Re: empty spectrum

Hi Rob - 

1) if dmextract fails, it's probably a bug, and you should report it.
   There should at least be noise, or even if no noise, you should
   still be able to make a histogram of value 0.

2) Sure, you can load an arf and rmf and assign to a non-existent
   histogram index.  This is how you fake data:

  a = load_arf("my.arf");
  r = load_rmf("my.rmf");
  assign_rsp( a, r, 1 ) ; % assuming no data have been loaded into 1.

But now the tricky part starts. What, exactly, should the histogram
be?  You need to have *something* to fit - such as background noise,
or a very weak source spectrum.  If you want to know, "How much flux
could I have and still have zero counts/all bins?", you could define a
model:

  fit_fun("Powerlaw(1)");

set your params and 

  fakeit;

to get a fake counts spectrum.  You could then fit this and the do the
conf on the normalization.  You could the high value of the conf is
then (hopefully) something like your upper limit.

However, if you are background limited, you probably need to extract
background counts (dmextract will work) and then 

() = define_back (1, "background.pha");

to associate the background pha w/ histogram 1.

Then you can set your model norm to 0.0, set the upper range to
something, and do the fit and conf to see what norm value is an upper
limit.


Is this something like what you are trying to do?


--Dave


    rgibson> In some cases, I wish to extract an ACIS spectrum for a
    rgibson> source which has zero counts in the extraction region.
    rgibson> (Maybe the source was detected at other wavelengths, and
    rgibson> I want to put upper limits on the X-ray flux.)

    rgibson> psextract fails for these guys, but one can create an ARF
    rgibson> and RMF manually.  But then I still don't have a spectrum
    rgibson> file (dmextract fails).

    rgibson> Is there a simple, robust way to associate the ARF and RMF with a zero-count
    rgibson> spectrum and analyze the data as a normal spectrum?

    rgibson> Thanks,
    rgibson> Rob



David Huenemoerder  617-253-4283 (o); -253-8084 (f); http://space.mit.edu/home/dph
MIT Kavli Institute for Astrophysics and Space Research
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