From: Chris Lidman (clidman@eso.org)
Date: Sun Jan 23 2005 - 10:09:40 PST
Hi Serena,
I've gone through the replies to the referees report and I provide
comments in the attached file. I've also reprocessed the SOFI data with
different software and I get the same result. The details are in the
attachment.
Cheers, Chris.
On Mon, 2005-01-10 at 12:30, Serena Nobili wrote:
> Dear all,
>
> here is the referee's report on the Iband paper. It sounds quite tough at
> the beginning, but it is not that terrible after all. One major point is
> on 1999Q. He seems to want us to state clearly how much is our magnitude
> of the SOFI point. I certainly need a quick feedback on this comment, while
> I work on the rest.
> Thank you in advance for you help.
> Cheers,
>
> Serena
>
> ---------- Forwarded message ----------
> Date: Fri, 07 Jan 2005 19:57:34 +0100
> From: aanda.paris@obspm.fr
> To: serena@lpnhep.in2p3.fr
> Subject: AA/2004/2463
>
> 07/01/2005
>
> Dr Serena Nobili
>
> serena@lpnhep.in2p3.fr
>
> Our Ref. : AA/2004/2463
>
> Dear Dr Nobili,
>
> Your paper "Restframe I-band Hubble diagram for type Ia supernovae up to z sim~0.5" was submitted to a competent referee who recommended publication after substantial revision (see enclosed report).
>
> Please take all of the referee's comments and suggestions into account in revising your work and send us the new version (in referee format) at your earliest convenience. Instructions for resubmission can be found at address https://mms-aanda.obspm.fr/is/aa. Your author ID number is 13552.
>
> In your cover letter, please indicate precisely all the changes made in the revised version, and mark them clearly (e.g., using boldface) in your manuscript.
>
> With best regards,
>
> Francoise Combes
> A&A Editor
>
> -----------------------------
> Referee Report
>
> The use of Type Ia Supernovae for cosmology has produced remarkable
> results. The checking of these results is of utmost importance and the
> paper 'Restframe I-band Hubble diagram for type Ia supernovae up to z
> sim~0.5' by Nobili et al. is trying to present such a check.
>
> The paper has two main topics: The necessary establishment of the
> I-band light curves of Type Ia Supernovae and the construction of an
> I-band Hubble diagram. The former is a precondition for the latter. The
> paper has to remain exploratory, since there are not enough data to
> populate the Hubble diagram at redshifts z>0.3 where the cosmological
> effects start to become significant.
>
> There are several shortcomings with this paper and it should be modified
> considerably before it can be recommended for publication. However,
> the paper can be brought to a stage where it can be published with some
> effort.
>
> The light curve fitting is rather ad hoc. The motivation to use B-band
> light curves to construct a surrogate I-band light curve is not
> explained anywhere. The authors should give a reason, why they chose
> this route and not another. There are obvious shortcomings from this
> approach that are not described in the paper. E.g. the late decline of
> the I band, which is typically faster than in B can not be described
> with this method at all. This means that after about 40 days past
> maximum, the I light curves can not be fit any longer. The paper does
> not use these late-phase data, but it should be made clear that the
> proposed model can only fit the maximum phase. By the way, the approach
> by Contardo et al. allowed them to fit the light curves, within the
> stated limitations, over a much longer time than what is proposed here.
> Another issue are the rise times. The paper states that it is 'assumed
> that the rising part of the light curve in I-band is the same as in
> B-band' to then on the next page say 'the trend in the residuals' in the
> fit to SN 1994D 'shows the limitations of the model.' An inspection of
> Figure 1 shows that this is evident in all cases where data in the rise
> is available. In all cases the fit has strong systematic deviations,
> when the peak is defined through pre- and post-maximum data. SN 1994D is
> just the case in point. It seems important to make the paper consistent
> in this respect.
> The above draws the result of Figure 2 into question. This figure is
> remarkably different from what was found in Contardo et al. (their
> Figure 4). This discrepancy should be discussed in the paper.
> Another example is the exclusion of two (SNe 1997br and
> 1998ab; both 'spectroscopically peculiar') supernovae due to the Monte
> Carlo fits. They happen to be the two with the smallest ratios between
> peak and dip in the light curves. The fact that the Monte Carlo fits
> fail to reproduce these light curves consistently is probably an
> indication that the model fails here as well.
>
> Table 2 should list the number of data points used in the fit. This
> would help the reader to assess the goodness of the fit as well.
> Especially, since these light curves are then later used to fit the
> distant supernovae, it would be good to restrict the construction of
> light curve 'templates' to only objects that have an over-constrained
> fit, i.e. more than let's say 10 phases spread over at least over the
> two peaks. Establishing a template with 6 data points appears unreliable
> given the model.
>
> Light curves are stretched according to values derived from the B light
> curves. What would be the rationale to apply the same stretch in I as
> well?
>
> The fits are done in flux units. Does that mean the magnitudes are
> converted into flux, fit and then converted back into magnitudes? Why is
> this done like this? How are the errors treated? This is an important
> detail that needs to be explained.
>
> The authors should decide what they want to do with SN 1999Q. As it is
> right now, they introduce the data for this supernova to quickly reject
> them as non-consistent. That would be OK, if they then would refrain
> from presenting all the analyses as with the other two objects. I
> suggest to either accept the published (Riess et al.) data as they are
> or, if the data can not be used, reject them and leave it at that. It
> does not make sense to fit a light curve, calculate a brightness and put
> the point into the Hubble diagram, if the data can not be trusted. The
> authors should indicate, why the data can not be trusted. Just saying 'a
> re-analysis of the publicly available SofI data suggest that the
> published J-band magnitude may be too faint' is not good enough. The
> public data contain all the information necessary to derive a reliable
> value. If it differs from the published result the difference should be
> stated explicitly.
>
> In several places the paper is confusing in the way it presents the
> samples. The abstract quotes 26 local supernovae, but the reader is
> confronted with different numbers (42, 28) in the text. It might be useful
> to state the sample selection (redshift range and exclusion of
> spectroscopically peculiar supernovae) in the abstract so that this is
> clear from the beginning. Maybe the selection criteria could be
> summarised again at the beginning of section 3.
>
> The use of references is sometimes unclear and appears also occasionally
> arbitrary. It would be good to carefully go through the paper again and
> sort these out. Suggestions are made below as well.
>
> Specific comments:
>
> 1. 'spectroscopic peculiar' should be changed to 'spectroscopically
> peculiar' throughout the text.
>
> 2. add Schmidt et al. (1998) to the reference list to the first sentence
> in the introduction.
>
> 3. add '(similar to Riess et al. 2000)' as a reference to the first
> sentence of the last paragraph of section 1.
>
> 4. The references for SN 1991T (section 2.2) are rather strange. It
> appears as if there is a mix-up with the following sentence. The
> references for SN 1991T should be Filippenko et al. 1991, Ruiz-Lapuente
> et al. 1991 and Phillips et al. 1992. The references for Li et al. 2001,
> Howell 2001 and Garavini et al. 2004 presumably refer to SNe 1995bd,
> 1997br, etc.
>
> The sentence starting with 'However, not all of these show ...' is
> unclear. Which supernovae are meant with 'these'? SN 1986G, SN 1993H or
> others?
>
> 5. In one paragraph Type Ia Supernovae are called 'standard candles' and
> the next (only two lines down) they are described to show 'a variety of
> properties.' It is passages like these that confuse the reader.
>
> 6. What does 'somewhat deviant' mean (caption of figure 6)? This
> statement should be quantified.
>
> 7. There is a change from observed redshift to a redshift corrected to
> the CMB frame between Tables 2 and 3. This should be explicitly stated
> either in the text or the captions to Table 3.
>
> 8. Table 4 and its discussion in the text are meaningless. Since the
> fits in the Hubble diagram is done with all data sets the smallest
> sample one will have the least weight and hence largest uncertainty.
> With the larger uncertainty the 'offset' automatically can be larger.
> Not surprisingly, all samples are consistent with each other within the
> errors. This table and the corresponding section should be dropped.
>
> 9. The first sentence of the third paragraph in section 5 should start
> with 'The low statistics of the high redshifts sample is insufficient
> ...' No statement can be made whether the concordance model is favoured
> or not from the presented data.
>
> 10. In the same paragraph there is a garbled sentence, the meaning of
> which did not become clear. However, as suggested above the discussion
> of SN 1999Q should be dropped here anyway.
>
> 11. It appears from the fits that a dust model with R_V=9.5 is the most
> favoured case, given the data. For the Monte Carlo simulation to check
> for the sample size to reject a dust hypothesis the number of individual
> data points per individual supernova must be important as well. This
> should be described.
>
> 12. The fourth paragraph of section 8 should start: 'J-band measurements
> of one high-redshift supernova plus published data of one more were used
> to extend ...'
> Also, instead of saying 'seemingly inconsistent information' on SN 1999Q
> the authors should be more explicit. It is alright to say that the authors do
> not trust the data and hence discarded this object.
>
> 13. There is an interesting mismatch in the interpretation between
> concordance model and the analysis of potential dust in the conclusions.
> The 'concordance model ... is found in better agreement with the data
> than other models' but the dust analysis yielded 'no firm limits on the
> presence of grey dust.'
>
>
>
-- European Southern Observatory Alonso de Córdova 3107, Vitacura Casilla 19001, Santiago 19 CHILEPh. +56 2 463 3106 FAX +56 2 463 3001
RRR) The light curve fitting is rather ad hoc. The motivation to use B-band
light curves to construct a surrogate I-band light curve is not
explained anywhere. The authors should give a reason, why they chose
this route and not another. There are obvious shortcomings from this
approach that are not described in the paper. E.g. the late decline of
the I band, which is typically faster than in B can not be described
with this method at all. This means that after about 40 days past
maximum, the I light curves can not be fit any longer. The paper does
not use these late-phase data, but it should be made clear that the
proposed model can only fit the maximum phase. By the way, the approach
by Contardo et al. allowed them to fit the light curves, within the
stated limitations, over a much longer time than what is proposed here.
AAA) As correctly pointed out by the referee, the choice of the B-band
template is completely arbitrary. The B-band template has been chosen
only by the fact to be able to describe the overall data, and that has
been already published (Nugent et al.). Different templates could be
used (such as a different band or any other functions), that could
probably work as well or better than the one we decided to use. We
have modified the text of Section 2 to point out the insignificance of
the template chosen. Also following the suggestions of the referee, we
have pointed out the limitation on epoch range of the method, as
opposed to the one of Contardo et al., developed with the aim of
fitting only data up to day +40. We note however, that the stretch
factor method is usually applied to data only up to +40 days also in
the standard B-band lightcurve fit.
CLi) I guess that there was some motivation on your part in choosing
the B-band template over other templates. I can think of two reasons
why one would have chosen this template: 1) it is better characterised
than other lightcurve templates, and 2) it results in better fits
than other templates (a Gaussian for example) when the 5 parameter fit
is used. You might want to modify what you have written in the footnote
at the bottom of page 2.
In the bold type of the second paragraph of section 2, you say that
you use two templates. In fact, you use only one template. The paragraph
should be made clearer.
I'm not sure I understood the 2nd footnote at the bottom of page 2. Is
it really necessary? Note that if you substituded the B-band
light curve template for a Gaussian, the number of parameters is the
same - you simply replace the stretch of the lightcurve with the width
of the Gaussian.
RRR) Another issue are the rise times. The paper states that it is 'assumed
that the rising part of the light curve in I-band is the same as in
B-band' to then on the next page say 'the trend in the residuals' in the
fit to SN 1994D 'shows the limitations of the model.' An inspection of
Figure 1 shows that this is evident in all cases where data in the rise
is available. In all cases the fit has strong systematic deviations,
when the peak is defined through pre- and post-maximum data. SN 1994D is
just the case in point. It seems important to make the paper consistent
in this respect.
The above draws the result of Figure 2 into question. This figure is
remarkably different from what was found in Contardo et al. (their
Figure 4). This discrepancy should be discussed in the paper.
AAA) About 15 SNe of the whole sample show pre-maximum data. Six
supernovae out of these, are not well fitted by the model and present
a systematic trend in the residuals of the fit. We already pointed out
that this problem does not affect the Hubble diagram. However, the
referee is concerned about the possible consequences on the fitted
time of maximum, especially due to the disagreement of our Figure 2
with Figure 4 of Contardo et al. We note that if the fit is forced
through the pre-maximum data points, the date of maximum would be
shifted at slightly earlier times. We compared the histogram of
figure 2 with the histogram obtained by selecting the sub-sample of 17
SNe that we have in common with Contardo et al. (out of 22 SNe). We
note that only 4 of the 6 SNe under discussion are in common. The
comparison histogram (see attached file) show how the sub-sample used
by Contardo et al., shown in red, is more spread over broad values of
the time of maximum and generally before B-maximum, while the whole
sample is more picked. We conclude that the differences in the two
distribution are only due to statistical fluctuations.
CLi) The referee is partially wrong. Not all SNe that have data that cover
the rising part of the light curve show systematic residuals -
SN 1999dq is a good counter-example. Rather than giving specific
numbers (6 out of 15), it might be simpler to say that approximately
half of the SNe with data that cover the rising part of the light-curve
show systematic residuals.
RRR) Table 2 should list the number of data points used in the fit. This
would help the reader to assess the goodness of the fit as well.
Especially, since these light curves are then later used to fit the
distant supernovae, it would be good to restrict the construction of
light curve 'templates' to only objects that have an over-constrained
fit, i.e. more than let's say 10 phases spread over at least over the
two peaks. Establishing a template with 6 data points appears unreliable
given the model.
AAA) Following what suggested by the referee we have added a column in
Table 2 with the number of points used for the fit. The number of
points used together with the reduced chisq reported in Table 2,
should give the reader enough information to established the goodness
of the fit, e.g. an over-constrained fit will result in a very small
chisq value. We understand the concern of the referee about the
propagation of the uncertainty due to an over-constrained fit on the
nearby SNe into the high redshift light curve fit, through the use of
all 42 templates. We believe the systematic uncertainty on the fitted
I_max, as estimated in Section 4.4, takes into account also this
contribution. Moreover, we note that the template fitting the
high-redshift SNe with lesser points is the one of SN1996bl, determined
by 9 data points.
CLi) In table 2, you list the DOF, which is not quite what the same thing.
However, given that there are five parameters in the fit, most people
should be able to work out how many points were fitted.
One way to assess the impact of using light curves with less than 10
points is to remove points from one of the better-sampled light
curves, repeat the fit and see what happens. This is similar to a
suggestion we made last year concerning the possible systemmatic
effects that might occur when fitting these templates to the high z
SNe lightcurves, which have even fewer points.
RRR) The fits are done in flux units. Does that mean the magnitudes are
converted into flux, fit and then converted back into magnitudes? Why is
this done like this? How are the errors treated? This is an important
detail that needs to be explained.
AAA) The fits are done in flux units for simplicity, since a fit in
magnitude units should be done on a logarithmic scale. Uncertainties
should be asymmetric in magnitudes, however, many authors do not
follow this obvious transformation and incorrectly transform the
symmetric uncertainties from flux to symmetric uncertainties in
magnitudes. Since this work is using data taken from literature, we
have applied the transformation back to flux, and considered the
uncertainties symmetric for the fit. A sentence explaining all of this
has been added at the beginning of Section 2.2.
CLi) I do not think you need to explicitly state this in the paper, as
you do in section 2.2 I think that the reply you have given to the
referee is sufficient.
RRR) The authors should decide what they want to do with SN 1999Q. As it is
right now, they introduce the data for this supernova to quickly reject
them as non-consistent. That would be OK, if they then would refrain
from presenting all the analyses as with the other two objects. I
suggest to either accept the published (Riess et al.) data as they are
or, if the data can not be used, reject them and leave it at that. It
does not make sense to fit a light curve, calculate a brightness and put
the point into the Hubble diagram, if the data can not be trusted. The
authors should indicate, why the data can not be trusted. Just saying 'a
re-analysis of the publicly available SofI data suggest that the
published J-band magnitude may be too faint' is not good enough. The
public data contain all the information necessary to derive a reliable
value. If it differs from the published result the difference should be
stated explicitly.
AAA) SCP.
CLi) I've re-reduced the SOFI data on 1999Q and I confirm my earlier
measurement.
We measure J=22.63 +/- 0.15
Riess publishes J=23.00 +/- 0.14
There is a 2 sigma difference between our measurent and that of Riess et al.
Converting this to I-band magnitdues,
we derive I=23.47 (using Serena's k-corrections)
Riess publishes I=23.93 (directly from Riess' paper)
This is a big difference.
This archive was generated by hypermail 2.1.4 : Sun Jan 23 2005 - 10:10:04 PST