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

¹ ISDC Data Center for Astrophysics of the University of Geneva chemin d’Écogia, 16 1290 Versoix, Switzerland
e-mail: carlo.ferrigno@unige.ch
² International Space Science Institute (ISSI) Hallerstrasse 6, 3012 Bern, Switzerland
³ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00044 Rome, Italy
⁴ INAF - Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate ( LC), Italy
⁵ ESA, European Space Astronomy Centre (ESAC), PO Box 78, 28691 Villanueva de la Cañada ( Madrid), Spain
⁶ Universitá degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato ( CA), Italy
⁷ INAF Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra ( CA), Italy

Received: 24 May 2010
Accepted: 13 September 2010

Abstract

Context. The burst-only source Swift J1749.4−2807 was discovered in a high X-ray-active state, 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.

Aims. We obtain additional information on Swift J1749.4−2807 and other burst-only sources.

Methods. 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.

Results. 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 ≃ 3 × 10²² cm^-2, Γ ≃ 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 $\mathit{\tau }\mathrm{=}\mathrm{1}{\mathrm{2}}_{-3}^{\mathrm{+}\mathrm{7}}$ days).

Conclusions. 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.