Hironori> I found that ISIS assinged the error of 1.866 to zero count Hironori> channels, while XSPEC assigned 1.0 to them. Hironori> I corrected the ISIS error values of the zero counts Hironori> channels to 1.0 by put_data_counts, and then compered ISIS That's not a "correction". The gehrels fudge is sigma(N) = 1.0 + sqrt(N + 0.75) sigma(0) = 1.866 sigma(N>>1) = sqrt(N) (this is a well known and much used approximation for poisson weighting at low counts). Xspec should be identical if you choose that weighting. The difference you see could just be statistical/numerical. It doesn't look significant. Hironori> and XSPEC again by fitting the same spetrum with the Hironori> absorbed brems model. Here are the best-fit values: Hironori> ISIS XSPEC Hironori> NH(cm^-2) 10^22 (fix) 10^22 (fix) Hironori> kT(keV) 10 (fix) 10 (fix) Hironori> N(He)/N(H) 0.085 (fix) 0.085 (fix) Hironori> norm 4.30e-4 4.13e-4 Hironori> data bin 651 651 Hironori> chi^2 596.5 607.86 Hironori> If I use the XSPEC best-fit value for ISIS, Hironori> ISIS XSPEC Hironori> norm 4.13e-4 4.13e-4 Hironori> chi^2 598.5 607.86 Hironori> If I use the ISIS best-fit value for XSPEC, Hironori> ISIS XSPEC Hironori> norm 4.30e-4 4.30e-4 Hironori> chi^2 596.5 609.92 Hironori> Thus, there is still a difference in the best-fit chi^2 Hironori> values. Why? I think the difference of ~10 may be serious. Hironori> thanks, Hironori> ============================================================== Hironori> Hironori MATSUMOTO Hironori> Center for Space Research Hironori> Massachusetts Institute of Technology Hironori> NE80-6045 Hironori> 77 Massachusetts Avenue Hironori> Cambridge, MA02139-4307, USA Hironori> TEL: USA(1)-(617)253-8008 Hironori> FAX: USA(1)-(617)253-8084 Hironori> e-mail : matumoto_at_email.domain.hidden Hironori> ============================================================== Hironori> ---- Hironori> You received this message because you are Hironori> subscribed to the isis-users list. Hironori> To unsubscribe, send a message to Hironori> isis-users-request_at_email.domain.hidden Hironori> with the first line of the message as: Hironori> unsubscribe -- Dave David Huenemoerder (617-253-4283; fax: 253-8084) MIT Center for Space Research/Chandra X-ray Center 70 Vassar St., NE80-6023, Cambridge, MA 02139 [Admin. Asst.: Deborah Gage 617-253-0228, dgage_at_email.domain.hiddenReceived on Fri Dec 21 2001 - 17:06:16 EST
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