ACIS absorption model v1.1

NOTE: This model is now in the Xspec release and this page is superceded by the Xspec manual

Model description and caveats

This model by George Chartas and Konstantin Getman at PSU is a first attempt at correcting for the absorption due to the stuff accumulating on the ACIS optical blocking filter. The user needs to supply the number of days between Chandra launch and observation. The composition of the hydrocarbon and the rate of decay should not be modified. The present version of ACISABS is to be used for the analysis of bare ACIS I and ACIS S data only. It is not appropriate for grating 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 0.67 keV and 5.895 keV 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., and tauinf = 0.582 +/- 0.024 based on fits to the observed decay rate performed by Allyn Tennant and Steve O'Dell (MSFC) .

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 LBL.

Here are a few illustrations of the effects of this model.

ACISABS Parameters

par1 = Days between Chandra launch and ACIS observation
par2 = norm parameter in decay rate equation (not used in model)
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

Installation instructions

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. Then follow the instructions for XSPEC local models.


Keith Arnaud, Lab. for High Energy Astrophysics, NASA/Goddard Space Flight Center

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Last modified: Tuesday, 14-Feb-2023 14:31:56 EST