4. Suzaku Data Analysis Overview

This chapter provides a brief outline of the standard analysis steps. Details are explained in subsequent chapters. Analysis topics covered in this chapter are:

1. Spectral analysis of the XIS and the HXD data.
2. Timing analysis of the XIS and the HXD data.
3. Imaging analysis of the XIS data.

We assume that the user has downloaded and decrypted the Suzaku data, and has access to the latest versions of the Suzaku FTOOLS and CALDB.

4.1 Important Events and Processing Version

Users should check

http://www.astro.isas.jaxa.jp/suzaku/log/operation/,
http://www.astro.isas.jaxa.jp/suzaku/log/hxd/, and
http://www.astro.isas.jaxa.jp/suzaku/log/xis/ for any important event (operational or instrumental issues) that may affect your specific observation.

Users should also check the processing version of the data, as recorded in the PROCVER keyword in any of the FITS files. Users should then consult
http://suzaku.gsfc.nasa.gov/docs/suzaku/aehp_proc.html to see if there are any issues for data processed using that version of the pipeline.

4.2 Checking for Updates

Users should check for any software or calibration updates that may affect the data in question. The Suzaku GOF disseminates information in several ways.

1. By targeted e-mails to the PIs of proprietary data. This method is used to communicate important updates that affect a specific subset of observations. We expect the PIs to pass the information to any collaborators who may be analyzing the data.

2. By e-mails to the suzakunews exploder. All types of Suzaku related news items are included in occasional messages through this exploder. All who are interested in the Suzaku mission should subscribe to this list at
   http://suzaku.gsfc.nasa.gov/docs/heasarc/news.html. if they are not already on the list. Note that users who are registered under a defunct address will get dropped if the exploder receives error messages several times in a row. On the other hand, after every proposal review cycle, we add the e-mail addresses of all successful US PIs who appear not to be on the suzakunews list.

3. Through this guide. Chapter 5 provides a list of important caveats that users should be aware of.

4. Via the Suzaku GOF web site. The Suzaku Data Analysis page:
   http://suzaku.gsfc.nasa.gov/docs/suzaku/aehp_data_analysis.html and links therein are updated more frequently than this guide. Of particular importance is the ``Things to Watch Out For'' page:
   http://suzaku.gsfc.nasa.gov/docs/suzaku/analysis/watchout.html.

These updates may simply note a newly discovered calibration problem, instruct users on how to obtain a software patch or updated calibration file, or provide workarounds such as an ad-hoc procedure that should be run on specific datasets.

4.3 Unfiltered or Screened?

The pre-extracted spectra and light curves in the product subdirectories, both for the XIS and the HXD, are provided for quick-look purposes only. We recommend against using these files for actual data analysis.

Event FITS files are provided to the users in two flavors (Chapter 3), unfiltered (event_uf) and screened (event_cl). Although the pre-determined screening criteria can never be perfect for each individual observation, they produce convenient and generally reliable event lists that are convenient to use. Whenever possible, it is more convenient for beginning users to start with the screened event files.

However, there are many circumstances which forces the users to start with unfiltered event files. We will endeavor to provide specific recipes when it is necessary to do so.

The following chapters (6 and 7) do contain descriptions of how to re-create screened event files with a different set of screening criteria.

4.4 Spectral Analysis

The following are the steps in the spectral analysis of XIS data.

1. Extract source and background spectra.
2. Build the response files (RMF and ARF).
3. Combine the spectra taken with XIS0, XIS2 (if available), and XIS3.

Chapter 6 contains pointers on the size and shape of the source extraction region, and on the typical background extraction region for a point source. The particle background is a strong function of the location of the Suzaku spacecraft within the geomagnetic field, and therefore is variable in time. The X-ray background is a function of the pointing direction. Therefore, ideally, the background spectrum should be extracted from the neighboring source-free region(s) of the same CCD chip from the same observation. However, for the analysis of extended sources, it is sometimes necessary to consult other observations, including the non X-ray background (NXB) database compiled by the XIS team.

We also explain the RMF and ARF generators. The latter is based on ray-tracing and can be extremely time-consuming. We therefore describe several options for speeding up ARF generation, as well as subsequent spectral fits.

Three of the XIS units (each with a frontside illuminated, or FI, chip) are sufficiently similar that we recommend the spectra from these units be summed for spectral fitting under most circumstances. However, we never recommend combining the event files (this will lead to the loss of information critical to downstream software). Also, XIS1 (with a backside illuminated, or BI, chip) has a distinctly different response and so XIS1 data should not be combined with those from other units.

The following are the steps, explained in detail in Chapter 7, in the spectral analysis of HXD data.

1. Obtain the appropriate background files.
2. Create a joint good time interval (GTI) file, when both the data and background models are available.
3. Extract spectra from the observation data, and correct for dead-time.
4. Extract spectra from the background files, and correct (for PIN) for the factor of 10 oversampling.
5. Select the appropriate response file, and correct for off-axis location of the source if necessary.

Since the HXD is a non-imaging instrument, users need not/cannot consider ``extraction regions.'' Instead, it is necessary to subtract the particle and X-ray background from the observation data. This is done using the background files generated by the HXD team, who model the particle background based on the orbital location and other information. In normal situations, the PIN background files are prepared within a few weeks of the distribution of the processed data to the PI; GSO background files are made about a month after distribution.

There is a noticeable dead time even for faint sources because the particle background alone results in a high count rate. This must be corrected for in the data. The background files, on the other hand, do not need a dead-time correction. However, in the case of the PIN, background files have an artificially inflated count rate to ensure sufficient statistical accuracy, and this has to be taken into account.

The HXD team provides the response files for the PIN and GSO, rather than response generators. The PIN settings have been adjusted since the initial operation several times, including changes in the bias voltage used on-board, and in the low energy thresholds used in ground processing. Therefore it is necessary to select a response file appropriate for the epoch of the observation.

4.5 Timing Analysis

The following are the steps in the timing analysis of XIS data.

1. Extract source and background light curve.
2. Subtract the latter from the former, with appropriate scaling.
3. Combine the light curves taken with all XIS units.

Users who have become familiar with XIS spectral analysis should find little difficulties in performing XIS timing analysis. In this case, it is generally safe to add light curves from all XIS units.

The following are the steps in the timing analysis of HXD data.

1. Obtain the appropriate background files.
2. Create a joint good time interval (GTI) file, when both the data and background models are available.
3. Extract light curves from the observation data, and correct for dead-time.
4. Extract light curves from the background files, and correct (for PIN) for the factor of 10 oversampling.

Again, this process parallels that of the spectral analysis, but requires the correction of time-variable dead-time, as explained in Chapter 7.

4.6 Imaging Analysis

The following are the steps in the imaging analysis of XIS data.

1. Extract XIS images.
2. Generate corresponding exposure maps.
3. Create exposure-corrected XIS images, and apply further arithmetic as desired.

Chapter 6 contains a detailed description on how to generate exposure maps.

4.7 Recipe 1: XIS Spectral Analysis

1. Make sure that you have access to the latest software and calibration files (see Chapter2).
2. Download the data (see Chapter3).
3. Check for any updates (this chapter)
4. Apply updated CTI calibration, if necessary, using xispi on the unscreened files:

   example% xispi infile=ae101005070xi0_0_3x3n066z_uf.evt.gz \
                  outfile=ae101005070xi0_0_3x3n066z_uf_new.evt \
                  hkfile=../hk/ae101005070xi0_0.hk.gz
   

   then using the xselect command file, xisrepro.xco (and associated screening criteria files) that we provide:

   xsel > @xisrepro
   

   (see Chapter6 for further details)
5. Use xselect to make, display and save products from cleaned event files.

   xsel:SUZAKU-XIS-1-STANDARD > read event
     > Enter the Event file dir >[] .
     > Enter Event file list >[] ae101005040xi1_0_5x5n000a_cl.evt.gz
   xsel:SUZAKU-XIS-1-STANDARD > extract all
   xsel:SUZAKU-XIS-1-STANDARD > plot image
   xsel:SUZAKU-XIS-1-STANDARD > plot curve
   xsel:SUZAKU-XIS-1-STANDARD > plot spectrum
   xsel:SUZAKU-XIS-1-STANDARD > save spectrum
   

6. Build the response files, using xisresp (see Chapter6).
7. Use addascaspec to combine FI spectra and responses.

   example% addascaspec fi.add fi.pha fi.rsp fi_b.pha
   

   (see Chapter6).

4.8 Recipe 2: HXD/PIN Spectral Analysis

1. Make sure that you have access to the latest software and calibration files (see Chapter2).
2. Download the data (see Chapter3).
3. Check for any updates (this chapter)
4. Determine if the observation was performed using the XIS nominal or the HXD nominal pointing position. Determine the epoch of the observation (see Chapter7)
5. Obtain the appropriate response files -- one for the source, and one (``flat'') for the Cosmic X-ray background
6. Obtain the non X-ray background (NXB) files -- see
   http://suzaku.gsfc.nasa.gov/docs/suzaku/analysis/pinbgd.html.
7. Generate the GTI file common to data and NXB

   unix% mgtime "ae101005040hxd_0_pinno_cl2.evt+2,ae101005040hxd_0_pinbgd.evt+2" \
   ae101005040hxd_wel_pin.gti AND
   

8. Apply GTI and extract spectra

   xsel > read event
   xsel > ./
   xsel > ae101005040hxd_0_pinno_cl2.evt.gz
   xsel > filter time file ae101005040hxd_wel_pin.gti
   xsel > extract spec
   xsel > save spec ae101005040hxd_0_pinno_cl2.pha
   xsel > clear all
   xsel > yes
   xsel > read event
   xsel > ./
   xsel > ae101005040hxd_0_pinbgd.evt.gz
   xsel > filter time file ae101005040hxd_wel_pin.gti
   xsel > extract spec
   xsel > save spec ae101005040hxd_wel_pin_bgd.pha
   xsel > exit
   

   This creates the source spectrum (ae101005040hxd_0_pinno_cl2.pha) and background spectrum (ae101005040hxd_wel_pin_bgd.pha).
9. Correct for deadtime

   unix% hxddtcor event_fname="ae101005040hxd_0_pse_cl.evt" \
              pi_fname="ae101005040hxd_0_pinno_cl2.pha"
   

10. Correct the exposure of PIN background

    unix% cp ae101005040hxd_wel_pin_bgd.pha \
               ae101005040hxd_wel_pin_bgd_expcor.pha
    unix% fkeyprint infile=ae101005040hxd_wel_pin_bgd_expcor.pha keynam=EXPOSURE
    
    ----- output  ---
    # FILE: ae101005040hxd_wel_pin_bgd_expcor.pha
    # KEYNAME: EXPOSURE
     
    # EXTENSION:    0
    EXPOSURE= 1.755875832736492E+03 / Exposure time
    # EXTENSION:    1
    EXPOSURE= 1.755875832736492E+03 / Exposure time
    # EXTENSION:    2
    EXPOSURE= 1.755875832736492E+03 / Exposure time
    -----------------
    
    unix% fparkey value=1.755875832736492E+04 \
            fitsfile="ae101005040hxd_wel_pin_bgd_expcor.pha+0" keyword=EXPOSURE
    unix% fparkey value=1.755875832736492E+04 \
            fitsfile="ae101005040hxd_wel_pin_bgd_expcor.pha+1" keyword=EXPOSURE
    unix% fparkey value=1.755875832736492E+04 \
            fitsfile="ae101005040hxd_wel_pin_bgd_expcor.pha+2" keyword=EXPOSURE