X-Ray Nova GRS 0831-429

R. Sunyaev on behalf of the GRANAT Team (Space Research Institute, Moscow, and the Danish Space Research Institute, Lyngby) reports: "WATCH detector data, obtained during Feb. 7, 8, 23, and Mar. 2, 11, and 15, show the presence of a strong x-ray transient, situated 6 deg away from the well-known pulsar Vela X-1, at R.A. = 8h31m +/- 3m, Decl. = -42.9 +/- 0.5 deg (equinox 1950.0; determined using the rotation modulation collimator). The flux in the energy range 5- to 15-keV was comparable to that of the Crab Nebula and was greater than the average flux from Vela X-1. During Feb. 23 and Mar. 2 sessions, Crab and Vela X-1 were also detected in the field-of-view (1 pi sr solid angle) of the instrument, while during the Feb. 7, 8, and Mar. 11 sessions, only Vela X-1 and GRS 0831-429 were in the field-of-view. For observations made on 1990 Jan. 25-26, the upper limit for the flux of GRS 0831-429 was 45 mCrab. The flux was weak during Feb. 5-7. During 100 000 s of observations on Feb. 7 and 8, the flux from the discovered source rose from 70 to 250 mCrab. On Feb. 23 and Mar. 2 the flux was about 1 Crab; on Mar. 11, GRS 0831-429 was still brighter than Vela X-1. The nearest transient x-ray source, 4U 0836-42 (R.A. = 8h36m12s, Decl. = -42 36'.9), was always much weaker than the Crab. However its position is inside our 3-sigma error box for GRS 0831-429 (but outside the 1-sigma box). According to its lightcurve, GRS 0831-429 most likely is an x-ray nova. In that case, data on the archival optical plates taken in 1990 Feb. and Mar. might give a better location."