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Register a new userSupernova Asymmetries and Pulsar Kicks -- Views on Controversial Issues
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H.-Th. Janka
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Two- and three-dimensional simulations demonstrate that hydrodynamic instabilities can lead to low-mode (l=1,2) asymmetries of the fluid flow in the neutrino-heated layer behind the supernova shock. This provides a natural explanation for aspherical mass ejection and for pulsar recoil velocities even in excess of 1000 km/s. We propose that the bimodality of the pulsar velocity distribution might be a consequence of a dominant l=1 mode in case of the fast component, while higher-mode anisotropy characterizes the postshock flow and SN ejecta during the birth of the slow neutron stars. We argue that the observed large asymmetries of supernovae and the measured high velocities of young pulsars therefore do not imply rapid rotation of the iron core of the progenitor star, nor do they require strong magnetic fields to play a crucial role in the explosion. Anisotropic neutrino emission from accretion contributes to the neutron star acceleration on a minor level, and pulsar kicks do not make a good case for non-standard neutrino physics in the nascent neutron star.
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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 ${approx}0.2 M_odot$.
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.
We show that Majoron emission from a hot nascent neutron star can be anisotropic in the presence of a strong magnetic field. If Majorons carry a non-negligible fraction of the supernova energy, the resulting recoil velocity of a neutron star can explain the observed velocities of pulsars.
We consider the formation of low-mass X-ray binaries containing accreting neutron stars via the helium-star supernova channel. The predicted relative number of short-period transients provides a sensitive test of the input physics in this process. We investigate the effect of varying mean kick velocities, orbital angular momentum loss efficiencies, and common envelope ejection efficiencies on the subpopulation of short-period systems, both transient and persistent. Guided by the thermal-viscous disk instability model in irradiation-dominated disks, we posit that short-period transients have donors close to the end of core-hydrogen burning. We find that with increasing mean kick velocity the overall short-period fraction, s, grows, while the fraction, r, of systems with evolved donors among short-period systems drops. This effect, acting in opposite directions on these two fractions, allows us to constrain models of LMXB formation through comparison with observational estimates of s and r. Without fine tuning or extreme assumptions about evolutionary parameters, consistency between models and current observations is achieved for a regime of intermediate average kick magnitudes of about 100-200 km/s, provided that (i) orbital braking for systems with donor masses in the range 1-1.5 solar masses is weak, i.e., much less effective than a simple extrapolation of standard magnetic braking beyond 1.0 solar mass would suggest, and (ii) the efficiency of common envelope ejection is low.
For the first time, we have systematically explored the population of discrete X-ray sources in the outskirts of early-type galaxies. Based on a broad sample of 20 galaxies observed with Chandra we detected overdensity of X-ray sources in their outskirts. The overdensity appears as halos of resolved sources around the galaxies. These halos are broader than the stellar light, extending out to at least ~ 10 Re (Re is the effective radius). These halos are composed of sources fainter than ~5E38 erg/s, whereas the more luminous sources appear to follow the distribution of the stellar light, suggesting that the excess source population consists of neutron star binaries. Dividing the galaxy sample into four groups according to their stellar mass and specific frequency of globular clusters, we find that the extended halos are present in all groups except for the low-mass galaxies with low globular cluster content. We propose that the extended halos may be comprised of two independent components, low-mass X-ray binaries (LMXBs) located in globular clusters (GCs), which are known to have a wider distribution than the stellar light, and neutron star (NS) LMXBs kicked out of the main body of the parent galaxy by supernova explosions. The available deep optical and X-ray data of NGC 4365 support this conclusion. For this galaxy we identified 60.1+/-10.8 excess sources in the 4-10 Re region of which ~ 40% are located in GCs, whereas ~ 60% are field LMXBs. We interpret the latter as kicked NS LMXBs. We discuss the implications of these results for the natal kick distributions of black holes and neutron stars.
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