From: Greg Aldering (aldering@panisse.lbl.gov)
Date: Wed Apr 30 2003 - 20:22:45 PDT
Hi Andy,
I see, basically your program threw away everything below 4000A in the
restframe, i.e, the Ca feature that is present in the observed spectrum,
to obtain your Ic fit to 98104. I understand the constraints imposed by
the limited spectral coverage of the template spectra, but throwing away a
key portion of the spectrum is obviously not the approach to take in this
case.
Also, I don't know whether bluening was important in getting a Ic fit for
this spectrum, but I think it is important to impose some limits on the
amount of bluening allowed. The only mechanisms for bluening are flux
calibration errors or atmospheric dispersion combined with guiding at a
blue wavelength. I doubt that the flux calibration errors are very large,
and since the Keck guider response peaks in R-band, atmospheric dispersion
should not be bluening the spectra much at all. (A check I did for a pivot
star that was in the slit when we observed Albinoni, using LRIS just as
for many of these SNe, showed excellent agreement with a model spectrum
for the pivot star.)
I understand that it is hard to code for all the different types of
observational situations which may arise, but to remove from the Hubble
diagram a SN based on such hard-won and expensive data, without first
imposing every realistic constraint on the spectral fits, just doesn't
make sense to me.
Do you have some fits that are better in this regard?
Cheers,
Greg
On Wed, 30 Apr 2003, Andy Howell wrote:
> Greg,
> This mainly a psychological effect of the rebinning -- I rebinned the
> data to 20A.
> I put up on the web page the same plot rebinned to 3A -- you can see it
> looks like the
> untouched data.
>
> Also, the spectrum I show is cropped to only show where the template
> spectrum and
> the observation overlap. I put up another plot showing a different fit
> with more of the data.
>
> My program has the freedom to do other things to the data as well. In
> my plots I have alread
> subtracted host galaxy. Plus, my program either reddens or bluens the
> spectrum to find the best fit.
>
> So the spectrum I show may not correspond to reality -- I let it mangle
> the observed
> spectrum to find the best fit possible. Often it will find junk, but
> the point is that
> if there is a Ia signal there it should find it. Here I just chose one
> example to show,
> but I don't believe it -- I don't believe any of the fits.
>
> -Andy
>
> Greg Aldering wrote:
>
> >Hi Andy,
> >
> >I am looking at the spectral fits to 98104 on your website. I don't see a
> >relation between your spectrum and Isobel's. Am I even looking at the
> >right thing? (Yours is labeled "Observed: 98104cc_comb.Ic.asc" and
> >Isobel's is labeled "98104 (Keck) : 98104cc_comb.asc". Yours is a
> >power law and Isobel's have plenty of wriggles.
> >
> >
> >
> >>Due to the E's 4000A break, you subtrack off more light blueward of 4000A
> >>than redward, and this can mimic Ca from the SN.
> >>
> >>
> >
> >No. The effect of subtracting an elliptical is to decrease the strength
> >of Ca - unless you let the elliptical go negative! But I do agree that
> >9878 isn't convincing.
> >
> >- Greg
> >
> >
> >
>
>
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