We report on the detection and follow-up high cadence monitoring observations
of MAXI J1659-152, a bright Galactic X-ray binary transient with a likely
black-hole accretor, by Swift over a 27 day period after its initial outburst
detection. MAXI J1659-152 was discovered almost simultaneously by Swift and
MAXI on 2010 Sept 25, and was monitored intensively from the early stages of
the outburst through the rise to a brightness of ~0.5 Crab by the Swift XRT,
UVOT, and BAT. We present temporal and spectral analysis of the Swift
observations. The broadband light-curves show variability characteristic of
black-hole candidate transients. We present the evolution of thermal and
non-thermal components of the 0.5-150 keV combined X-ray spectra during the
outburst. MAXI J1659-152 displays accretion state changes typically associated
with black-hole binaries, transitioning from its initial detection in the Hard
State, to the Steep Power-Law State, followed by a slow evolution towards the
Thermal State, signified by an increasingly dominant thermal component
associated with the accretion disk, although this state change did not complete
before Swift observations ended. We observe an anti-correlation between the
increasing temperature and decreasing radius of the inner edge of the accretion
disk, suggesting that the inner edge of the accretion disk in-falls towards the
black-hole as the disk temperature increases. We observed significant evolution
in the absorption column during the initial rise of the outburst, with the
absorption almost doubling, suggestive of the presence of an evolving wind from
the accretion disk. We detect quasi- periodic oscillations that evolve with the
outburst, as well as irregular shaped dips that recur with a period of
2.42\pm0.09 hours, strongly suggesting an orbital period that would make MAXI
J1659-152 the shortest period black-hole binary yet known.Comment: Accepted for publication in ApJ. 9 pages, 7 figure
