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LAXPC Data Reduction Guideline

• LAXPC Data

  Light curves and Spectrum in the Processed Data

• Broad Band (BB) Counting mode

• Event Analysis (EA) mode

• Create Good Time Interval

• Create Light Curve

• Create Spectrum

• Create Phase Resolved Spectrum

• Avoid extra background in the light curve

• LAXPC tool to merge event mode data

• LAXPC Spectroscopy

• LAXPC Data

  Data from the three LAXPCs are processed independently.

  LAXPC archival users and guest observers should download level2 data from ISSDC

  You need to be familiar with the LAXPC level2 data structure

• Light curves and Spectrum in the Processed Data

  The level2 processed data includes light curves (BB and EA mode) and spectrum (EA mode). These are created from almost the entire data duration. These are useful indicators of the quantity (duration) and quality of data. However, these are not to be used for scientific analysis. Light curves and spectra are to be extracted separately using the processes described below for any scientific analysis. Importantly, good time intervals should be selected carefully as described below.

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  You may want to start by plotting various parameters in the mkf file using fv or fplot. The following parameters could be of interest to many users.

  ELV,
  ANG_OFFSET,
  CPM_Rate,
  Roll_RA,
  Roll_DEC,
  EARTHLAT,
  EARTHLON,
  LX_DET_*_HVM,
  LX_DET_*_HV_CORONA_STS,
  LX_DET_*_HV_SAA_STS.
  

• Broad Band (BB) Counting mode

  For each LAXPC, light curves can be extracted from modeBB data using lxplc:

  lxplc infile=infile_modeBB.dat gtifile=gtifile.gti outfile=outfile_modeBB.lc

  Two other parameters that can be set are the

  BB_binsize

  and

  BB_counters

  (see the parameter file lxplc.par in your pfiles directory).

  BB_binsize should be an integer and the light curve will be produced with a binsize that is larger than the default value in BB mode data by this factor.

  You can read description of the various counters in the BB mode data here, or in the header of the datafile and choose BB_counters (example: BB_counters=C12, C13) for light curve extraction accordingly. Make sure not to choose combinations that will add up the same energy band twice. Counter C4 gives the total genuine count rate in the LAXPC detectors.

  
  

• Event Analysis (EA) mode

  For each LAXPC, modeEA data is analysed in the following manner.

  The default processing creates light curves from all seven anodes (in five layers) of the detector with a binsize of 1 sec. Find the file with .lc extension and plot it. The light curve may look like this. Some part of this light curve contains source+background data and some part may contain background data when the spacecraft is facing the earth or looking at a blank part of the sky.

• Create Good Time Intervals

  Find the good time intervals from the light curve itself form the intervals 'source+background' and 'background' and create two GTI files for source and background. Or you can use this GTI creation tool and the MKF file to create good time intervals for source and background.

• Create Light Curve

  You can extract light curves from event mode data using the tool lxplc for the source and the background time intervals.

  lxplc infile=infile_modeEA.dat gtifile=gtifile.gti outfile=outfile_modeEA.lc

  The parameters that need to be specified are :

  EA_binsize: Binsize of the light curve (in seconds),
  EA_Layer_Energy: specify the anodes and the corresponding channel range to be used (L1 0-1023,L2 0-1023,L3 0-1023,L4 0-1023,L5 0-1023,L6 0-1023,L7 0-1023),
  EA_Event_Type: Event Type (single, double or all)
  

  (see the parameter file lxplc.par in your pfiles directory).

  Plot the original lightcurve, the source light curve, and the background light curve to satisfy yourself that the GTI selections are appropriate. The light curve would look like this.

  [If this observation does not have background data, you can use background data from a different observation. There is a few percent difference. LAXPC model background will be available in future.

• Create Spectrum

  Similarly, extract the spectrum using the tool 'lxpspec' for the source and the background time intervals.

  lxpspec infile=infile_modeEA.dat gtifile=gtifile.gti outfile=outfile_modeEA.pha

  The parameters that need to be specified are :

  EA_Layer_Id: specify the anodes to be used (L1, L2, L3, L4, L5, L6, L7)
  EA_Event_Type: Event Type (single, double or all)
  

  (see the parameter file lxpspec.par in your pfiles directory).

  Plot the source and the background spectra together with XSPEC. This sample figure shows the channel (PHA) range over which the source is detected over background, 17-520 in this figure. This will depend on the brightness of the sources and the hardness of the spectrum.

  You can extract the light curve using any binsize you desire from any of the anodes over any PHA range. You are set to do timing analysis with LAXPC light curves. You can also extract spectra from anode ranges of your choice and you are ready to do spectral analysis.

• Create Phase Resolved Spectrum

  Phase resolved spectra (pulse phase resolved spectra for a pulsar or orbital phase resolved spectra for a binary) can also be extracted with lxpspec. For this, set:

  flag_phase_spectra=yes
  

  and then specify the Epoch (To), Period, StartingPhase and EndingPhase (see the parameter file lxpspec.par in your pfiles directory). StartingPhase should be smaller than EndingPhase and both should be within 0.0 and 1.0.

  lxpspec infile=infile_modeEA.dat gtifile=gtifile.gti outfile=outfile_modeEA.pha flag_phase_spectra=yes To=****, Period,=**** StartingPhase=**** EndingPhase=****

  The above should be done for each of the phase ranges.

• Avoid extra background in the light curve

  However, in LAXPC, the soft and medium energy X-rays do not reach the bottom layers of the detectors. In addition, upto what highest energy the X-rays are detected in each layer depends on the source brightness and hardness of the source spectrum. You can therefore, choose the PHA range suitably for each layer while extracting the light curve and spectra. This will ensure that you have the minimum background.

  Here are the steps to get minimum background:

  Extract source and background spectra for each anode separately. Plot the same using XSPEC.

  XSPEC> data src_01.pha back bkg_01.pha

  XSPEC> plot

  For each anode, note down the PHA range over which the source is detected. Here is a sample plot of the spectrum from five layers (from the seven anodes) of LAXPC shown from top to bottom. It shows that the source is detected in channel range 10-360 in layer-1, 80-360 in layer-2 and so on.

  Now extract light curve for the useful anodes over these PHA ranges.

  Do the same for the background.

  Now here is a plot of the source and background light curves from selected PHA ranges for different anodes. The background, in this figure is reduced by a factor of a few compared to the earlier figure while the source count has remained same, increasing the signal to noise ratio.

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