43 research outputs found
QED can explain the non-thermal emission from SGRs and AXPs : Variability
Owing to effects arising from quantum electrodynamics (QED),
magnetohydrodynamical fast modes of sufficient strength will break down to form
electron-positron pairs while traversing the magnetospheres of strongly
magnetised neutron stars. The bulk of the energy of the fast mode fuels the
development of an electron-positron fireball. However, a small, but potentially
observable, fraction of the energy ( ergs) can generate a
non-thermal distribution of electrons and positrons far from the star. This
paper examines the cooling and radiative output of these particles. Small-scale
waves may produce only the non-thermal emission. The properties of this
non-thermal emission in the absence of a fireball match those of the quiescent,
non-thermal radiation recently observed non-thermal emission from several
anomalous X-ray pulsars and soft-gamma repeaters. Initial estimates of the
emission as a function of angle indicate that the non-thermal emission should
be beamed and therefore one would expect this emission to be pulsed as well.
According to this model the pulsation of the non-thermal emission should be
between 90 and 180 degrees out of phase from the thermal emission from the
stellar surface.Comment: 7 pages, 5 figures, to appear in the proceedings of the conference
"Isolated Neutron Stars: from the Interior to the Surface" (April 2006,
London), eds. D. Page, R. Turolla, & S. Zane, Astrophysics & Space Scienc
Light bending by nonlinear electrodynamics under strong electric and magnetic field
We calculate the bending angles of light under the strong electric and
magnetic fields by a charged black hole and a magnetized neutron star according
to the nonlinear electrodynamics of Euler-Heisenberg interaction. The bending
angle of light by the electric field of charged black hole is computed from
geometric optics and a general formula is derived for light bending valid for
any orientation of the magnetic dipole. The astronomical significance of the
light bending by magnetic field of a neutron star is discussed.Comment: drastically revised with conclusion change, reference added, version
to appear in JCA
Is Vacuum Decay Significant in Ekpyrotic and Cyclic Models?
It has recently been argued that bubble nucleation in ekpyrotic and cyclic
cosmological scenarios can lead to unacceptable inhomogeneities unless certain
constraints are satisfied. In this paper we show that this is not the case. We
find that bubble nucleation is completely negligible in realistic models.Comment: 3 pages, 1 figure, minor revision
QED and the High Polarization of the Thermal Radiation from Neutron Stars
The thermal emission of strongly magnetized neutron-star atmospheres is
thought to be highly polarized. However, because of the different orientations
of the magnetic field over the surface of the neutron star (NS), it is commonly
assumed that the net observed polarization will be significantly reduced as the
polarization from different regions will cancel each other. We show that the
birefringence of the magnetized QED vacuum decouples the polarization modes in
the magnetosphere; therefore, the direction of the polarization follows the
direction of the magnetic field up to a large distance from the stellar
surface. At this distance, the rays that leave the surface and are destined for
our detectors pass through only a small solid angle; consequently, the
polarization direction of the emission originating in different regions will
tend to align together. The net observed polarization of the thermal radiation
of NSs should therefore be very large. Measurement of this polarization will be
the first direct evidence of the birefringence of the magnetized vacuum due to
QED and a direct probe of behavior of the vacuum at magnetic fields of order of
and above the critical QED field of 4.4 x 10 13 G. The large observable
polarization will also help us learn more about the atmospheric properties of
NSs.Comment: 6 pages, 2 figures, minor changes to reflect accepted versio
Variational Approach to Hydrogen Atom in Uniform Magnetic Field of Arbitrary Strength
Extending the Feynman-Kleinert variational approach, we calculate the
temperature-dependent effective classical potential governing the quantum
statistics of a hydrogen atom in a uniform magnetic at all temperatures. The
zero-temperature limit yields the binding energy of the electron which is quite
accurate for all magnetic field strengths and exhibits, in particular, the
correct logarithmic growth at large fields.Comment: Author Information under this
http://www.physik.fu-berlin.de/~kleinert/institution.html Latest update of
paper also at this http://www.physik.fu-berlin.de/~kleinert/30
Differential rotation of nonlinear r-modes
Differential rotation of r-modes is investigated within the nonlinear theory
up to second order in the mode amplitude in the case of a slowly-rotating,
Newtonian, barotropic, perfect-fluid star. We find a nonlinear extension of the
linear r-mode, which represents differential rotation that produces large scale
drifts of fluid elements along stellar latitudes. This solution includes a
piece induced by first-order quantities and another one which is a pure
second-order effect. Since the latter is stratified on cylinders, it cannot
cancel differential rotation induced by first-order quantities, which is not
stratified on cylinders. It is shown that, unlikely the situation in the
linearized theory, r-modes do not preserve vorticity of fluid elements at
second-order. It is also shown that the physical angular momentum and energy of
the perturbation are, in general, different from the corresponding canonical
quantities.Comment: 9 pages, revtex4; section III revised, comments added in Introduction
and Conclusions, references updated; to appear in Phys. Rev.
Evidence for a Binary Companion to the Central Compact Object 1E 1207.4-5209
Unique among neutron stars, 1E 1207.4-5209 is an X-ray pulsar with a spin
period of 424 ms that contains at least two strong absorption features in its
energy spectrum. This neutron star has been identified as a member of the
radio-quiet compact central objects in supernova remnants. It has been found
that 1E 1207.4-5209 is not spinning down monotonically suggesting that this
neutron star undergoes strong, frequent glitches, contains a fall-back disk, or
possess a binary companion. Here, we report on a sequence of seven XMM-Newton
observations of 1E 1207.4-5209 performed during a 40 day window in June/July
2005. Due to unanticipated variance in the phase measurements beyond the
statistical uncertainties, we could not identify a unique phase-coherent timing
solution. The three most probable timing solutions give frequency time
derivatives of +0.9, -2.6, and +1.6 X 10^(-12) Hz/s (listed in descending order
of significance). We conclude that the local frequency derivative during our
XMM-Newton observing campaign differs from the long-term spin-down rate by more
than an order of magnitude, effectively ruling out glitch models for 1E
1207.4-5209. If the long-term spin frequency variations are caused by timing
noise, the strength of the timing noise in 1E 1207.4-5209 is much stronger than
in other pulsars with similar period derivatives. Therefore, it is highly
unlikely that the spin variations are caused by the same physical process that
causes timing noise in other isolated pulsars. The most plausible scenario for
the observed spin irregularities is the presence of a binary companion to 1E
1207.4-5209. We identified a family of orbital solutions that are consistent
with our phase-connected timing solution, archival frequency measurements, and
constraints on the companions mass imposed by deep IR and optical observations.Comment: 8 pages, 4 figures. To be published in the proceedings of "Isolated
Neutron Stars: from the Interior to the Surface" (April 24-28, 2006) - eds.
D. Page, R. Turolla & S. Zan
Electromagnetic transitions of the helium atom in superstrong magnetic fields
We investigate the electromagnetic transition probabilities for the helium
atom embedded in a superstrong magnetic field taking into account the finite
nuclear mass. We address the regime \gamma=100-10000 a.u. studying several
excited states for each symmetry, i.e. for the magnetic quantum numbers
0,-1,-2,-3, positive and negative z parity and singlet and triplet symmetry.
The oscillator strengths as a function of the magnetic field, and in particular
the influence of the finite nuclear mass on the oscillator strengths are shown
and analyzed.Comment: 10 pages, 8 figure
QED Effective Action at Finite Temperature: Two-Loop Dominance
We calculate the two-loop effective action of QED for arbitrary constant
electromagnetic fields at finite temperature T in the limit of T much smaller
than the electron mass. It is shown that in this regime the two-loop
contribution always exceeds the influence of the one-loop part due to the
thermal excitation of the internal photon. As an application, we study light
propagation and photon splitting in the presence of a magnetic background field
at low temperature. We furthermore discover a thermally induced contribution to
pair production in electric fields.Comment: 34 pages, 4 figures, LaTe
How does the electromagnetic field couple to gravity, in particular to metric, nonmetricity, torsion, and curvature?
The coupling of the electromagnetic field to gravity is an age-old problem.
Presently, there is a resurgence of interest in it, mainly for two reasons: (i)
Experimental investigations are under way with ever increasing precision, be it
in the laboratory or by observing outer space. (ii) One desires to test out
alternatives to Einstein's gravitational theory, in particular those of a
gauge-theoretical nature, like Einstein-Cartan theory or metric-affine gravity.
A clean discussion requires a reflection on the foundations of electrodynamics.
If one bases electrodynamics on the conservation laws of electric charge and
magnetic flux, one finds Maxwell's equations expressed in terms of the
excitation H=(D,H) and the field strength F=(E,B) without any intervention of
the metric or the linear connection of spacetime. In other words, there is
still no coupling to gravity. Only the constitutive law H= functional(F)
mediates such a coupling. We discuss the different ways of how metric,
nonmetricity, torsion, and curvature can come into play here. Along the way, we
touch on non-local laws (Mashhoon), non-linear ones (Born-Infeld,
Heisenberg-Euler, Plebanski), linear ones, including the Abelian axion (Ni),
and find a method for deriving the metric from linear electrodynamics (Toupin,
Schoenberg). Finally, we discuss possible non-minimal coupling schemes.Comment: Latex2e, 26 pages. Contribution to "Testing Relativistic Gravity in
Space: Gyroscopes, Clocks, Interferometers ...", Proceedings of the 220th
Heraeus-Seminar, 22 - 27 August 1999 in Bad Honnef, C. Laemmerzahl et al.
(eds.). Springer, Berlin (2000) to be published (Revised version uses
Springer Latex macros; Sec. 6 substantially rewritten; appendices removed;
the list of references updated