X-Ray Bursts from the Accreting Millisecond Pulsar XTE J1814-338

Abstract

Since the discovery of the accreting millisecond pulsar XTE J1814-338, a total of 27 thermonuclear bursts have been observed from the source with the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. Spectroscopy of the bursts, as well as the presence of continuous burst oscillations, suggests that all but one of the bursts are sub-Eddington. The remaining burst has the largest peak bolometric flux of 2.64 × 10-8 ergs s-1 cm-2, as well as a gap in the burst oscillations, similar to that seen in Eddington-limited bursts from other sources. Assuming that this burst was Eddington limited, we obtain a source distance of ≈8 kpc. All the bursts show coherent oscillations at the 314.4 Hz spin frequency. The burst oscillations are strongly frequency and phase locked to the persistent pulsations. Only two bursts show evidence for frequency drift in the first few seconds following burst onset. In both cases, the initial drift corresponds to a spin-down of a few tenths of a hertz. The large oscillation amplitude during the bursts confirms that the burst flux is modulated at the spin frequency. We detect, for the first time, a significant first harmonic component in burst oscillations. The ratio of count rate in the first harmonic to that in the fundamental can be greater than 0.25 and is, on average, less than that of the persistent pulsations. If the pulsations result from a single bright region on the surface, the harmonic strength suggests that the burst emission is beamed, perhaps because of a stronger magnetic field than in nonpulsing low-mass X-ray binaries. Alternatively, the harmonic content could result from a geometry with two bright regions.