GRS 0834-430

S. Grebenev and R. Sunyaev, on behalf of the GRANAT team, report: "The GRS 0831-429 (IAUC 5122) field was observed on May 28.7 UT during a 1-hr exposure. The ART-P telescope detected only GRS 0834-430, the southern of two sources observed in that field by ROSAT, GINGA, and GRANAT (IAUC 5139, 5142, 5180). This source was found in a bright state; its 6- to 30-keV flux was at the level of 250 mCrab. The source spectrum was hard, with photon index about 1.65. We detected strong pulsations with a barycentric period of 12.3167 +/- 0.0006 s. It is obvious that GRS 0834-430 was the source of these pulsations, as first found by GINGA (IAUC 5139, 5148). The period derivative, estimated in the assumption of a steady spin-up during the interval between these and previous (IAUC 5180) ART-P observations, was equal to -(7.8 +/- 0.4) x 10E-10. The 3-sigma upper limit on the flux from the northern source (MX 0836-42) in the same field was equal to 30 mCrab. The 1991 May outburst was the third bright outburst observed from the GRS 0834-430 field during 1990-1991 (cf. IAUC 5122, 5180)."