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A double outburst in the accretion-powered millisecond pulsar IGR J00291+5934

ATel #1786; Duncan K. Galloway (Monash), Jacob M. Hartman (NRL), Deepto Chakrabarty (MIT), Edward H. Morgan (MIT), and Jean H. Swank (NASA/GSFC)
on 17 Oct 2008; 11:54 UT
Credential Certification: Duncan K. Galloway (duncan@space.mit.edu)

Subjects: Infra-Red, Optical, X-ray, Binary, Neutron Star, Pulsar

We report on Rossi X-ray Timing Explorer Proportional Counter Array observations of the accreting millisecond pulsar IGR J00291+5934. The system was detected in outburst on 2008 Aug 13 (ATel #1660), and was subsequently observed in X-rays with Swift as well as in the optical and UV (ATel #1664, #1665, #1666, #1668). The X-ray flux measured by RXTE/PCA was at a maximum during the initial measurements between Aug 13 and 15, of 6.3e-10 erg/cm^2/s (2.5-25 keV; the Crab flux in this band is 3.3e-8 erg/cm^2/s). The X-ray intensity decreased to around 7e-11 erg/cm^2/s by 21 Aug, where it remained until 21 Sep. This low-level flux includes a contribution from the nearby (17 arcmin away) source V709 Cas (see also ATel #1664), and we note that pulsations were not detected between 21 Aug and 21 Sep. The upper limit on the pulsed fraction is difficult to quantify because the contribution by IGR 00291+5934 to the count rate during this interval is unknown.

The optical counterpart had also faded significantly by Aug 24, but was subsequently reported to have re-brightened in the optical and X-ray bands on Sep 18 (ATel #1726). The RXTE/PCA observation on 21 Sep found the 2.5-25 keV flux had increased to 2.3e-10 erg/cm^2/s, and pulsations were detected once more. The flux continued to increase to a second peak of around 5e-10 erg/cm^2/s between Sep 24 and 26. Subsequently the intensity faded again, reaching the level likely corresponding to quiescence for the pulsar on MJD 54743.

The present outburst began 3.68 yr after the previous one, in 2004 Dec. Two earlier outbursts were detected retroactively in the RXTE/ASM lightcurve of the source, with intervals of 2.80 and 3.23 yr (ATel #357). The outburst interval has thus been steadily increasing with time. The previous outburst was well studied (Galloway et al. 2005, ApJ 622, L45) and consisted of a single peak, in contrast to the recent behaviour. We expect that the system has now returned to quiescence, although it is possible that activity may resume as it did in September. We encourage continued X-ray and optical monitoring.