21. Neutron Stars Neutron stars were proposed in the 1930s Pulsars were discovered in the 1960s Pulsars are rapidly rotating neutron stars Pulsars slow down as they age Neutron stars are superfluid & superconductive The fastest pulsars are in close binary systems Pulsating X-ray sources are also neutron stars White dwarfs & neutron stars make novae & bursters Neutron stars have upper mass limits

Neutron Stars Proposed in the 1930s The neutron is discovered 1932 Discovered by James Chadwick Basic properties of the neutron No electrical charge ~ 1,837 times the mass of an electron Mass of 1 proton + Mass of 1 electron = Mass of 1 neutron Charge of proton + Charge of electron = Charge of neutron The neutron star is proposed Hypothesized by Fritz Zwicky & Walter Baade Proposed the neutron star as supernovae leftovers Two possible stellar corpses… White dwarfs & neutron stars Supported by degenerate neutron pressure Basic properties at 1.0 MSun Diameter of ~ 30 km Escape velocity of ~ 0.5 . c

Pulsars Discovered in the 1960s Radio telescope array constructed 1967 Jocelyn Bell et al. search for random radio twinkling Jocelyn Bell et al. discover regular radio pulsing Pulses 1.3373011 seconds apart Others have periods ranging from ~ 0.25 to ~ 1.5 seconds Three possible explanations rejected Eclipsing binary stars Star edges would have to overlap to orbit fast enough Variable stars Rapid diameter changes would tear apart any star Rotating white dwarfs with hot spots Rapid rotation rate would tear apart any white dwarf One conclusion accepted Radio source must be much smaller than a white dwarf

The Crab Pulsar: “On” & “Off”

Intensity Variations of a Pulsar

Pulsars: Rapidly Rotating Neutron Stars Important questions about neutron stars Why should neutron stars emit any radiation? Why should neutron stars emit radio wavelengths? Why should neutron stars emit pulses of radiation? Why should neutron stars emit pulses so fast? Important physical aspects of neutron stars They are very small Their mass exceeds the Chandrasekhar limit Their surface area is 10–10 times their ZAMS surface area They rotate very fast Conservation of angular momentum insures they rotate fast They have intense magnetic fields Their magnetic field is 10+10 times their ZAMS magnetic field Approximately 1012 Gauss

Different Rotational & Magnetic Axes Prior observations The Sun’s rotational & magnetic axes are not aligned No planet’s rotational & magnetic axes are aligned Basic physical processes No fundamental reason why they should be aligned Electric generators Rotation in a strong magnetic field Neutron stars should be huge electric generators Spontaneous production of e– & e+ pairs One example of the conversion of energy into mass Magnetic field lines accelerate the e– & e+ pairs These behave just like a radio antenna Narrow beams of radio energy leave both magnetic poles Typically ~ 2° wide

A Pulsar’s Rotational & Magnetic Axes

Pulsars Do Not Pulse; They Rotate! Observations These objects are observed to blink on & off These objects were assumed to turn on & off Underlying processes These objects actually emit radiation constantly These objects behave like a lighthouse beam The light is on constantly The light is focused into a narrow beam The beam of light rotates & is seen only intermittently

Radio-Wavelength View: Crab Nebula

Optical/X-Ray View: The Crab Nebula © NASA (2002)

Pulsars Emit at Multiple Wavelengths Basic physical process No fundamental reason to emit only radio l’s Wide variety of energy levels exist Wide range of l’s should be emitted Additional observations The Crab pulsar X-ray l’s Recorded by orbiting Einstein Observatory Visible l’s Recorded by optical telescopes Other pulsars > 1,000 identified since 1968 > 100,000 probably exist in the Milky Way galaxy Probable source Type II supernovae Core collapse of massive stars

A Pulsar Seen at Three Wavelengths

Ejected Pulsars Basic observations Basic conclusions Many fast-moving pulsars have been identified Basic conclusions Many Type II supernovae must be asymmetric The resultant neutron star cannot remain within the remnant The resultant neutron star penetrates the supernova remnant

Neutron Star Ejected by a Supernova

Pulsars Slow Down As They Age Basic observations Supernova remnants emit huge amounts of energy Rotation rate of neutron stars gradually decreases Energy expended is same as energy emitted by the remnant Many supernova remnants emit unusual blue light Synchrotron radiation similar to particle accelerators Relativistic electrons in a powerful magnetic field Basic physical process Energy transferred to electrons from the magnetic field Similar to the Sun’s coronal heating

Superfluidity & Superconductivity Some basic properties of neutron stars Thought to have a solid crust Thought to have a fluid interior of degenerate neutrons Two special properties of neutron stars Superfluidity Matter can flow without friction under some conditions Superfluids can flow up the side of their container Convection in a neutron star can continue indefinitely Superconductivity Current can flow without friction under some conditions This assumes there is no “load” on the current Protons & electrons in a neutron star produce electric currents Interactions in a neutron star Glitches occur Sudden increase in rotational speed Faster spinning fluid core “grabs onto” slowing solid crust

The Internal Structure of a Neutron Star

Glitches in Neutron Stars

Protons & Electrons In Neutron Stars Basic physical processes Pressure & temperature are extremely high Great majority of p+ & e– are forced to join as neutrons Pressure & temperature are average properties A few particles will have quite low actual values These particles can remain separated as free particles Consequences Some p+ & e– are free to generate a magnetic field

Fastest Pulsars in Close Binary Systems Discovery of the fastest pulsar 1982 Period of 1.558 milliseconds Rotates ~ 642 times per second One implication Fast rotation should lead to fast energy loss & rapid slow-down The reality The slow-down rate is far less than expected The cause This pulsar is part of a very close binary system The two stars were of substantially different mass The high-mass star evolved quickly & died in a supernova The low -mass star survived to the red giant phase The low -mass star over-fills its Roche lobe Mass transfer “spins up” the companion neutron star Other millisecond pulsars Some are not part of binary systems A mystery

The Black Widow Eclipsing Pulsar

X-Ray Binary Pulsars Discovered in 1971 by the Uhuru spacecraft These high-energy pulsars are in close binary systems Deduced from cyclical Doppler shift every 1.7 days Pulsing period of ~ 1.24 seconds More than 20 have been discovered Basic physical processes Mass transfer from ordinary star to neutron star Channeled by magnetic field to the magnetic poles Accelerated by gravity to ~ 0.5 . c Hot spots form at ~ 108 K Intense X-ray emission ~ 105 . LSun Pulsar beam sweeps past the Earth

X-Ray Pulses From Centaurus X-3 Height variations are an artifact of sensor orientation.

Model of an X-Ray Binary Pulsar

Novae & Bursters Novae X-Ray Bursters Brighten by a factor of 104 to 108 in hours to days Reach a peak luminosity of ~ 109 . LSun White dwarfs in close binary systems Gradual mass transfer of H onto the white dwarf’s surface Highly compressed & heated to ~ 107 K by strong gravity Runaway surface H fusion is the nova X-Ray Bursters Brighten by a factor of 101 for ~ 20 seconds Neutron stars in close binary systems Relatively weak magnetic field allows surface H accumulation Fusion converts some H into He Runaway surface He fusion is the burster

Novae & Type Ia Supernovae Similarities Both occur in close binary systems One star is always a white dwarf Differences Novae are much less energetic than supernovae Novae ~ 1037 joules Supernovae ~ 1044 joules Novae accrete relatively small amounts of gas Runaway fusion occurs on the surface The white dwarf is not destroyed This event can happen repeatedly Supernovae accrete relatively large amounts of gas Runaway fusion occurs in the interior The white dwarf is destroyed This event can happen only once

The Light Curve of Nova Cygni 1975

The Light Curve of an X-Ray Burster

Neutron Stars Have Upper Mass Limits Degenerate electron pressure Capable of supporting < ~1.4 . MSun Chandrasekhar limit End result is a white dwarf Escape velocity < c Degenerate neutron pressure Capable of supporting <~ 3.0 . MSun End result is a neutron star Escape velocity ~ c

Important Concepts Pulsars are discovered 1967 Strongly emits at radio l’s Periods from ~ 0.25 to ~ 1.5 seconds Object much smaller than white dwarf Pulsars must be neutron stars Supported by degenerate n pressure Very small diameter Very rapid rotation Very strong magnetic field Basic physical processes Offset rotational & magnetic axes Behave just like a lighthouse beam Magnetic field channels e– & e+ pairs Produces ~ 2° wide radio beam Pulsars rotate, not turn “on” & “off” Pulsars emit at multiple l’s X-Ray & visible l’s Pulsars gradually slow down Special properties of neutron stars Superfluidity & superconductivity Interact to produce glitches P+ & n– can exist in neutron stars Generate the magnetic field Millisecond pulsars Accelerated by accreting gas Other unusual phenomena X-Ray binary pulsars Neutron stars in close binary systems Accretion causes radiating hot spots Novae White dwarfs in close binary systems Runaway surface H fusion Bursters Runaway surface He fusion Novae & Type IA supernovae Repeatable vs. non-repeatable events