2008ApJ...672.1079T

We report on optical and near-infrared observations obtained during and after the 2004 December discovery outburst of the X-ray transient and accretion-powered millisecond pulsar IGR J00291+5934. Our observations monitored the evolution of the brightness and the spectral properties of IGR J00291+5934 during the outburst decay toward quiescence. We also present optical, near-infrared, and Chandra observations obtained during true quiescence. Photometry of the field during outburst reveals an optical and near-infrared counterpart that brightened from R≃23 to R≃17 and from K=19 to K≃16. Spectral analysis of the RIJHK broadband photometry shows excess in the near-infrared bands that may be due to synchrotron emission. The Hα emission line profile suggests the orbital inclination is ≃22°-32°. The preferred range for the reddening toward the source is 0.7≤E(B-V)≤0.9, which is equivalent to 4.06x10²¹/cm2≤N_H≤5.22x10²¹/cm2. The Chandra observations of the pulsar in its quiescent state gave an unabsorbed 0.5-10 keV flux for the best-fitting power-law model to the source spectrum of (7.0±0.9)x10^–14 ergs/cm2/s (adopting a hydrogen column of 4.6x10²¹/cm2). The fit resulted in a power-law photon index of 2.4^+0.5_–0.4. The (R-K)₀ color observed during quiescence supports an irradiated donor star and accretion disk. We estimate a distance of 2-4 kpc toward IGR J00291+5934 by using the outburst X-ray light curve and the estimated critical X-ray luminosity necessary to keep the outer parts of the accretion disk ionized. Using the quiescent X-ray luminosity and the spin period, we constrain the magnetic field of the neutron star to be <3x10⁸ G.