Pulsar kicks
Encyclopedia
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| Distance
| 7.700 Ly
A pulsar kick refers to the observed phenomenon that the neutron star
remnants of many supernovae do not move with the velocity
of their progenitor star
, but rather with a substantially greater speed. The cause of pulsar kicks is unknown, but many astrophysicists believe that it must be due to an asymmetry in the way a supernova explodes. If true, pulsar kicks would give us insight into the supernova mechanism.
leading it out of the galaxy
. An extremely convincing example of a pulsar kick can be seen in the guitar nebula, where the bow shock
generated by the pulsar moving relative to the supernova remnant nebula has been observed and confirms a velocity of 800 km/s.
Of particular interest is whether the magnitude or direction of the pulsar kick has any correlation
with other properties of the pulsar, such as the spin axis, magnetic moment
, or magnetic field
strength. To date, no correlation has been found between the magnetic field strength and the magnitude of the kick. However, there is some contention over whether a correlation between spin axis and kick direction has been observed. For many years, it was believed that no correlation existed. In studies of the Vela
and Crab
pulsars, jets have been observed which are believed to align with the spin axis of the pulsar. Since these jets align very closely with the bow shock as well as the directly measured velocity of the pulsars, this is considered strong evidence that these pulsars have kicks aligned with their spin axis. It is also possible to measure the spin axis of a pulsar using the polarization of its radiation
, and a recent study of 24 pulsars has found a strong correlation between the polarization and kick direction. Such studies have always been fraught with difficulty, however, since uncertainties associated with the polarization measurement are very large, making correlation studies troublesome.
There is a possibility that the distribution of kick speeds is bimodal
. Strong evidence for this possibility comes from the "neutron star retention problem". Most globular clusters in our galaxy have an escape velocity
under 50 km/s, so that few pulsars should have any difficulty in escaping. In fact, with the directly measured distribution of kick velocities, we would expect less than 1% of all pulsars born in a globular cluster to remain. But this is not the case—globular clusters contain many pulsars, some in excess of 1000. The number can be improved somewhat if one allows a fraction of the kick momentum
to be transferred to a binary
partner. In this case, perhaps 6% ought to survive, but this is not sufficient to explain the discrepancy. This appears to imply that some large set of pulsars receive virtually no kick at all while others receive a very large kick. It would be difficult to see this bimodal distribution directly because many speed measurement schemes only put an upper limit on the object's speed. If it is true that some pulsars receive very little kick, this might give us insight into the mechanism for pulsar kicks, since a complete explanation would have to predict this possibility.
using convection or mechanical instabilities in the presupernova star. Perhaps the easiest to understand is the "overstable g-mode". In this theory, we first assume that the core is pushed slightly to one side, off center from the star. This increases the pressure
in the nearby silicon
and oxygen
shells of the star. Since the rate of nuclear reactions in these shells is very sensitively dependent on pressure, the added pressure results in a large release of energy, and the core is pushed back the other way. This in turn adds greater pressure on the other side, and we find that the core begins to oscillate. It has been shown that many such modes are overstable in heavy stars, that is, a small perturbation becomes large over time. When the star explodes, the core has additional momentum in some direction, which we observe as the kick. It has been proposed that hydrodynamical models can explain the bimodal distribution, through a "dichotomous kick scenario" in which the envelope of the presupernova star is stolen by a binary companion, damping mechanical instabilities and thus reducing the resulting kick.
There are two main neutrino
driven kick scenarios, relying on the parity violation of neutrino interactions to explain an asymmetry in neutrino distribution. The first uses the fact that in the presence of a magnetic field, the direction that a neutrino is scattered off a nucleus
is biased in some direction. So if neutrino emission happened in the presence of a strong magnetic field, we might expect the average neutrino drift to align in some way with that field, and thus the resulting explosion would be asymmetric. A main problem with this theory is that to have sufficient asymmetry the theory requires fields of order
G
, much stronger than is expected in a heavy star. Another neutrino based theory uses the fact that the cross section
for neutrino scattering depends weakly on the strength of the ambient magnetic field. Thus, if the magnetic field is itself anisotropic, then there could be dark spots which are essentially opaque
to neutrinos. This however requires anisotropies of order
G, which is even more unlikely.
The final main proposal is known as the electromagnetic rocket scenario. In this theory, we assume the pulsar's magnetic dipole
to be offcenter and offaxis from the pulsar's spin axis. This results in an asymmetry in the magnitude of the dipole oscillations, as seen from above and below, which in turn means an asymmetry in the emission of radiation
. The radiation pressure
then slowly rockets the pulsar away. Notice that this is a postnatal kick, and has nothing to do with asymmetries in the supernova itself. Also notice that this process steals energy from the pulsar's spin, and so a main observational constraint on the theory is the observed rate of rotation for pulsar's throughout the galaxy. A major bonus to this theory is that it actually predicts the spin-kick correlation. However, there is some contention as to whether this can generate sufficient energy to explain the full range of kick velocities.
| Distance
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...
| 7.700 Ly
A pulsar kick refers to the observed phenomenon that the neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
remnants of many supernovae do not move with the velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of their progenitor star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
, but rather with a substantially greater speed. The cause of pulsar kicks is unknown, but many astrophysicists believe that it must be due to an asymmetry in the way a supernova explodes. If true, pulsar kicks would give us insight into the supernova mechanism.
Observation
It is generally accepted today that the average pulsar kick ranges from 200–500 km/s. However, some pulsars have a much greater velocity. For example, the hypervelocity star B1508+55 has been reported to have a speed of 1100 km/s and a trajectoryTrajectory
A trajectory is the path that a moving object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass...
leading it out of the galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
. An extremely convincing example of a pulsar kick can be seen in the guitar nebula, where the bow shock
Bow shock
A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
generated by the pulsar moving relative to the supernova remnant nebula has been observed and confirms a velocity of 800 km/s.
Of particular interest is whether the magnitude or direction of the pulsar kick has any correlation
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
with other properties of the pulsar, such as the spin axis, magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
, or magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
strength. To date, no correlation has been found between the magnetic field strength and the magnitude of the kick. However, there is some contention over whether a correlation between spin axis and kick direction has been observed. For many years, it was believed that no correlation existed. In studies of the Vela
Vela (constellation)
Vela is a constellation in the southern sky. Its name is Latin for the sails of a ship, and it was originally part of a larger constellation, the ship Argo Navis, which was later divided into three parts, the others being Carina and Puppis.-Stars:...
and Crab
Crab Nebula
The Crab Nebula is a supernova remnant and pulsar wind nebula in the constellation of Taurus...
pulsars, jets have been observed which are believed to align with the spin axis of the pulsar. Since these jets align very closely with the bow shock as well as the directly measured velocity of the pulsars, this is considered strong evidence that these pulsars have kicks aligned with their spin axis. It is also possible to measure the spin axis of a pulsar using the polarization of its radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
, and a recent study of 24 pulsars has found a strong correlation between the polarization and kick direction. Such studies have always been fraught with difficulty, however, since uncertainties associated with the polarization measurement are very large, making correlation studies troublesome.
There is a possibility that the distribution of kick speeds is bimodal
Bimodal distribution
In statistics, a bimodal distribution is a continuous probability distribution with two different modes. These appear as distinct peaks in the probability density function, as shown in Figure 1....
. Strong evidence for this possibility comes from the "neutron star retention problem". Most globular clusters in our galaxy have an escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
under 50 km/s, so that few pulsars should have any difficulty in escaping. In fact, with the directly measured distribution of kick velocities, we would expect less than 1% of all pulsars born in a globular cluster to remain. But this is not the case—globular clusters contain many pulsars, some in excess of 1000. The number can be improved somewhat if one allows a fraction of the kick momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
to be transferred to a binary
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
partner. In this case, perhaps 6% ought to survive, but this is not sufficient to explain the discrepancy. This appears to imply that some large set of pulsars receive virtually no kick at all while others receive a very large kick. It would be difficult to see this bimodal distribution directly because many speed measurement schemes only put an upper limit on the object's speed. If it is true that some pulsars receive very little kick, this might give us insight into the mechanism for pulsar kicks, since a complete explanation would have to predict this possibility.
Theories
Many hydrodynamical theories have been proposed, all of which attempt to explain the asymmetry in supernovaSupernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
using convection or mechanical instabilities in the presupernova star. Perhaps the easiest to understand is the "overstable g-mode". In this theory, we first assume that the core is pushed slightly to one side, off center from the star. This increases the pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
in the nearby silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
shells of the star. Since the rate of nuclear reactions in these shells is very sensitively dependent on pressure, the added pressure results in a large release of energy, and the core is pushed back the other way. This in turn adds greater pressure on the other side, and we find that the core begins to oscillate. It has been shown that many such modes are overstable in heavy stars, that is, a small perturbation becomes large over time. When the star explodes, the core has additional momentum in some direction, which we observe as the kick. It has been proposed that hydrodynamical models can explain the bimodal distribution, through a "dichotomous kick scenario" in which the envelope of the presupernova star is stolen by a binary companion, damping mechanical instabilities and thus reducing the resulting kick.
There are two main neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
driven kick scenarios, relying on the parity violation of neutrino interactions to explain an asymmetry in neutrino distribution. The first uses the fact that in the presence of a magnetic field, the direction that a neutrino is scattered off a nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
is biased in some direction. So if neutrino emission happened in the presence of a strong magnetic field, we might expect the average neutrino drift to align in some way with that field, and thus the resulting explosion would be asymmetric. A main problem with this theory is that to have sufficient asymmetry the theory requires fields of order
GGauss (unit)
The gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...
, much stronger than is expected in a heavy star. Another neutrino based theory uses the fact that the cross section
Cross section (physics)
A cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....
for neutrino scattering depends weakly on the strength of the ambient magnetic field. Thus, if the magnetic field is itself anisotropic, then there could be dark spots which are essentially opaque
Opacity (optics)
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
to neutrinos. This however requires anisotropies of order
G, which is even more unlikely.The final main proposal is known as the electromagnetic rocket scenario. In this theory, we assume the pulsar's magnetic dipole
Magnetic dipole
A magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the dimensions of the source are reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the electric dipole, but the analogy is not complete. In particular, a magnetic...
to be offcenter and offaxis from the pulsar's spin axis. This results in an asymmetry in the magnitude of the dipole oscillations, as seen from above and below, which in turn means an asymmetry in the emission of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
. The radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
then slowly rockets the pulsar away. Notice that this is a postnatal kick, and has nothing to do with asymmetries in the supernova itself. Also notice that this process steals energy from the pulsar's spin, and so a main observational constraint on the theory is the observed rate of rotation for pulsar's throughout the galaxy. A major bonus to this theory is that it actually predicts the spin-kick correlation. However, there is some contention as to whether this can generate sufficient energy to explain the full range of kick velocities.