[vsnet-chat 2734] Re: SN2000B in NGC 2320

[Hitoshi YAMAOKA <yamaoka@rc.kyushu-u.ac.jp>]

Dear Odd and all,

David Bishop (probably) wrote:
>
> Odd Trondal wrote:
> >
> > I took a look at the SN2000B webpage and  I see that G. A. Sala  and
> > L. Robinson has magnitudes 16.2 and 16.1 the night before and the night
> > after my observation. On both of their picture the supernova is fainter
than
> > the 17.6 magnitude star indicated on my picture. Interesting, isn't it ?
>
> Thanks for the images.
>
> yes, I have noticed that you consistantly come out about 1.0 mag below
> everybody else.  Any ideas as to why?
>

Odd Trondal wrote:
>Hello,
>   yes,  but first a short introduction :
>   I am a variable star observer, and when a variable star is too
>   faint one have to identify the faintest comparison star, say A.
>   Than one can tell that it is fainter than this ( <A ). In the new 
>   picture in this letter the supernova is allmost not visible,
>   but the the star A with Rmag.=176 Bmag.=180 is clearly visible.
>   Then it is fainter than A ( more close to the star indicated by
>   the thick white arrow. )

# This picture can be seen at 
# http:/www.rc.kyushu-u.ac.jp/~yamaoka/trondal/ngc2320.jpg .

  I have looked at several images on D. Bishop's page taken during
Feb. 20 -- 26 , and I guess it would be caused by the difference and
the curious magnitudes of reference stars.

  For the first, the caption of F. Ewalt's image tells that he used
three GSC stars and their GSC magnitudes for reference.  These are
quite brighter than USNO magnitudes as:  

    GSC 1.1/ACT   GSC1.2        USNO_A1.0          USNO_A2.0
    name     mag   mag      name       rmag bmag     name       rmag bmag
B 3399.105  14.79 14.77  1350_07125783 15.1 16.7  1350_06971063 15.0 16.1
C 3399.385  15.24 15.22  1350_07121835 15.5 16.5  1350_09697115 15.4 15.8
D 3399.599  14.42 14.42  1350_07120205 14.4 15.8  1350_09695483 14.3 15.2

  So, if we use the USNO magnitudes for reference, the SN magnitude
will become about 0.2 mag (using rmag) or 0.5 mag (using vsnet-mag = 
0.375*bmag + 0.625*rmag of USNO_A1.0) dimmer than one derived by Dr.
Ewalt (16.65 +/- 0.25 at Feb. 22.104 UT).  SN was somewhat brighter
than star "A" named by Dr. O. Trondal ( = USNO_A1.0 1350_07123976,
rmag 17.6 bmag 18.8 = USNO_A2.0 1350_06969254, rmag 17.6 bmag 18.0) at
that time.

  On the other images, namely on the images taken by R. Poncy, P. Re',
G.A. Sala, and L. Robinson, SN 2000B is aproximately the same
magnitude as star "A".  There seems to be some trouble (they reported
16.1 - 16.2 for each image).  I guess it is possible due to the
curious magnitudes for the stars quite near to the galaxy and for the
galaxy itself in GSC or USNO catalogs.  These magnitudes are largely
affected by the glow of the galaxy, so they are *never* reliable and
could not be used for reference stars.  For example, USNO_A1.0 
1350_07123142 (18"W, 2"S from the nucleus of NGC 2320, mag about 17.5)
is catalogued as rmag 13.1 bmag 19.1, and also USNO_A1.0 1350_07123333
(2"W, 22"S mag about 17.5) as rmag 12.6 bmag 14.7.  These stars can
lead the estimate to be very bright.  

  For the solution of this confusion, I guess somebody should 
re-estimate with these images using the same reference sequence (and
the same technique).  Could you do it, Odd?  And could the observers
agree for it?

Sincerely Yours,
Hitoshi Yamaoka, Kyushu Univ., Japan
yamaoka@rc.kyushu-u.ac.jp

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