin_event_dtcor.lc ERROR "ERROR/DTCOR" clobber=yes
    
    The above steps were: calculate the live time in the DTCOR column of a temporary file, pin_pseudo_div4.lc; copy that column into the light curve file, pin_event.lc; create a new light curve filre pin_event_dtcor.lc in which the RATE column is dead time corrected; dead time-correct the ERROR column in that file.

  4. Extract the background light curve, and divide it by 10.
    fcalc pin_bgd.lc+1 pin_bgd_div10.lc RATE "RATE/10"
    fcalc pin_bgd_div10.lc+1 pin_bgd_div10.lc ERROR "ERROR/10" clobber=yes
    
    Note that, in addition to this light curve, the observed light curve contains the cosmic X-ray background component, which can be treated as a constant.


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This file was last modified on Tuesday, 10-Jun-2014 19:24:45 EDT

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