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Progress towards metal-free radical alkylations of quinones under mild conditions
Discovery of pulsations in the X-ray transient 4U 1901+03
We describe observations of the 2003 outburst of the hard-spectrum X-ray
transient 4U 1901+03 with the Rossi X-ray Timing Explorer. The outburst was
first detected in 2003 February by the All-Sky Monitor, and reached a peak
2.5-25 keV flux of 8x10^-9 ergs/cm^2/s (around 240 mCrab). The only other known
outburst occurred 32.2 yr earlier, likely the longest presently known
recurrence time for any X-ray transient. Proportional Counter Array (PCA)
observations over the 5-month duration of the 2003 outburst revealed a 2.763 s
pulsar in a 22.58 d orbit. The detection of pulsations down to a flux of
3x10^-11 ergs/cm^2/s (2.5-25 keV), along with the inferred long-term accretion
rate of 8.1x10^-11 M_sun/yr (assuming a distance of 10 kpc) suggests that the
surface magnetic field strength is below ~5x10^11 G. The corresponding
cyclotron energy is thus below 4 keV, consistent with the non-detection of
resonance features at high energies. Although we could not unambiguously
identify the optical counterpart, the lack of a bright IR candidate within the
1' RXTE error circle rules out a supergiant mass donor. The neutron star in 4U
1901+03 probably accretes from the wind of a main-sequence O-B star, like most
other high-mass binary X-ray pulsars. The almost circular orbit e=0.036
confirms the system's membership in a growing class of wide, low-eccentricity
systems in which the neutron stars may have received much smaller kicks as a
result of their natal supernova explosions.Comment: 7 pages, 6 figures, accepted by ApJ. Very minor addition in response
to referee's comment; updated author affiliatio
Synthetic biology: advancing biological frontiers by building synthetic systems
Advances in synthetic biology are contributing
to diverse research areas, from basic biology to
biomanufacturing and disease therapy. We discuss the
theoretical foundation, applications, and potential of
this emerging field
The Cosmic Censor Forbids Naked Topology
For any asymptotically flat spacetime with a suitable causal structure
obeying (a weak form of) Penrose's cosmic censorship conjecture and satisfying
conditions guaranteeing focusing of complete null geodesics, we prove that
active topological censorship holds. We do not assume global hyperbolicity, and
therefore make no use of Cauchy surfaces and their topology. Instead, we
replace this with two underlying assumptions concerning the causal structure:
that no compact set can signal to arbitrarily small neighbourhoods of spatial
infinity (``-avoidance''), and that no future incomplete null geodesic is
visible from future null infinity. We show that these and the focusing
condition together imply that the domain of outer communications is simply
connected. Furthermore, we prove lemmas which have as a consequence that if a
future incomplete null geodesic were visible from infinity, then given our
-avoidance assumption, it would also be visible from points of spacetime
that can communicate with infinity, and so would signify a true naked
singularity.Comment: To appear in CQG, this improved version contains minor revisions to
incorporate referee's suggestions. Two revised references. Plain TeX, 12
page
On the Gannon-Lee Singularity Theorem in Higher Dimensions
The Gannon-Lee singularity theorems give well-known restrictions on the
spatial topology of singularity-free (i.e., nonspacelike geodesically
complete), globally hyperbolic spacetimes. In this paper, we revisit these
classic results in the light of recent developments, especially the failure in
higher dimensions of a celebrated theorem by Hawking on the topology of black
hole horizons. The global hyperbolicity requirement is weakened, and we expand
the scope of the main results to allow for the richer variety of spatial
topologies which are likely to occur in higher-dimensional spacetimes.Comment: 13 pages, no figures, to appear in Class. Quantum Gra
Thermonuclear burst physics with RXTE
Recently we have made measurements of thermonuclear burst energetics and
recurrence times which are unprecedented in their precision, largely thanks to
the sensitivity of the Rossi X-ray Timing Explorer. In the "Clocked Burster",
GS 1826-24, hydrogen burns during the burst via the rapid-proton (rp) process,
which has received particular attention in recent years through theoretical and
modelling studies. The burst energies and the measured variation of alpha (the
ratio of persistent to burst flux) with accretion rate strongly suggests solar
metallicity in the neutron star atmosphere, although this is not consistent
with the corresponding variation of the recurrence time. Possible explanations
include extra heating between the bursts, or a change in the fraction of the
neutron star over which accretion takes place. I also present results from 4U
1746-37, which exhibits regular burst trains which are interrupted by "out of
phase" bursts.Comment: 4 pages, 2 figures, AIP conference proceedings format. To appear in
the proceedings of the "X-ray Timing 2003: Rossi and Beyond" meeting held in
Cambridge, MA, November, 200
Torque Saturation in Bipedal Robotic Walking through Control Lyapunov Function Based Quadratic Programs
This paper presents a novel method for directly incorporating user-defined
control input saturations into the calculation of a control Lyapunov function
(CLF)-based walking controller for a biped robot. Previous work by the authors
has demonstrated the effectiveness of CLF controllers for stabilizing periodic
gaits for biped walkers, and the current work expands on those results by
providing a more effective means for handling control saturations. The new
approach, based on a convex optimization routine running at a 1 kHz control
update rate, is useful not only for handling torque saturations but also for
incorporating a whole family of user-defined constraints into the online
computation of a CLF controller. The paper concludes with an experimental
implementation of the main results on the bipedal robot MABEL
Soliton Solutions to the Einstein Equations in Five Dimensions
We present a new class of solutions in odd dimensions to Einstein's equations
containing either a positive or negative cosmological constant. These solutions
resemble the even-dimensional Eguchi-Hanson--(anti)-de Sitter ((A)dS) metrics,
with the added feature of having Lorentzian signatures. They provide an
affirmative answer to the open question as to whether or not there exist
solutions with negative cosmological constant that asymptotically approach
AdS, but have less energy than AdS. We present
evidence that these solutions are the lowest-energy states within their
asymptotic class.Comment: 9 pages, Latex; Final version that appeared in Phys. Rev. Lett; title
changed by journal from original title "Eguchi-Hanson Solitons
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