1101_23_24_neutron_stars_black_holesIandII

Credit: Subaru, Gendler Neutron Stars and Black Holes I & II 20 Neutron Stars are the collapsed cores left after core- collapse supernovae of massive stars. 8 < M initial < 15 or 25 or 30 M sun (?) Held up against gravity by Degenerate Neutron Pressure. Shine only by residual heat : no fusion or contraction Mass: ~1.2 – 2.1 M sun Radius: ~12 km Density: ~2x10 14 g/cc! Escape Speed: ~0.7c! Rotation periods: 0.002 – 10 s! Magnetic fields: 10 10 – 10 15 G! Properties: 21 Neutron stars were accidentally discovered in 1967. Jocelyn Bell (a Cambridge grad student) and Anthony Hewish (her PhD adviser) discovered pulsating radio sources while looking for something else. Pulsars = Pulsa ting R adio S ources 22 Radiation Beam Magnetic Field Spin Axis Pulsars are rapidly spinning, magnetized neutron stars. Lighthouse Model of Pulsars: Strong magnetic field and rotation create beams of intense radiation. Pulsar slow its rotation as it ages. It “spins down.” Spinning magnet sheds energy. They emit sharp millisecond-long pulses every spin period from radio to X-rays. Hundreds of pulsars known, spin periods of 2 ms to > 10 s. 23

Lighthouse model for pulsars Video by Michael Kramer (MPIfR/JBCA) 24 ~1 supernova explodes every 100 years. After 10 billion years: ~100 million supernovae have exploded. ~1 billion M sun of heavy elements is produced. ~100 million neutron stars have formed. In a galaxy like the Milky Way, there are more than 200 billion stars. ~1 Sun-like star is born every year. ~1 star >8 M sun is born every 100 years. 25 The race to explosion or black hole … The “proto”-neutron star is supported by: Normal gas pressure (P ~ density x temp) Neutron degeneracy pressure (P is temp independent) Neutron star mass is growing because of infalling matter. On a timescale of 1 second The core goes from about 1.4 M Sun à larger than 2.5 M Sun . Neutrinos cool the core. Less thermal pressure. If M > 2.5 M sun (or so), strong force fails, degeneracy pressure fails, collapse to BH formation… 27 Vartanyan et al. 2018 Some failures! 28

Gravity’s Final Victory A star more massive than ~15, or 20-30M sun may not explode as a supernova Proto-neutron star becomes M > 2.5 M sun about 1 second after collapse: • Neutron degeneracy pressure fails, strong force fails, and nothing can stop gravitational collapse. Core collapses to a singularity , an object with • Zero radius • Infinite density: Density = Mass/Volume In “classical” General Relativity 34 Black Holes The ultimate extreme object: • Gravity so strong not even light escapes. • Infalling matter gets shredded by powerful tides & crushed to infinite density. • V esc exceeds the speed of light Black : It neither emits nor reflects light. (But … Hawking radiation) Hole : Nothing entering can ever escape. 35 Schwarzschild Radius Light cannot escape from a Black Hole if it comes from a radius less than the Schwarzschild Radius : V esc 2 = 2 GM R = c 2 ⇒ R S = 2 GM c 2 M = Mass of the Black Hole For M = 1 M sun , R S ~ 3 km (Recall, for Neutron star: M = 1-2 M sun , R~10 km) 36 Neutron Star vs. Black Hole Neutron Star M=1.5 M sun R=10 km Manhattan Black Hole M = 1.5 M sun R S = 4.5 km 37

1915: General Relativity, Einstein ’ s Theory of Gravity 1916: Schwarzschild ’ s Discovery of BHs in GR BHs only understood & accepted in the 1960s (Term “ Black Hole ” coined by John Wheeler in 1967) Karl Schwarzschild Albert Einstein 38 R S defines the Event Horizon : Events that happen inside R S are invisible to the outside Universe. Things that get inside R S can never leave the black hole. The “ Point of No Return ” for a Black Hole. But, R S is not the “ singularity ” . The singularity is a single point at the center of the object. 39 Space and time are curved and warped by gravity. 40 Space and time are curved and warped by gravity. 41

Light is bent/deflected by a strong gravitational field. 46 Jill’s View from far away Jack’s view from 10R S Light is bent/deflected by a strong gravitational field. 47 Circling a Black Hole. 48 10 M sun Black Hole at a distance of 600 km. Light is strongly bent by the strong gravity field around a black hole. Gravitational Lensing is a generic effect seen around all massive objects. Important prediction of Einstein’s General Relativity. It is most extreme in proximity to a black hole. 49

A journey down a black hole Andrew Hamilton (JILA) vimeo.com/8818891 50 Down the hole... Jill Sees: • Sees one last laser flash after a long delay • Flash is faint and at long radio wavelengths • She never sees another flash from Jack… Jack Sees: • Universe warped as he crosses the Event Horizon • Gets shredded by strong tides near the singularity and crushed to infinite density. • Atoms break down into constituents, then nuclei, then protons & neutrons, then … 51 Jill’s Conclusions: The powerful gravity of a black hole warps space and time around it: 1.Time appears to stand still at the event horizon as seen by a distant observer. 2.Time flows as it always does as seen by an infalling astronaut. Time Dilation. “Time” is relative. 3.Light emerging from near the black hole is Gravitationally Redshifted to longer (redder) wavelengths Yes, Jack could “hover” very close to the event horizon for a few hours, days, or weeks, and Jill could age by 1, 10, 100, 1000, … years 52 The black hole from the movie Interstellar. MURPH! 53

Simulation by Armitage & Reynolds Matter falling into a black hole settles into a hot accretion disk that radiates IR to X-ray light. Accretion onto BHs emits a lot of light! E radiated ≈ 0.1 m accreted c 2 Matter is heated by “friction” as it spirals into the black hole. Inner parts are moving faster than outer parts. We see the photons from the hot gas disk. V circ = ( GM / r ) 1/2 59 We see accretion disks. “X-ray binaries” are bright, variable X-ray sources powered by mass accretion. Binary system with a star and a Neutron star or Black Hole Estimate mass by measuring the orbital period and orbital speed . (Just like Jupiter and Galilean moons!) Black hole candidates have masses bigger than the maximum neutron star mass ~2.5 M sun There are currently about 25 good black hole candidates in X-ray systems. All about 5-15 M sun. 60 Can we further test this picture? At 1.5 R S photons orbit in a circle. Long-predicted “ring” of light at 27 2 ࠵? ! Work by Page & Thorne (1974), Bardeen (1973), Luminet (1979) Luminet (1979) 61 Luminet (1979) 62

The black hole from the movie Interstellar. Not quite right. 63 Observations of M87 supermassive black hole by the Event Horizon Telescope confirm photon ring and black hole “shadow”. Too small to see for stellar- mass black holes in our galaxy. It’s bright on one side because of “Doppler beaming:” the bright side is coming at you! 64 First LIGO event says a binary BH 39 + 26 M sun existed, merged. All in distant galaxies. Rare events, but many seen. All predictions of GR confirmed. Mergers! General Relativity predicts BHs in binary systems could merge by emitting gravitational waves. Ripples in spacetime. 65 BH-BH binary mergers BH X-ray binaries Pulsar binaries NS-NS mergers 67