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Register a new userTheoretical Support for the Hydrodynamic Mechanism of Pulsar Kicks
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J. Nordhaus
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The collapse of a massive stars core, followed by a neutrino-driven, asymmetric supernova explosion, can naturally lead to pulsar recoils and neutron star kicks. Here, we present a two-dimensional, radiation-hydrodynamic simulation in which core collapse leads to significant acceleration of a fully-formed, nascent neutron star (NS) via an induced, neutrino-driven explosion. During the explosion, a ~10% anisotropy in the low-mass, high-velocity ejecta lead to recoil of the high-mass neutron star. At the end of our simulation, the NS has achieved a velocity of ~150 km s$^{-1}$ and is accelerating at ~350 km s$^{-2}$, but has yet to reach the ballistic regime. The recoil is due almost entirely to hydrodynamical processes, with anisotropic neutrino emission contributing less than 2% to the overall kick magnitude. Since the observed distribution of neutron star kick velocities peaks at ~300-400 km s$^{-1}$, recoil due to anisotropic core-collapse supernovae provides a natural, non-exotic mechanism with which to obtain neutron star kicks.
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Glitches are sudden jumps in the spin frequency of pulsars believed to originate in the superfluid interior of neutron stars. Superfluid flow in a model neutron star is simulated by solving the equations of motion of a two-component superfluid consisting of a viscous proton-electron plasma and an inviscid neutron condensate in a spherical Couette geometry. We examine the response of our model neutron star to glitches induced in three different ways: by instantaneous changes of the spin frequency of the inner and outer boundaries, and by instantaneous recoupling of the fluid components in the bulk. All simulations are performed with strong and weak mutual friction. It is found that the maximum size of a glitch that originates in the bulk decreases as the mutual friction strengthens. It is also found that mutual friction determines the fraction of the frequency jump which is later recovered, a quantity known as the healing parameter. These behaviours may explain some of the diversity in observed glitch recoveries.
Observations of binary pulsars and pulsars in globular clusters suggest that at least some pulsars must receive weak natal kicks at birth. If all pulsars received strong natal kicks above unit[50]{kms}, those born in globular clusters would predominantly escape, while wide binaries would be disrupted. On the other hand, observations of transverse velocities of isolated radio pulsars indicate that only $5pm2%$ have velocities below unit[50]{kms}. We explore this apparent tension with rapid binary population synthesis modelling. We propose a model in which supernovae with characteristically low natal kicks (e.g., electron-capture supernovae) only occur if the progenitor star has been stripped via binary interaction with a companion. We show that this model naturally reproduces the observed pulsar speed distribution and without reducing the predicted merging double neutron star yield. We estimate that the zero-age main sequence mass range for non-interacting progenitors of electron-capture supernovae should be no wider than ${approx}0.2 M_odot$.
We have observed the remnant of supernova SN~1987A (SNR~1987A), located in the Large Magellanic Cloud (LMC), to search for periodic and/or transient radio emission with the Parkes 64,m-diameter radio telescope. We found no evidence of a radio pulsar in our periodicity search and derived 8$sigma$ upper bounds on the flux density of any such source of $31,mu$Jy at 1.4~GHz and $21,mu$Jy at 3~GHz. Four candidate transient events were detected with greater than $7sigma$ significance, with dispersion measures (DMs) in the range 150 to 840,cm$^{-3},$pc. For two of them, we found a second pulse at slightly lower significance. However, we cannot at present conclude that any of these are associated with a pulsar in SNR~1987A. As a check on the system, we also observed PSR~B0540$-$69, a young pulsar which also lies in the LMC. We found eight giant pulses at the DM of this pulsar. We discuss the implications of these results for models of the supernova remnant, neutron star formation and pulsar evolution.
We present results from a suite of axisymmetric, core-collapse supernova simulations in which hydrodynamic recoil from an asymmetric explosion produces large proto-neutron star (PNS) velocities. We use the adaptive-mesh refinement code CASTRO to self-consistently follow core-collapse, the formation of the PNS and its subsequent acceleration. We obtain recoil velocities of up to 620 km/s at ~1 s after bounce. These velocities are consistent with the observed distribution of pulsar kicks and with PNS velocities obtained in other theoretical calculations. Our PNSs are still accelerating at several hundred km/s at the end of our calculations, suggesting that even the highest velocity pulsars may be explained by hydrodynamic recoil in generic, core-collapse supernovae.
Two entwined problems have remained unresolved since pulsars were discovered nearly 50 years ago: the orientation of their polarized emission relative to the emitting magnetic field and the direction of putative supernova ``kicks relative to their rotation axes. The rotational orientation of most pulsars can be inferred only from the (``fiducial) polarization angle of their radiation, when their beam points directly at the Earth and the emitting polar fluxtube field is $parallel$ to the rotation axis. Earlier studies have been unrevealing owing to the admixture of different types of radiation (core and conal, two polarization modes), producing both $parallel$ or $perp$ alignments. In this paper we analyze the some 50 pulsars having three characteristics: core radiation beams, reliable absolute polarimetry, and accurate proper motions. The ``fiducial polarization angle of the core emission, we then find, is usually oriented $perp$ to the proper-motion direction on the sky. As the primary core emission is polarized $perp$ to the projected magnetic field in Vela and other pulsars where X-ray imaging reveals the orientation, this shows that the proper motions usually lie $parallel$ to the rotation axes on the sky. Two key physical consequences then follow: first, to the extent that supernova ``kicks are responsible for pulsar proper motions, they are mostly $parallel$ to the rotation axis; and second that most pulsar radiation is heavily processed by the magnetospheric plasma such that the lowest altitude ``parent core emission is polarized $perp$ to the emitting field, propagating as the extraordinary (X) mode.
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