OBSERVATIONS OF THE 599 Hz ACCRETING X-RAY PULSAR IGR J00291+5934 DURING THE 2004 OUTBURST AND IN QUIESCENCE

摘要

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 approx= 23 to R approx= 17 and from K = 19 to K approx= 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 approx= 22°-32°. The preferred range for the reddening toward the source is 0.7 ≤ E(B - V) ≤ 0.9, which is equivalent to 4.06 × 10~(21) cm~(-2) ≤ N_H ≤ 5.22 × 10~(21) cm~(-2). The Chandra observations of the pulsar in its quiescent state gaveanunabsorbed 0.5-10keV flux for the best-fitting power-law model to the source spectrum of (7.0 ± 0.9) × 10~(14) ergs cm~(-2) s~(-1) (adopting a hydrogen column of 4.6 × 10~(21) cm~(-2)). The fit resulted in a power-law photon index of 2.4_(-0.4)~(+0.5). 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 ＜ 3 × 10~8 G.