Neutron Star Kicks

Neutron Star KicksChris FryerAimee HungerfordFrank 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 J0045-7319 (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 protonsCore collapses nearly at freefall!

Core reaches nuclear densities Nuclear forces and neutron degeneracy increase pressure

Bounce!

Radius (km)

Vel

ocit

y (c

)

Vel

ocit

y (c

)

Radius (km)

Neutrino-Driven Supernova Mechanism: Convection

Fryer 1999

Upflow

Downflow

Proto-NeutronStar

AnatomyOf the ConvectionRegion

Fryer & Warren 2002

AccretionShock

The convective engine also explains why, even though the collapse releases 1053 ergs of energy, the observed explosion is only a few times 1051 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 KicksAsym. Collapse

Rotating SN

SASI Sterile 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 2007Fryer 2004

KickAsym. Collapse

Rotating SN

SASI Sterile Neutrino

Neutrino-sphere

Dipole B Depend.

Yes? No

Kick along ejecta Asym

? Yes

Kick along spin axis

Yes Not Nec.

Bimodal Kick

B field Low Mass Stars

AsymmetriesIn convectionAnd neutrino HeatingCause AsymmetriesIn theSupernovaExplosion!

ExplosionVelocitiesCan be Twice as Strong alongThe rotationEquator thanAlong the Pole!

Asym

metries from

Rotation

KickAsym. Collapse

Rotating SN

SASI Sterile Neutrino

Neutrino-sphere

Dipole B Depend.

Yes? No No

Kick along ejecta Asym

? Yes Yes

Kick along spin axis

Yes Not Nec. Yes

Bimodal Kick

B field Low 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. 2003

• 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

KickAsym. Collapse

Rotating SN

SASI Sterile Neutrino

Neutrino-sphere

Dipole B Depend.

Yes No No No

Kick along ejecta Asym

? Yes Yes Yes – Opposite Direction

Kick along spin axis

Yes Not Nec. Yes Not Nec.

Bimodal Kick

B field Low Mass Stars

High spin only

Massive Stars

Asymmetries from Anisotropic Neutrino Emission

Fuller et al. 2003 Fryer & Kusenko 2006

Neutrino oscillation to sterile neutrinos in a

highly magnetized core can produce kicks.

KickAsym. Collapse

Rotating SN

SASI Sterile Neutrino

Neutrino-sphere

Dipole B Depend.

Yes No No No No?

Kick along ejecta Asym

? Yes Yes Yes – Opposite Direction

Yes – Same Direction

Kick along spin axis

Yes Not Nec. Yes Not Nec. Not Nec.

Bimodal Kick

B field Low 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)

KickAsym. Collapse

Rotating SN

SASI Sterile Neutrino

Neutrino-sphere

Dipole B Depend.

Yes? No No No No? No?

Kick along ejecta Asym

? Yes Yes Yes – Opposite Direction

Yes – Same Direction

Not Nec.

Kick along spin axis

Yes Not Nec. Yes Not Nec., but likely in fast spinning models

Yes? Yes?

Bimodal Kick

B field Low Mass Stars

High spin only

Massive Stars

B field? B field?