From: Alexander Conley (AJConley@lbl.gov)
Date: Thu Sep 16 2004 - 11:51:03 PDT
Do you guys remember exactly how you did the V band fits in P99
in which you forced the V stretch and date of maximum to be the
same as the B band fits?
I've been trying to figure out how to do this, since Saul asked me to
for my paper, and it turns out to not be as simple as it looks. If it
is
done in what seems like the obvious way it actually does something
different, which may have affected the P99 fits.
When you do a snmin fit, the output contains the best fit
parameter t of B max. A typical value is around 0. This value
can be fixed by editing the control file to the value you want and
then adding a 'fix 1' line.
The subtlety arises when you realize that this number is in a system
that is relative only to the lightcurve being fit -- that is, fixing
t of B max to -11 will mean different things to a B only and V only
fit to the same supernova because the B and V lightcurves are not
identical. To first order this date system is relative to the brightest
point in the lightcurve in question.
A simple example may help clarify what I mean. Imagine a
SN with B observations on day 1,2,3,4,5,6,7 and V band observations
on days 5,6 and 7. The brightest B point is on day 2, and the brightest
V on day 5. Say that the snmin fit returns t = +0.2, which would mean
that in real days the date of maximum is 2+0.2 = 2.2. Next imagine that
you want to do the V band fit, and you edit the control file to fix the
first
parameter at 0.2, which is what you got from the B fit. The problem is
that in the V frame t=0.2 actually means 5+0.2 = 5.2, which is not
at all the same.
So, you might wonder, if this is the case, shouldn't it have been
obvious
from the fits? Not necessarily, because most of the time if there is
color information is is taken around peak, so the brightest point will
probably have both B and V. It isn't quite this simple because of
random fluctuations and what not, but for most lightcurves the B and
V band frame that snminuit works in will be pretty close -- but not
always.
In addition, I suspect that this piece of code doesn't understand the
difference between the B and V peak dates, so you can expect to be
systematically off by 1-2 days, depending on whose SN template you
are using.
Note that this should not affect the primary results of P99, since the
B max is not affected. It should affect the color estimates, so the fit
with extinction correction will be affected. It also doesn't affect K03
because Rob would have nothing to do with this fitting method.
So -- do either of you remember the details of how you did these fits,
and if you were aware of the potential difference in the date systems
in different filters for the same supernova?
Alex
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