57,202 research outputs found
Where is SGR1806-20?
We apply a statistical method to derive very precise locations for soft gamma
repeaters using data from the interplanetary network. We demonstrate the
validity of the method by deriving a 600 arcsec^2 error ellipse for SGR1900+14
whose center agrees well with the VLA source position. We then apply it to
SGR1806-20, for which we obtain a 230 arcsec^2 error ellipse, the smallest
burst error box to date. We find that the most likely position of the source
has a small but significant displacement from that of the non-thermal core of
the radio supernova remnant G10.0-0.3, which was previously thought to be the
position of the repeater. We propose a different model to explain the changing
supernova remnant morphology and the positions of the luminous blue variable
and the bursting source.Comment: 12 pages and 2 color figures, accepted for publication in
Astrophysical Journal Letter
Outburst Morphology in the Soft X-ray Transient Aquila X-1
We present optical and near-IR (OIR) observations of the major outbursts of
the neutron star soft X-ray transient binary system Aquila X-1, from summer
1998 -- fall 2007. The major outbursts of the source over the observed timespan
seem to exhibit two main types of light curve morphologies, (a) the classical
Fast-Rise and Exponential-Decay (FRED) type outburst seen in many soft X-ray
transients and (b) the Low-Intensity State (LIS) where the
optical-to-soft-X-ray flux ratio is much higher than that seen during a FRED.
Thus there is no single correlation between the optical (R-band) and soft X-ray
(1.5-12 keV, as seen by the ASM onboard RXTE) fluxes even within the hard state
for Aquila X-1, suggesting that LISs and FREDs have fundamentally different
accretion flow properties. Time evolution of the OIR fluxes during the major
LIS and FRED outbursts is compatible with thermal heating of the irradiated
outer accretion disk. No signature of X-ray spectral state changes or any
compact jet are seen in the OIR, showing that the OIR color-magnitude diagram
(CMD) can be used as a diagnostic tool to separate thermal and non-thermal
radiation from X-ray binaries where orbital and physical parameters of the
system are reasonably well known. We suggest that the LIS may be caused by
truncation of the inner disk in a relatively high mass accretion state,
possibly due to matter being diverted into a weak outflow.Comment: Accepted for publication in ApJ. Uses aastex. 27 pages and 10 figure
Scalable Co-Optimization of Morphology and Control in Embodied Machines
Evolution sculpts both the body plans and nervous systems of agents together
over time. In contrast, in AI and robotics, a robot's body plan is usually
designed by hand, and control policies are then optimized for that fixed
design. The task of simultaneously co-optimizing the morphology and controller
of an embodied robot has remained a challenge. In psychology, the theory of
embodied cognition posits that behavior arises from a close coupling between
body plan and sensorimotor control, which suggests why co-optimizing these two
subsystems is so difficult: most evolutionary changes to morphology tend to
adversely impact sensorimotor control, leading to an overall decrease in
behavioral performance. Here, we further examine this hypothesis and
demonstrate a technique for "morphological innovation protection", which
temporarily reduces selection pressure on recently morphologically-changed
individuals, thus enabling evolution some time to "readapt" to the new
morphology with subsequent control policy mutations. We show the potential for
this method to avoid local optima and converge to similar highly fit
morphologies across widely varying initial conditions, while sustaining fitness
improvements further into optimization. While this technique is admittedly only
the first of many steps that must be taken to achieve scalable optimization of
embodied machines, we hope that theoretical insight into the cause of
evolutionary stagnation in current methods will help to enable the automation
of robot design and behavioral training -- while simultaneously providing a
testbed to investigate the theory of embodied cognition
- …