GCN Circular 190

GRB 981226 Optical Observations

B. Lindgren, J. Hjorth, H. Pedersen, (University of Copenhagen), 
M. I. Andersen (NOT), A. O. Jaunsen (University of Oslo), 
J. Sollerman (Stockholm Observatory), 
J. Smoker, C. Mooney (Queens University, Belfast) and
E. Palazzi (Istituto TeSRE, Bologna; on behalf of the SAX GRB team)
report:

"R band images of parts of the BeppoSAX WFC error circle of GRB 981226 
(Piro, GCN Circ. 174
Not found
 
 
), covering the NFI error circle (Di Ciolo et al., 
IAU Circ. 7074; Frontera et al. IAU Circ. 7078), were obtained on 1998 
December 26.82 UT (1440 sec, seeing FWHM = 1.7"), Dec 27.85 UT (2400 sec, 
FWHM = 1.6") and Dec 28.83 UT (2400 sec, FWHM = 1.0") with the 2.5-m 
Nordic Optical Telescope (NOT) on La Palma and on Dec 27.06 UT (500 sec, 
FWHM = 1.5") and Dec 28.09 UT (500 sec, FWHM = 1.5") with the 1.5-m Danish 
telescope, La Silla. An I band image was obtained with the NOT on 
Dec 28.86 UT (2100 sec, FWHM = 1.2").

Down to a limiting magnitude of R ~ 23 we have found no significant evidence 
for a variable source inside the NFI error circle. Finding charts of the 
field are posted at http://www.astro.ku.dk/~brian_j/grb981226

The results we obtained for two field reference stars (A,B) and the
three candidate optical counterparts to GRB 981226, reported by Galama
et al. (GCN Circ. 172
Not found
 
 
), Castro-Tirado et al. (GCN Circ. 173
Not found
 
 
) and Wozniak
(GCN Circ. 177
Not found
 
 
), respectively, are presented in the table below. 

The photometric zero point is tied to that of Galama et al. (GCN Circ. 172
Not found
 
 
), 
assuming R = 21.04 for their reported object. All photometry is based on PSF 
photometry of the NOT R images, and assumes unresolved objects. The data 
obtained at the Danish 1.5-m telescope are consistent with these results. The 
quoted photometric uncertainties include formal fitting errors only. The 
astrometry refers to the USNO-A2.0 system (J2000) and is accurate to +- 0.23" 
(mean absolute deviation).

               RA (J2000)   Dec (J2000)  Dec 26.83    Dec 27.85    Dec 28.83
star A         23 29 29.28  -23 57 36.9  16.79+-0.00  16.79+-0.00  16.79+-0.00
star B         23 29 29.57  -23 57 45.6  18.79+-0.01  18.77+-0.01  18.80+-0.00
Galama         23 29 34.94  -23 56 30.5  21.01+-0.05  21.09+-0.07  21.07+-0.04
Castro-Tirado  23 29 34.92  -23 55 41.3  22.47+-0.14  22.36+-0.16  23.05+-0.18
Wozniak        23 29 27.31  -23 56 53.5 (21.31+-0.06)(21.58+-0.11)(21.60+-0.05)

Galama's object is constant within +- 0.05 mag during our observations.
This is consistent with the findings of Castro-Tirado et al. (GCN Circ.
173
Not found
 
 
), Bloom et al. (GCN Circ. 182
Not found
 
 
) and Schaefer et al. (GCN Circ. 185
Not found
 
 
). 

Castro-Tirado's object is close to the detection limit in our images,
and we conclude that there is no evidence for variability of this object. 
In a combined image of all our R band data the object has R = 22.9 +- 0.1.
If Castro-Tirado's variable (J band) object is the optical afterglow to 
GRB 981226, then the object we have detected is the likely host galaxy. It 
would be among the brightest host galaxies detected so far.

In our combined R image (FWHM = 1.2") Wozniak's object is clearly extended.
While consistent with that found by Schaefer et al. (GCN Circ. 185
Not found
 
 
), our PSF 
photometry is therefore biased and seeing dependent. We conclude that this 
object is a galaxy with a constant R = 21.16 +- 0.05 and that any contribution
from a variable source must be small in R.

For our Dec 28.86 I band NOT image we find I = 18.1 (based on an
approximate photometric zeropoint accurate to an estimated +- 0.2 mag)
for the reference star reported to have i = 20.47 (Bloom et al., GCN
Circ. 182
Not found
 
 
). In what follows we adopt our own preliminary zero point and
offset the Bloom et al. values to this system for comparison.

Galama's object has I = 20.35 +- 0.05, i.e., R - I = 0.7. Castro-Tirado's 
object is not detected down to I = 22. Wozniak's object is close to the 
detection limit; we find I = 21.9 +- 0.5 on Dec 28.86. This value should be 
compared to I = 20.3 on Dec 27.1 UT (Wozniak, GCN Circ. 185
Not found
 
 
), and to the 
corrected values I = 20.55 on Dec 27.1 UT and I = 21.05 on Dec 28.1 UT (based
on Bloom et al., GCN Circ. 182
Not found
 
 
). Thus, Wozniak's object has continued to fade.

In the simplest scenario, Wozniak's object is a fading red source 
(R - I > 2.6) superposed on an extended blue (R - I < -0.7) object. It is 
tempting to interpret this as a rapidly fading red optical afterglow 
originating in a bright, blue host galaxy. However, given its position more 
than one error radius (70") outside the NFI error circle, its relation to 
GRB 981226 remains questionable.

The results reported in this circular are preliminary.  A more detailed
analysis is ongoing." 

This report may be cited.