2004MNRAS.351L..78F

In the past two years, tremendous progress in understanding X-ray flashes has been made. Now it is widely believed that X-ray flashes and gamma-ray bursts are intrinsically the same, and that their very different peak energy and flux may be merely due to our different viewing angles to them. Here we analytically calculate the very early afterglow of X-ray flashes, i.e. the reverse shock emission powered by the outflows interacting with the interstellar medium. Assuming z ∼ 0.3, we have shown that typically the R-band flux of reverse shock emission can be bright to ∼16-17th magnitude (the actual values are model-dependent and sensitive to the initial Lorentz factor of the viewed ejecta). That emission is bright enough to be detected by the telescope on work today such as Robotic Optical Transient Search Experiment (ROTSE-III) or the upcoming Ultraviolet and Optical Telescope (UVOT) carried by the Swift satellite, planned for launch in late 2004.