From: Chris Lidman (clidman@eso.org)
Date: Sun Feb 29 2004 - 19:48:51 PST
Hi Serena,
Good work! I've read your replies to the points made at the
meeting and I've added some comments.
I'd like to address a point which was not addressed in the
meeting but is interesting and is partially related to points 8 and 9.
In the lower half of figure
http://www.physto.se/~snova/internal/papers/iband/figs/Imax_sb_00fr.jpg
I note that the best fit template to 2000fr is also the template that
makes it the brightest. All other fits with poorer Chi-sq (but still
with chi_sq < chi_sq_min + 3) are systematically fainter.
We should always be wary of choosing an analysis method which makes
the data look better, but we may want to reconsider whether or not
we should use the fit with the minimum chi_sq to define the peak magnitude
of the SNe.
If one was to define the peak magnitude as the average of all fits
that satisfied chi_sq < chi_sq_min + 3, then 2000fr would be 0.15 to
0.2 magnitudes fainter.
We might want to think of this difference as a systematic error
induced by choosing a different analysis.
I can give you an example of where the chi_sq minimum value is not used.
When Andy determines the SN epoch, he takes the average of the best five(?)
fits. I've cc'ed this e-mail to Andy so he can explain to us why
he chooses this method of analysis.
Similarly, the fit to 1999ff is the second brightest of all fits with
chi_sq < chi_sq_min + 3.
http://www.physto.se/~snova/internal/papers/iband/figs/Imax_sb_99ff.jpg
Note that these plots are quite flat, which would argue that the
light curve shape is not all that important in determining the peak
magnitude.
In relation to the numbered items.
1) There was concern for the way the dispersion in the Hubble diagram
is computed. Serena should compute the weighted r.m.s.
Serena's reply ...
I have done this. The weighted r.m.s. gives about the same values as
the non-weighted r.m.s. due to the uncertainties, which are about the same
for all the points. However, Table 4 now includes the weighted r.m.s. as
well as the weighted average for each of the samples. I have also added a
discussion in section 3 about the differences between the data sets.
Chris' comment ...
I think that we were also interested in seeing the RMS scatter about
the individual Hubble lines and not the scatter about the Hubble line
that is derived from the entire data set. Was the RMS calculated in
this way? It is not 100% clear in the paper.
2) Lifan computes different K-corrections than I. We should try to
compare them and figure out why there are differences. Serena uses
the spectral templates published in Nobili et al 2003. Lifan uses a
sample of nearby spectral templates for which he computes the
K-corrs and average them out. Lifan will try to compare them.
As Ariel pointed out, we would like to use templates for computing
K-corrections, as this is used for the high-z SNe, in order not to
treat them differently.
Chris' comment ...
For this paper, I tend to agree. We should treat high and low redshift
SNe equally. Nevertheless, the comparison should be done.
I think that it is worth investigating if we can better constrain the
spectral energy distribution from some observable, for example the
B-band stretch, and hence derive a more accurate k-correction. This
might reduce the scatter about the Hubble line. I am not arguing
that it should be in this paper, but it is something that the
collaboration should do.
10) Serena will try to add the restframe B,V-band for 1999Q, reduced by
Vitaliy, in the analysis presented in Section 6.
Serena's reply ...
This is interesting! It turned out that the fit obtained by SNMINUIT
fixing the time of B-maximum to Tonry value, would give a color excess
E(B-V) = 0.2 mag (see email by Vitaliy in the archive on Feb 12). Given
that this SN is host-less, this value is very surprising. I have been
playing with numbers and plots. If the B-band stretch factor was sB=0.82
the colors would make perfectly sense (even in I-band). Unfortunately the
stretch fitted by SNMINUIT is 1.061 instead. Check plots and discussion
in:
http://www.physto.se/~snova/internal/papers/iband/index.new.html#99q
Giving all of this, I do not think we should add the B-V colors of SN1999Q
in the analysis of section 6.
Chris's comments
Why is the horizontal axis ins these plots scaled with the stretch?
I've seen this with the B-band lightcurve, but it is the first time I
have seen this done with colour.
Would the colour of 99Q be normal if the date of maximum derived by
Vitaliy was used instead?
Cheers, Chris.
Serena Nobili wrote:
> One more thing. The new version of the paper draft is linked in the web
> page:
>
> http://www.physto.se/~snova/internal/papers/iband/
>
> Cheers
>
> Serena
>
> ----------------------------------------------------------------------
> www.physto.se/~serena
> Tel +46 8 55378661
>
> Give free food at:
> http://www.porloschicos.com/
> http://www.thehungersite.com/
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