3 Discussion and conclusion

Optical and IR images of the RXTE error box were obtained starting 106 hours after the event. No variable optical counterpart was found within the $\rm A\,\char93 \,1$ and $\rm A\,\char93 \,4$ X-ray source error circles in the RXTE/IPN annulus, being the variation $\Delta B \leq 0.3$, $\Delta V \leq 0.2$, $\Delta R \leq 0.2$ and $\Delta i \leq 0.3$ for optical objects with $B \leq 23$, $V \leq 23$, $R \leq 24$ and $i \leq 22$. In fact, no objects brighter than R = 24.2 are seen within the error box of the ROSAT $\rm R\,\char93 \,2$ source detected at the ASCA position, consistently with the magnitudes reported by Groot et al. (1997). Therefore, either there is a considerable intrinsic absorption (as in the case of GRB 970828, Yoshida et al. 1999) or the transient optical emission displayed a fast decline, as seen in GRB 980326 (Groot et al. 1998) and GRB 980519 (Hjorth et al. 1999).

No IR variability has been found in a $5^{\prime} \times 4^{\prime}$ region centered at the position of the candidate proposed by Galama et al. (1997). This object was observed at $J = 18.3 \pm 0.2$, and $K^{\prime} = 17.4 \pm 0.3$ and did not show any obvious IR/optical variation in the $K^{\prime}$-band. The color is consistent with this of a late-type star, as suggested by Pahre et al. (1998) and it seems to be a non-variable star (Groot et al. 1997). Any fading or increase was $\leq 0.5$ mag for $K^{\prime} \leq 17.0$.

Concerning the above mentioned cluster of galaxies (the long arrow in Fig. 1), it is probably related to the X-ray source $\rm A\,\char93 \,2$, which is outside the IPN band, and consequently not related to the GRB.

 

Figure:1 A coadded R-band image of the field of GRB 970616 obtained with the Danish 1.54-m telescope. The total exposure time is 19.5 hours and the limiting magnitude R = 24.2. The straight thick lines represent the IPN arc determined by Ulysses and BATSE. The two circles fully included in the IPN band are $\rm A\,\char93 \,4$ (large, at the top) and $R\,\char93 \,2$ (small at the bottom), whereas the circles partially included are $\rm A\,\char93 \,3$ (top) and $\rm A\,\char93 \,1$ (bottom). The circle totally outside the IPN band is $\rm A\,\char93 \,2$. $\rm R\,\char93 \,1$ and $\rm R\,\char93 \,3$ are outside the field of view. The two short arrows show the variable objects proposed by Galama et al. (1997), -inside the IPN band- and Udalski (1997), -outside the IPN band-. The field of view is $13\hbox{$.\mkern-4mu^\prime$}6 \times 13\hbox{$.\mkern-4mu^\prime$}6$ and is totally within the RXTE/ASM error box. North is to the top and east to the left. The long arrow indicates the massive cluster of galaxies found by Benítez et al. (1999)

Acknowledgements

We wish to thank A. Yoshida for fruitful conversations and the referee N. Masetti for useful suggestions. This work has been partially supported by Spanish CICYT grant ESP95-0389-C02-02.