95 research outputs found
Combined Spectral and Timing Analysis of the Black Hole Candidate MAXI J1659-152 Discovered by MAXI and Swift
We report on X-ray spectral and timing results of the new black hole
candidate (BHC) MAXI J1659-152 with the orbital period of 2.41 hours (shortest
among BHCs) in the 2010 outburst from 65 Rossi X-ray Timing Explorer (RXTE)
observations and 8 simultaneous Swift and RXTE observations. According to the
definitions of the spectral states in Remillard & McClintock (2006), most of
the observations have been classified into the intermediate state. All the
X-ray broadband spectra can be modeled by a multi-color disk plus a power-law
with an exponential cutoff or a multi-color disk plus a Comptonization
component. During the initial phase of the outburst, a high energy cutoff was
visible at 30-40 keV. The innermost radius of the disk gradually decreased by a
factor of more than 3 from the onset of the outburst and reached a constant
value of 35 d_10 cos i^-1/2 km, where d_10 is the distance in units of 10 kpc
and is the inclination. The type-C quasi-periodic oscillation (QPO)
frequency varied from 1.6 Hz to 7.3 Hz in association with a change of the
innermost radius, while the innermost radius remained constant during the
type-B QPO detections at 1.6-4.1 Hz. Hence, we suggest that the origin of the
type-B QPOs is different from that of type-C QPOs, the latter of which would
originate from the disk truncation radius. Assuming the constant innermost
radius in the latter phase of the outburst as the innermost stable circular
orbit, the black hole mass in MAXI J1659-152 is estimated to be 3.6-8.0 M_solar
for a distance of 5.3-8.6 kpc and an inclination angle of 60-75 degrees.Comment: 27 pages, 14 figures, accepted for publication in PAS
X-Ray Flashes in Recurrent Novae: M31N 2008-12a and the Implications of the Swift Non-detection
Models of nova outbursts suggest that an X-ray flash should occur just after
hydrogen ignition. However, this X-ray flash has never been observationally
confirmed. We present four theoretical light curves of the X-ray flash for two
very massive white dwarfs (WDs) of 1.380 and 1.385 M_sun and for two recurrence
periods of 0.5 and 1 years. The duration of the X-ray flash is shorter for a
more massive WD and for a longer recurrence period. The shortest duration of 14
hours (0.6 days) among the four cases is obtained for the 1.385 M_sun WD with
one year recurrence period. In general, a nova explosion is relatively weak for
a very short recurrence period, which results in a rather slow evolution toward
the optical peak. This slow timescale and the predictability of very short
recurrence period novae give us a chance to observe X-ray flashes of recurrent
novae. In this context, we report the first attempt, using the Swift
observatory, to detect an X-ray flash of the recurrent nova M31N 2008-12a (0.5
or 1 year recurrence period), which resulted in the non-detection of X-ray
emission during the period of 8 days before the optical detection. We discuss
the impact of these observations on nova outburst theory. The X-ray flash is
one of the last frontiers of nova studies and its detection is essentially
important to understand the pre-optical-maximum phase. We encourage further
observations.Comment: 12 pages, including 9 figures and 3 tables. To appear in the
Astrophysical Journa
Swift observations of SAX J1808.4-3658: monitoring the return to quiescence
The transient accreting millisecond pulsar SAX J1808.4-3658 has shown several
outbursts to date but the transition from outburst to quiescence has never been
investigated in detail. Thanks to the Swift observing flexibility, we monitored
for the first time the decay to quiescence during the 2005 outburst. At
variance with other transients, wide luminosity variations are observed. In
addition, close to quiescence, SAX J1808.4-3658 seems to switch between two
different states. We interpret them in terms of the accretion states accessible
to a magnetized, fast rotating neutron star.Comment: Accepted for publication in ApJL. 4 pages 3 figure
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