D. Chakrabarty (MIT), J. H. Swank (NASA/GSFC), C. B. Markwardt (U.Md./CRESST/NASA/GSFC), E. Smith (WIS/NASA/GSFC)

ATel #1660; Accreting Millisecond Pulsar IGR J00291-5934 in Outburst Again
on 14 Aug 2008; 20:26 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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We report the detection of a new outburst of the millisecond X-ray pulsar IGR J00291+5934.

IGR J00291+5934 was previously found during an outburst in 2004 to be an accreting millisecond X-ray pulsar with a pulse period of 1.67 ms and an orbital period of 2.46 hr (ATEL #352, #353, #354, #355, #356, #357,#360,#361, #363, #364, #365, #366, #369). The outburst lasted about 2 weeks as observed in the X-ray and optical bands (ATEL #363, #365). In retrospect, RXTE/ASM data suggested a 3.2 yr recurrence time (ATEL #357), and on that basis it is overdue for an outburst. Recently, brief RXTE/PCA monitoring observations of the source have been made every 3 days since 2008 May.

In a PCA monitoring observation on Aug 13, 22:29 UT, the source was detected with a flux of about 16 mCrab (2-10 keV) in an 18 minute observation. We confirm the detection of pulsations, with a mean barycentered pulse frequency of 598.90 Hz (not accounting for the binary orbital motion). These millisecond pulsations confirm that IGR J00291+5934 is the source in outburst. The flux is highly variable, with ~50% r.m.s. fluctuations below frequencies of 1 Hz. A previous observation on Aug 10, 10:18 UT did not detect the source.

Observations at other wavelengths are encouraged as the source was detected in the radio, IR, and optical bands during its 2004 outburst (ATEL #354, #355, #356, #363, #366). More RXTE observations are planned.