HXD/PIN Background Files for V2.x Processed Data
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 is 500V or 400V.
(Amended on 2007 Dec 27) There are two periods during which the current V2 NXB files are known to show ~10% systematic offset, caused by minor changes in operational parameters. They are:
- Before 2005 September 1, when the HXD was in the initial operation phase.
- 2006 March 23 - 2006 May 13, during which some GSO parameters were changed.
1. Usage for Spectral Analysis:
- 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.
- 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, except for the bgd_d files.
- 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.
- 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
- The accuracy of the background model is expected to reach as good as
5-10 % of the average background. The background modeling, however,
is still under development, and the evaluation of the systematic errors
has not been completed yet.
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 threshold, users must now choose PIN response matrices that are appropriate for the epoch of observation.
| Epoch | File(s) |
|---|---|
| 2005.8.17 -- 2006.5.13 | ae_hxd_pinXXXXXe1_20070914.rsp (XXXXX=xinom,hxnom,flat) |
| 2006.5.13 -- 2006.10.2 | ae_hxd_pinXXXXXe2_20070914.rsp (XXXXX=xinom,hxnom,flat) |
| 2006.10.2 -- 2007.7.28 | ae_hxd_pinXXXXXe3_20070914.rsp (XXXXX=xinom,hxnom,flat) |
| 2007.7.28 -- ** | ae_hxd_pinXXXXXe4_20070914.rsp (XXXXX=xinom,hxnom,flat) |
These files are available from the Suzaku CALDB (2007-09-15 version or later).
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.
- Merge the GTIs (see Usage for Spectral Analysis, Step 1).
- 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=yesExtract lightcurve from this "pure" pseudo event file, while applying the merged GTI file, and save it as pin_pseudo.lc, for example. - 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. - 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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