From: Alexander Conley (AJConley@lbl.gov)
Date: Wed Oct 13 2004 - 14:16:51 PDT
Hello all,
I have finished (I hope) trying to answer all of the questions that
you
have asked me about my analysis. What I need from you is for you all
to tell me if you are satisfied with my attempt at an answer, or if I
need
to look at a particular issue more deeply.
There is a new version of the paper (1.32) available in the usual place
( http://panisse.lbl.gov/~aconley/cmag ) which incorporates many of the
changes you suggested, plus a few more.
The biggest change is that probcut is no more. I discovered a subtle
bias that probcut was introducing in the analysis (it tended to punish
extremely well observed low redshift SNe with slopes that were a bit
away from the central value). I ended up excluding 2 low-z SNe by hand
that probcut had been catching: 97br, 97bp. Their data appears to
be internally inconsistent, with a much larger degree of scatter than
can be accounted for by their quoted photometric errors.
There is also a (blinded) w fit. I haven't combined it with the other
priors, but there isn't much point in doing so in blind mode anyways.
Finally, I started putting the systematic errors together, although the
job isn't done yet.
Below I have listed all of you in alphabetical order and tried to give
a brief response to each of the questions you asked of me.
Ariel:
Original message:
http://supernova.lbl.gov/collab/archive/cmag/0006.html
Q: Have you tried not applying the MW extinction correction and
seeing
if that behaves as you expect?
A: As expected, the m_B residuals (as measured by the RMS) got much
worse and the B_BV residuals did not.
More info:
http://supernova.lbl.gov/collab/archive/cmag/0010.html
Q: Why do you set s_B = s_V for some fits but not for others?
A: I think it would be internally inconsistent to force s_B=s_V
given what
we know from CMAGIC -- namely, that when there is a bump, s_V <
s_B.
Note that I have only allowed s_B ne s_V when there is a bump.
And it's not like the s_B = s_V fits to SNe with bumps are good
-- most
of them are absolutely awful, clearly missing the shape of the
light curve.
On a related note, on Saul's urging I did redo all fits so that
s_B was fit
to the B data only, and found that using these values for
stretch, etc. made
very little difference. See Saul's section below.
More info:
http://supernova.lbl.gov/collab/archive/cmag/0007.html
Q: If the split in the high redshift residuals (between the Knop and
Barris sample)
is really due to dust, can you see this in the residuals of
m_B vs B_BV?
A: Yes, the matched residuals are consistent with this story. This
is probably
my single greatest area of concern with my result -- but I'm
not sure what
to do about it. Note that if a 'by-hand' extinction
correction is applied, the
sample no longer splits.
More info:
http://supernova.lbl.gov/collab/archive/cmag/0007.html
Q: Have you tried to set limits on intergalactic dust?
A: No, but we can talk about this after the blindness comes off.
Chris:
Original email:
http://supernova.lbl.gov/collab/archive/cmag/0001.html
Q: Could you add a stretch vs. scriptM plot?
A: Done
Q: You seem less sensitive to the U-enhanced K-correction
systematic
than K03. Could you emphasize this more?
A: Am I? K03 was completely insensitive to changing the U-B
color of
the uberspectrum as long as an extinction correction was not
applied,
and very sensitive when one was. I don't apply a extinction
correction,
and my sensitivity lies somewhere between the two K03 values.
See the latest version of the paper for actual numbers.
Q: s_B ne s_V?
A: Ariel also asked this (although you beat him to the punch).
See the
answer in his section.
Greg:
Greg's comments:
http://supernova.lbl.gov/collab/archive/cmag/0013.html
Greg had many comments. I won't be able to hit them all here. See
http://supernova.lbl.gov/collab/archive/cmag/0013.html for many
of my responses.
Some highlights, and things I didn't get to in that email.
Q: The first 3 high redshift SNe all seem high. Is their anything
funny with them?
A: Nothing I can find. They come from 3 different reductions
(P99, K03 and Riess 98).
Applying a by-hand extinction correction does not save them.
They are also
all high in a maximum magnitude fit. There was a problem
with one of the
lightcurve points for SN1995ba (a photometry error -- I have
informed Rob),
but that doesn't really 'save' it.
Q: Can you break up the residuals by various subsamples and see if
they
differ.
A: By photometry source: The SCP and hi-z team samples are quite
compatible
in terms of mean and scatter. SCP mean: 0.025 avdev:
0.183 rms: 0.218
high-z team mean: 0.006 avdev: 0.183 rms: 0.233. The
Hawaii (basically
Barris) sample has an offset mean and lower rms -- mean:
-0.13
avdev: 0.11 rms: 0.19. The offset is well documented
(the split in the
residuals) -- see Ariel's section.
By redshift: Split into three chunks: [0,0.45),
[0.45,0.55),[0.55) (to get roughly
equal numbers of SNe), they are consistent. The RMS gets
marginally
smaller at the high redshift end (0.23 -> 0.17), but I
expect this to some
extent because the mean extinction will be lower because
of selection effects.
Summary: I see no evidence for any real problems, except
the already noted
shift in the mean between the Barris sample and everybody
else (which seems
to arise because the Barris sample has unusually little
host galaxy extinction).
Q: In the matched residuals (m_B vs B_BV) is the distribution
narrowing as you
move towards positive residuals? Do bump SNe have anything
to do with this?
A: Splitting the sample in half, the lower RMS is 0.16, the upper
is 0.15, so it is
slightly narrower, but not by much. Removing all of the
SNe with bumps
decreases the RMS values slightly to lower: 0.16 upper:
0.14.
On a related note, of the 8 SNe with bumps, 6 have
positive residuals.
This is significant at the 1.4 sigma (equiv: 14.4%) level,
according to
the binomial distribution. Bottom line: This doesn't seem
significant.
Q: What does the chisquare distribution for the fits to the CMAG
relations look like?
A: Too good -- as expected. This is why W03 rescaled their error
bars -- people
are clearly underestimating them. I decided that the
rescaling was far too
dangerous given the quality of the high-z data. The plot of
the chisq probability
(the prob that the chisq should be higher than it is
observed to be) is
attached. It is very difficult to say what this
distribution should look like because
it is different for every number of points in the CMAG
region, but it's fair
to say it should be peaked around 0.5. The big spike at
around 1 is a bunch
of SNe that are too well fit.
Mark:
Original email:
http://supernova.lbl.gov/collab/archive/cmag/0011.html
Q: Could you include the intrinsic error in the stretch vs scriptM
plot?
A: Done
Q: How about propogating the z error into the vertical error bars
in the
Hubble residual plots?
A: Done
Q: On the resid vs. resid plot, do you take the errors into
account when
calculating the pearson's coefficient?
A: Not yet, but I think this can wait until after the blindness is
removed
(since that shouldn't affect these values).
Saul:
Q: Could you do a blinded w fit?
A: Done. See the most recent version of the paper. The blindness
scheme
is a bit rough given the almost pathological shape of the
error contours.
I have not combined these contours with the other priors yet,
but there is
no point in doing so now. Adding the priors and then blinding
would probably allow the blindness scheme to be violated, so I
won't do it.
More info:
http://supernova.lbl.gov/collab/archive/cmag/0021.html
Q: Could you redo all of the lightcurve fits using the prescription
of P99 --
that is, do a B only fit, then fix the stretch and date of
maximum to those values
for the V fit.
A: Done. Didn't make a difference (see the most recent version of
paper). As
detailed at great length in
http://supernova.lbl.gov/collab/archive/deepnews/1361.html
a very long and boring email which nobody probably read, this
is (a) harder
than it looks, and (b) wasn't done right in P99. Luckily it
only affects the V
magnitudes, so doesn't change fit C. Note that I didn't
follow this prescription
for the 8 bump SNe, since the quality of the fit in that case
is god-awful.
Q: What effect does applying an extinction correction have?
A: It shifts the contours almost purely along the long axis inward
(by 0.4ish).
Note that this fit can NEVER be published. It's a piece of
junk found by
combining lots of extinction values that other people
calculated, and so
is horribly internally inconsistent.
Q: What effect does removing all SNe with bumps have?
A: A shift of 0.016 along the short axis and -0.14 along the long
axis, giving
lower values of om, ol, but a slightly higher value (+0.016)
for om in a flat
universe. Not a really large systematic.
Vitaliy:
Original email:
http://supernova.lbl.gov/collab/archive/cmag/0015.html
Q: Are you going to combine the shifts into a final systematic error?
A: Yes. I have started doing this in the latest paper draft.
Okay -- hopefully I didn't miss anything. Please let me know if you are
happy with my answers, and if not, what I can do to try and answer your
questions better. Then, the blindness can be removed.
Alex
This archive was generated by hypermail 2.1.4 : Wed Oct 13 2004 - 14:17:04 PDT