9,785 research outputs found
Variability in GRBs - A Clue
We show that external shocks cannot produce a variable GRB, unless they are
produced by an extremely narrow jets (angular opening of < ~10^{-4}) or if only
a small fraction of the shell emits the radiation and the process is very
inefficient. Internal shocks can produce the observed complex temporal
structure provided that the source itself is variable. In this case, the
observed temporal structure reflects the activity of the ``inner engine'' that
drives the bursts. This sets direct constraints on it.Comment: 15 page latex file with 5 PS figure. Complete uuencoded compressed PS
file is available at ftp://shemesh.fiz.huji.ac.il or at
http://shemesh.fiz.huji.ac.il/papers/SaP_aclue.u
Assesment of dial data collection and analysis techniques
The key issues in all areas of Differential Absorption Lidar (DIAL) data collection and analysis techniques were examined. This included consideration of the practical and theoretical limitations of DIAL and the range of possible DIAL measurements
Magnetar-like Emission from the Young Pulsar in Kes 75
We report detection of magnetar-like X-ray bursts from the young pulsar PSR
J1846-0258, at the center of the supernova remnant Kes 75. This pulsar, long
thought to be rotation-powered, has an inferred surface dipolar magnetic field
of 4.9x10^13 G, higher than those of the vast majority of rotation-powered
pulsars, but lower than those of the ~12 previously identified magnetars. The
bursts were accompanied by a sudden flux increase and an unprecedented change
in timing behavior. These phenomena lower the magnetic and rotational
thresholds associated with magnetar-like behavior, and suggest that in neutron
stars there exists a continuum of magnetic activity that increases with
inferred magnetic field strength.Comment: 17 pages, 2 figures, accepted for publication in Science. Note: The
content of this paper is embargoed until February 21, 200
IMET shipboard systems, operations and software user manual
This report has two parts. The first is a copy of the Operations and Software User Manual prepared for use with the IMET
shipboard software distribution. It describes the programs used to acquire and record data from IMET systems installed on R/V Knorr and R/V Oceanus. The second part adds appendix material that contains the documentation pages for programs and
subroutines used in the IMET shipboard software system. These items are available through network or diskette access. This report
has been prepared to give this information broader visibility and circulation.Funding was provided by the National Science Foundation under Grant Nos. OCE-92-04034 and OCE-87-09614
Asymptotically exact probability distribution for the Sinai model with finite drift
We obtain the exact asymptotic result for the disorder-averaged probability
distribution function for a random walk in a biased Sinai model and show that
it is characterized by a creeping behavior of the displacement moments with
time, ~ t^{\mu n} where \mu is dimensionless mean drift. We employ a
method originated in quantum diffusion which is based on the exact mapping of
the problem to an imaginary-time Schr\"{odinger} equation. For nonzero drift
such an equation has an isolated lowest eigenvalue separated by a gap from
quasi-continuous excited states, and the eigenstate corresponding to the former
governs the long-time asymptotic behavior.Comment: 4 pages, 2 figure
Normal ground state of dense relativistic matter in a magnetic field
The properties of the ground state of relativistic matter in a magnetic field
are examined within the framework of a Nambu-Jona-Lasinio model. The main
emphasis of this study is the normal ground state, which is realized at
sufficiently high temperatures and/or sufficiently large chemical potentials.
In contrast to the vacuum state, which is characterized by the magnetic
catalysis of chiral symmetry breaking, the normal state is accompanied by the
dynamical generation of the chiral shift parameter . In the chiral
limit, the value of determines a relative shift of the longitudinal
momenta (along the direction of the magnetic field) in the dispersion relations
of opposite chirality fermions. We argue that the chirality remains a good
approximate quantum number even for massive fermions in the vicinity of the
Fermi surface and, therefore, the chiral shift is expected to play an important
role in many types of cold dense relativistic matter, relevant for applications
in compact stars. The qualitative implications of the revealed structure of the
normal ground state on the physics of protoneutron stars are discussed. A
noticeable feature of the parameter is that it is insensitive to
temperature when , where is the chemical potential, and
{\it increases} with temperature for . The latter implies that the
chiral shift parameter is also generated in the regime relevant for heavy ion
collisions.Comment: 28 pages, 6 figures; v2: title changed in journa
Anapole moment of an exotic nucleus
We demonstrate that there is no appreciable enhancement of the anapole moment
of Be. The effect of small energy intervals is compensated for by a
small overlap of the halo neutron wave function with core.Comment: 5 pages, LaTe
Yet Another Model of Gamma-Ray Bursts
Sari and Piran have demonstrated that the time structure of gamma-ray bursts
must reflect the time structure of their energy release. A model which
satisfies this condition uses the electrodynamic emission of energy by the
magnetized rotating ring of dense matter left by neutron star coalescence; GRB
are essentially fast, high field, differentially rotating pulsars. The energy
densities are large enough that the power appears as an outflowing equilibrium
pair plasma, which produces the burst by baryon entrainment and subsequent
internal shocks. I estimate the magnetic field and characteristic time scale
for its rearrangement, which determines the observed time structure of the
burst. There may be quasi-periodic oscillations at the rotational frequencies,
which are predicted to range up to 5770 Hz (in a local frame). This model is
one of a general class of electrodynamic accretion models which includes the
Blandford and Lovelace model of AGN, and which can also be applied to black
hole X-ray sources of stellar mass. The apparent efficiency of nonthermal
particle acceleration is predicted to be 10--50%, but higher values are
possible if the underlying accretion flow is super-Eddington. Applications to
high energy gamma-ray observations of AGN are briefly discussed.Comment: 21pp, latex, uses aaspp4.st
X-ray Timing of PSR J1852+0040 in Kesteven 79: Evidence of Neutron Stars Weakly Magnetized at Birth
The 105-ms X-ray pulsar J1852+0040 is the central compact object (CCO) in SNR
Kes 79. We report a sensitive upper limit on its radio flux density of 12 uJy
at 2 GHz using the NRAO GBT. Timing using XMM and Chandra over a 2.4 yr span
reveals no significant change in its spin period. The 2 sigma upper limit on
the period derivative leads, in the dipole spin-down formalism, to an energy
loss rate E-dot < 7e33 ergs/s, surface magnetic field strength B_p < 1.5e11 G,
and characteristic age tau_c = P/2P-dot > 8 Myr. This tau_c exceeds the age of
the SNR by 3 orders of magnitude, implying that the pulsar was born spinning at
its current period. However, the X-ray luminosity of PSR J1852+0040, L(bol) ~
3e33(d/7.1 kpc)^2 ergs/s is a large fraction of E-dot, which challenges the
rotation-powered assumption. Instead, its high blackbody temperature,
0.46+/-0.04 keV, small blackbody radius ~ 0.8 km, and large pulsed fraction, ~
80%, may be evidence of accretion onto a polar cap, possibly from a fallback
disk made of supernova debris. If B_p < 1e10 G, an accretion disk can penetrate
the light cylinder and interact with the magnetosphere while resulting torques
on the neutron star remain within the observed limits. A weak B-field is also
inferred in another CCO, the 424-ms pulsar 1E 1207.4-5209, from its steady spin
and soft X-ray absorption lines. We propose this origin of radio-quiet CCOs:
the B-field, derived from a turbulent dynamo, is weaker if the NS is formed
spinning slowly, which enables it to accrete SN debris. Accretion excludes
neutron stars born with both B_p 0.1 s from radio pulsar
surveys, where B_p
40 Myr) or recycled pulsars. Finally, such a CCO, if born in SN 1987A, could
explain the non-detection of a pulsar there.Comment: 8 pages, 3 figures, to appear in The Astrophysical Journa
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