INTEGRAL, Swift, and RXTE observations of the 518 Hz accreting transient pulsar Swift J1749.4-2807

Abstract

The burst-only Swift J1749.4-2807 source was discovered in a high X-ray-active state, while during an {INTEGRAL observations of the Galactic bulge on 2010 April 10. Pulsations at 518 Hz were discovered in the RXTE data, confirming previous suggestions of possible associations between burst-only sources and accreting millisecond X-ray pulsars. The subsequent discovery of X-ray eclipses made Swift J1749.4$-$2807 the first eclipsing accreting millisecond X-ray pulsar. We obtain additional information on Swift J1749.4-2807 and other burst-only sources. We report on the results of a monitoring campaign on the source, carried out for about two weeks with the Swift, INTEGRAL, and RXTE satellites. The observations showed that the X-ray spectrum (energy range 0.5-40 keV) of Swift J1749.4-2807 during the entire event was accurately modeled by an absorbed power-law model (N_H~3e2 cm^-2, Gamma~1.7). X-ray eclipses were also detected in the Swift data and provides a clear evidence of a dust-scattering halo located along the line of sight to the source. Only one type-I X-ray burst was observed throughout the two-weeks long monitoring. The X-ray flux of Swift J1749.4-2807 decayed below the detection threshold of Swift/XRT about 11 days after the discovery, in a exponential fashion (e-folding time of tau=12^+7_-3 days). We compare the properties of the outburst observed from Swift J1749.4-2807 with those of the previously known millisecond X-ray pulsars and other transient low mass X-ray binaries.