Compute LETG/HRC-S Grating ARFs

Background Information

mkgarf incorporates the telescope dither explicitly and averages over the response of the detector region which is exposed to a point in the sky. Hence, plate gaps and other detector non-uniformities, as well as the mirror vignetting function, are included. Arbitrary dither, off-axis response, or off-nominal source positions are not accounted for in the pre-computed effective area files.

The tool computes the efficiency for a full aperture in the cross-dispersion direction. The effect of the finite region width is calculated by mkgrmf, since the line-spread-function is physically coupled to the extraction width. This aperture efficiency factor is accounted for in analysis when using an ARF and RMF jointly to model the instrument. Sherpa, ISIS, and XSPEC will include this factor automatically when using both the ARF and RMF. ISIS also provides a factor_rsp function with which this energy-dependent term can be extracted, if it is explicitly needed.

Get Started

Sample ObsID used: 460 (LETG/HRC-S, 3C 273)

File types needed: evt2; pha2; asol1; bpix1; dtf1

It is assumed that you have created gRMFs for your observation by running the Create Grating RMFs for HRC-S Observations thread.

If you created a new bad pixel file by running the Creating a New Observation-Specific HRC Bad Pixel File thread, make sure that you have set up ardlib to use the same bad pixel file.

Determine the orders to model

Because of the low energy resolution in the HRC-S, the PHA2 file contains two rows (negative and postive)  containing all the spectral orders. While it is not possible to separate the overlapping orders, CIAO can make response files (gRMFs and gARFs) for higher orders, allowing you to model and fit them.

We choose to focus on the first three positive and negative orders of the spectra. The Creating Higher-order Responses for HRC-S/LETG Spectra thread provides some advice on determing the number of orders to model.

Compute the Aspect Histograms (asphist)

The aspect solution files are used to create a binned histogram detailing the aspect history of the observation. An aspect histogram must be created for each plate of the detector you are going to use.

To create a histogram for each of the three plates - S1 (chip_id=1), S2 (chip_id=2), and S3 (chip_id=3) - the tool is run three times:

unix% punlearn asphist
unix% pset asphist infile=pcadf063792009N003_asol1.fits
unix% pset asphist dtffile=hrcf00460_000N004_dtf1.fits

unix% foreach f ( 1 2 3 )
foreach? asphist evtfile="hrcf00460N004_evt2.fits[chip_id=$f]" \
         outfile=460_asphist_s{$f}.fits mode=h
foreach? end

# asphist (CIAO 3.4): WARNING: skipping 41 livetime correction records (from time: 63792195.736228 to time: 63792277.736231)
...

unix% ls -1 *asphist*
460_asphist_s1.fits
460_asphist_s2.fits
460_asphist_s3.fits

In many cases there will be more than one asol1.fits file for an observation. All the files must be input to the infile parameter, either as a list or as a stack (see ahelp stack for more information).

Many "skipping N livetime correction records" may be printed to the screen. These warnings are common and do not indicate a problem.

Get Source Position (dmlist)

The source position is required as one of the inputs to mkgarf. This information can be easily obtained from the PHA2 file with dmlist:

unix% dmlist hrcf00460N004_pha2.fits"[cols x,y]" opt=data
 
--------------------------------------------------------------------------------
Data for Table Block SPECTRUM
--------------------------------------------------------------------------------
 
ROW    X                    Y
 
     1           32842.6250      32638.082031250
     2           32842.6250      32638.082031250

The source is located at (32842.6250, 32638.082031250).

Run mkgarf

Now we have all the information needed to run mkgarf. The tool needs to be run once per plate for each order; each order falls on two of the plates. Since we are interested in three positive and three negative orders, we need to run the tool twelve times:

• S1: +1, +2, +3
• S2: -1, -2, -3, +1, +2, +3
• S3: -1, -2, -3

Again, refer to the HRC-S plates and LETG orders section if you need assistance determining how many times to run mkgarf. Remember that if you run it for the "wrong" plate, the tool will just issue a warning and create an empty file.

unix% punlearn mkgarf
unix% pset mkgarf asphistfile="460_asphist_s1.fits[ASPHIST]"
unix% pset mkgarf detsubsys=HRC-S1
unix% pset mkgarf grating_arm=LEG
unix% pset mkgarf sourcepixelx=32842.6250 sourcepixely=32638.082031250

unix% foreach f ( 1 2 3 )
foreach? mkgarf outfile=garf_460_S1_LEG_{$f}.fits order=$f \
         engrid="grid(460_leg_$f.grmf[cols ENERG_LO,ENERG_HI])" \
         mode=h
foreach? end

unix% pset mkgarf asphistfile="460_asphist_s2.fits[ASPHIST]"
unix% pset mkgarf detsubsys=HRC-S2

unix% foreach f ( -1 -2 -3 1 2 3 )
foreach? mkgarf outfile=garf_460_S2_LEG_{$f}.fits order=$f \
         engrid="grid(460_leg_$f.grmf[cols ENERG_LO,ENERG_HI])" \
         mode=h
foreach? end

unix% pset mkgarf asphistfile="460_asphist_s3.fits[ASPHIST]"
unix% pset mkgarf detsubsys=HRC-S3

unix% foreach f ( -1 -2 -3 )
foreach? mkgarf outfile=garf_460_S3_LEG_{$f}.fits order=$f \
         engrid="grid(460_leg_$f.grmf[cols ENERG_LO,ENERG_HI])" \
         mode=h
foreach? end

The history of each file, stored in the header, contains the complete mkgarf command used to create that gARF:

unix% dmhistory garf_460_S2_LEG_3.fits mkgarf
mkgarf asphistfile="460_asphist_s2.fits[ASPHIST]"
outfile="garf_460_S2_LEG_3.fits" order="3" sourcepixelx="32842.625"
sourcepixely="32638.08203125" engrid="grid(460_leg_3.grmf[cols
ENERG_LO,ENERG_HI])" obsfile="460_asphist_s2.fits[ASPHIST]"
osipfile="NONE" maskfile="NONE" mirror="hrma" detsubsys="HRC-S2"
grating_arm="LEG" ardlibparfile="ardlib.par" geompar="geom"
verbose="0" clobber="no"  

Combine the gARFs (dmarfadd)

dmarfadd is used to combine the individual gARFs for each order into a single response file. The tool is run six times, once for each order:

unix% punlearn dmarfadd
unix% dmarfadd infile="garf_460_S2_LEG_-1.fits,garf_460_S3_LEG_-1.fits" \
      outfile=460_LEG_-1.garf

unix% dmarfadd infile="garf_460_S2_LEG_-2.fits,garf_460_S3_LEG_-2.fits" \
      outfile=460_LEG_-2.garf

unix% dmarfadd infile="garf_460_S2_LEG_-3.fits,garf_460_S3_LEG_-3.fits" \
      outfile=460_LEG_-3.garf

unix% dmarfadd infile="garf_460_S1_LEG_1.fits,garf_460_S2_LEG_1.fits" \
      outfile=460_LEG_1.garf

unix% dmarfadd infile="garf_460_S1_LEG_2.fits,garf_460_S2_LEG_2.fits" \
      outfile=460_LEG_2.garf

unix% dmarfadd infile="garf_460_S1_LEG_3.fits,garf_460_S2_LEG_3.fits" \
      outfile=460_LEG_3.garf

The "ARF was computed to be zero at all energies" Warning

When running mkgarf, a warning of this form may be printed:

*** WARNING: The ARF was computed to be zero at all the specified energies.
             This is probably due to an incorrect source position

There are a few possible situations that are causing this:

1. An incorrect source position was input to the sourcepixelx and sourcepixely parameters. Confirm that the source position is correct, and re-run the tool if it's not.

2. You are attempting to create a gARF for an order that does not fall on that plate, e.g. a positive order on HRC-S3. Refer to the HRC-S plates and LETG orders section for more information.

If neither of these seems to fix the problem, contact the Helpdesk for assistance.

Note on the Effective Area

The grating ARFs as generated by mkgarf are appropriate for events falling within the full 2pi steradian solid angle behind the grating. They do not include a correction for the fraction of these events that fall outside the "bowtie" extraction region whence the first-order spectra are selected from the L2 event file (tgextract). This fraction is of order 10%, and a correction for this is included in the RMF generated by mkgrmf, specifically in the LSFPARM calibration file.

Use of ARF and RMF files combined, e.g. within Sherpa, XSPEC, or ISIS, will result in the correct effective area being applied. Should they be required for analysis of LETG+HRC-S observations using methods or software that cannot easily combine RMF and ARF files, pre-generated on-axis LETG+HRC-S ARF files that include the bow-tie "extraction efficiency" are available on the LETG instrument team web page.