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GCN Circular 2944

Subject
SGR1806: Optical Transient Candidates in SOHO/LASCO Images
Date
2005-01-08T02:54:09Z (20 years ago)
From
AAVSO GRB Network at AAVSO <aavso@aavso.org>
P. Schnoor and A. Price, on behalf of the AAVSO International High Energy
Network, report on the detection of two transients close to the location of the
recent superflare of SGR1806-20 (GCN #2920Loading...
 
 ; Borkowski, et. al)  in archival
images of the SOHO spacecraft.

The C3 telescope, a part of the Large Angle and Spectrometric Coronagraph
(LASCO)  instrument aboard the SOHO spacecraft, imaged the field of view
of the SGR1806-20 superflare before and after detection of the flare on
December 27, 2004 at 21:30:26 UT.

Two transient objects have been detected in the region near the SGR.  The
first transient ("A") is nearly coincident with the radio afterglow
position reported in GCN #2929Loading...
 
  (Gaensler, et al.) of 18h08m39.3s, Dec
-20o24'39.7" +/- 0.1" but is only discernable in a single image. The
second transient ("B") is discernable in two images (thus is not a cosmic
ray) and exhibits variability between the images but its location is
further away from the radio afterglow position.

Transient A 
Location: RA 18h08m Dec. -20o26' +/- 2 arc minutes in both directions
        Date        Time (UT)             Visual Estimate (v mag) 
     2004/12/27      23:18             No object down to <9.5
     2004/12/27      23:42             6.2
     2004/12/28      00:18             No object down to <9.5

Transient B
Location: RA 18h06m Dec. -20o31' +/- 2 arc minutes in both directions
        Date        Time (UT)             Visual Estimate (v mag) 
     2004/12/27      22:18             No object down to <9.5
     2004/12/27      23:18             7.5           
     2004/12/27      23:42             6.1
     2004/12/28      00:18             No object down to <9.5

The image from 12/27 at 22:42 is not available via the online archive.  
Astrometry is by visual estimate. The images were taken with no filter
and no polarizer. The quantum efficiency of the CCD is about 0.3-0.5 from
500 to 700 nm. Brightness estimates for transient A were made by visual
comparison to GSC 6276-1797 (Tycho Vt=6.6) and GSC 6272-0089 (Tycho
Vt=6.0). Brightness estimates for transient B were made by visual
comparison with GSC 6276-1797 and GSC 6263-0713 (Tycho-2 Vt=8.0). We
estimate visual precision to +/- 0.3 mag. Limiting magnitude for all
images is estimated with GSC 6276-0854 (Tycho Vt=9.5).

This area of the C3 telescope is subject to "strong vignetting" as reported in
the LASCO Handbook (1) so astrometry and photometry must be judged as suspect.
The authors of this GCN are not trained in analysis of LASCO images so careful
calibration by LASCO experts is needed for confirmation. A GCN may be issued in
the future when such analysis becomes available.

A web page with images, finder chart and an animated, annotated GIF of the LASCO
frames is here:
         http://www.aavso.org/observing/programs/lascosgr.shtml

Courtesy of SOHO/LASCO consortium. SOHO is a project of international
cooperation between ESA and NASA.

1)  http://lasco-www.nrl.navy.mil/handbook/hndbk_6.html