From: Gaston Folatelli (gaston@physto.se)
Date: Tue Jan 13 2004 - 13:33:49 PST
Dear collaborators,
There's yet a new version of the paper draft on EW measurements. This has
to do with the very helpful comments from Chris and Isobel on v 7.1.
You can retrieve v 7.2 from the usual place (usual SCP access):
http://www.physto.se/~snova/private/internal.html
In the following, I answer to some of the points raised by Chris.
Below, I address Isobel's suggestions.
FROM CHRIS:
> General
> =======
>
The terms used to name specific elements or concepts in the text are now
more coherent. The confusion had to do, to a great extent, with my limited
knowledge of English.
The issue of "indices", "indicators", "quantities" and "features" is
solved (I hope) by the following conventions:
- I use "spectral quantities" to refer to the equivalent width-like
measurements before they are defined.
- Afterward, they are referred to as "EW".
- I save the use of "feature" to mention the absorptions, emissions, P
Cygni profiles, etc seen on the SN spectra.
- Finally, I use the word "parameter" for anything that works as a
parameter. Therefore, Section 5 deals with the spectral parameters that
are derived from the EW's.
I hope this makes the whole text, and the abstract in particular, more
clear. No more "indices" or "indicators", unless somebody complains
strongly. The title was modified accordingly ("indices" --> "quantities").
Also, I took away all the "fast decliner" and "slow decliner", except for
the firts ones (when I define the SN~Ia subtypes in the Intro). I only
talk about "1991bg-like SNe" and "1991T-like SNe". And I stick to "Branch
normal SNe" (should it be "Branch-normal SNe"?).
> Abstract
> ========
>
I took Chris' recommendations into account for this new version of the
abstract, which I hope is better than the one before. The text has been
changed in accordance to the terminology mentioned above.
> Introduction
> ============
>
> The first paragraph is difficult to follow; there are two many ideas
> in it. As a general rule, each new idea or concept should have its own
> paragraph.
The idea of the first paragraph is just to give a very brief description
the status of SN~Ia cosmology. I don't intend to explain the things
mentioned. That's why I wouldn't write more paragraphs on those topics. I
just want to say that we measured a non-vanishing \Lambda and that there are
concerns about the systematics. This paper aims to provide with some tools
that would improve the use of SN measurements and help in controling the
systematics.
> Data Description
> ================
>
> What does "integer number of days since maximum" mean? Were the real numbers
> rounded to the nearest integer?
Yes. Because in many cases the date of max is not precise enough to try
anything better.
>
> Feature Definitions
> ===================
>
> What does "The spectrum is normalised by the continuum" mean?
I changed it to "divided by the pseudo-continuum".
>
> Systematic Effects
> ==================
>
> I do not like the way the systematic error is computed. If I have
> understood the text correctly, you shift the region by 1/4 of the
> region size and you recompute the EW. Why 1/4. Why not 1/10th? This
> is arbitrary and I do not think it is a valid method in estimating the
> systematic error.
>
The 1/4 is a rough number arising from the 1-sigma uncertainty of a
uniform distribution. For such a distribution in the range
a < x < b, the sigma (sqrt of the second moment) is
(b-a)/sqrt(12) ~ (b-a)/4. This can be taken
as the uncertainty in the position of the fitting region. I added a
footnote in the text explaining this but maybe this is going too much into
details. This was the simplest way to account for the uncertainties due
to the choice of the fitting region. I tried several
reasonable amounts by which to shift the regions (i. e. without going into
a neighbouring absorption) and the resulting estimates didn't change much.
I should note that for very badly sampled spectra, I had to shift by larger
amounts in order to include other pixels. This happened in very few
cases. In those cases, the systematic
errors estimated were obviouslu larger.
> Remove ", given by the point-to-point ratio ... wavelength range,"
I kept a part of the phrase to emphasize that these EW measurements
involve a limited wavelength range and thus long range effects like
reddening are not expected to affect them.
> Table 5 and other tables like it
> ================================
>
> What is the error (1 sigma uncertainty) for columns 3,6 and 8. Is
> it related to delta? Saul asked this question, but I have not seen this
> issue addressed in your reply to him.
Sorry, I skipped this point when answering to Saul. I had added a footnote
to explain what things are, but because of a wrong LaTex coding, the
footnote didn't manage to appear in version 7.1. Now it is there.
>
> Table 9.
> ========
>
> Perhaps the footnote "e" should label the Delta m15 values and not Delta
> M_B.
>
I changed the text of that footnote and left it for Delta M_B.
> EW^{max}_{(2+3)} (I've used Latex notation here) should be EW_{(2+3)} at
> B band maximum.
I changed the notation to emphasize that I don't refer to the maximum of
EW but to the EW at B max. Thus, I changed the superscript from "max" to
"t_max" hoping this is explicit enough.
> Section 5.1.2
> =============
>
> In paragraph six, you state that we should expect to see a correlation
> between t_br and absolute magnitude. Why? They are measuring different
> quantities. The fact that they measure these quantities over a similar
> wavelength region is not sufficient for me.
The location of that statement in the text was wrong. It was meant to
refer to the correlation between t_br and Delta m_15 (and not M_B). This
is fixed now and an additional sentence was added to explain what I mean.
> Section 4.1.3
> =============
>
> Paragraph 3.
>
> I'd suggest that you delete the clause "especially ... 0.3 mag."
Done. I realized that this paragraph contained some relics from some too
old version.
>
> Paragraph 7.
>
> This paragraph is not clear. I do not understand how you have computed
> the affect of contamination on alpha(2+3).
>
I hope it is clearer now.
> I do not understand figure 13. If half the B band flux is host, then,
> according to figure 13, alpha changes by 30%. If alpha was initially 10, then
> alpha becomes 7. This is a change of 3, i.e 30%. According to equation 6,
> the estimate of the peak magnitude would then change by 0.103 * 3, which
> is 0.3 magnitudes and not 0.1 magnitudes as one would infer from the
> lower panel in figure 13.
I checked this and my numbers are OK. The reason for this is that alpha
should be taken in ABSOLUTE terms when going to Eqn. 6 because the lower
panel of figure 13 has ordinates which are in absolute terms. In this
example, alpha changes from ~3 to ~2 (~30%), but that's not the same as
changing from 10 to 7 when it comes to M_B.
------------------------------------------------------
ISOBEL'S COMMENTS:
With respect to Isobel's suggestion of bringing back the plots of
deviations from the average EW evolution curves versus M_B (I was using
Delta m_15 at that time), I didn't have time to work on this. I'll see
how things look. For the moment,
I hand the draft over to everybody as it is. I will put the data together
for a web based table. With respect to a plot of EW(2+3) versus epoch, I
know it's hard to read out but that info is present in Figures 4 and 5.
------------------------------------------------------
Finally, I'd like to say that more or less any day would be fine for me to
have a telecon. I would suggest that we make an agenda before the meeting
in order to be more efficient.
Have a nice reading,
-Gastón
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