THE DISCOVERY OF 8.7 2ND PULSATIONS FROM THE ULTRASOFT X-RAY SOURCE 4U-0142+61

We discovered an X-ray periodicity at approximately 8.7 s from the direction of the two sources 4U 0142+61 and RX J0146.9+6121. The pulsations are visible only in the 1-4 keV range, during an observation obtained with EXOSAT in 1984 August. In the same data, periodic oscillations at 25 minutes had been previously found in an additional hard spectral component above 4 keV. The newly discovered periodicity most likely originates from the optically unidentified source 4U 0142+61, previously considered a possible black hole candidate on the basis of its ultrasoft spectrum. Marginal evidence for the approximately 8.7 s pulsations is found in the two 1985 EXOSAT observations and in a 1991 ROSAT high-resolution imager pointing; if true, these measurements imply a spin-up timescale of approximately 530 yr. Although the very high (> 10(4)) X-ray to optical flux ratio of 4U 0142+61 is compatible with models based on an isolated neutron star, the simplest explanation involves a low-mass X-ray binary with a faint companion, similar to 4U 1626-67. A search for delays in the pulse arrival times caused by orbital motion gave negative results. The discovery of periodic pulsations from 4U 0142+61 weakens the phenomenological criterion that an ultrasoft spectral component is a signature of accreting black holes.