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Pulsars. http://www.cv.nrao.edu/course/astr534/Pulsars.html High Energy Astrophysics jlc@mssl.ucl.ac.uk http://www.mssl.ucl.ac.uk/ Enrico Massaro notes. Pulsed emission; Rotation and energetics; Magnetic field; Neutron star structure; Magnetosphere and pulsar models;
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Pulsars http://www.cv.nrao.edu/course/astr534/Pulsars.html High Energy Astrophysics jlc@mssl.ucl.ac.uk http://www.mssl.ucl.ac.uk/ Enrico Massaro notes
Pulsed emission; • Rotation and energetics; • Magnetic field; • Neutron star structure; • Magnetosphere and pulsar models; • Radiation mechanisms; • Age and population
Period interpulse pulse (~P/10) Introduction • Known radio pulsars appear to emit short pulses of radio radiation with pulse periods between 1.4 ms and 8.5 seconds (P usually ≤ 1sec, dP/dt > 0). Even though the word pulsar is a combination of "pulse" and "star," pulsars are not pulsating stars. Their radio emission is actually continuous but beamed, so any one observer sees a pulse of radiation each time the beam sweeps across his line-of-sight. Since the pulse periods equal the rotation periods of spinning neutron stars, they are quite stable.
Introduction • Radio observations of pulsars have yielded a number of important results because: • Neutron Stars – supported by degeneracy pressure; Fermi exclusion principle restricts position hence Heisenberg uncertainty principle allows large momentum/high pressure - are physics laboratories providing extreme conditions (deep gravitational potentials, densities exceeding nuclear densities, magnetic field strengths as high as B~1014-15 gauss) not available on Earth. • Pulse periods can be measured with accuracies approaching 1 part in 1016, permitting exquisitely sensitive measurements of small quantities such as the power of gravitational radiation emitted by a binary pulsar system or the gravitational perturbations from planetary-mass objects orbiting a pulsar. Period increases quasi-regularly 10-20<dP/dt<10-12 s/s. However in some cases “glitches” are observed (abrupt decreasing of dP/dt).
Discovery • The radical proposal that neutron stars exist was made with trepidation by Baade & Zwicky in 1934: "With all reserve weadvance the view that a supernova represents the transition of anordinary star into a new form of star, the neutron star, whichwould be the end point of stellar evolution. Such a star maypossess a very small radius and an extremely high density." Pulsars provided the first evidence that neutron stars really do exist. They tell us about the strong nuclear force and the nuclear equation of state in new ranges of pressure and density, test general relativity and alternative theories of gravitation in both shallow and relativistically deep (GM/rc2>>0) potentials, and led to the discovery of the first extrasolar planets. • Pulsars were discovered serendipidously in 1967 on chart-recorder records obtained during a low-frequency (=81 MHz) survey of extragalactic radio sources that scintillate in the interplanetary plasma, just as stars twinkle in the Earth's atmosphere.
Discovery • Pulsar signals "had been recorded but not recognized" several years earlier with the 250-foot Jodrell Bank telescope. Most pulses seen by radio astronomers are just artificial interference from radar, electric cattle fences, etc., and short pulses from sources at astronomical distances imply unexpectedly high brightness temperatures T~1023–1030 K >>1012 K, the upper limit for incoherent electron-synchrotron radiation set by inverse-Compton scattering • brightness temperature: Tb=Fc2/ k2, the temperature for which if F is given by the RJ formula (F~KT2) T=Tb. • Cambridge University graduate student Jocelyn Bell noticed pulsars in her scintillation survey data because the pulses appeared earlier by about 4 minutes every solar day, so they appeared exactly once per sidereal day and thus came from outside the solar system. • The sources and emission mechanism were originally unknown, and even intelligent transmissions by LGM ("little green men") were seriously suggested as explanations for pulsars.
Discovery Astronomers were used to slowly varying or pulsating emission from stars, but the natural period of a radially pulsating star depends on its mean density and is typically days, not seconds.
Basic properties There is a lower limit to the rotation period P of a gravitationally bound star, set by the requirement that the centrifugal acceleration at its equator not exceed the gravitational acceleration. If a star of mass M and radius R rotates with angular velocity =2/P
Basic properties The canonical neutron star has M~ 1.4MSun and R~ 10 km,depending on the equation-of-state of extremely dense mattercomposed of neutrons, quarks, etc. The extreme density andpressure turns most of the star into a neutron superfluid that is asuperconductor up to temperatures T~109 K. Any star ofsignificantly higher mass (M~3MSun in standard models) mustcollapse and become a black hole. The masses of several neutronstars have been measured with varying degrees of accuracy, andall turn out to be very close to 1.4MSun.
White dwarf, neutron stars and black holes From the Salpeter IMF (number of stars formed each year per cubic Mpc with mass between M and M+dM): (M,M+dM)=2 10-12 M-2.35 dm 98% of the stars will end forming a white dwarf, i.e. the total number of WD in a typical galaxy is ~1010 Neutron stars will form if Mns>1.4MSun. They are thought to form form from the collapse of the core of stars with mass between 8 and 35 MSun. The total number of neutron stars in a typical galaxy is ~109. Black holes will form is MBH>3MSun. They are thought to form from the collapse of the core of stars with M>35 MSun. The total number of black holes in a typical galaxy is ~106-107
Basic properties The Sun and many other stars are known to possess roughlydipolar magnetic fields. Stellar interiors are mostly ionized gas andhence good electrical conductors. Charged particles areconstrained to move along magnetic field lines and, conversely,field lines are tied to the particle mass distribution. When the coreof a star collapses from a size 1011cm to 106 cm, its magnetic flux is conserved and the initial magneticfield strength is multiplied by 1010, the factor by which thecross-sectional area a falls. An initial magnetic field strength ofB~100 Gauss becomes B~1012 Gauss after collapse, so young neutronstars should have very strong dipolar fields.
Magnetic induction Magnetic flux, Radius collapses from 7 x 10 m to 10 m constant RNS surface R 8 4 Surface changegives
The Sun has magnetic fields of several different spatial scales and strengths but its general polar field varies with solar cycle and is ≈ 0.01 Tesla. • Thus the field for the neutron star: B ~ 5 x 10 Tesla = 5 x 10 Gauss • If the main energy loss from rotation is through magnetic dipole radiation then: B ~ 3.3 x 1015 (P P)½ Tesla or ~ 106 to 109Tesla for most pulsars 7 11 ns •
Pulsar energetics The traditionalmagnetic dipole model of a pulsar (Pacini 1966) Light cylinder (the cylinder centered on the pulsar andaligned with the rotation axis at whose radius the co-rotatingspeed equals the speed of light). RL=c/=(c/2)P= 4.775109 cm
Pulsar energetics Where P is the pulsar period. This electromagnetic radiation will appear at the very low frequency=1/P<1kHz, so low that it cannot be observed, or even propagate through the ionized ISM. The huge power radiated is responsible for pulsar slowdown as it extracts rotational kinetic energy from the neutron star. The absorbed radiation can also light up a surrounding nebula, the Crab nebula for example.
Emission mechanism The neutron star is surrounded by a magnetosphere with free charges that produce intense electric currents. The neutron star is a spinning magnetic dipole, it acts as a unipolar generator.There are two main regions: The closed magnetosphere, defined as the region containing the closed field lines within the light cilinder The open magnetosphere, the region where the field lines cannot close before RL and extend above this radius. If gravity is negligible, The total force acting on a charged particle is:
Pulsar Magnetospheres Forces exerted on particles Particle distribution determined by - gravity - electromagnetism e- Gravity Newton
Magnetic force RNS Newton PNS 13 This is a factor of 10 larger than the gravitational force and thus dominates the particle distribution.
Emission mechanism The particles moves along the field lines and at the same time rotate with them. Charges in the magnetic equatorial region redistribute themselves by moving along closed field lines until they build up an electrostatic field large enough to cancel the magnetic force and give F=0 . The voltage induced is about 106 V in MKS units. However, the co-rotating field lines emerging from the polar caps cross the light cylinder (the cylinder centered on the pulsar and aligned with the rotation axis at whose radius the co-rotating speed equals the speed of light) and these field lines cannot close. Electrons in the polar cap are magnetically accelerated to very high energies along the open but curved field lines, where the acceleration resulting from the curvature causes them to emit curvature radiation that is strongly polarized in the plane of curvature. As the radio beam sweeps across the line-of-sight, the plane of polarization is observed to rotate by up to 180 degrees, a purely geometrical effect. High-energy photons produced by curvature radiation interact with the magnetic field and lower- energy photons to produce electron-positron pairs that radiate more high-energy photons. The final results of this cascade process are bunches of charged particles that emit at radio wavelengths.
Magnetosphere Charge Distribution • Rotation and magnetic polar axes shown co-aligned • Induced E field removes charge from the surface so charge and • currents must exist above the surface – the Magnetosphere • Light cylinder is at the radial distance at which rotational velocity of • co-rotating particles equals velocity of light • Open field lines pass through the • light cylinder and particles stream • out along them • Feet of the critical field lines are at • the same electric potential as the • Interstellar Medium • Critical field lines divide regions of • + ve and – ve current flows from • Neutron Star magnetosphere
Radio Emission Velocity- of - Light Cylinder Radio Emission A more realistic model... • For pulses, magnetic and rotation axes • cannot be co- aligned. • Plasma distribution and magnetic field • configuration complex for Neutron Star • For r < rc, a charge-separated co-rotating magnetosphere • Particles move only along field lines; • closed field region exists within field-lines • that touch the velocity-of-light cylinder • Particles on open field lines can flow out of • the magnetosphere • Radio emission confined to these open-field • polar cap regions
Radio beam A better picture r=c/w Open magnetosphere Light cylinder B Closed magnetosphere Neutron star mass = 1.4 M radius = 10 km B = 10 to 10 Tesla 4 9
Curvature radiation The extremely high brightness temperatures are explained by coherent radiation. The electrons do not radiate as independent charges e; instead bunches of N electrons in volumes whose dimensions are less than a wavelength emit in phase as charges Ne. Since Larmor's formula indicates that the power radiated by a charge q is proportional to q2, the radiation intensity can be N times brighter than incoherent radiation from the same total number N of electrons. Because the coherent volume is smaller at shorter wavelengths, most pulsars have extremely steep radio spectra. Typical (negative) pulsar spectral indices are ~1.7 (S-1.7 ), although some can be much steeper (>3) and a handful are almost flat (~0.5).
Radiation Mechanisms in Pulsars Emission mechanisms Total radiation intensity coherent exceeds incoherent does not exceed Summed intensity of spontaneous radiation of individual particles
Incoherent emission - example For radiating particles in thermodynamic equilibrium i.e. thermal emission. Blackbody => max emissivity So is pulsar emission thermal? Consider radio: n~108 Hz or 100MHz; l~3m
Use Rayleigh-Jeans approximation to find T: Watts m Hz ster -2 -1 -1 Crab flux density at Earth, F~10 watts m Hz Source radius, R~10km at distance D~1kpc then: -25 -2 -1 (1)
So - 6 -1 -1 -2 In = 10 watts m Hz ster From equation (1): this is much higher than a radio blackbody temperature!
Models of Coherent Emission high-B sets up large pd => high-E particles e- e- e+ electron-positron pair cascade B = 1.108Tesla R = 104 m 1.1018V cascades results in bunches of particles which can radiate coherently in sheets
Emission processes in pulsars • Important processes in magnetic fields : - cyclotron - synchrotron • Curvature radiation => Radio emission Optical & X-ray emission in pulsars => B High magnetic fields; electrons follow field lines very closely, pitch angle ~ 0o
Curvature vs Synchrotron Synchrotron Curvature B B
Spectrum of curvature radiation (c.r.) - similar to synchrotron radiation, • For electrons: intensity from curvature radiation << cyclotron or synchrotron • Ifradio emission produced this way, need coherence Flux n 1/3 exp(-n) n n m
Incoherent X-ray emission? • In some pulsars, eg. Crab, there are also pulses at IR, optical, X-rays and g-rays. • - Are these also coherent? • Probably not – brightness temperature of X-rays is about 1011 K, equivalent to electron energies 10MeV, so consistent with incoherent emission. IR, optical, X-rays, g-rays incoherent radio coherent
Beaming of pulsar radiation • Beaming => radiation highly directional • Take into account - radio coherent, X-rays and Optical incoherent - location of radiation source depends on frequency - radiation is directed along the magnetic field lines - pulses only observed when beam points at Earth • Model: - radio emission from magnetic poles - X-ray and optical emission from light cylinder
Pulsar population TheP-Pdot Diagram is useful forfollowing the lives of pulsars, playing a role similar to theHertzsprung-Russell diagram for ordinary stars. It encodes atremendous amount of information about the pulsar populationand its properties, as determined and estimated from two of theprimary observables, P and Pdot Using those parameters, one canestimate the pulsar age, magnetic field strength B, and spin-downpower dE/dt. (From the Handbook of Pulsar Astronomy, by Lorimerand Kramer)
Pulsar population Pulsars are born in supernovae and appear in the upper left corner of the PP diagram. If B is conserved, they gradually move to the right and down, along lines of constant B and crossing lines of constant characteristic age. Pulsars with characteristic ages <105yr are often found in SNRs. Older pulsars are not, either because their SNRs have faded to invisibility or because the supernova explosions expelled the pulsars with enough speed that they have since escaped from their parent SNRs. The bulk of the pulsar population is older than 105 yr but much younger than the Galaxy (1010 yr). The observed distribution of pulsars in the PPdot diagram indicates that something changes as pulsars age. One possibility is that the magnetic fields of old pulsars decays on time scales 107 yr, causing pulsars to move straight down in the diagram until they fall below into the graveyard below the death line.
Pulsar population The death line in the PPdot diagram corresponds toneutron stars with sufficiently low B and high P that the curvatureradiation near the polar surface is no longer capable of generatingparticle cascades (Prad scales with B2 and P-4). Almost all short-period pulsars below the spin-up line near log[Pdot/P(sec)]~-16 are in binary systems, as evidenced by periodic (i.e. orbital) variations in their observed pulse periods. These recycled pulsars have been spun up by accreting mass and angular momentum from their companions, to the point that they emit radio pulses despite their relatively low magnetic field strengths B 108 G (the accretion causes a substantial reduction in the magnetic field strength). The magnetic fields of neutron stars funnel ionized accreting material onto the magnetic polar caps, which become so hot that they emit X-rays. As the neutron stars rotate, the polar caps appear and disappear from view, causing periodic fluctuations in X-ray flux; many are detectable as X-ray pulsars.
Pulsar population Millisecond pulsars (MSPs) with low-mass (M~0.1-1 MSun) white-dwarf companions typically have orbits with small eccentricities. Pulsars with extremely eccentric orbits usually have neutron-star companions, indicating that these companions also exploded as supernovae and nearly disrupted the binary system. Stellar interactions in globular clusters cause a much higher fraction of recycled pulsars per unit mass than in the Galactic disk. These interactions can result in very strange systems such as pulsar-main-sequence-star binaries and MSPs in highly eccentric orbits. In both cases, the original low-mass companion star that recycled the pulsar was ejected in an interaction and replaced by another star. (The eccentricitye of an elliptical orbit is defined as the ratio of the separation of the foci to the length of the major axis. It ranges between e for a circular orbit and e for a parabolic orbit.) A few millisecond pulsars are isolated. They were probably recycled via the standard scenario in binary systems, but the energetic millisecond pulsars eventually ablated their companions away.
Neutron Stars • General parameters: - R ~ 10 km (10 m) - r ~ 10 kg m = 10 g cm - M ~ 1.4 - 3.2 M - surface gravity, g = GM/R2 ~ 10 m s • We are going to find magnetic induction, B, for a neutron star. 4 18 -3 15 -3 inner 12 -2
Neutron star structure crust inner outer Heavy nuclei (Fe) find a minimum energy when arranged in a crystalline lattice Neutron star segment neutron liquid 1. solid core? Superfluid neutrons, superconducting p+ and e- 2. 17 -3 2x10 kg m 1km 14 -3 crystallization of neutron matter 4.3x10 kg m 9km 9 -3 10 kg m 18 -3 10km 10 kg m
