Andy's suggestions and my answers. (See the file 'andy_comm.txt' for his list of suggestions.) I divide Andy's comments into the same 5 categories he used: ***************************************************************** I. General problems / questions ***************************************************************** THINGS TO DISCUSS: ----------------- AH: * My biggest problem is that your definitions of features is essentially physically meaningless for most features. Yes, that's true. I intended to get empirical measurements from the spectra. I based this on the fact that in general we see greater absorption (or pseudo-absorption if we consider P-Cygni profiles and emission lines) for subluminous SNe. EW's gave the means to quantify this. AH: Photospheric and nebular spectra are physically distinct -- considering them as part of the same definition of features muddles the interpretation of the results. I mention that in section 3.1. The empirical fact that EW measurements get more homogeneous after a month or so from maximum light is in fact telling us something. AH: 1) Different SNe enter the nebular phase at different times, and this is correlated with stretch. So at say day 25 you might be comparing absorption from one SN to a gap in emission from another. This was exactly the idea: I try to compare what's happening on roughly the same wavelength region for different SNe at equal epochs. And it's pointed out especially in the case of MgII 4300. AH: 2) It adds noise that may affect the plots like Fig. 4, when you average delta EW. I cut the measurements at day 30 for building Fig.4 because of that reason. AH: * Your results may be dominated by emission near an absorption feature rather than than the true EW of the line itself. Again, this was the idea in order not to have to make a careful determination of the actual continuum. THINGS TO DO: ------------ AH: * You should mention that EW as you define it in SNe is very different than the EW over a single narrow line for a star. I'll point it out more clearly. AH: * Fig. 1 is not sufficient to show your line definitions. You should show this for multiple epochs and types of spectra. Yes! I've been working a bit on such a plot. I'll go back to it. AH: * How does reddening, or just a difference in the color of the SN affect the results? I'll do the test. AH: Your results for some lines are dominated by different ions coming into the spectrum. This happens at a different rate for different stretch SNe. (...) . Please add a table giving the rest and observed wavelength of various lines and some rough idea of when they affect the spectra of a normal Ia. I'll try to see how to add this information. AH: * You should have a table of the resolution of each spectrum I'll test the effects of resolution. A table with those figures may be hard to build for public spectra. I'll check the range of resolutions and make the tests on that range. AH: It would be useful to see S/N in this table as well. I will also do this test. ***************************************************************** II. Text-specific comments ***************************************************************** In general, these comments are very useful. I'll do the changes. A few points that I'd like to answer to: AH: p. 5-6 I don't believe the trend shown for Fe II 4800 in Fig. 4. Yes! I changed that. Now I don't discard any SNe. The straight line may be removed. AH: p. 11, sec 4.5, par 2. SN 1991T is known for having essentially no Ca II. I measured what's there. Where CaII-H&K normally is there's a shallow, double line on early spectra. I just checked that Fisher marginally identifies it as NiII using SYNOW. On CaII-IR I measured a very faint feature before max. ***************************************************************** III. New things to try: ***************************************************************** AH: (1) Blueshift of the absorption minima in km/s. (2) The Full Width at Half Maximum (FWHM) of the feature. These are of course interesting measurements. I have done an amount of them. I thought this can be part of a future work. I wanted here first to present a new kind of measurement and second to be able to use all the features, including non-single element ones. AH: * Are there linear combinations of parameters that produce a stonger correlation than a single parameter alone? I tried several things: ratios of EW's and sums of EW's from several features. I wanted to limit the amount of work on the data in order to have this article ready at some point. I'll investigate if any of those combinations is worth adding. AH: * How do the EWs that you calculated compare to EWs determined by fitting a gaussian in IRAF? I recall having tried this sanity checks in the beginning of the developing of my code. Things matched pretty nice then. I would've used IRAF if it were not for its incomplete error treatment. I'll redo the checks now. AH: Is it possible to make any plots that show absolute magnitude rather than delta-m15 (I don't know how many have a reliable distance determination)? This would be possible presently only for some of the public SNe. For our SNe we are still waiting for the photometry to come out and then this will be possible. We would like to use the absolute magnitudes instead of the delta-m15's in order to have a direct correlation to luminosity. Delta-m15 was chosen because we were able to remeasure it ourselves from the lightcurves available, giving consistent results for all SNe. AH: * You may be able to find more SNe to add to your sample at the SuSPECT archive. I know they have data for SN 1996X and SN 1999ee. I am aware of that. I simply didn't have the time to measure those SNe. SuSPECT data involves the additional work of controlling reddening, de-redshifting or possible calibration problems which we don't know about a priori. ***************************************************************** IV. Style: ***************************************************************** Again, these are very fair and useful comments. A few points: AH: You might also try connecting the points for a given SN with lines. I was doing that in the beginning but it proved to mess up the plots. And then I even had less data points. I'll make a try though. AH: * I would change the title to "Spectroscopic Homogeneity of Type Ia Supernovae" Why not! ***************************************************************** V. A sample of papers to investigate. ***************************************************************** Here I should say that I didn't have the time to compile the papers that are appropriate to each case. I sometimes put a sign saying "references" and left it to be finished in the end, with your help and others'. It is hard for me to find out which are the sources of the public spectra we got from Peter and you. That's why these authors are not yet cited. I'd be very glad to have that information.