Create a Phase-binned Spectrum

Get Started

Sample ObsID used: 133 (ACIS-I, PSR B0540-69)

File types needed: evt2

The pulsar used here has a frequency of 19.794885 Hz (period of 50.5181 ms).

The standard event files have not been barycenter corrected. We have already run the Apply Barycenter Correction thread to generate barycenter-corrected times, accounting for the difference in photon arrival times as Chandra orbits the Earth and the Earth moves around the Sun. This barycenter correction should precede any phase calculation.

Hereafter we will call the level=2 event file:

acis_133_bary_evt2.fits

It is important to note that you should not fold a spectrum on a period that is close to or smaller than the temporal resolution of the data. The majority of ACIS observations are done in timed exposure (TE) mode, collecting photons and reading them out periodically. Each readout is called a "frame" and every event in a given frame has the same time assigned to it. The default resolution is 3.2 s, plus 0.04104 seconds frame transfer time; the exact value used, including the frame transfer time, is stored in the TIMEDEL header keyword:

unix% dmkeypar acis_timed_evt2.fits TIMEDEL echo+
3.24104

Trying to analyze this TE-mode observation on the 50 ms timescale used in this thread would result in meaningless output.

The benefit of continuous clocking (CC) mode observations, such as the one used here, is that they have a much shorter time resolution since the data is continually read out:

unix% dmkeypar acis_133_bary_evt2.fits TIMEDEL echo+
0.00285

Define a Source Region

In subsequent steps of this thread, we are only interested in results from the source region, not the full field of view. Here we show how to define the source region.

Load the file into ds9:

unix% ds9 acis_133_bary_evt2.fits &

Since this is a CC-mode observation, there is only one spatial dimension (see the Continuous Clocking Mode why topic for details), so the image looks like Figure 1 .

Outline the source on the image. Figure 2  shows the rotated box used in this example; for information on how to rotate shapes, see this FAQ.

Then save the region from ds9:

• Region -> File Format-> Ciao
• Region -> File Coordinate System -> Physical
• Region -> Save Regions... -> Save As "source.reg"

The resulting region file will look something like this:

unix%  more source.reg 
# Region file format: CIAO version 1.0
rotbox(4095.125,4054.625,19.815731,5.6090815,333.70372)

If you are interested, you can use this region file as input to dmcopy to create a new, smaller event list. If you choose to do this, you will not need to specify a source region in the Run dmextract step.

Create PHASE column

When working with large files, you can save some time by combining the two dmtcalc steps that are used in this section. Here we give the short-and-sweet example; for a full explanation of the steps, jump ahead to the Convert Time to Phase step.

unix% punlearn dmtcalc
unix% dmtcalc infile=acis_133_bary_evt2.fits outfile=acis_133_PHASE.fits \
      expression=@dmtcalc.expr

where

unix% cat dmtcalc.expr 
GPHASE=(time-TSTART)*19.794885
PHASE=GPHASE-(long)GPHASE

Note that this step does not take in account the frequency derivative; see the end of the first section for information on how to do so.

After completing this step, you can go right to the Extract the Spectrum section of this thread.

unix% punlearn dmtcalc
unix% pset dmtcalc infile=acis_133_bary_evt2.fits
unix% pset dmtcalc outfile=acis_133_GPHASE.fits
unix% pset dmtcalc expression="GPHASE=(time-TSTART)*19.794885"
unix% dmtcalc 
Input file (acis_133_bary_evt2.fits): 
Output file (acis_133_GPHASE.fits): 
expression(s) to evaluate (GPHASE=(time-TSTART)*19.794885): 

unix% pset dmtcalc expression="GPHASE=(time-TSTART)*(f0 +(fdot * (time-TSTART)))"

unix% pset dmtcalc expression="GPHASE=(time-TSTART)/(P0 +(Pdot * (time-TSTART)))"

unix% punlearn dmtcalc
unix% pset dmtcalc infile=acis_133_GPHASE.fits
unix% pset dmtcalc outfile=acis_133_PHASE.fits
unix% pset dmtcalc expression="PHASE=GPHASE-(long)GPHASE"
unix% dmtcalc
Input file (acis_133_GPHASE.fits): 
Output file (acis_133_PHASE.fits): 
expression(s) to evaluate (PHASE=GPHASE-(long)GPHASE):

Make a Lightcurve (optional)

At this point, it is possible to make a lightcurve of the phase-folded data. Use dmextract to bin on the PHASE column while simultaneously filtering on the source region:

unix% dmextract "acis_133_PHASE.fits[sky=region(source.reg)][bin PHASE=0:1:0.1]" \
      acis_133_lightcurve.fits opt=generic

The resultant lightcurve can be displayed in ChIPS:

unix% chips

Welcome to ChIPS, version CIAO 3.4
Copyright (C) 1999-2003, Smithsonian Astrophysical Observatory

chips> curve "acis_133_lightcurve.fits[cols phase,counts,stat_err]" x 1 y 2 yerr 3
chips> xlabel PHASE
chips> xlabel size 1.5
chips> ylabel COUNTS  
chips> ylabel size 1.5

chips> quit

Figure 3  shows the plot, which clearly illustrates the counts as a function of phase.

IMPORTANT NOTE: make sure that an interesting feature that may appear in your lightcurve is not the result of a false period; for more information, see the Timing Analysis with Lightcurves why topic. An unfortunate choice in the width of the phase bin may lead to features that are not real. This is due in part to the fact that events need not be distributed uniformly in the phase histogram even if they are distributed uniformly in phase from the source.

Also, note that exposure times are not modified and that the counts histogram has no GTI information. This is the same limitation that is mentioned in the Caveats section.

The contents of the parameter file may be checked using plist dmextract.

Extract the Spectrum

unix% punlearn dmextract
unix% pset dmextract \
      infile="acis_133_PHASE.fits[sky=region(source.reg),phase=0.2:0.4][bin pi]"
unix% pset dmextract outfile=acis_133_phase_bin_spectrum.fits
unix% dmextract 
Input event file (acis_133_PHASE.fits[sky=region(source.reg),phase=0.2:0.4][bin pi]): 
Enter output file name (acis_133_phase_bin_spectrum.fits): 

unix% sherpa
 
-----------------------------------------------------
Welcome to Sherpa: CXC's Modeling and Fitting Program
-----------------------------------------------------
Version: CIAO 3.4

Type AHELP SHERPA for overview.
Type EXIT, QUIT, or BYE to leave the program.

Notes:
    Temporary files for visualization will be written to the directory: 
    /tmp
    To change this so that these files are not deleted when you exit Sherpa,
    edit $ASCDS_WORK_PATH in your 'ciao' setup script.

    Abundances set to Anders & Grevesse

sherpa> data acis_133_phase_bin_spectrum.fits 
The inferred file type is PHA.  If this is not what you want, please 
specify the type explicitly in the data command.

sherpa> ploty counts
sherpa> sherpa.dataplot.symbolstyle="none"     
sherpa> sherpa.dataplot.curvestyle="simpleline"
sherpa> lplot data

Caveats

1. This thread does not correctly account for the data in the GTI tables when binning on phase. That is, the EXPOSURE keyword in the header of the new file IS NOT the sum of the new time bins. It follows the usual guidelines: filtering on one column doesn't update any information in the other columns. There are future plans to update this thread with a method that properly modifies the GTIs (through use of dmgti).

2. For fast timing of ACIS CC data, there are other corrections that may be important, such as read-out time and removing the dither and motion of the SIM . There are planned modifications to acis_process_events to take these calculations into account. Until the updated tool is available, please see Zavlin, et al. 2000 (ApJ, 540, L25) for a discussion of correcting the event arrival times.

   Note that the Zavlin paper does not describe how to remove the readout time, only the dither and SIM motion. As a result, the events will still have some phase offset, but it will be the same for all of them. A comparison of the absolute phase of Chandra ACIS CC data to data from another mission can not be performed until this effect is also removed.