Basic Lightcurves
[CIAO 3.4 Science Threads]
OverviewLast Update: 1 Dec 2006 - updated for CIAO 3.4: CHIPS version Synopsis: The CIAO lightcurve tool is no longer actively maintained or enhanced. Users instead are encouraged to create lightcurves with the new functionality of dmextract, which more carefully and accurately applies good time interval (GTI) information. Purpose: To create lightcurves for use in a variety of analyses. Related Links:
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Contents
- Get Started
- Tool: dmextract vs. lightcurve
- ACIS Lightcurves
- HRC Lightcurves
- Caveats
- Parameter files:
- History
- Images
Get Started
Sample ObsIDs used: 953 (ACIS-I, 47 Tuc); 461 (HRC-I, 3C 273)
File types needed: evt2; dtf1 (ObsID 461 only)
Tool: dmextract vs. lightcurve
Here we compare the syntax of the two tools; more information is available from the help files for dmextract and lightcurve.
This lightcurve command:
unix% lightcurve infile="acis_evt2.fits[ccd_id=3,sky=region(src.reg)]" \
outfile="lightcurve.fits" binlength="259.28"
becomes the following for dmextract:
unix% dmextract \
infile="acis_evt2.fits[ccd_id=3,sky=region(src.reg)][bin time=::259.28]" \
outfile="lightcurve.fits" opt="ltc1"
ACIS Lightcurves
The most common lightcurve is made from a point source observed with the ACIS detector. This may be done to get an idea of the variability of the source or to help identify periods of high background.
To begin, we define the regions - two source and one
background - which will be used to create the
lightcurves. For instructions on how to create regions in
ds9, see the Using
CIAO Region Files thread. The regions used here
are shown in Figure 1 ![[Link to Image 1: Image of 47 Tuc with extraction regions]](../../imgs/imageicon.gif){kind=small}
:
unix% more src1.reg # Region file format: CIAO version 1.0 circle(4008.5,4035.5,8) unix% more src2.reg # Region file format: CIAO version 1.0 circle(4032.5,4032.5,8) unix% more bkg.reg # Region file format: CIAO version 1.0 circle(3875.5,3972,54.5)
Determine which chips are being used
dmextract uses a ccd_id filter on the input file ensure that the proper GTIs are used. Use dmstat to determine the correct chip:
unix% dmstat "acisf00953N002_evt2.fits[sky=region(src1.reg)][cols ccd_id]"
ccd_id
min: 3 @: 1
max: 3 @: 1
mean: 3
sigma: 0
sum: 6975
good: 2325
null: 0
unix% dmstat "acisf00953N002_evt2.fits[sky=region(src2.reg)][cols ccd_id]"
ccd_id
min: 3 @: 1
max: 3 @: 1
mean: 3
sigma: 0
sum: 5622
good: 1874
null: 0
unix% dmstat "acisf00953N002_evt2.fits[sky=region(bkg.reg)][cols ccd_id]"
ccd_id
min: 3 @: 1
max: 3 @: 1
mean: 3
sigma: 0
sum: 660
good: 220
null: 0
The regions with which we are working are all located on chip 3 (ACIS-I3); see Figure 6.1 of the POG for an illustration of the focal plane)
Create a background-subtracted lightcurve
First we extract a background-subtracted lightcurve for "src2":
unix% punlearn dmextract unix% pset dmextract \ infile="acisf00953N002_evt2.fits[ccd_id=3,sky=region(src2.reg)][bin time=::2000]" unix% pset dmextract outfile="src2_sub_lc.fits" unix% pset dmextract bkg="acisf00953N002_evt2.fits[ccd_id=3,sky=region(bkg.reg)]" unix% pset dmextract opt="ltc1" unix% dmextract Input event file (acisf00953N002_evt2.fits[ccd_id=3,sky=region(src2.reg)][bin time=::2000]): Enter output file name (src2_sub_lc.fits):
You can check the parameter file that was used with plist dmextract.
The lightcurve may be plotted using ChIPS:
unix% chips Welcome to ChIPS, version CIAO 3.4 Copyright (C) 1999-2003, Smithsonian Astrophysical Observatory chips> curve "src2_sub_lc.fits[cols time,net_rate,err_rate]" yerr 3 chips> curve simpleline chips> curve red chips> exit
This lightcurve is shown in Figure
2 . Notice that there is a significant drop in
count rate near 6.961x107 seconds. This
information is used again in the next section.
Looking for variability
It is also valuable to compare the lightcurves of sources in the same field when looking for variation. Here we extract a lightcurve for each of the "src1" and "src2" regions (intentionally including the background counts):
unix% punlearn dmextract
unix% dmextract outfile="curve_1.fits" opt="ltc1" \
infile="acisf00953N002_evt2.fits[ccd_id=3,sky=region(src1.reg)][bin time=::2000]"
unix% dmextract outfile="curve_2.fits" opt="ltc1" \
infile="acisf00953N002_evt2.fits[ccd_id=3,sky=region(src2.reg)][bin time=::2000]"
and compare the results in ChIPS:
unix% chips Welcome to ChIPS, version CIAO 3.4 Copyright (C) 1999-2003, Smithsonian Astrophysical Observatory chips> split 2 chips> split gap 0.05 chips> d 1 curve "curve_1.fits[cols time,count_rate]" chips> d 2 curve "curve_2.fits[cols time,count_rate]" chips> d all curve simpleline chips> d all curve red chips> d 1 ylabel "curve\_1.fits" chips> d 1 ylabel "curve\_1.fits" chips> d 2 ylabel "curve\_2.fits" chips> d all ylabel size 1.5 chips> exit
These commands produce Figure
3 . Comparing this to the background-subtracted
lightcurve created in the previous section proves two
things:
-
This is not an intrumental effect, since it shows up in curve_2.fits but not curve_1.fits. An instrumental feature would appear in both sources, as they are close together, on the same chip, etc.
-
This is not a background feature, since it is present in both the subtracted (src2_sub_lc.fits) and unsubtracted (curve_2.fits) lightcurves.
It is highly likely, therefore, that the dip in count rate is an indication of a variable star. In the case of 47 Tuc, this is due to a binary system; see the Chandra Photo Album entry for 47 Tucanae for more information.
High background levels
The technique for identifying periods of high background from a lightcurve - and subsequently filtering them out - is explained in detail in the Filtering Lightcurves thread.
HRC Lightcurves
The proper method of creating an HRC lightcurve requires accounting for the Dead Time Factor (DTF). The DTF describes the detector's deviation from the standard detection efficiency. This time-dependent change is due to the physical effect of an event striking the micro-channel plate, as illustrated in the MCP Principles page. The DTF is evaluated roughly every 2 seconds and the data are stored in the "dtf1.fits" file. The DTF is correlated with the count rate, requiring careful attention for proper analysis.
As of CIAO 3.3, the DTF file can be input to the exp parameter of dmextract, which greatly simplifies the process of creating HRC lightcurves. The average DTF value within the time bin is used to correct the exposure time and count rate in the lightcurve.
The source region for this example has been saved in the file hrc_src.reg. Again, for instructions on how to create regions in ds9, see the Using CIAO Region Files thread.
unix% more hrc_src.reg # Region file format: CIAO version 1.0 circle(16478,16292,19.5) unix% punlearn dmextract unix% pset dmextract infile="hrcf00461N003_evt2.fits[sky=region(hrc_src.reg)][bin time=64938947.367:64959159.548:1000]" unix% pset dmextract outfile=hrc_lc.fits unix% pset dmextract opt=ltc1 unix% pset dmextract exp=hrcf00461_001N003_dtf1.fits unix% dmextract Input event file (hrcf00461N003_evt2.fits[sky=region(hrc_src.reg)][bin time=64938947.367:64959159.548:1000]): Enter output file name (hrc_lc.fits):
You can check the parameter file that was used with plist dmextract.
Plotting the lightcurve in ChIPS:
unix% chips Welcome to ChIPS, version CIAO 3.4 Copyright (C) 1999-2003, Smithsonian Astrophysical Observatory chips> curve "hrc_lc.fits[cols time,count_rate]" chips> limits y 10.7 auto chips> c 1 simpleline chips> c 1 red
creates Figure 4 .
Refer to the Filtering Lightcurves thread for information on how to handle the low point at ~6.496E+07 seconds.
Caveats
There are a number of subtleties that it is important to be aware of when using lightcurves for timing analysis. These issues are described in the Timing Analysis with Lightcurves why topic; please read that document before continuing with the analysis.
Parameters for /home/username/cxcds_param/dmextract.par
#--------------------------------------------------------------------
#
# DMEXTRACT -- extract columns or counts from an event list
#
#--------------------------------------------------------------------
infile = acisf00953N002_evt2.fits[ccd_id=3,sky=region(src2.reg)][bin time=::2000] Input event file
outfile = src2_sub_lc.fits Enter output file name
(bkg = acisf00953N002_evt2.fits[ccd_id=3,sky=region(bkg.reg)]) Background
region file or fixed background (counts/pixel/s) subtraction
(error = gaussian) Method for error determination(poisson|gaussian|<variance file>)
(bkgerror = gaussian) Method for background error determination(poisson|gaussian|<variance file>)
(bkgnorm = 1.0) Background normalization
(exp = ) Exposure map image file
(bkgexp = ) Background exposure map image file
(sys_err = 0) Fixed systematic error value for SYS_ERR keyword
(opt = ltc1) Output file type: pha1
(defaults = ${ASCDS_CALIB}/cxo.mdb -> /soft/ciao/data/cxo.mdb) Instrument defaults file
(wmap = ) WMAP filter/binning (e.g. det=8 or default)
(clobber = no) OK to overwrite existing output file(s)?
(verbose = 0) Verbosity level
(mode = ql)
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Parameters for /home/username/cxcds_param/dmextract.par
#--------------------------------------------------------------------
#
# DMEXTRACT -- extract columns or counts from an event list
#
#--------------------------------------------------------------------
infile = hrcf00461N003_evt2.fits[sky=region(hrc_src.reg)][bin time=64938947.367:64959159.548:1000] Input event file
outfile = hrc_lc.fits Enter output file name
(bkg = ) Background region file or fixed background (counts/pixel/s) subtraction
(error = gaussian) Method for error determination(poisson|gaussian|<variance file>)
(bkgerror = gaussian) Method for background error determination(poisson|gaussian|<variance file>)
(bkgnorm = 1.0) Background normalization
(exp = hrcf00461_001N003_dtf1.fits) Exposure map image file
(bkgexp = ) Background exposure map image file
(sys_err = 0) Fixed systematic error value for SYS_ERR keyword
(opt = ltc1) Output file type
(defaults = ${ASCDS_CALIB}/cxo.mdb -> /soft/ciao/data/cxo.mdb) Instrument defaults file
(wmap = ) WMAP filter/binning (e.g. det=8 or default)
(clobber = no) OK to overwrite existing output file(s)?
(verbose = 0) Verbosity level
(mode = ql)
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History
| 03 Jan 2005 | reviewed for CIAO 3.2: no changes |
| 21 Dec 2005 | updated for CIAO 3.3: default value of dmextract error and bkgerror parameters is "gaussian"; dmextract can now accept a DTF file in the exp parameter, which simplifies the process of creating HRC Lightcurves |
| 01 Dec 2006 | updated for CIAO 3.4: CHIPS version |
