Pulse Delay Observations of GRO J1744−28

Abstract

The bursting pulsar GRO J1744-28 exhibits a unique combination of persistent X-ray pulsations (with a pulse period Ppulse ≈ 0.467 s) and X-ray bursts. The pulsations are also present (at an enhanced amplitude) during the bursts, but the arrival times of the pulses during the burst are delayed with respect to those of the persistent emission. We present the results of a detailed study of the pulse delays using data obtained with the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory. We find that the average delay, as measured during a 1.5 s interval at the peak of the burst, is independent of energy in the energy range ~25-75 keV and has a magnitude Δt = 74±13 ms. We also find that the phase delay measured near the peak of the bursts remained approximately constant throughout the first outburst of the source, although the peak flux of the bursts varied by a factor of ~3.3.