Ultraviolet/optical emission accompanying gamma-ray bursts

We discuss the possible simultaneously ultraviolet (UV)/optical emission accompanying gamma-ray bursts (GRBs). We show that as long as the intrinsic spectrum of GRBs can extend to similar to10 GeV or higher, there are large amounts of relativistic e(+/-) pairs generated due to the annihilation of soft gamma-rays with very energetic photons, which dominates over the electrons/positrons associated with the fireball, whether the fireball is highly magnetized or not (for the highly magnetized fireball, the magnetic field is ordered, high linear polarization of multiwavelength emission is expected). We find that these e pairs can power a UV flash with m similar or equal to 12-13 mag, and the corresponding optical emission can be up to m(R) similar or equal to 15-16 mag. Such bright UV emission will be able to be detected by the satellite Swift, planned for launch in 2004. The behaviour of the optical-UV spectrum (F(v) proportional to v(5/2)) differs significantly from that of the reverse shock emission (F(v) proportional to v(-beta/2), beta similar or equal to 2.2), which is a signature of the emission accompanying the GRB. Mild optical emission can be detected with the ROTSE-IIIa telescope system, if the response to the GRB alert is fast enough.