Re: Chi2 calculation for rebinned spectra in ISIS

Hi Maria,

can you send the code you use for rebinning and fitting in isis? I'm not 
100% sure, but I know that there are some pitfalls in there that I 
myself fell in on the way ...

Cheers,
Victoria

On 4/29/14 5:44 AM, María Díaz Trigo wrote:
> Dear ISIS experts,
>
> I have realised that the chi2 value for rebinned spectra is very
> different when calculated by ISIS and XSPEC. The chi2 is exactly the
> same when I use non-binned spectra, so the difference must arise with
> the way of calculating errors after rebinning.
>
> I am using the default fitting method (which seems to be the same for
> XSPEC and ISIS) and rebin the HEG spectra to have a minimum of 25
> cts/bin. Before rebinning, a simple power law fit gives me a chi2 of
> 838.37 for 2362 dof both in ISIS and XSPEC. After rebinning, I get:
>
> XSPEC: Chi2 of 217.90 (460)
> ISIS: Chi2 of 420.46 (460)
>
> So the difference is almost a factor of two, and the parameters of the
> fit are also different.
>
> Do you know why am I getting such a difference?
>
> Best,
>
> Maria
>
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