From: Robert A. Knop Jr. (robert.a.knop@vanderbilt.edu)
Date: Wed Feb 19 2003 - 14:25:15 PST
On Wed, Feb 19, 2003 at 01:59:59PM -0800, Alex Conley wrote:
> I've refit a couple of Rob's troublesome low-z sne with and without
OK, here's one example 1992ag -- my fits, my K-corrections. I've
attached two plots, one is with floting zero, the other is without
floating zero. (Look at the plot to figure out which is which.) You
can't tell me that the fixed-zero fits are reasonable. Using that in
cosmology would be inexcusable. Notice that m_B goes from 17.38 to
16.57 when I go from a fixed to a floating zero. That's huge. Looking
at the data and the fits, it's *very* clear which of the two of these I
ought to believe-- the one with the floating zero.
(The zero offset is only 0.03 compared to the flux height of the
lightcurve, so it's not a big offset being fit here; it's just all those
low-down points distorting the lightcurve. Why didn't Alex see this
huge difference? He eliminated points later than 30 days after max.
However, for purposes of this paper, it would not be a good idea to just
eliminate those later points, because on some of our high-z supernovae,
we have points, including HST points, that late; for instance, consider
SN9855, whose lightcurve plot I've also attached-- and that's not even
the one with the latest HST points.)
(Part of the problem also is that I suspect Hamuy has lied about his
error bars, by quoting them as magnitude error bars, giving the low
points more signifiance than they observe. But, we can do no better
than to trust the error bars published.)
I have similar trouble in the other direction if I try to *float* every
zero-- plus the issue that I discussed in the meeting, that I've already
implicitly assumed that the zero has been proprely subtracted when
combining HST and ground data.
There is no "right" way to do this that is utterly pure as the driven
snow statistically. Treating every supernova in a purely consistent
manner gives results that are obviously plain wrong, as the attached
supernova demonstrates. So I picked the fit procedure that worked best
for most of them, and then changed it for those few supernovae where it
was necessary to do so. This is the best we can do.
-Rob
-- --Prof. Robert Knop Department of Physics & Astronomy, Vanderbilt University robert.a.knop@vanderbilt.edu
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