Model Description and Caveats

This model by George Chartas and Konstantin Getman at PSU is a first attempt at correcting
for the absorption caused by molecular contamination of the ACIS optical blocking filters.
The user needs to supply the number of days between Chandra launch and observation.
The ACISABS parameters related to the composition of the hydrocarbon and the rate of
decay should be frozen and not modified. The present version of ACISABS is to be used for
the analysis of bare ACIS I and ACIS S data. The present version of ACISABS
requires the standard qe file vN0003 instead of the optional vN0004 file be used in mkarf.

ACISABS assumes a decrease in time of the ACIS quantum efficiency based on the observed
decay in the ratio of the 0.67keV and 5.895keV line complexes of the external calibration source
as observed with ACIS S3. The decay-rate measurements were performed by Catherine Grant (MIT).
The decay rate is modelled by the function:

R(t) = norm*double(exp(-tauinf*(1.d0 - exp(-t/tefold))))

with              norm = 0.00722 +/- 0.00007
                      tefold = 620. +/- 66.
                      tauinf = 0.582 +/- 0.024

based on Allyn Tennant's and Steve O'Dells (MSFC) fits to the observed decay rate.

The composition of the contaminant was inferred from simple fits to grating
observations of PKS2155 and probably  will need to be
updated  when the composition of the contaminant is better constrained .  For
the purposes of bare ACIS spectral analysis the model ACISABS appears to produce reasonable
results based on fits to several test-sources.

Because of the present large uncertainty in the ACIS gain at energies below 350eV we
recommend that events in the 0-350eV range be ignored in the spectral analysis
until the gain issue is resolved.

ACISABS calculates the mass absorption coefficients of the
contaminant from atomic scattering factor files
provided at :
http://www-cxro.lbl.gov/optical_constants/asf.html

ACISABS Parameters:

par1 = Days between Chandra launch and ACIS observation
par2 = norm parameter in decay rate equation
par3 = tauinf parameter in decay rate equation
par4 = tefold parameter in decay rate equation
par5 = Number of carbon atoms in hydrocarbon
par6 = Number of hydrogen atoms in hydrocarbon
par7 = Number of oxygen atoms in hydrocarbon
par8 = Number of nitrogen atoms in hydrocarbon
 
 
 

Instructions for Installing the ACISABS Model

The files required by ACISABS are available in a gzipped tar file.
This should be untarred in the user's local model directory (ie whatever you set LMODDIR to).
The files extracted will be acisabs.f, the source code, c.nff, h.nff, n.nff, o.nff, the mass absorption
coefficients, and acisabs_lmodeldat, the model description for the lmodel.dat file. This last file
should either be renamed to lmodel.dat or appended to your current lmodel.dat file if you already
have other local models in use. For information on using xspec local models please, see page 155
(Adding models to XSPEC) at :
http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/xspec11.2_manual.pdf
and xspec installation instructions at:
http://heasarc.gsfc.nasa.gov/docs/software/lheasoft/install_src.html .
 

acisabs.pro : An Equivalent Approach

An equivalent approach to using the xspec model ACISABS
to account for the ACIS qe decay is to
modify the arf file created by the CXC tool mkarf
using the software tool acisabs.pro written in IDL.
With this approach there is no need to add an additional model
in the spectral fits.

The user needs to supply the number of days between Chandra
launch and observation.
The present version V1.1 of acisabs.pro is to be used for
the analysis of bare ACIS I and ACIS S data. The present
version V1.1 of acisabs.pro requires the standard qe file vN0003
instead of the optional vN0004 file be used in mkarf.

The user edits the parameter file acisabs.par and
inputs the number of days between Chandra launch
and observation, and the name of the original arf
file produced by mkarf.

acisabs.pro creates a new arf file with a corrected effective
area column.
 

EXAMPLE :

In the following example we show how to use acisabs.pro
to produce a new arf file from an existing one
produced by mkarf
 

step 1. Download acisabsv1.1_idl.tar.gz from the site:

http://www.astro.psu.edu/users/chartas/xcontdir/acisabsv1.1_idl.tar.gz

and place it in your local working directory.
Unpack the gzipped tar file:

gunzip acisabsv1.1_idl.tar.gz
tar xvf acisabsv1.1_idl.tar
 

step 2.
Edit the parameter file acisabs.par
You need to supply the number of days between Chandra
launch and observation and the name of the original
arf file that you want to modify.
Place the original arf file in your working directory
or provide the complete path to the original arf file.
You can edit acisabs.par using vi or emacs.
 

step 3.
run the IDL program acisabs.pro

idl
.r acisabs

A plot will appear with the correction applied
to the original effective area.
acisabs.pro creates a modified arf file in your working
directory.