Previous outbursts of U Sco were recorded in 1863, 1906, 1936, 1979 and 1987 (cf. Sekiguchi et al. 1988, hereafter S88). They were all characterized by a very fast evolution ( days) and a large amplitude (from mag in quiescence to mag at maximum). Several others have been quite possibly missed because U Sco lies just 4o from the ecliptic. In the following the comparison will be limited to the 1979 and 1987 events, because these are the only ones for which useful spectroscopic and photometric data have been obtained (S88, Barlow et al. 1981, Williams et al. 1981 and Warner 1995; hereafter B81, W81 and W95 respectively).
The system in quiescence shows eclipses (Schaefer & Ringwald 1995, hereafter SR95) with a period of 1.2305631 days. The bright prospects to derive the masses of the components from spectroscopic orbits have been however hampered by the faintness in quiescence and devoted attempts with 4-m class telescopes have so far provided only contradicting results (Johnston & Kulkarni 1992, Duerbeck et al. 1993, SR95).
Thanks to an immediate notification by the outburst discoverer (P.Schmeer, Belgium) and the VSNET network (cf. http://www.kusastro.kyoto-u.ac.jp/vsnet/ ), we were able to begin the monitoring of the 1999 outburst within a few hours from the maximum. In this letter we present the results of our all-out spectroscopic campaign and report about the photometric evolution as long as U Sco has remained brighter than V=15 mag. Modeling of the data and additional observations of U Sco once it will have returned to flat quiescence will be presented elsewhere (Selvelli et al. 1999, in preparation). Similarly, a detailed discussion of the reddening as inferred from the interstellar absorption lines visible in our Echelle spectra will be addressed in detail elsewhere (Munari and Zwitter 1999, in preparation).
© European Southern Observatory (ESO) 1999
Online publication: June 6, 1999