2,271 research outputs found
Modeling high-energy pulsar lightcurves from first principles
Current models of gamma-ray lightcurves in pulsars suffer from large
uncertainties on the precise location of particle acceleration and radiation.
Here, we present an attempt to alleviate these difficulties by solving for the
electromagnetic structure of the oblique magnetosphere, particle acceleration,
and the emission of radiation self-consistently, using 3D spherical
particle-in-cell simulations. We find that the low-energy radiation is
synchro-curvature radiation from the polar-cap regions within the light
cylinder. In contrast, the high-energy emission is synchrotron radiation that
originates exclusively from the Y-point and the equatorial current sheet where
relativistic magnetic reconnection accelerates particles. In most cases,
synthetic high-energy lightcurves contain two peaks that form when the current
sheet sweeps across the observer's line of sight. We find clear evidence of
caustics in the emission pattern from the current sheet. High-obliquity
solutions can present up to two additional secondary peaks from energetic
particles in the wind region accelerated by the reconnection-induced flow near
the current sheet. The high-energy radiative efficiency depends sensitively on
the viewing angle, and decreases with increasing pulsar inclination. The
high-energy emission is concentrated in the equatorial regions where most of
the pulsar spindown is released and dissipated. These results have important
implications for the interpretation of gamma-ray pulsar data.Comment: 14 pages, 11 figures, Accepted for publication in MNRA
Ab-initio pulsar magnetosphere: the role of general relativity
It has recently been demonstrated that self-consistent particle-in-cell
simulations of low-obliquity pulsar magnetospheres in flat spacetime show weak
particle acceleration and no pair production near the poles. We investigate the
validity of this conclusion in a more realistic spacetime geometry via
general-relativistic particle-in-cell simulations of the aligned pulsar
magnetospheres with pair formation. We find that the addition of frame-dragging
effect makes local current density along the magnetic field larger than the
Goldreich-Julian value, which leads to unscreened parallel electric fields and
the ignition of a pair cascade. When pair production is active, we observe
field oscillations in the open field bundle which could be related to pulsar
radio emission. We conclude that general relativistic effects are essential for
the existence of pulsar mechanism in low obliquity rotators.Comment: 5 pages, 4 figure, submitted to ApJLetter
Electromagnetic precursors to black hole - neutron star gravitational wave events: Flares and reconnection-powered fast-radio transients from the late inspiral
The presence of magnetic fields in the late inspiral of black hole -- neutron
star binaries could lead to potentially detectable electromagnetic precursor
transients. Using general-relativistic force-free electrodynamics simulations,
we investigate pre-merger interactions of the common magnetosphere of black
hole -- neutron star systems. We demonstrate that these systems can feature
copious electromagnetic flaring activity, which we find depends on the magnetic
field orientation but not on black hole spin. Due to interactions with the
surrounding magnetosphere, these flares could lead to Fast Radio Burst-like
transients and X-ray emission, with as an upper bound for
the luminosity, where is the magnetic field strength on the surface of
the neutron star.Comment: 13 pages, 6 figures; accepted version, to appear in ApJ
Reconnection-powered fast radio transients from coalescing neutron star binaries
It is an open question whether and how gravitational wave events involving
neutron stars can be preceded by electromagnetic counterparts. This work shows
that the collision of two neutron stars with magnetic fields well below
magnetar-level strengths can produce millisecond Fast-Radio-Burst-like
transients. Using global force-free electrodynamics simulations, we demonstrate
that electromagnetic flares, produced by overtwisted common flux tubes in the
binary magnetosphere, collide with the orbital current sheet and compress it,
resulting in enhanced magnetic reconnection. As a result, the current sheet
fragments into a sequence of plasmoids, which collide with each other leading
to the emission of coherent electromagnetic waves. The resulting
millisecond-long burst of radiation should have frequencies in the range of
for magnetic fields of at the
stellar surfaces.Comment: 10 pages, 4 figures, version accepted by PR
Electromagnetic precursor flares from the late inspiral of neutron star binaries
The coalescence of two neutron stars is accompanied by the emission of
gravitational waves, and can also feature electromagnetic counterparts powered
by mass ejecta and the formation of a relativistic jet after the merger. Since
neutron stars can feature strong magnetic fields, the non-trivial interaction
of the neutron star magnetospheres might fuel potentially powerful
electromagnetic transients prior to merger. A key process powering those
precursor transients is relativistic reconnection in strong current sheets
formed between the two stars. In this work, we provide a detailed analysis of
how the twisting of the common magnetosphere of the binary leads to an emission
of electromagnetic flares, akin to those produced in the solar corona. By means
of relativistic force-free electrodynamics simulations, we clarify the role of
different magnetic field topologies in the process. We conclude that flaring
will always occur for suitable magnetic field alignments, unless one of the
neutron stars has a magnetic field significantly weaker than the other.Comment: 19 pages, 10 figure
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