Neutron Stars and Black Holes Remnant cores of

Neutron Stars and Black Holes Remnant cores of massive stars: produce pulsars, jets and gamma-ray bursts Relativity theory is needed for full understanding, but …

NEUTRON STARS • CORE REMNANTS of stars with masses between about 8 and 30 M . • Outer layers blasted off in SN explosion. • Remaining NS masses are about 1. 4 to 2 M • Radii of about 10 -12 km (city sized) • Densities above 1014 g/cm 3 or 108 tons/teaspoon • Immense surface gravity: you would weigh billions of tons

Properties of Neutron Stars • Interior composition: 7/8 ths neutrons + p's and e's • The inner core may have lots of PIONS and be a superfluid and superconductor. • Neutron stars have an "atmosphere" only a few centimeters thick and a "crust" a few meters thick, both mostly made of iron. • Conservation of ANGULAR MOMENTUM means NSs are formed spinning very fast, with PERIODS of < 10 ms (~ 100 rotations/sec!) • Conservation of MAGNETIC FIELD means B above 1012 Gauss!

Isolated NS and X-ray Emission • NS found by Xrays and then HST moving > 100 km/s • Accretion disk around NS fed by companion • SS 433: radio maps show jets ejected from disk around NS or BH

Finding Neutron Stars • Isolated NSs are hard to detect by thermal X-ray emission from very hot surfaces. They are so small that their black-body emission, L = 4πR 2 T 4, is weak. • However, NSs in binaries will accrete from companions, just like WDs do. • Since NS's are much smaller, gas falls in further through the accretion disk, and gets much hotter: disk emits LOTS of X-RAYS. • Nuclear burning on NS surface can X -RAY BURSTERS

X-ray Bursters • Globular cluster Terzian 2 w/ central dot indicating location of X-ray bursts • X-ray images showing great increase during burst

NASA Neutron Star Resources • X-rays from the Supernova Remnant Cas A: a neutron star inside • Chandra monitors Crab Nebula during powerful Gamma-ray flare Chandra X-ray Observatory Education Resources http: //chandra. harvard. edu/formal/age_snr/ds 9. html A very nice lab, but only works on Macs or Linux.

Thought Question According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star? A. The neutron star’s rotation would speed up. B. The neutron star’s rotation would slow down. C. Nothing, the directions would cancel each other out.

Thought Question According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star? A. The neutron star’s rotation would speed up. B. The neutron star’s rotation would slow down. C. Nothing, the directions would cancel each other out.

Discovery of Neutron Stars: Pulsars • NS's were discovered only in 1967, since some of them are PULSARS. Jocelyn Bell & Ant. Hewish discovered them; only Hewish shared Nobel Prize (with Martin Ryle) in 1974. • But they were predicted to exist in the 1930’s by nuclear physicists: astronomers scoffed back then! • Strong B field plus fast rotation generates powerful forces: accelerate particles (mainly e's) close to speed of light. • Fast acceleration in magnetic fields SYNCHROTRON RADIATION

Pulsar Light Curves and Periods • Pulsars are excellent clocks, with very accurately measured periods between ~1 ms and ~10 s. Some pulses stronger than others. • Young, single pulsars start out spinning very fast, but slow down to periods of a few seconds over 106 years. WHY? • Rotational energy and angular momentum are radiated away with photons.

Pulsars are Beamed • A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis

Pulsars, Rare • The radiation beams sweep through space like lighthouse beams as the neutron star rotates • So, visible from only certain directions: many unseen ones should be out there!

Why Pulsars Must be Neutron Stars Circumference of NS = 2π (radius) ~ 60 km Spin Rate of Fast Pulsars ~ 1000 cycles per second Surface Rotation Velocity ~ 60, 000 km/s ~ 20% speed of light ~ escape velocity from NS Anything else would be torn to pieces!

The Crab Pulsar • • 33 ms period (30 flashes/second) At center of Crab nebula SNR Seen in optical, X-ray Shows inter-pulse, probably from 2 nd beam

Gamma-Ray Pulsars • Geminga and Crab both emit gamma-rays but Geminga is much weaker in optical and invisible in radio; its 0. 24 s period is shown on top

Binary Pulsars • A few pairs of pulsar binaries have been found. Accurate clocks mean Doppler shifts measured remarkably well. • Their orbits are decaying because of gravitational radiation and allow for sensitive tests of General Relativity • Led to Nobel prize for Joe Taylor and Russell Hulse in 1993 for 1974 discovery of the first binary pulsar

Pulsar(s) with Planets? • The first (2) extrasolar planets detected were found in 1992 around a pulsar (of all places!) • They caused tiny changes in the radial-velocity because of their tugs on the NS, showing periods of 67 and 98 days (at 0. 4 and 0. 5 AU) • Could planets survive a SN? Form from debris after? Perhaps NS was in an exchange encounter with a MS binary; could allow easier ms pulsar spin-up; maybe planet capture too?

Millisecond Pulsars • Old pulsars in binary systems can be rejuvenated into MILLISECOND pulsars, if they accrete enough mass and angular momentum from companion. • Typically much lower magnetic fields (so pulsar is old)

A (Very) Short Course in Relativity • In 1905 Einstein published the Special Theory of Relativity (along with photoelectric effect proving light was a photon; and Brownian motion proving atoms exist!) • An improvement on Newton’s laws of motion when things move close to c • Key postulates: 1) speed of light is constant in a vacuum and the same in all directions; and nothing can go faster than light 2) equations of physics should be the same in all inertial frames (those moving relatively with constant velocities)--the Principle of Relativity • TOGETHER THESE LEAD TO IMPORTANT RESULTS: • 3) There is no absolute frame of reference -- no preferred observer AND • 4) Space and time can’t be considered independently: we have SPACE-TIME: different observers, different values

Proof of Constancy of c • Michelson & Morley (1887) used an interferometer to see how much faster light was moving with and against the earth’s motion • Answer: NO DIFFERENCE!

Adding Velocities Relativistically

Lorentz Contraction and Time Dilation • A moving object appears shorter • A moving clock appears to tick slower • Lorentz factor,

Special Relativity Works! • E=mc 2 : tested in nuclear fission and fusion • Lifetimes of cosmic ray muons: they decay in 2. 0 microseconds at rest, but travel big distances, implying longer lives (like 44 s) in our frame if they move at 0. 999 c. • Effective mass increases from rest mass as v c: meff = m • So it’s harder to accelerate a particle that is moving faster (a = F/meff), explaining why so much energy is needed in cyclotrons and other “atom smashers”.

GENERAL RELATIVITY • In 1916 Einstein published the final form of the General Theory of Relativity • Equivalence between gravity and acceleration: you are weightless in a plummeting elevator • Improves on Newtonian gravity and motion laws when masses are big

Space-Time Warped Near Masses • In GR, matter warps space-time, so that the straightest and shortest path (geodesic) looks like a curve to us. • Mass tells space how to curve. • Space tells matter how to move. • Analogy: weight on a tight rubber sheet depresses it, so a ball is deflected

General Relativity Works Too! • GR predicts that light will appear to bend as it follows a curved path near a mass • Measure small displacement of stellar positions near Sun during a solar eclipse (done in 1919): 1. 75” at limb • Made Einstein world famous since it agreed very nicely!

Other Tests of GR • Mercury’s perihelion was found to advance some 574”/century but planetary perturbations explained only 531”/cent • GR perfectly explained the excess 43”/century • Later tests: radar ranging to planets; Global Positioning Satellite (GPS) system; dragging of inertial frames by rotating earth (Gravity Probe B)

Gravity Waves: a GR Prediction • Gravity radiates energy away as waves, causing orbits to shrink: perfect fit to binary pulsar orbit decay (Noble Prize to Hulse and Taylor in 1993) • Detectors (LIGO now; LISA in space may happen in your lifetimes) may “see” : • NS-NS mergers, NS -BH collisions, Supernova explosions; providing a new “window on the universe” (not photons or neutrinos or cosmic rays)

BLACK HOLES • A part of space-time divorced from the rest of the universe. • Not even light can escape if emitted too close to a black hole (BH); inside event horizon or Schwarzschild radius.

General Relativity and BHs • A BH is a singularity: finite amount of mass at a point, so • Density there is (nominally) INFINITE • The BH is surrounded by an event horizon or infinite redshift surface or Schwarzschild radius So a BH with Earth’s mass has RS = 1 cm! 100, 000 Msun BH has Rs = 300, 000 km or 3 x 108 km = 10 -5 parsec = 1000 light-seconds

Too much mass in too little volume! • Warping of space-time can be so severe that the region effectively pinches off • Space-time curvature becomes extremely strong in the vicinity of a BH’s event horizon

If the Sun shrank into a black hole, its gravity would be different only near the event horizon So black holes don’t really suck!

Light waves take extra time to climb out of a deep hole in spacetime leading to a gravitational redshift

Redshifted Emission • Photons lose energy as they climb out of the gravitational pit established by a BH. • We observe longer (redder) wavelengths (lower frequencies) compared to those emitted. • Time freezes for a distant observer watching something fall past event horizon

Black Hole Applets • • Escape Velocity and Radius Schwarzschild Radii and Mass Time Near BH Spacetime Orbits

Black Holes have no Hair! A BH is characterized by only: 1. Mass 2. Electric charge (astrophysically unimportant) 3. Angular momentum (spin) ergosphere

Rotating Black Holes • A rotating (Kerr) BH will have a SMALLER EVENT HORIZON than the same mass non-rotating (Schwarzschild) BH. • BUT, outside the Event Horizon there will be an ellipsoidal STATIONARY LIMIT: inside of it, everything MUST rotate w/ BH; outside the Stationary Limit, a powerful enough rocket could stand still. • The region between the Event Horizon and the Stationary Limit is called the ERGOSPHERE: (it is sort of donut shaped) In principle (and maybe in practice too!)

More About Kerr BH’s • In principle (and maybe in practice too!) the ROTATIONAL ENERGY of a BH can be EXTRACTED by PARTICLES or MAGNETIC FIELDS that penetrate the ERGOSPHERE (Penrose effect). • A way to make a great garbage dispos. ALL plus power plant! • If the SPIN of a BH is too large it could become a NAKED SINGULARITY, with no EVENT HORIZON; but the COSMIC CENSORSHIP HYPOTHESIS argues this never happens and BH's stay clothed with horizons.

Tidal Stretching & Hawking Radiation • Large gravity differences (tides): “toothpaste tube effect” • Quantum gravity effect: Hawking temperature T=h/16 2 k. GM=6 10 -8 K(M /M) • Hawking power: L R 2 T 4 M 2/M 4 1/M 2 • Incredibly small if BH mass > 1017 g (rules out stars/galaxies)

It’s Hard to Find Black Holes • They don’t emit (significant) radiation • Light bending means they don’t even show up as dark spots: • Unless distance is close to RS, gravity is close to that of a regular star of the same mass

Origin of Black Holes • Collapse of very massive stars (>30 M ) can lead to BHs of ~3 -25 M (neutron stars must have masses below about 2 M ). • A NS could accrete more gas from a binary companion, kicking it over the upper mass limit • Collapse of densest regions of forming galaxies, either directly or through merger of stars in dense clusters can yield BHs with M > 1000 M . • Quantum fluctuations in the early universe could give primordial BHs of a wide range of masses.

NASA Black Hole Resources • http: //nasa. gov/audience/foreducators/912/features/F_Black_Hole_Extreme_Exploration. html • • • Black Hole Videos X-rays from a microquasar: GRS 1915 How a black hole can form in a supernova explosion of a massive star

Accretion Disks • Form when gas spirals down into a massive object. Seen in: • Stars (and planetary systems) being born • Binary stellar systems with compact component: white dwarf neutron star black hole • Active Galactic Nuclei (later)

In an Accretion Disk • • Mass moves inward Angular momentum is carried outward Friction (viscosity) in the gas heats it up Usually most of this heat is radiated from the disk surface giving: Ultraviolet radiation from white dwarfs X-rays from neutron stars and stellar mass BHs Mostly visible and UV from AGN BHs • Most logical way to launch jets

One famous X-ray binary with a very likely black hole is in the constellation Cygnus

Cyg X-1: Radio Image & X-ray light curve Combining observations: optical of blue giant; Doppler shifted lines of star and gas stream we conclude star has M>15 M and X-ray emitting companion has M>10 M , so

Cygnus X-1 is a Black Hole Binary

Accretion Disks are Efficient • E = mc 2 • Complete conversion of mass to energy is only possible in matter-antimatter annihilation • But normal accretion disks can convert > 5. 7% but probably < 32% of mass to energy (the absolute upper limit is ~42%) • This is far better than chemical reactions (~ 0. 0001 %) or even nuclear fusion (~0. 7 %) • Full conversion of 1 M /year = 5. 7 1039 W

Jets Launched From Disks Artist’s rendition of jets launched from vicinity of BH in the center of an AD.

Gamma-Ray Bursts • Tremendous powers in high energy photons emitted in just a few seconds • First discovered by spy satellites in 1960 s looking for atomic bomb tests: isotropic in the sky • Usually have “afterglows”: emission in X-rays, optical and radio bands that decay more slowly • Generic model: a “fireball” of very hot plasma, bursting out as a very relativistic jet ( ~100) • This makes it look even brighter if jet points at us, but still involves great powers, since many are very distant

Where do gamma-ray bursts come from?

GRB Light Curves and Locations

GRBs are Far Away • Since 1997 many have had galaxies identified as their hosts; at large cosmological redshifts, therefore billions of parsecs away

Competing Models: NS-NS Mergers or Hypernovae (or both)?

Some GRBs are Hypernovae • Light curves brightening and looking like SN have been seen in a few cases, making it likely that some (many? , all? ) GRBs are exceptionally powerful SN • But could just be long GRBs, w/ short ones NS-NS mergers.

End of Compact Stellar Remnants • We now turn to collections of stars • Galaxies, starting with our Home Galaxy • Focus on Active Galactic Nuclei