[Aavso-photometry] GJ 436 update and upcoming transit times

[Geir Klingenberg]

Sorry for not posting the data yet.

The report was made before the campaign was announced. Following the
suggestions in the announcement I have used ensemble techniques on the
data, and I have tried to transform. But I can't seem to get accurate
V-mag estimates for GJ 436, and I am running out of ideas as to what
might be the problem. Except the obvious, of course, which is me :-)

>From Alert Notice 350 GJ436 has a baseline V-mag of 10.702.
Untransformed, I get the following:

Comp  GJ436 V-mag
_106    10.64
_107    10.53
_114    10.64
_116    10.53
_127a   10.56

Average 10.58, std 0.056. That is about 0.12 mags away, which seems
pretty much to me. These results are consistent over three different
nights.

Transforming does not help much either. With the transformation
coefficients that I have established on several occasions - using both
NGC7790, M67 and some Landolt fields - the difference compared with
the untransformed ones is negligible.

You find info about the transformation coefficients here:

http://www.remote-astronomical-society.org/Tables%20and%20Files/Transformation%20Coefficients%20for%20ARE01.pdf

(I used a method based on differential photometry, but I have checked
that the method in the Henden book gives similar results). I use the
following equation to calculate the transformed V-mag, using the known
B-V of GJ 436:

Vt = Vc + (vt – vc) + Tv * ( (Bt - Vt) – (Bc - Vc) )

I also got some B and V frames a couple of days ago to find the color
myself, without any significant change in the results.

Finally I used these B and V images to obtain transformation
coefficients from the stars in the GJ436 field, including GJ436
itself, and that gave me the following coefficients: Tbv = 0.957, Tv =
0.09. That helped and gives an avg. V mag of 10.65. But using GJ436 in
the solution is kind of cheating, I guess.

I have noted earlier that I get the best result by finding
transformation coefficients using the comp stars in the field of the
variable. Is this common?

Also, could it be a flat issue? We can not make our own flats for this
robotic telescope, instead we use the one that is supplied. It is not
of highest quality when it comes to photometry, but I didn't think it
was *that* bad.

If anyone want to have a look at some of the images I will be glad to
send them to you.

Please let me know if you see any calculation errors or similar.

Best regards,
Geir Klingenberg

On 6/15/07, Aaron Price <aaronp at aavso.org> wrote:
>  We have our first set of data, reported by Bill Goff (GFB). I've placed a light curve
> of his data on the web page: http://www.aavso.org/news/gj436.shtml .  There seems to
> be some disagreement about Geir's reported transit on May 31. The data has not been
> submitted to the AAVSO, but his report is also available via that web page.
>
>  Here are some predicted transits (UT) for the next week. As you can see, they are
> fast (62 min.)!
>
>  Begin                                    End
>  JD               Mon. Day  Hour Min
>  2454267.54 2007  6    16    0   53      2454267.59 2007  6 16  2  3
>  2454270.18 2007  6    18    16  20      2454270.23 2007  6 18 17 30
>  2454272.82 2007  6    21    7   47      2454272.87 2007  6 21  8 57
>  2454275.47 2007  6    23    23  14      2454275.52 2007  6 24  0 24
>  2454278.11 2007  6    26    14  41      2454278.16 2007  6 26 15 52
>
> >From http://www.ucolick.org/%7elaugh/GJ436____b.transits.txt
>
>  But remember we need data all the time, not just during transit windows.
>
>
> Aaron
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