[vsnet-chat 5629] [vsnet-alert 7572] SGR 1900+14 /bright opticalcounterpart

["Berto Monard" <LAGMonar@csir.co.za>]

Hi Arne,

thanks for all the advice and indeed let's avoid those affected
regions. 

And therefore still no optical countepart for SGR 1900+14.

One point though: even if the peaks (of the planckian curves) ly in the
IR, the effective wavelength of the observing window of an unfiltered
CCD camera for these reddened objects is still only around I, perhaps a
bit further than it . That is the point I made (not like you've put it
that the stars peak at Ic..). 

You can actually work these effective wavelenghts out if spectra and
response functions are known, but it is far more easy to let the filters
do the work for you. But since the filters are not monochromatic,
effective wavelenghts even through them do change with the target
spectrum. In addition they cut out far to many photons.

For the time being I'll stick to unfiltered, but with avoidance of the
above and below situations.


Kind regards,

Berto


>>> Arne Henden <aah@nofs.navy.mil> 11/07/02 05:13PM >>>
Berto,
   You (and most everyone else!) are not used to working with
heavily reddened regions.  If you read the Vrba et al. papers,
you will find that the M stars lie in a region with Av=19.  Because
of this, the stars "peak" in the infrared, not at Ic as you suggest.
You cannot observe this kind of field with an unfiltered system
and be able to transform it at all.  I will say one more time that
unfiltered observations are prone to error for most astronomical
objects; they often work ok for CVs only because those stars have zero
color
in outburst.  Starting out in photometry doing unfiltered observations
is fine, but when you want to start contributing scientifically
useful data, you need to seriously consider buying a couple of
filters.  If you don't, then at least understand the limitations
of your unfiltered camera.

 >But there is also the possibility that star A has brightened over 10
B
 >magnitudes. The analysis would be completely different but the ST-7E
 >would see this blue brightness increase.
 >Without an optical filter (V or B and even R would be OK) I can not
 >improve my findings with new observations, but if this star A were to
be
 >measured and found to be now brighter than 17 V or B, I bet it is
the
 >counterpart.
   You have to remember that the position of the SGR is well-known,
far better than an arcsec, because the source is seen at other
wavelengths
than the optical.  These M stars are 12 arcsec away.  They
cannot be the counterpart.  Please read the relevant papers.
   This is *not* to say that your observations are worthless.  As I
mentioned
earlier, no simultaneous optical observation has been made of an SGR
outburst.
There is a small chance that a simultaneous observation would be able
to detect some optical emission, and that would be an exciting
discovery.  Since these bursts are random, much like a flare star,
you can monitor for a *long* time before you are lucky; we were not
successrul in many hours of open shutter time with the 1.0m and 1.55m
telescopes.
Arne


Berto Monard wrote:
> Thanks Taichi,
> 
> at least I got one article and will perhaps see some 'light' in
there.
> 
> You also seem to suggest that E(R-I) might be close to 3...
> 
> 
> But it would be more conclusive if a B (or V) band measurement or
> observation were done on this field..
> 
> Regards,
> 
> Berto
> 
> 
> 
>>>>Taichi Kato <tkato@kusastro.kyoto-u.ac.jp> 11/07/02 11:12AM >>>
>>>
> Re: [vsnet-alert 7572] SGR 1900+14 / bright optical counterpart
> 
> Dear Berto,
> 
> 
>>I have tried to get the refered articles but to no avail. So I try
> 
> to
> 
>>respond without knowing the findings in those reports, which could
>>possibly refute the assumptions I gonna make.
> 
> 
>    Please try the ADS abstract service,
> 
>    http://adsabs.harvard.edu/abstract_service.html 
> 
>    And enter "SGR 1900" for the title query form, and select "and"
> operator.
> Then you can obtain what you would want.  The old articles
(including
> Vrba et al. 1996) have been scanned, and are available to anyone.
> 
> 
>>I estimate 20.0R (+or- 1) for A from the DSS plate of which I do not
>>exactly know the wavelength response function.
>>Taking R-I= 2 for very red stars (worst case), star A would then be
>>18.0I (+or- 2) on the DSS image.
> 
> 
>    You need to introduce interstellar reddening.  The R-I = 2 value
> corresponds to a *unreddened* star.  The key terminoligies are Av
and
> E(B-V), E(R-I) etc.  You will probably be able to reach a correct
and
> necessary reference :-)
> 
> Regards,
> Taichi Kato
> 
> 
>

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