Pulse Arrival Time Glitches in GRO J1744−28

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Abstract
We observe glitches in the arrival times of X-ray pulses from GRO J1744-28 associated with the large X-ray bursts observed from this source. In early observations, the pulse arrival time becomes delayed by 28 ms (6% of the 467 ms pulsar period) following bursts while the shape of the nearly sinusoidal light curve is essentially unchanged. This arrival-time lag decays exponentially over the next ~1000 s. In later observations, the observed phase lags are smaller, and eventually, some bursts appear to be accompanied by a small phase advance. While explanations of these observations that rely upon changes in the postburst accretion flow are natural, our observations present some problems for these explanations. It may be that this behavior can be understood by analogy with glitches in spinning-down radio pulsars, though this explanation has its own problems. Explained as a glitch, the phase shift is the result of coupling of the neutron star's crust to the crustal superfluid, which is spun up more slowly than the rest of the star, and the recovery is due to coupling between the crust and the core of the neutron star.