A Redshift Determination for XRF 020903: First Spectroscopic Observations of an X-Ray Flash

We report the discovery of optical and radio afterglow emission from the extremely soft X-ray flash XRF 020903. Our spectroscopic observations provide the first redshift for an X-ray flash, thereby setting the distance scale for these events. At z = 0.251, XRF 020903 is one of the nearest cosmic explosions ever detected, second only to the recent GRB 030329 and the unusual GRB 980425/SN 1998bw. Moreover, XRF 020903 is the first X-ray flash for which we detect an optical afterglow. The luminosity of the radio afterglow of XRF 020903 is 1000 times greater than that of Ibc supernovae but similar to those of GRB afterglows. From broadband afterglow modeling we show that the explosion energy of XRF 020903 is not dissimilar to values inferred for typical gamma-ray bursts, suggesting that these cosmological explosions may derive from a similar mechanism.