From: Hironori Matsumoto <matumoto>
Date: Wed, 19 Dec 2001 17:15:59 -0500 (EST)

I fitted an ACIS spectrum with both ISIS and XSPEC, and
found that the chi^2 value of ISIS is different from that of
XSPEC. Does anyone know the reason?

Let me show what I did in detail. I used an ACIS spectrum
taken without HETG, and the spectrum is not binned. I used
an absorbed brems model to fit the spectrum in the 0.5 -- 10
keV band. I fixed the column density, temperature, and ratio
of He to H. That means, only the normalization is free. I
used a standard method for the error estimation
(i.e. sqrt(N)) in XSPEC, which I think is the same method as
used in ISIS. I show the results below:

               ISIS                    XSPEC
NH(cm^-2)      10^22 (fix)             10^22 (fix)
kT(keV)        10 (fix)                10 (fix)
N(He)/N(H)     0.085 (fix)             0.085 (fix)
norm           4.47e-4                 4.13e-4
data bin       651                     651
chi^2          550                     607.86

Why the best-fit chi^2 values are so different between ISIS
and XSPEC? 


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Received on Wed Dec 19 2001 - 17:16:03 EST

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