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Chapter 13

Chapter 13 - neutron stars and black holes.docx

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McMaster University
Daniel Harris

Chapter 13: Neutron Stars and Black Holes 13.1 Neutron Stars - for a type I (carbon-detonation) supernova, it unlikely that anything is left behind after the explosion - the entire star is shattered by the blast - for a type II (core-collapse) supernova, part of the star survives the explosion - iron core of a massive star collapses until its neutrons effectively come into contact with one another - at that point, central portion of the core rebounds, creating a powerful shock wave that races outward through the star, violently expelling matter into space *shock wave does not start at the very centre of the collapsing core* - the innermost part of the core (which rebounds) remains intact as the shock wave it produces destroys the rest of the star - neutron star - an ultracompressed ball of neutrons that remains after a type II supernova has subsided - extremely small and very massive; incredibly dense 7 8 3  average density can reach 10 or even 10 kg/m ; nearly a billion tomes denser than a white dwarf - composed purely of neutrons packed together in a tight ball about 20 km across  a typical neutron star is not much bigger than a small asteroid or a terrestrial city, yet its mass is greater than that of the sun - solid objects with very strong gravity  70-kg human would weigh the Earth equivalent of about 1 billion kg (1 million tons)  severe pull of neutron star’s gravity would flatten you much thinner than a piece of paper - rotate extremely rapidly with periods measure in fractions of a second (law of conservation of angular momentum) - have very strong magnetic fields  original field of parent star is amplified as the collapsing core squeezes the magnetic field lines closer together, creating a magnetic field trillions of times stronger than Earth’s - in time (theoretically) a neutron star will spin more and more slowly as it radiates its energy into space, and its magnetic field will diminish 13.2 Pulsars - pulsar - each has its own characteristic pulse shape and period - periods are generally quite short, ranging from about a few milliseconds to about a second  correspond to flashing rates btwn 1 and several 100 times per second  in some cases, periods are extremely stable - constant to within a few seconds in a million years  this makes pulsars the most accurate natural clocks known in the universe The pulsar model - upon its discovery, no one knew what a pulsar was:  Anthony Hewish reasoned that the only physical mechanism consistent with such precisely timed pulses is a small rotating source of radiation  only rotation can cause a high degree of regularity of the observed pulses and only a small object can account for the sharpness of each pulse - current model of pulsar (lighthouse model)  a compact, spinning neutron star that periodically flashes radiation toward Earth  2 ‘hot spots’ either on the surface of the neutron star or in the magnetosphere just above it, continuously emit radiation in a narrow beam - hotspots - most likely localized regions near the neutron star’s magnetic poles where charged particles, accelerated to extreme high energies by the star’s rotating magnetic field, emit radiation along the star’s magnetic axis - radiate more or less steadily through space like a revolving lighthouse beacon as the neutron star rotates pulses - pulses can be seen if neutron star is oriented such that one of the rotating beams sweeps across Earth - pulses at different frequencies do not necessarily all occur at the same instant in the pulse cycle - most pulsars emit pulses in form of radio radiation; some also pulse in visible, X-ray, and gamma-ray - whatever type of radiation produced, electromagnetic flashes at different frequencies all occur at regular, repeated intervals - period of a pulse = star’s rotating period - periods of most pulsars are quite short, ranging from about 0.03 s to 0.3 s (flashing btwn 3 and 30 times/sec) - by observing the speed + direction of ejected matter from pulsar, astronomers can work backward to pinpoint the location in space at which explosion must have occurred (this location corresponds to location of pulsar) - neutron star’s strong magnetic field + rapid rotation channel high-energy particles from near the star’s surface into the surrounding nebula  result is an energetic pulsar wind that flows outward at almost the speed of light, primarily in the star’s equilateral plane - most known pulsars are observed to have high speeds  much greater than the typical speeds of stars in our Galaxy  neutron stars may receive substantial "kicks" due to asymmetries in the supernovae in which they formed  such asymmetries are generally not very pronounced, but if the supernova's enormous energy is channeled even slightly in one direction, the newborn neutron star can recoil in the opposite direction with a speed of many tens or even hundreds of km/s Neutron stars and pulsars - all pulsars are neutron stars, but not all neutron stars are observed as pulsars - 2 ingredients that make neutron star pulse  rapid rotation and a strong magnetic field  both diminish with time  pulses gradually weaken + become less frequent - even a young, bright neutron star is not necessarily detectable as a pulsar from our vantage point on Earth - pulsar beam is relatively narrow-perhaps as little as a few degrees across in some cases; only if the neutron star happens to be oriented in just the right way do we actually see pulses - most cataloged neutron stars have been detected either as pulsars or by their interaction with a ‘normal’ stellar companion in a binary system - our observations of pulsars are consistent with the ideas that (1) every high-mass star dies in a supernova explosion (2) most supernovae leave a neutron star behind (few result in black holes) (3) all young neutron stars emit beams of radiation, just like pulsars we actually detect 13.3 Neutron Star Binaries - many pulsars are known to be isolated; at least some do have binary companions, and the same is true of neutron stars in general  consequence of this pairing is that the masses of some neutron stars have been determined quite accurately  all the measured masses are fairly close to 1 .4 times the mass of the Sun X- Ray Sources - numerous X-ray sources were found near central regions of our Galaxy + near centers of few rich star clusters - X-ray bursters - emit much of their energy in violent eruptions, each thousands of times more luminous than our Sun, but lasting only a few seconds - this X-ray emission arises on or near neutron stars that are members of binary systems - matter torn from the surface of (main-sequence or giant) companion by neutron star's strong gravitational pull accumulates on neutron star's surface.  gas forms an accretion disk around the neutron star, then slowly spirals inward  inner portions of the accretion disk become extremely hot, releasing a steady stream of X rays  as gas builds up on neutron star's surface, its temperature rises due to pressure of overlying material  soon, temperature becomes hot enough to fuse H  result is a sudden period of rapid nuclear burning that releases a huge amount of energy in a brief, but intense, flash of X rays (X-ray burst)  after several hours of renewed accumulation, a fresh layer of matter produces the next burst (thus, an X- ray burst is much like a nova on a white dwarf) Millisecond pulsars - millisecond pulsars - class of very rapidly rotating objects - speed is about as fast as a typical neutron star can spin ,without flying apart (in some cases, star's equator is moving at more than 20% of speed of light) - many of them are found in globular clusters* * - this is odd, since globular clusters are known to be very old-- 10 billion years, at least - no stars have formed in any globular cluster since the cluster itself came into being  no new neutron star has been produced in a globular duster in a very long time - pulsar produced by a supernova is expected to slow down in only a few million years, and after 10 billion years its rotation should have all but ceased - thus, rapid rotation of pulsars found in globular clusters cannot be a relic of their birth  these objects must have been "spun up" ( had their rotation rates increased) by some other, much more recent, mechanism - neutron star has been spun up by drawing in matter from a companion star - as matter spirals down onto the star's surface in an accretion disk, it provides a "push" that makes it spin faster - accretion onto a neutron star from a binary companion is the same scenario that we just used to explain existence of X-ray bursters  both phenomena are closely linked  many X-ray bursters may be on their way to becoming millisecond pulsars, and many millisecond pulsars are X-ray sources, powered by the trickle of material still falling onto them from their binary companions. Pulsar planets - pulse period of a millisecond pulsar lying some 500 pc from Earth exhibits tiny but regular fluctuations on 2 distinct time scales  67 and 98 days  most likely explanation: caused by the Doppler effect as the pulsar wobbles back and forth in space under the combined gravitational pulls of not one, but two, small bodies, each about three times the mass of Earth  one orbits the pulsar at a distance of 0.4 AU and the other at a distance of 0.5 AU. Their orbital periods are 67 and 98 days, respectively  presence of a third body, with mass comparable to Earth's Moon, orbiting only 0.2 AU from the pulsar was also revealed - first definite evidence of planet-sized bodies outside our solar system. - any planetary system orbiting the pulsar's progenitor star was almost certainly destroyed in supernova explosion that created the pulsar  scientists are still unsure about how these planets came into being  one possibility: the binary companion that provided the matter necessary to spin the pulsar up to millisecond speeds  possibly, the pulsar's intense radiation + strong gravity destroyed the companion and then spread its matter out into a disk (a little like the solar nebula) in whose cool outer regions the planets might have condensed 13.4 Gamma-Ray Bursts - gamma-ray bursts - discovered serendipitously in the late 1960s by military satellites looking for violators of the Nuclear Test Ban Treaty - consist of bright, irregular flashes of gamma rays typically lasting only a few seconds Distances and Luminosities - bursts do not originate within our own galaxy - gamma-ray observations in and of themselves do not provide enough info to tell us how far away a burst is  to determine the distance, the burst must be associated with some other object in the sky (burst counterpart) whose distance can be measured by other means  studying counterparts involve observations in optical or X-ray parts of electromagnetic spectrum  resolution of a gamma-ray telescope is quite poof, so the burst positions are uncertain by about a degree and a relatively large region of the sky must be scanned in search of a counterpart  most successful searches for burst counterparts have been carried out by satellites combining gamma- ray detectors with X-ray and/or optical telescopes - all distances to the bursts are very large  bursts must be extremely energetic, otherwise they wouldn’t be detectable by our equipment - assume gamma-rays are emitted in all directions (a big assumption)  total energy being emitted can be calculated using the inverse square law - we find that each burst generates more energy  in some cases, hundreds of times more energy  than a typical supernova explosion, all in a matter of seconds What causes the bursts? - gamma-ray burst sources are extremely energetic and very small - all of their energy must come from a volume no larger than a few hundred kilometers across  if the emitting region were, say, 300,000 km  1 light-second  across, even an instantaneous change in intensity at the source would be smeared out over a time interval of 1 s as seen from Earth, because light from the far side of the object would take 1 s longer to reach us than light from the near side  for the gamma-ray var
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