neut_gamm_bh - NEUTRON STARS, GAMMA RAY BURSTS, and BLACK...

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NEUTRON STARS, GAMMA RAY BURSTS, and BLACK HOLES (chap. 22 in textbook) We will review the classes of remnants that can be left behind a star at the end of its life. We have already discussed the remnants of low-mass stars: white dwarfs. The following diagram may clarify, and is a useful review of stellar evolution. So we will discuss “neturon stars” and “black holes,” but in between we discuss the still mysterious “gamma-ray bursts.”
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Neutron Stars For carbon detonation SN probably no remnant . The entire white dwarf explodes and disperses. A large fraction of it is turned into iron, and in fact this is believed to be the main supply of iron everywhere in the universe. (Recall that the light curves could be interpreted as the radioactive decay of two elements: nickel and cobalt. These will become iron.) For core-collapse SN remnant is a neutron-degenerate core neutron star Densities ~ 10 14 to 10 15 g/cm 3 ~ billion times denser than water Cubic centimeter contains ~ 100 million tons! Like a single enormous nucleus, all neutrons nearly touching. Gravity at surface is huge. e.g. a human would weigh ~ million tons Rotation period ~ fraction of a second when first formed (conservation of angular momentum) Magnetic field is huge, amplified by the collapse (~10 12 x Earth’s field strength). Most extreme of these are called “magnetars”—100s now known. Observed as pulsars (discovered 1967). This was one of the most important astronomical discoveries of all time, by graduate student Jocelyn Bell, although her advisor was given the Nobel Prize for the totally accidental or serendipitous discovery.
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The fact that a pulsar is observed right in the center of the Crab Nebula (see picture below) is taken as strong evidence that the picture of neutron stars as collabsed cores of massive stars that exploded as supernova remnants. Each pulsar has different pulse period (0.03 to 0.3 sec for most) but most are very stable. (Fig. 22.2, and below. ) Here is another pulsar time series (“light curve”): A few pulsars are seen within supernova remnants (e.g. Crab Nebula, Fig. 22.4, 22.5 ), which is very strong evidence that pulsars are neutron stars are remnants of supernova explosions, but not all remnants have a detectable pulsar. Reason explained below. To understand neutron stars as “pulsars”, need to understand “ accretion disks .” Mass transferred from companions star has orbital angular momentum, and can’t fall right onto the star, but must for a rotating disk, whose gas gradually loses angular momentum by not-very-understood processes: an “accretion disk.” These occur on every scale in the universe, around many objects (protostellar disks were one variety). Here is a picture of an accretion disk forming by mass transfer onto a white dwarf. This is in fact how we claimed white dwarfs could become supernovae, carbon detonation supernovae.
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Interpretation: rotating “lighthouse model.” (Fig. 22.3 ) Rotation slows down with time, on a timescale of about a million years. Understand why, in this model, not all
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neut_gamm_bh - NEUTRON STARS, GAMMA RAY BURSTS, and BLACK...

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