The SXT science data can be stored in two modes (i) Photon Counting (PC) mode and (ii) Fast Windowed Photon Counting (FW) mode. In the PC mode, data from the entire 600 × 600 pixels of CCD is collected with a read-out time of ∼ 2.4 s. On the other hand, in the FW mode 150 × 150 pixel window in the centre of CCD is used with readout time (∼ 0.278 s) one order of magnitude lower than the PC mode.
Here we demonstrate how to extract source spectra and make them suitable for fitting in XSPEC. The PC mode observation of GRS 1915+105 is used as an example. The observation id is G06_033T01_9000001116. The test data used here is available in the public domain and can be downloaded from the astrobrowser portal.
$ cd results
$ xselect
** XSELECT V2.4j **
$ Enter session name >[ ] xsel
$ xsel:NICER > read event GRS_1915_sxt_merged_cl.evt
$ Enter the Event file dir >[ ] ./
Got new mission: ASTROSAT
$ Reset the mission? >[ ] yes
Notes: XSELECT set up for ASTROSAT
Time keyword is TIME in units of s
Default timing binsize = 5.0000
Setting...
Image keywords = X Y with binning = 1
WMAP keywords = X Y with binning = 1
Energy keyword = PI with binning = 1
Getting Min and Max for Energy Column...
Got min and max for PI: 0 1023
Got the minimum time resolution of the read data: 2.3775
MJDREF = 5.5197000000000E+04 with TIMESYS = UTC
Number of files read in: 1
******************** Observation Catalogue ********************
Data Directory is: /path_to_present_directory/
HK Directory is: /path_to_present_directory/
TELESCOP INSTRUME DATAMODE DATE-OBS OBJECT ORB_NUM
1 ASTROSAT SXT PC 2017-03-29 GRS 1915+105 8118
$ xsel:ASTROSAT-SXT-PC > extract image
extractor v5.36 26 Feb 2020
Getting FITS WCS Keywords
Doing file: /path_to_present_directory/GRS_1915_sxt_merged_cl.evt
100% completed
Total Good Bad: Time Phase Grade Cut
166868 166868 0 0 0 0
=================================================================
Grand Total Good Bad: Time Phase Grade Cut
166868 166868 0 0 0 0
in 5068.8 seconds
Image has 166868 counts for 32.92 counts/sec
$ xsel:ASTROSAT-SXT-PC > plot image
$ xsel:ASTROSAT-SXT-PC > filter region ds9.reg
$ xsel:ASTROSAT-SXT-PC > show filters
*** SELECTIONS ***
NONE
*** FILTERS ***
Spatial selections have been read in from: ds9.reg
$ xsel:ASTROSAT-SXT-PC > extract spectrum
extractor v5.36 26 Feb 2020
Getting FITS WCS Keywords
Doing file: /path_to_present_directory/GRS_1915_sxt_merged_cl.evt[regfilter("xsel_region.xsl",X,Y)]
100% completed
Total Good Bad: Time Phase Grade Cut
138211 138211 0 0 0 0
=================================================================
Grand Total Good Bad: Time Phase Grade Cut
138211 138211 0 0 0 0
in 5068.8 seconds
Spectrum has 138211 counts for 27.27 counts/sec
... written the PHA data Extension
$ xsel:ASTROSAT-SXT-PC > save spectrum sxt_spectra.pha
Wrote spectrum to sxt_spectra.pha
$ xsel:ASTROSAT-SXT-PC > exit
Save this session? >[ ] no
This spectrum can then be passed through grppha to make it suitable for use in XSPEC. The recent Ancillary Response File (ARF), Response Matrix Files (RMF), and background spectrum can be found here: https://www.tifr.res.in/~astrosat_sxt/dataanalysis
$ grppha
$ Please enter PHA filename[ ] sxt_spectra.pha
$ Please enter output filename[ ] sxt_spectra_grp.pha
-------------------------
MANDATORY KEYWORDS/VALUES
-------------------------
--------------------------------------------------------------------
--------------------------------------------------------------------
EXTNAME - SPECTRUM Name of this BINTABLE
TELESCOP - ASTROSAT Mission/Satellite name
INSTRUME - SXT Instrument/Detector
FILTER - NONE Instrument filter in use
EXPOSURE - 1364.7 Integration time (in secs) of PHA data
AREASCAL - 1.0000 Area scaling factor
BACKSCAL - 9.58510E-02 Background scaling factor
BACKFILE - none Associated background file
CORRSCAL - 1.0000 Correlation scaling factor
CORRFILE - none Associated correlation file
RESPFILE - none Associated redistribution matrix file
ANCRFILE - none Associated ancillary response file
POISSERR - TRUE Whether Poissonian errors apply
CHANTYPE - PI Whether channels have been corrected
TLMIN1 - 0 First legal Detector channel
DETCHANS - 1024 No. of legal detector channels
NCHAN - 1024 No. of detector channels in dataset
PHAVERSN - 1.2.0 OGIP FITS version number
STAT_ERR - FALSE Statistical Error
SYS_ERR - FALSE Fractional Systematic Error
QUALITY - TRUE Quality Flag
GROUPING - FALSE Grouping Flag
--------------------------------------------------------------------
--------------------------------------------------------------------
$ GRPPHA [ ] group min 30
$ GRPPHA [ ] chkey backfile SkyBkg_comb_EL3p5_Cl_Rd16p0_v01.pha
$ GRPPHA [ ] chkey ancrfile sxt_pc_excl00_v04_20190608.arf
$ GRPPHA [ ] chkey respfile sxt_pc_mat_g0to12_gmod.rmf
$ GRPPHA [ ] exit
... written the PHA data Extension
...... exiting, changes written to file : sxt_spectra_grp.pha
** grppha 3.1.0 completed successfully
$ xspec
XSPEC version: 12.11.0
Build Date/Time: Fri Jul 24 21:58:05 2020
$ XSPEC12>data sxt_spectra_grp.pha
1 spectrum in use
Spectral Data File: sxt_spectra_grp.pha Spectrum 1
Net count rate (cts/s) for Spectrum:1 2.719e+01 +/- 7.335e-02 (99.7 % total)
Assigned to Data Group 1 and Plot Group 1
Noticed Channels: 1-677
Telescope: ASTROSAT Instrument: SXT Channel Type: PI
Exposure Time: 5069 sec
Using fit statistic: chi
Using Background File SkyBkg_comb_EL3p5_Cl_Rd16p0_v01.pha
Background Exposure Time: 2.447e+05 sec
Using Response (RMF) File sxt_pc_mat_g0to12_gmod.rmf for Source 1
Using Auxiliary Response (ARF) File sxt_pc_excl00_v04_20190608.arf
$ XSPEC12>cpd /xw
$ XSPEC12>pl ld
$ XSPEC12>ig **-0.5 7.0-**
20 channels (1-20) ignored in spectrum # 1
78 channels (600-677) ignored in spectrum # 1
$ XSPEC12>pl
$ XSPEC12>setpl en
$ XSPEC12>pl
$ XSPEC12>ipl
$ PLT> hardcopy spectra.ps/ps
$ PLT> exit
$ XSPEC12>gain fit
Input parameter value, delta, min, bot, top, and max values for ...
1 0.01( 0.01) 0.01 0.5 1.5 5
Resp source_1_spec_1: slope>1,-1
Input parameter value, delta, min, bot, top, and max values for ...
0 0.01( 0.01) -1 -1 1 1
Resp source_1_spec_1: offset>
One should use fixed gain slope = 1. Only gain offset should be kept free. Once you have decided appropriate gain offset value (best fit) then these values can be frozen in the model so that the model fitting doen’t get affected.
$ XSPEC> systematic 0.02
The users are supposed to use 3% (as suggested by LAXPC team) systematic when fitting LAXPC and SXT data together so in that case no need to add extra 2% because of SXT only.
$ XSPEC12>save all spectra.xcm
$ XSPEC12>exit
$ XSPEC: quit