Chi2 calculation for rebinned spectra in ISIS

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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