2,411 research outputs found
XMM-Newton view of Swift J1834.9-0846 and its Magnetar Wind Nebula
We report on the analysis of two XMM-Newton observations of the recently
discovered soft gamma repeater Swift J1834.9-0846, taken in September 2005 and
one month after the source went into outburst on 2011 August 7. We performed
timing and spectral analyses on the point source as well as on the extended
emission. We find that the source period is consistent with an extrapolation of
the Chandra ephemeris reported earlier and the spectral properties remained
constant. The source luminosity decreased to a level of 1.6x10^34 erg s^-1
following a decay trend of . Our spatial analysis of the
source environment revealed the presence of two extended emission regions
around the source. The first (Region A) is a symmetric ring around the point
source, starting at 25arcsec and extending to ~50arcsec. We argue that Region A
is a dust scattering halo. The second (Region B) has an asymmetrical shape
extending between 50arcsec and 150arcsec, and is detected both in the pre- and
post-outburst data. We argue that this region is a possible magnetar wind
nebula (MWN). The X-ray efficiency of the MWN with respect to the rotation
energy loss is substantially higher than those of rotation powered pulsars:
. The
higher efficiency points to a different energy source for the MWN of Swift
J1834.9-0846, most likely bursting activity of the magnetar, powered by its
high magnetic field, B=1.4x10^14 G.Comment: 10 pages, 10 figures, accepted for publication in Ap
COLD-SAT: An orbital cryogenic hydrogen technology experiment
The COLD-SAT spacecraft will perform subcritical liquid hydrogen storage and transfer experiments under low-gravity conditions to provide engineering data for future space transportation missions. Consisting of an experiment module mated to a spacecraft bus, COLD-SAT will be placed in an initial 460 km circular orbit by an Atlas I commercial launch vehicle. After deployment, the three-axis-controlled spacecraft bus will provide electric power, experiment control and data management, communications, and attitude control along with propulsive acceleration levels ranging from 10(-6) to 10(-4)g. These accelerations are an important aspect of some of the experiments, as it is desired to know the effects that low gravity levels might have on the heat and mass transfer processes involved. The experiment module will contain the three liquid hydrogen tanks, valves, pressurization equipment, and instrumentation. At launch all the hydrogen will be in the largest tank, which has helium-purged MLI and is loaded and topped off by the hydrogen tanking system used for the Centaur upper stage of the Atlas. The two smaller tanks will be utilized in orbit for performing some of the experiments. The experiments are grouped into two classes on the basis of their priority, and include six regarded as enabling technology and nine regarded as enhancing technology
Onset of collective and cohesive motion
We study the onset of collective motion, with and without cohesion, of groups
of noisy self-propelled particles interacting locally. We find that this phase
transition, in two space dimensions, is always discontinuous, including for the
minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for
which a non-trivial critical point was previously advocated. We also show that
cohesion is always lost near onset, as a result of the interplay of density,
velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let
- …