A minor problem is found in the header keyword (MJDREF) of PIN background. The HXD team has corrected it to the appropriate keywords (MJDREFI and MJDREFF), and also updated the METHODV keyword to "2.0pre20080404" for all NXB data before 2008-03-15 20:45 (ZETA_OPH). Old files are moved into the "old20080527" directory.

Observation date	Directory	METHOD *	METHODV **
After 2005-09-02	pinnxb_ver2.0	PINUDLCUNIT	2.0preYYYYMMDD

*: FITS header keyword to distinguish the background modeling methods.
**: FITS header keyword to distinguish the minor revisions of the same model. (YYYYMMDD corresponds to the creation date of that file.)

The directory is 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.

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. The HXD team defines 4 different epochs (Epoch-1 to 5) referring to the major change of operation modes (i.e. the PIN bias voltage). Current NXB release automatically supports these change. However, there are two time regions (in epoch-1) in which they are known to show some (~10%) systematic offset caused by minor operation parameter changes. They are

- 2005-07-20 to 2005-09-01 :
"The initial operation period", in which the HXD was in the initial operation phase.
- 2006-03-23 to 2006-05-13 :
"The GSO threshold change period", in which some GSO parameters were changed.

In these two periods, users are suggested to use the "tuned" (METHD=LCFITDT) NXB files, which can be found in the tuned PIN-NXB directory.

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.

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. 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_20080129.rsp (XXXXX=xinom,hxnom,flat)
2006.5.13 -- 2006.10.2	ae_hxd_pinXXXXXe2_20080129.rsp (XXXXX=xinom,hxnom,flat)
2006.10.2 -- 2007.7.28	ae_hxd_pinXXXXXe3_20080129.rsp (XXXXX=xinom,hxnom,flat)
2007.7.28 -- 2008.8.31	ae_hxd_pinXXXXXe4_20080129.rsp (XXXXX=xinom,hxnom,flat)
2008.9.1 -- 2009.9.30	ae_hxd_pinXXXXXe5_20080716.rsp (XXXXX=xinom,hxnom,flat)
2009.10.1 -- 2010.1.16	ae_hxd_pinXXXXXe6_20090826.rsp (XXXXX=xinom,hxnom,flat)
2010.1.16 -- 2010.2.1	ae_hxd_pinXXXXXe7_20100731.rsp (XXXXX=xinom,hxnom,flat)
2010.2.2 -- 2010.4.3	ae_hxd_pinXXXXXe8_20100731.rsp (XXXXX=xinom,hxnom,flat)
2010.4.3 -- 2010.11.30	ae_hxd_pinXXXXXe9_20100731.rsp (XXXXX=xinom,hxnom,flat)
2010.12.1 -- 2011.5.24	ae_hxd_pinXXXXXe10_20101013.rsp (XXXXX=xinom,hxnom,flat)
2011.5.25 --	ae_hxd_pinXXXXXe11_20110601.rsp (XXXXX=xinom,hxnom,flat)

These files are available from the Suzaku CALDB (2009-12-07 version or later).

If you have any questions, please contact Suzaku Help Desk.