TITLE: GCN GRB OBSERVATION REPORT NUMBER: 190 SUBJECT: GRB 981226 Optical Observations DATE: 99/01/06 12:56:10 GMT FROM: Jens Hjorth at U.Copenhagen 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), 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), Castro-Tirado et al. (GCN Circ. 173) and Wozniak (GCN Circ. 177), respectively, are presented in the table below. The photometric zero point is tied to that of Galama et al. (GCN Circ. 172), 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), Bloom et al. (GCN Circ. 182) and Schaefer et al. (GCN Circ. 185). 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), 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). 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), 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). 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.