The Main Characteristics of GRO J1744-28 Observed by the Proportional Counter Array Experiment on the Rossi X-Ray Timing Explorer
In this Letter, we provide an introduction to the main features seen in a series of observations of the bursting pulsar GRO J1744-28. The observations were made from 1996 January through May with the proportional counter array on the Rossi X-Ray Timing Explorer spacecraft. In the 2--10 keV band, GRO J1744-28 emitted large bursts of ~10 s duration at a rate of about 2 per hour. The peak flux during these bursts was ~6--40 times greater than that in the quiescent, or nonbursting, periods. For the earliest bursts, the inferred peak luminosity approaches 100 times the Eddington limit, which is suggestive of some kind of beaming mechanism. A range of smaller bursts and quasi-periodic oscillation features were also seen. All this activity was superposed on an almost perfect sinusoidal modulation at a frequency of 2.14 Hz with an amplitude of ~10% of the nonbursting flux. The source's persistent flux declined in a roughly linear trend from late January until mid-May, by which time its intensity was confused with several other sources.