2 The observations

GRB 971227 was detected by BeppoSAX on December 27.349 UT (Coletta et al. 1997). Two X-ray sources were soon after identified in the GRB error circle (radius of 10'). One of these, 1SAX J1257.3+5924, disappeared approximately 22 hours after the $\gamma$ burst and therefore was proposed as the best candidate for the X-ray afterglow (Piro et al. 1997). Its position ($\alpha_{2000} = 12^{\rm h} \,57^{\rm m}\, 15^{\rm s}$, $\delta_{2000} = +59^{\circ} 24'\, 02''$, error circle radius of $1\hbox{$.\mkern-4mu^\prime$}5$) was at the northern boundary of the error box of GRB 971227.

Among the images of the GRB 971227 field taken by Castro-Tirado et al. (1997) at the 2.2-m German-Spanish telescope at Calar Alto, only one on December 27.90 UT (about 13 hours after the $\gamma$ flare) included the entire error circle of 1SAX J1257.3+5924. Inside this circle an object was detected at $\alpha_{2000} = 12^{\rm h}\, 57^{\rm m}\, 10^{\rm s}$,$\delta_{2000} = +59^{\circ}$ 24' 43'' (error box radius of 3'', Fig. 1). It was near the border of the frame, where the instrumental background is higher; however, the reliability of the detection is better than $2\sigma$. Other fainter stars can be clearly detected in the same part of the frame. It should also be noted that this field was imaged at very high airmass (secz=4.7). The object was not present in two summed R frames (limiting magnitude $\sim22$,Fig. 2) and in one I frame (limiting magnitude $\sim21$) taken by Bartolini et al. (1997) on December 30.16 UT at 1.5-m telescope of the Bologna Observatory in Loiano. Therefore it was proposed as the optical counterpart of GRB 971227 (Castro-Tirado et al. 1997; Bartolini et al. 1997).

 

Table 1: Journal of the optical observations of the part of the GRB 971227 field including the OT

Table 1 reports the journal of observations of the field of 1SAX J1257.3+5924 presented in this paper; some more frames not including this field have been obtained at Calar Alto and NOT telescopes. Magnitudes were calibrated referring them to the PG 0942-029 sequence (Landolt 1992). For the OT we found $R=20.0\pm0.3$.On December 30.8, Djorgovski et al. (1998) did not find the candidate above R = 24 (with a limiting magnitude $R\sim25.5$ for the frame). It was also not visible above R = 20.5 on December 31.44 (Bond et al. 1998), and above R = 22.3 on January 4.24 at Calar Alto.

 

Figure 2: Part of the two summed R frames taken at Loiano on Dec. 30.16, centered on the position of the OT. Size of the field: $4\hbox{$.\mkern-4mu^\prime$}5 \times 4\hbox{$.\mkern-4mu^\prime$}5$. The limiting magnitude is $R\sim22$. North is at top, East to the left