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We present ground-based and Hubble Space Telescope optical observations of the X-ray flash ( XRF) 020903, covering 300 days. The afterglow showed a very rapid rise in the first day, followed by a relatively slow decay in the next few days. There was a clear bump in the light curve after similar to 25 days, accompanied by a drastic change in the spectral energy distribution. The light curve and the spectral energy distribution are naturally interpreted as describing the emergence and subsequent decay of a supernova ( SN), similar to SN 1998bw. At peak luminosity, the SN is estimated to be 0.8 +/- 0.1 mag fainter than SN 1998bw. This argues in favor of the existence of a SN associated with this XRF. A spectrum obtained 35 days after the burst shows emission lines from the host galaxy. We use this spectrum to put an upper limit on the oxygen abundance of the host at [O/H] <= 0.6 dex. We also discuss a possible trend between the softness of several bursts and the early behavior of the optical afterglow, in the sense that XRFs and X-ray-rich gamma- ray bursts ( GRBs) seem to have a plateau phase or even a rising light curve. This can be naturally explained in models in which XRFs are similar to GRBs but are seen off the jet axis.
In the favoured core-accretion model of formation of planetary systems, solid planetesimals accumulate to build up planetary cores, which then accrete nebular gas if they are sufficiently massive. Around M-dwarf stars ( the most common stars in our Galaxy), this model favours the formation of Earth-mass (M+) to Neptune-mass planets with orbital radii of 1 to 10 astronomical units (AU), which is consistent with the small number of gas giant planets known to orbit M-dwarf host stars(1-4). More than 170 extrasolar planets have been discovered with a wide range of masses and orbital periods, but planets of Neptune's mass or less have not hitherto been detected at separations of more than 0.15 AU from normal stars. Here we report the discovery of a 5.5(-2.7)(+5.5)M(+) planetary companion at a separation of 2.6(- 0.6)(+1.5) AU from a 0.22(-0.11)(+0.21)M(.) M-dwarf star, where M-. refers to a solar mass. (We propose to name it OGLE- 2005-BLG-390Lb, indicating a planetary mass companion to the lens star of the microlensing event.) The mass is lower than that of GJ876d (ref. 5), although the error bars overlap. Our detection suggests that such cool, sub-Neptune-mass planets may be more common than gas giant planets, as predicted by the core accretion theory.