The Discovery of a Neutron Star with a Spin Frequency of 530 Hz in A1744-361

We report the detection with the Rossi X-ray Timing Explorer (RXTE)Proportional Counter Array (PCA) of 530 Hz burst oscillations in athermonuclear (type I) burst from the transient X-ray source A1744-361. This isonly the second burst ever observed from this source, and the first to be seenin any detail. Our results confirm that A1744-361 is a low mass X-ray binary(LMXB) system harboring a rapidly rotating neutron star. The oscillations arefirst detected along the rising edge of the burst, and they show evidence forfrequency evolution of a magnitude similar to that seen in other burst sources.The modulation amplitude and its increase with photon energy are also typicalof burst oscillations. The lack of any strong indication of photospheric radiusexpansion during the burst suggests a 9 kpc upper limit of the source distance.We also find energy dependent dips, establishing A1744-361 as a highinclination, dipping LMXB. The timescale between the two episodes of observeddips suggests an orbital period of ~ 97 minutes. We have also detected a 2 - 4Hz quasi-periodic-oscillation (QPO) for the first time from this source. ThisQPO appears consistent with ~ 1 Hz QPOs seen from other high-inclinationsystems. We searched for kilohertz QPOs, and found a suggestive 2.3 sigmafeature at 800 Hz in one observation. The frequency, strength, and qualityfactor are consistent with that of a lower frequency kilohertz QPO, but therelatively low significance argues for caution, so we consider this a tentativedetection requiring confirmation.Comment: 8 pages, 4 figures, published in ApJ Letter