719 research outputs found
Revisiting the Parallax of the Isolated Neutron Star RX J185635-3754 Using HST/ACS Imaging
We have redetermined the parallax and proper motion of the nearby isolated
neutron star RX~J185635-3754. We used eight observations with the high
resolution camera of the HST/ACS taken from 2002 through 2004. We performed the
astrometric fitting using five independent methods, all of which yielded
consistent results. The mean estimate of the distance is 123 (+11, -15) pc (1
sigma), in good agreement with our earlier published determination
Computation of Neutron Star Structure Using Modern Equation of State
Using the modern equations of state derived from microscopic calculations, we
have calculated the neutron star structure. For the neutron star, we have
obtained a minimum mass about which is nearly independent
of the equation of state, and a maximum mass between and
which is strongly dependent on the equation of state. It
is shown that among the equations of state of neutron star matter which we have
used, the stiffest one leads to higher maximum mass and radius and lower
central density. It is seen that the given maximum mass for the Reid-93
equation of state shows a good consistency with the accurate observations of
radio pulsars. We have indicated that the thickness of neutron star crust is
very small compared to the predicted neutron star radius.Comment: 16 pages, 6 figure
Effects of Symmetry Energy in the Reaction 40Ca+124Sn at 140 MeV/nucleon
The density-dependent symmetry energy is a hot topic in nuclear physics. Many
laboratories over the world are planning to perform related experiments to
probe the symmetry energy. Based on the semiclassical
Boltzmann-Uehling-Uhlenbeck (BUU) transport model, we study the effects of
nuclear symmetry energy in the central reaction 40Ca+124Sn at 140MeV/nucleon in
the laboratory system. It is found that the rapidity distribution of free
nucleon's neutron-to-proton ratio is sensitive to the symmetry energy,
especially at large rapidities. The free neutron-to-proton ratios at small or
large rapidities may reflect high or low density behavior of nuclear symmetry
energy. To probe the density dependence of nuclear symmetry energy, it is
better to give the kinetic distribution and the rapidity distribution of
emitted nucleons at the same time.Comment: 4 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1204.085
The effects of r-process heating on fall-back accretion in compact object mergers
We explore the effects of r-process nucleosynthesis on fall-back accretion in
neutron star(NS)-NS and black hole-NS mergers, and the resulting implications
for short-duration gamma-ray bursts (GRBs). Though dynamically important, the
energy released during the r-process is not yet taken into account in merger
simulations. We use a nuclear reaction network to calculate the heating (due to
beta-decays and nuclear fission) experienced by material on the
marginally-bound orbits nominally responsible for late-time fall-back. Since
matter with longer orbital periods t_orb experiences lower densities, for
longer periods of time, the total r-process heating rises rapidly with t_orb,
such that material with t_orb > 1 seconds can become completely unbound. Thus,
r-process heating fundamentally changes the canonical prediction of an
uninterrupted power-law decline in the fall-back rate dM/dt at late times. When
the timescale for r-process to complete is > 1 second, the heating produces a
complete cut-off in fall-back accretion after ~ 1 second; if robust, this would
imply that fall-back accretion cannot explain the late-time X-ray flaring
observed following some short GRBs. However, for a narrow, but physically
plausible, range of parameters, fall-back accretion can resume after ~ 10 s,
despite having been strongly suppressed for ~ 1-10 s after the merger. This
suggests the intriguing possibility that the gap observed between the prompt
and extended emission in short GRBs is a manifestation of r-process heating.Comment: 7 pages; 4 figures; submitted to MNRA
Probing the symmetry energy with isospin ratio from nucleons to fragments
Within the framework of ImQMD05, we study several isospin sensitive
observables, such as DR(n/p) ratios, isospin transport ratio (isospin
diffusion), yield ratios for LCPs between the projectile region and
mid-rapidity region for the reaction systems Ni+Ni, Zn+Zn, Sn+Sn at
low-intermediate energies. Our results show that those observables are
sensitive to the density dependence of symmetry energy, and also depend on the
cluster formation mechanism. By comparing these calculations to the data, the
information of the symmetry energy and reaction mechanism is obtained.Comment: Talk given by Yingxun Zhang at the 11th International Conference on
Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1,
2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference
Series (JPCS
Probing nuclear symmetry energy with the sub-threshold pion production
Within the framework of semiclassical Boltzmann-Uehling-Uhlenbeck (BUU)
transport model, we investigated the effects of symmetry energy on the
sub-threshold pion using the isospin MDI interaction with the stiff and soft
symmetry energies in the central collision of Ca + Ca at the
incident beam energies of 100, 150, 200, 250 and 300 MeV/nucleon, respectively.
We find that the ratio of of sub-threshold charged pion
production is greatly sensitive to the symmetry energy, particularly around 100
MeV/nucleon energies. Large sensitivity of sub-threshold charged pion
production to nuclear symmetry energy may reduce uncertainties of probing
nuclear symmetry energy via heavy-ion collision.Comment: 5 pages, 5 figures, typo corrections, submitted to Chinese Physics
Letter
Rapid Cooling of the Neutron Star in Cassiopeia A Triggered by Neutron Superfluidity in Dense Matter
We propose that the observed cooling of the neutron star in Cassiopeia A is
due to enhanced neutrino emission from the recent onset of the breaking and
formation of neutron Cooper pairs in the 3P2 channel. We find that the critical
temperature for this superfluid transition is ~0.5x10^9 K. The observed
rapidity of the cooling implies that protons were already in a superconducting
state with a larger critical temperature. Our prediction that this cooling will
continue for several decades at the present rate can be tested by continuous
monitoring of this neutron star.Comment: Revised version, to be published in Phys. Rev. Let
The effect of neutrinos on the initial fireballs in gamma-ray bursts
We investigate the fate of very compact, sudden energy depositions that may
lie at the origin of gamma-ray bursts. Following on from the work of Cavallo
and Rees (1978), we take account of the much higher energies now believed to be
involved. The main effect of this is that thermal neutrinos are present and
energetically important. We show that these may provide sufficient cooling to
tap most of the explosion energy. However, at the extreme energies usually
invoked for gamma-ray bursts, the neutrino opacity suffices to prevent dramatic
losses, provided that the heating process is sufficiently fast. In a generic
case, a few tens of percent of the initial fireball energy will escape as an
isotropic millisecond burst of thermal neutrinos with a temperature of about 60
MeV, which is detectable for nearby gamma-ray bursts and hypernovae. For
parameters we find most likely for gamma-ray burst fireballs, the dominant
processes are purely leptonic, and thus the baryon loading of the fireball does
not affect our conclusions.Comment: 10 pages, 4 figures. To be submitted to MNRA
Compression load failure of aluminum plates due to fire
An experimental study was performed to quantify the response and failure of 5083-H116 and 6082-T6 aluminum plates under compression load while being subjected to a constant heat flux representing a fire exposure. Using an intermediate scale loading frame with integrated heating, the study evaluated the effects of geometry, aluminum type, fire exposure, load, and fire protection. Intermediate scale aluminum panels which were more than 0.7 m high and 0.2 m wide were used to gain insights into the structural behavior of large structural sections exposed to fire. Failure temperatures were measured to range from 100 to 480 C and were dependent on applied stress and aluminum type. This indicates that the use of a single temperature criterion in fire resistance without load as typically done is not sufficient for evaluating structural response during fire. An empirical failure model was developed to account for fire exposure conditions, aluminum type, and geometr
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