[vsnet-chat 5625] (fwd) [vsnet-alert 7572] SGR 1900+14 / bright optical counterpart

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

(fwd) [vsnet-alert 7572] SGR 1900+14 / bright optical counterpart

Date: Thu, 07 Nov 2002 10:52:59 +0200
From: "Berto Monard" <LAGMonar@csir.co.za>
Subject: Re: [vsnet-alert 7572] SGR 1900+14 / bright optical
	counterpart

any further follow up from my side on this will be posted to
vsnet-chat..

Dear Arne and Taichi,

Before I start blushing and acknowledge another amateur mistake, I
would like to discuss this observation.

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.

Taichi cites V=24.5 (and slightly variable) for one of the red stars.
But actually both stars are faintly visible on the DSS2/red image (but
not on the /blue image). The problem was that the printouts I made and
took home, did not show them. 

Since it shows most prominently on my image and the least bright on the
DSS2/red image, I will further deal with the Eastern star of the two,
which I will name A. 

Since I measured 15.0 CR for the combined light (using nearby USNO-A2.0
R magnitudes), we could safely derive an upper limit of 15.5CR for A. 
But in view of the redness and/or reddening of the stars, the quoted
'unit' CR will have to be redefined. The effective wavelength of the
unfiltered ST-7E camera for cool red stars (with approx BB function of
2500K / peaking at 1.2 micron) will be close to that of the I filter, so
CR is actually CI. Assuming R-I=0.5 for the reference star used, the
measure would be 15.0CI for A. The coarsely estimated uncertainty could
be 1I.

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.


If the above assumptions hold, this star A could have brightened over
3I, which is a significant brightening. Considering the uncertainty
limits it is also possible that the star remained constant, although
this is unlikely.

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. 


Regards,

Berto Monard
Bronberg Observatory.

PS last night's image can be made available
  

>>> Arne Henden <aah@nofs.navy.mil> 11/07/02 02:56AM >>>
Please don't get too anxious about this observation.  If you
look at the literature on SGR1900+14 (especially Vrba et al.
2000, ApJ 533, 17 and Vrba et al. 1996, ApJ 468, 225), you
will find that there is a close pair of highly reddened M stars
located
at approximately
   19:07:15.2 +9:19:21.3 J2000
Since this location matches Berto's coordinates, I am sure that
he is just seeing these two stars blended, especially since he is
observing
unfiltered.  You must be very careful in reddened regions.
Arne

B Monard wrote:
> Hi,
> 
> In response to alerts from several GCN circulars, the starfield of
the
> suspected active transient and gamma ray burster SGR 1900+14 was
imaged this
> evening at the Bronberg Observatory (South Africa), using the 12"
f/4.2 SCT
> with CCD camera ST-7E.
> 
> More than 30 images with exposures of 45 sec were taken (in extremely
windy
> conditions) and the best images were inspected.
> 
> By comparison to images of the Digitized Sky Survey of generation 2 /
red
> and blue, a new and bright object was visible at position 19 07 15.18
+09 19
> 22.6, astrometrically traceable to the USNO-A2.0 frame.
> 
> The brightness of this new object and its neareness to the published
> coordinates (19 07 14.33 +09 19 21.1 from radio interferometry) make
this
> object the probable optical counterpart of the active gamma ray
burster.
> 
> Photometry of the object showed no obvious fluctuations of the
brightness.
> The unfiltered CCD observation:
> SGR 1900+14   20021106.737   15.0CR
> 
> Note: on most images the object looked distinctly elongated in the
E-W
> direction.
> 
> If the weather allows, follow-up observations will be done at the
Bronberg
> Observatory tomorrow.
> However, the longer nights in the Northern hemisphere ought to
enable
> timeseries of much longer duration on this Northern target.
> 
> This notice may be cited and communicated to the GCN network.
> 
> Berto Monard / MLF
> Bronberg Observatory / CBA Pretoria
> 
> 
> 
>

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