From: Chris Lidman (clidman@eso.org)
Date: Sat Mar 27 2004 - 19:46:11 PST
Hi Serena,
During the last SCP Exec meeting we discussed ways in which we might
be
able to reflect the uncertainties in some parts of your analysis. I have
written some sample text which you should consider.
k-corrections (note the lower case k)
============================
The first uncertainty concerns k-corrections. The following text
might
be used to replace second paragraph in section 2.2.
"A potential source of systematic error is the I-band k-correction.
The spectral region around 8000\AA\ is dominated by a broad Ca-II
triplet, which can vary in strength and shape between supernovae. The
k-corrections in this paper use the spectral templates described in
Nobili et al. (2003), which do not take this variation into account.
Hence, for individual SN~Ia, the k-correction may be systematically
wrong by $\sim 0.1$ magnitudes and this uncertainty contributes to the
scatter in the I-band Hubble diagram. The accuracy of the
cross-filter, J- to I-band k-corrections of SN~Ia at $z \sim 0.5$ are
also similarly affected. With the current nearby data sets (Folatelli
et al. in preparation), there does not appear to be a clear
correlation between the strength of this feature and other
observables, such as SN~Ia subtype (vis. Branch-normal, 91T- or
91bg-like SN~Ia), so it is not yet possible to reduce the uncertainty.
However, there soon will be a large number of well-observed SNe~Ia
with accurate spectrophotmetry (refer to the supernova factory here),
and
this will lead to a significant improvement in the I-band k-corrections
of SN~Ia."
If you include this text, you can delete future references to the
uncertainty as
is done in the 7th paragraph of section 4.1.
The 0.1 magnitudes comes from Lifan's analysis of ~500 spectra - no
don't jump!
I am assuming that Lifan got this part of the analysis correct. The
0.05 magnitudes
we had used before came from comparing a much smaller number of
spectra, basically
yours and Rob's. I feel that the 0.1 magnidues estimated by Lifan is
more secure
estimate of the scatter.
IR-Optical photmetry
================
I'd like you to expand this section. Perhaps you can replace paragraph
8 in section 4.1
with the following text.
"The cross-filter, J- to I-band k-correction crosses the boundary
that separates IR and optical photometric systems. Since the systems
used on
large telescopes are generally removed from the primary standard
(Vega, which is used for computing the cross filter k-correcions) by
several generations, it not unreasonable to expect a significant
offset between IR and optical photometric systems.
To estimate this offset, we select a subsample of A0 stars from the
Hipparcos catalogue with ground-based Johnson photoelectric
photometry. We further limit the subsample to stars that are
non-variable, more than 30 degrees from the Galactic plane and with
|B-V| < 0.5 (note the symmetric bound). This subsample is then matched
to the 2MASS catalogue and further limited to stars that have the
"AAA" 2MASS quality flag. The final catalogue contains 75 stars with
excellent optical and IR photometry. The median (B-V)_J, (V_J-J_2MASS)
colours are 0.002 and 0.043 respectively. The latter is indicative of
an offset between the IR and optical zero-points and we apply this
offset as a correction to all the J-band photometry in this paper."
I think that you should remove the last sentence in the 4th paragraph
in section 5.
With 2MASS, the IR systems are now very well defined.
Systematics in the light curve fitting of z=0.5 SNe
===================================================
The following sentences should be included towards the end of section
5 (perhaps in the last paragraph) as a caveat. I am assuming that you
will look
into the issue of possible systematic effects when the paper is being
refereed.
"For SN 2000fr and SN 1999ff, we note that the peak magnitudes
that are derived from the best fitting nearby SN~Ia are the brightest
of all the peak magnitudes that are derived from the fits in which the
reduced chi-square is 3 or less. This may be a sign of a systematic
error
in the fitting technique."
When your analysis of the systemmatics is complete, we may
decide to expand on this point.
1999Q
=====
I do not know whatto think about this one.
Let's have a look at where it lands in figures 13 and 14. If E(B-V) is
0.2,
then it is certainly lands outside the 99.7% contour. However,
the optical photometry of Vitaliy needs to be verified.
That's all for now. More later.
Cheers, Chris.
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