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Steve Willner wrote:
[snip] Sorry to step into your discussion, but I've also got a problem with the time dilation of SN light curves, and you seem to be quite knowledgeable about this. There is a paper on the preprint archive: astro-ph/0404207 by a Jerry W. Jensen, where he argues that the stretch in the light curves is not due to time dilation, but due to the supernovae having higher magnitudes and therefore longer decay times (if I understand him correctly). He also seems to suggest that what is assumed to be distant supernovae are actually hypernovae (thus explaining their higher magnitudes). Some of the things he proposes look rather cranky, but others seem to be valid. This was also discussed in some detail on the badastronomy.com bulletin board: http://www.badastronomy.com/phpBB/viewtopic.php?p=227836&highlight=malmquist+bias+su pernova&sid=dc9b062af8a301dd9f0be4c7a6c78c10#22783 6 But most of his arguments there were not refuted by the posters, and I am unsure if he has a point or not. Could you please help me out? Bye, Bjoern |
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In article ,
(sean) writes: [Referring to Knop et al., 2003 ApJ 598, 102 or the preprint thereof] There are 13 readings averaged out to 4.8 by my calculations so I`m not sure whether the single peak measurement point at 1.2 on the graph represents the `averaged` reading of 4.8 or the one peak reading of 5.89. It's pretty obvious if you compare the table and the graph and note the open/filled circles, as the figure caption explains. The open circle just below 1.2 represents the 13 "BTC" measurements, the filled circles at 1.0 are the two HST measurements (3.83, 3.89 in Table 11), and the open circle at 0.8 with the big error bar represents the WIYN measurements. Thus "1.0" on the figure is something like 3.9 in the table (probably the 3.89). One magnitude fainter is 1.56 in the table, very close to 1.54 measured at day 50846.7. (I didn't pick this SN as an example at random!) this is shown on the graph as 1.2 mag at day 50817 Not "mag"; linear flux density. and my guess is 1.2 probably represents the averaged lower mag value of 4.8. Close enough; I get 5.1. Did you weight the measurements when you took the average? One problem I have is that the 1.2 `reading` on the graph does not decay by one mag to 0.4 as you suggest but rather to 0.48 linear (using the calculation 1.2/2.5=0.48) . Look at the error bars! The "1.2" is highly uncertain. The two HST measurements around the same time have much smaller uncertainties. Those are what establish the peak of the light curve, which conveniently is put at 1.0 in the graph. Giving a time in days for 0.48 mag is difficult to do accurately on the graph, but from the graph 0.48 occurs at about 21-22 days after peak, not 29 days! Again not "mag," and you are looking for 0.40, not 0.48. Unfortunately 1997ek is one of only a couple where there is *any* chance of defining a peak reading that has been observed rather than inferred by the template. There's nothing wrong with using the templates. In fact that's the "right" way to do things because the time of maximum, where the light curve is flat, is hard to determine. Fortunately, the authors have done the work for you. If you don't like the template, look at the time to decline an additional 0.78 mag (from 1.54 to 0.75 in Table 11), 12.1 days. The corresponding time for 1995E is about 5 days. There are many more supernovae published than in the Knop et al. paper. All we are looking for here is a sanity check. Probably a hundred or more astronomers spend most of their time working on SN light curves. If there were no time dilation, don't you think one of them would have noticed by now? And the templates are not an accurate enough guide as one only has to see how in SN1997eq and ek the actual peak measurements are well above the inferred template peak of 1 Only if you ignore the error bars. Incidentally,are you sure that 1995E is 0.01 redshift? It seems to be in the mid range of `heliocentric redshifts` (on table 3 page 51 Reiss) at 3.54 in a range between 3.1 - 4.5 (or 0.01 - 0.1) but unfortunately I dont know how to convert `heliocentric` to z. The table is labelled "log cz." Take 10^(cz) -- sometimes called "antilog" -- then divide by c=3E5. You have to know the units meant are km/s. You can check by looking up the galaxy velocity (NGC 2441, 3470 km/s). So yes, I'm sure, z=0.012. I didn't pick this SN at random, either. In every post, you have made elementary mistakes. I am not sure what to suggest, but you need to learn how to interpret data if you want to pursue this project (or indeed any other in astronomy). -- Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA (Please email your reply if you want to be sure I see it; include a valid Reply-To address to receive an acknowledgement. Commercial email may be sent to your ISP.) |
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Craig Markwardt wrote in message ...
In a given analysis, there is only one light curve template, and it is based on the nearby supernova remnant sample. The "stretch factor" s is introduced to match an individual light curve to the template, with the understanding that individual supernova light curves can be slightly slower or faster than the mean. There would be evidence of bias if the fitting of the light curve profiles treated the nearby and high-z supernovae separately. BUT, they were NOT. Both the samples were fitted with the same function, including a stretch factor. [ref. 1, Tables 1 & 3] There would be evidence of bias if the fitting of the light curve profiles allowed only a stretch but not a compression. BUT, they did NOT. Both stretched and compressed light curves were permitted in the fitting, and in fact some moderate-z light curves evolved faster than local ones as a result. [ref. 1, Tables 1 & 3] However, on *average*, the high-z supernova light curves evolve much more slowly. [ref. 1, References 1. Goldhaber, et al. 2001, ApJ, 558, 359 It is Goldhaber himself who has put the time dilation into the SN data by introducing a stretch s proportional to redshift. Take out the stretch from the data (and the k correction to B,and the template fits to R or parab-18) and the apparent time dilation will disappear. The closest available data that I can find, supplied by Goldhaber seems to be fig 1a and 1b which do not have the k correction or stretch but do have a template fit to R.? If there were a lightcurve or table of the SN data without this template fit to parab-18 for both the C-T and SCP SN data, I am sure that the apparent `time dilation` would disappear and what would remain would be only a range of lightcurves reflecting the various different restframe emission wavelengths of the SN`s studied. However, its not that clear to me what the filter bands the observations were made in but I believe that its R band observed for all the SCP SN`S and B band observed for all the C-T SN`s. These then appear to have been fitted to R and B band templates respectively. You may possibly be able to clarify this part of the process a bit more. One question here regarding 2 SN`s from Knop may provide an answer as to why even with the s factored in, Knops` results still favor no time dilation...Why do you 1998ay and 1998be have different `s` values applied? The two lightcurves graphs are distinctinctly different from each other with 1998be having faster decays in both I and R. The stretch value s in the table 3 Knop is also different for each. 1998ay is s=1.04 while 1998be is s=0.8. Yet they are both the same redshift at z=0.64 . I would have thought that the same stretch factor s would be applied to both SN`s seeing as they both have the same redshift. Sean (My apologies to Steve W. In my last post I incorrectly stated that 1998as is z=0.85 when in fact its 0.35. Sorry) |
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