350 research outputs found
A software toolbox for robotics
A method for programming cooperating manipulators, which is guided by a geometric description of the task to be performed, is given. For this a suitable language must be used and a method for describing the workplace and the objects in it in geometric terms. A task level command language and its implementation for concurrently driven multiple robot arm is described. The language is suitable for driving a cell in which manipulators, end effectors, and sensors are controlled by their own dedicated processors. These processors can communicate with each other through a communication network. A mechanism for keeping track of the history of the commands already executed allows the command language for the manipulators to be event driven. A frame based world modeling system is utilized to describe the objects in the work environment and any relationships that hold between these objects. This system provides a versatile tool for managing information about the world model. Default actions normally needed are invoked when the data base is updated or accessed. Most of the first level error recovery is also invoked by the database by utilizing the concepts of demons. The package can be utilized to generate task level commands in a problem solver or a planner
The pulsar wind nebula of the Geminga pulsar
The superb spatial resolution of Chandra has allowed us to detect a 20''-long
tail behind the Geminga pulsar, with a hard spectrum (photon index 1.0+/-0.2)
and a luminosity (1.3+/-0.2) 10^{29} ergs/s in the 0.5 - 8 keV band, for an
assumed distance of 200 pc. The tail could be either a pulsar jet, confined by
a toroidal magnetic field of about 100 microGauss, or it can be associated with
the shocked relativistic wind behind the supersonically moving pulsar confined
by the ram pressure of the oncoming interstellar medium. We also detected an
arc-like structure 5'' - 7'' ahead of the pulsar, extended perpendicular to the
tail, with a factor of 3 lower luminosity. We see a 3-sigma enhancement in the
Chandra image apparently connecting the arc with the southern outer tail that
has been possibly detected with XMM-Newton. The observed structures imply that
the Geminga's pulsar wind is intrinsically anisotropic.Comment: Revised version: data analysis described in more detail, Figure 2
replaced; 6 pages, 2 color figures; accepted for publication in ApJ (v.643,
2006 June 1
Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197
Three observations of the 5.54 s Transient Anomalous X-ray Pulsar XTE
J1810-197 obtained over 6 months with the Newton X-Ray Multi-Mirror Mission
(XMM-Newton) are used to study its spectrum and pulsed light curve as the
source fades from outburst. The decay is consistent with an exponential of time
constant 300 days, but not a power law as predicted in some models of sudden
deep crustal heating events. All spectra are well fitted by a blackbody plus a
steep power law, a problematic model that is commonly fitted to anomalous X-ray
pulsars (AXPs). A two-temperature blackbody fit is also acceptable, and better
motivated physically in view of the faint optical/IR fluxes, the X-ray pulse
shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting
areas that are less than or equal to the surface area of a neutron star. The
fitted temperatures remained the same while the flux declined by 46%, which can
be interpreted as a decrease in area of the emitting regions. The pulsar
continues to spin down, albeit at a reduced rate of (5.1+/-1.6)x10^{-12} s
s^{-1}. The inferred characteristic age Tau_c = P/2Pdot ~17,000 yr, magnetic
field strength B_s ~1.7x10^{14} G, and outburst properties are consistent with
both the outburst and quiescent X-ray luminosities being powered by magnetic
field decay, i.e., XTE J1810-197 is a magnetar.Comment: 10 pages, 5 figures, accepted by Ap.
Phase Structure of Compact Star in Modified Quark-Meson Coupling Model
The K condensation and quark deconfinement phase transitions are
investigated in the modified quark-meson coupling model. It is shown that K
condensation is suppressed because of the quark deconfinement when
202.2MeV, where is the bag constant for unpaired quark matter.
With the equation of state (EOS) solved self-consistently, we discuss the
properties of compact stars. We find that the EOS of pure hadron matter with
condensed K phase should be ruled out by the redshift for star EXO0748-676,
while EOS containing unpaired quark matter phase with being about
180MeV could be consistent with this observation and the best measured mass of
star PSR 1913+16. We then probe into the change of the phase structures in
possible compact stars with deconfinment phase as the central densities
increase. But if the recent inferred massive star among Terzan 5 with
M1.68M is confirmed, all the present EOSes with condensed phase
and deconfined phase would be ruled out and therefore these exotic phases are
unlikely to appear within neutron stars.Comment: 11 pages, 5 figure
Temporally variable recurrence regimes of mega-tsunamis in the 6500 years prior to the 2004 Indian Ocean event
The analyses of cores retrieved from three sites near Port Blair (South Andaman) revealed out-of-sequence deposits at various depths. They are identified as previous episodes of tsunami by their sediment characteristics and microfossil content, using the 2004 event deposition as a template. These deposits have median ages of 601 cal. yr BP, 837 cal. yr BP, 1440 cal. yr BP, 3018 cal. yr BP, 3591 cal. yr BP, 4712 cal. yr BP, 5607 cal. yr BP, and 6357 cal. yr BP and are chronologically equivalent of those from the far-field locations in the Indian Ocean region. The distant deposits that are correlated with the South Andaman sites most likely owe their origin to the 2004-type events, as indicated by tsunami simulations in the study region. The long-term record presented here is characterized by an early phase of a quasiperiodic recurrence regime that transitions into a distinct interval of temporally clustered events. The quasiperiodic regime that appears around the mid-Holocene with an inter-event interval of 980 ± 385 years is followed by a sizable quiescent period of 1605 ± 245 years, before being succeeded by a regime of temporally clustered events. The chronology of nine tsunami events in the last 6500 years from the Indian Ocean region, thus implies a nonlinear pattern for the causative earthquakes. As demonstrated in the subduction zones elsewhere, the temporal variability of tsunamigenic great earthquakes is supported by the theoretical models espousing the characteristics of long-term stress recycling processes active within the subduction zones and transfer processes between the lower viscoelastic layer and the upper seismogenic crust
X-ray observations of the compact central object in supernova remnant G347.3-0.5
We present Chandra, XMM-Newton and RXTE observations of 1WGA J1713.4-3949, a
compact source at the center of the galactic supernova remnant (SNR)
G347.3-0.5. The X-ray spectrum of the source is well-fitted by the sum of a
blackbody component with a temperature of about 0.4 keV plus a power law
component with photon index about 4. We found no pulsations down to 4% in the
0.01-0.16 Hz range and down to 25% in the 0.01-128 Hz range. This source
resembles other compact central objects (CCOs) in SNRs, and we suggest that
1WGA J1713.4-3949 is the associated neutron star for G347.3--0.5. We also
measured the properties of the adjacent radio pulsar PSR J1713-3945 with a 392
ms period and show that it is not associated with 1WGA J1713.4-3949 nor, most
probably, with SNR G347.3-0.5 as well.Comment: 8 pages, 2 figures, accepted for publication in ApJ Letter
Spot-like Structures of Neutron Star Surface Magnetic Fields
There is growing evidence, based on both X-ray and radio observations of
isolated neutron stars, that besides the large--scale (dipolar) magnetic field,
which determines the pulsar spin--down behaviour, small--scale poloidal field
components are present, which have surface strengths one to two orders of
magnitude larger than the dipolar component. We argue in this paper that the
Hall--effect can be an efficient process in producing such small--scale field
structures just above the neutron star surface. It is shown that due to a
Hall--drift induced instability, poloidal magnetic field structures can be
generated from strong subsurface toroidal fields, which are the result of
either a dynamo or a thermoelectric instability acting at early times of a
neutron star's life. The geometrical structure of these small--scale surface
anomalies of the magnetic field resembles that of some types of
``star--spots''. The magnetic field strength and the length--scales are
comparable with values that can be derived from various observations.Comment: 4 pages, 2 figures, accepted by Astronomy & Astrophysics Letters;
language improved, 2nd para of Sect. 3 change
Phase-resolved Crab Studies with a Cryogenic TES Spectrophotometer
We are developing time- and energy-resolved near-IR/optical/UV photon
detectors based on sharp superconducting-normal transition edges in thin films.
We report observations of the Crab pulsar made during prototype testing at the
McDonald 2.7m telescope with a fiber-coupled transition-edge sensor (TES)
system. These data show substantial (d[alpha]~0.3), rapid variations in the
spectral index through the pulse profile, with a strong phase-varying IR break
across our energy band. These variations correlate with X-ray spectral
variations, but no single synchrotron population can account for the full
Spectral Energy Distribution (SED). We also describe test
spectrophotopolarimetry observations probing the energy dependence of the
polarization sweep; this may provide a new key to understanding the radiating
particle population.Comment: 12 pages, 10 figures -- to appear in ApJ V56
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