Torres, M. A. P.; Jonker, P. G.; Steeghs, D.; Roelofs, G. H. A.; Bloom, J. S.; Casares, J.; Falco, E. E.; Garcia, M. R.; Marsh, T. R.; Mendez, M.; Miller, J. M.; Nelemans, G.; Rodríguez-Gil, P.
Bibliographical reference
The Astrophysical Journal, Volume 672, Issue 2, pp. 1079-1090.
Description
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 4.06×1021
cm-2<=NH<=5.22×1021
cm-2. 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)×10-14 ergs
cm-2 s-1 (adopting a hydrogen column of
4.6×1021 cm-2). 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 <3×108 G.
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