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Faulkes Telescope monitoring of the current outburst of IGR J00291+5934

ATel #1666; D. M. Russell (Univ. of Amsterdam), F. Lewis (Las Cumbres Observatory Global Telescope Network (LCOGTN)/Open Univ.), M. Linares (Univ. of Amsterdam), P. Roche (LCOGTN/Cardiff Univ.), D. Maitra (Univ. of Amsterdam)
on 17 Aug 2008; 14:47 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

Subjects: Infra-Red, Optical, Neutron Star, Transient, Pulsar

Referred to by ATel #: 1667, 1668, 1726, 1786, 7837

As part of an optical monitoring project of low-mass X-ray binaries (Lewis et al. 2008), we report on recent observations just prior to, and during the current outburst of the millisecond X-ray pulsar IGR J00291+5934 (ATel #1660, #1664, #1665). The field of the source was observed with the 2-m robotic Faulkes Telescope North, at Haleakala on Maui on August 3, 9, 15 and 16, so far. Exposures were 200 seconds on source per filter. We report the following magnitudes:

2008-08-03 (MJD 54681.57): i > 19.4
2008-08-09 (MJD 54687.53): i > 20.1
2008-08-15 (MJD 54693.56): i = 17.17 +- 0.07; R = 17.90 +- 0.06; V = 19.51 +- 0.08; B = 20.49 +- 0.15
2008-08-16 (MJD 54694.56): i = 17.22 +- 0.07

The source was also detected in y-band (1.0 micron) on Aug 15 and 16; flux calibration will be achieved at a later date for this filter. The non-detection on Aug 9 and solid detection on Aug 15 confirms the optical and X-ray brightenings were approximately contemporaneous (ATel #1660). The source has risen by ~ 5 mag in i-band from its reported quiescent level of I ~ 22.4 (D'Avanzo et al. 2007, A&A, 472, 881; Jonker et al. 2008, ApJ, 680, 615). The system peaked at R ~ 17.4 during its 2004 outburst (ATel #354) so either the current outburst is fainter than the last, or the source is still brightening.

We converted the BVRi magnitudes on Aug 15 to intrinsic (de-reddened) flux densities adopting an interstellar extinction of Av = 2.5 (Torres et al. 2008, ApJ, 672, 1079) and obtain 0.59 mJy, 0.55 mJy, 1.1 mJy and 1.5 mJy in B, V, R and i, respectively. The excess towards the infrared wavebands is likely due to synchrotron emission from outflowing jets. An infrared-excess has been reported near the peak of almost all millisecond X-ray pulsar outbursts, which (like black hole X-ray binaries) likely has a jet origin (e.g. Wang et al. 2001, ApJ, 563, L61; Giles et al. 2005, MNRAS, 361, 1180; Russell et al. 2007, MNRAS, 379, 1108).

Further multi-wavelength monitoring is encouraged, especially in the radio and infrared regimes. Our light curves will be posted online at the link below. 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