Neutron Star Kicks Chris Fryer Aimee Hungerford Frank Timmes  Observational Evidence and constraints on kicks  Theoretical kick mechanisms

NS Kicks required for some binary systems Be-X-ray Binaries (van den Heuvel & Rappaport 1987; Martin et al. 2009) Misalignment of PSR J (Kaspi et al. 1996) DNS progenitor orbital separations require kicks (Fryer & Kalogera 1997) BH systems also have kicks (Fragos et al. 2009) Clusters argue for low kicks as well (Fryer et al. 1998; Pfahl et al. 2002)

Large Kicks and Possible Bimodality from Pulsar Velocities With new, larger, distances, Lyne and Lorimer (1994) argued for mean velocities of roughly ~450km/s. Arzoumanian et al. (2002) confirm the high velocities and confirmed the better fit of a bimodal distribution suggested by binary and cluser issues. Fauchier-Giguere & Kaspi (2006) found a single Maxwellian + tail fit to the data But see Hobbs et al. (2005)

Alignment of Kick and Explosion Asymmetry Some kicks appear to be aligned with the asymmetries in the ejecta, others do not. The magnitude of the kick doesn’t seem to correlate with magnetic field strength.

Alignment of Kick and Spin There appears to be a bimodal alignment of kick and spin (the kick is parallel and perpendicular to the spin axis). More than one kick mechanism (Johnston et al. 2005, Ng & Romani 2007, Rankin 2007, Kuranov et al. 2009). Willems et al. (2004) place further constraints on kick/spin alignment from DNS systems.

NS Kicks: Observational Summary Large velocities observed in the pulsar population implying large kicks at/near birth. Pulsar + binary data suggests bimodal distribution likely Some kicks along ejecta asymmetry axis, some not Some kicks along spin axis, some not (bimodal?) No obvious correlation between dipole magnetic field strength and kick velocity.

NS Kick Mechanisms Pulsar Rocket – a post- formation kick model. Asymmetric Collapse Convective asymmetries – rotation Convective asymmetries – e.g. standing accretion shock instability Asymmetric neutrinos – sterile neutrinos in core Asymmetric neutrinos – neutrino emission at neutrinosphere

Neutrino-Driven Supernova Mechanism Temperature and Density of the Core Becomes so High that: Iron dissociates into alpha particles Electrons capture onto protons Core collapses nearly at freefall! Core reaches nuclear densities Nuclear forces and neutron degeneracy increase pressure Bounce! Radius (km) Velocity (c) Radius (km)

Neutrino-Driven Supernova Mechanism: Convection Fryer 1999

Upflow Downflow Proto- Neutron Star Anatomy Of the Convection Region Fryer & Warren 2002 Accretion Shock

The convective engine also explains why, even though the collapse releases ergs of energy, the observed explosion is only a few times ergs. SASI instabilities occur later, and hence will produce weaker explosions and more massive remnants.

NS Kick Mechanisms – Pulsar Rocket Harrison and Tademaru (1975) argued that asymmetric pulsar emission would then produce a net motion along a pulsars spin axis. Advantages: steady acceleration (may explain runaway OB stars) Disadvantages: requires long-lived quadrupole moments or some such asymmetry.

Summary of Kicks Asym. Collapse Rotating SN SASISterile Neutrino Neutrino- sphere Dipole B Depend. Yes? Kick along ejecta Asym ? Kick along spin axis Yes Bimodal Kick B field One mechanism does not explain all the data!

Asymmetries in the Collapse may cause kicks Large-scale mixing in the Oxygen/Silicon burning can cause asymmetries that can be magnified in the collapse and cause kicks. Meakin & Arnett 2007 Fryer 2004

Kick Asym. Collapse Rotating SN SASISterile Neutrino Neutrino- sphere Dipole B Depend. Yes?No Kick along ejecta Asym ?Yes Kick along spin axis YesNot Nec. Bimodal Kick B fieldLow Mass Stars

Asymmetries In convection And neutrino Heating Cause Asymmetries In the Supernova Explosion! Explosion Velocities Can be Twice as Strong along The rotation Equator than Along the Pole! Asymmetries from Rotation

Kick Asym. Collapse Rotating SN SASISterile Neutrino Neutrino- sphere Dipole B Depend. Yes?No Kick along ejecta Asym ?Yes Kick along spin axis YesNot Nec.Yes Bimodal Kick B fieldLow Mass Stars High spin only

Asymmetries from Single-Lobe Convection Convection Drives explosion. The convective cells merge with time. With sufficient time, Low- Mode convection develops. Scheck et al Neutron Star Kicks for Slow Explosions

In 3-dimensions, the asymmetry is not quite as big. These instabilities are evident in 3- dimensions, but the kick and the explosion asymmetry is not so dramatic (Fryer & Young 2007).

Convective instabilities: Spin and kick are not aligned Fryer & Young 2007

Kick Asym. Collapse Rotating SN SASISterile Neutrino Neutrino- sphere Dipole B Depend. YesNo Kick along ejecta Asym ?Yes Yes – Opposite Direction Kick along spin axis YesNot Nec.YesNot Nec. Bimodal Kick B fieldLow Mass Stars High spin only Massive Stars

Asymmetries from Anisotropic Neutrino Emission Fuller et al Fryer & Kusenko 2006 Neutrino oscillation to sterile neutrinos in a highly magnetized core can produce kicks.

Kick Asym. Collapse Rotating SN SASISterile Neutrino Neutrino- sphere Dipole B Depend. YesNo No? Kick along ejecta Asym ?Yes Yes – Opposite Direction Yes – Same Direction Kick along spin axis YesNot Nec.YesNot Nec. Bimodal Kick B fieldLow Mass Stars High spin only Massive Stars B field

Magnetic fields near the neutrinosphere can also produce asymmetric neutrino emission, producing neutron star kicks (not necessarily aligned with the explosion ejecta). Socrates et al. (2005)

Kick Asym. Collapse Rotating SN SASISterile Neutrino Neutrino- sphere Dipole B Depend. Yes?No No? Kick along ejecta Asym ?Yes Yes – Opposite Direction Yes – Same Direction Not Nec. Kick along spin axis YesNot Nec.YesNot Nec., but likely in fast spinning models Yes? Bimodal Kick B fieldLow Mass Stars High spin only Massive Stars B field?