ISIS vs XSPEC

Hello, 

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? 

thanks,
Hironori

==============================================================
Hironori MATSUMOTO
Center for Space Research
Massachusetts Institute of Technology
NE80-6045
77 Massachusetts Avenue
Cambridge, MA02139-4307, USA
TEL: USA(1)-(617)253-8008
FAX: USA(1)-(617)253-8084
e-mail : matumoto_at_email.domain.hidden==============================================================
----
You received this message because you are
subscribed to the isis-users list.
To unsubscribe, send a message to
isis-users-request_at_email.domain.hiddenwith the first line of the message as:
unsubscribe