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Renewed optical and X-ray activity in IGR J00291+5934

ATel #1726; F. Lewis (Las Cumbres Observatory Global Telescope Network (LCOGTN)/Open Univ.), M. Linares, D. M. Russell, R. Wijnands (Univ. of Amsterdam), P. Roche (LCOGTN/Cardiff Univ.)
on 20 Sep 2008; 14:04 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

Subjects: Optical, X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 1786, 7836

We report a re-brightening of the millisecond X-ray pulsar IGR J00291+5934. In August the source performed an outburst which was detected in X-ray, UV and optical (ATel #1660, #1664, #1665, #1666, #1667, #1668). Since then we have continued to monitor this source with the Faulkes Telescope North in many optical filters as it appeared to be fading towards its previous quiescent levels (our light curves are found at the link below). Exposures were 200 seconds on source per filter. On 18th September 12:21 UT the source was detected at i' = 17.38 +- 0.07; just ~0.2 mag fainter than the outburst peak on 15th August.

We subsequently acquired a ~1 kilosecond Swift-XRT observation on 20th September at 1:08 UT and clearly detect IGR J00291+5934, confirming the renewed activity. The X-ray spectrum (corrected for pile-up) can be fitted with an absorbed power law of index ~1.6 and a column density ~0.5*10^22 cm^-2, giving a 2-10 keV flux of ~1.1*10^-10 erg/cm2/s. This flux is about a factor of 2 lower than that measured near the previous peak of the outburst in August (ATel #1667). By comparison, the i'-band flux on 18th September is a factor ~1.2 fainter than the previous peak on 15th August.

During the Faulkes Telescope monitoring of the outburst, the source was observed to undergo a fast exponential decay (fading by 3.6 mag in i'-band in 8.0 days) followed by a small re-flare of ~ 0.8 mag on 25th August (MJD 54703) and a slower decline. The most recent magnitudes are:

2008-09-06 = MJD 54715.51: i' = 21.41 +- 0.29
2008-09-13 = MJD 54722.46: i' = 21.74 +- 0.65
2008-09-16 = MJD 54725.55: i' > 19.06
2008-09-18 = MJD 54727.51: i' = 17.38 +- 0.07

The re-brightening in i' band is > 1.6 mag in 2 days and 4.4 +- 0.7 mag in 5 days. The source probably did not fade to its reported quiescent level of I ~ 22.4 (D'Avanzo et al. 2007, A&A, 472, 881; Jonker et al. 2008, ApJ, 680, 615) before this re-brightening. Its magnitude near the time of discovery in 2004 was R ~ 17 (ATel #354). We will update the light curves as data come in. The separation of the two brightenings of the current outburst is ~36 days, which has implications for the duty cycles of both millisecond X-ray pulsars and low-mass X-ray binaries. The millisecond X-ray pulsar HETE J1900.1-2455 also re-flared during its decline from outburst (ATel #1106).

Observations at all wavelengths are encouraged. We thank the Swift team for the rapid scheduling of the observation. The Faulkes Telescope observations are part of an optical monitoring project of low-mass X-ray binaries (Lewis et al. 2008). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). FL acknowledges support from the Dill Faulkes Educational Trust.

IGR J00291+5934 optical light curves