Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode). Chapter 14 Neutron Stars and Black Holes Outline I. Neutron Stars A. Theoretical Prediction of Neutron Stars B. The Discovery of Pulsars C. A Model Pulsar D. The Evolution of Pulsars E. Binary Pulsars F. The Fastest Pulsars G. Pulsar Planets II. Black Holes A. Escape Velocity B. Schwarzschild Black Holes C. Black Holes Have No Hair D. A Leap into a Black Hole E. The Search for Black Holes 1
Outline (continued) III. Compact Objects with Disks and Jets A. X-Ray Bursters B. Accretion Disk Observations C. Jets of Energy from Compact Objects D. Gamma-Ray Bursts Neutron Stars A supernova explosion of a M > 8 M sun star blows away its outer layers. The central core will collapse into a compact object of ~ a few M sun. Formation of Neutron Stars Compact objects more massive than the Chandrasekhar Limit (1.4 M sun ) collapse further. Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object: p + e - n + ν e Neutron Star 2
Properties of Neutron Stars Typical size: R ~ 10 km Mass: M ~ 1.4 3 M sun Density: ρ ~ 10 14 g/cm 3 Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!! Discovery of Pulsars Angular momentum conservation => Collapsing stellar core spins up to periods of ~ a few milliseconds. Magnetic fields are amplified up to B ~ 10 9 10 15 G. (up to 10 12 times the average magnetic field of the sun) => Rapidly pulsed (optical and radio) emission from some objects interpreted as spin period of neutron stars Pulsars / Neutron Stars Neutron star surface has a temperature of ~ 1 million K. Cas A in X-rays Wien s displacement law, λ max = 3,000,000 nm / T[K] gives a maximum wavelength of λ max = 3 nm, which corresponds to X-rays. 3
Pulsar Periods Over time, pulsars lose energy and angular momentum => Pulsar rotation is gradually slowing down. Lighthouse Model of Pulsars A Pulsar s magnetic field has a dipole structure, just like Earth. Radiation is emitted mostly along the magnetic poles. Neutron Star (SLIDESHOW MODE ONLY) 4
Images of Pulsars and Other Neutron Stars The vela Pulsar moving through interstellar space The Crab nebula and pulsar The Crab Pulsar Pulsar wind + jets Remnant of a supernova observed in A.D. 1054 The Crab Pulsar (2) Visual image X-ray image 5
Light Curves of the Crab Pulsar Proper Motion of Neutron Stars Some neutron stars are moving rapidly through interstellar space. This might be a result of anisotropies during the supernova explosion forming the neutron star Binary Pulsars Some pulsars form binaries with other neutron stars (or black holes). Radial velocities resulting from the orbital motion lengthen the pulsar period when the pulsar is moving away from Earth... and shorten the pulsar period when it is approaching Earth. 6
Neutron Stars in Binary Systems: X-ray Binaries Example: Her X-1 2 M sun (F-type) star Star eclipses neutron star and accretion disk periodically Neutron star Orbital period = 1.7 days Accretion disk material heats to several million K => X-ray emission Pulsar Planets Some pulsars have planets orbiting around them. Just like in binary pulsars, this can be discovered through variations of the pulsar period. As the planets orbit around the pulsar, they cause it to wobble around, resulting in slight changes of the observed pulsar period. Black Holes Just like white dwarfs (Chandrasekhar limit: 1.4 M sun ), there is a mass limit for neutron stars: Neutron stars can not exist with masses > 3 M sun We know of no mechanism to halt the collapse of a compact object with > 3 M sun. It will collapse into a single point a singularity: => A Black Hole! 7
Escape Velocity Velocity needed to escape Earth s gravity from the surface: v esc 11.6 km/s. Now, gravitational force decreases with distance (~ 1/d 2 ) => Starting out high above the surface => lower escape velocity. If you could compress Earth to a smaller radius => higher escape velocity from the surface. v esc v esc v esc The Schwarzschild Radius => There is a limiting radius where the escape velocity reaches the speed of light, c: R s = 2GM c 2 V esc = c G = Universal const. of gravity M = Mass R s is called the Schwarzschild Radius. Schwarzschild Radius and Event Horizon Event horizon No object can travel faster than the speed of light => nothing (not even light) can escape from inside the Schwarzschild radius We have no way of finding out what s happening inside the Schwarzschild radius. 8
Schwarzschild Radius of Black Hole (SLIDESHOW MODE ONLY) Black Holes in Supernova Remnants Some supernova remnants with no pulsar / neutron star in the center may contain black holes. Schwarzschild Radii 9
Black Holes Have No Hair Matter forming a black hole is losing almost all of its properties. Black Holes are completely determined by 3 quantities: Mass Angular Momentum (Electric Charge) General Relativity Effects Near Black Holes An astronaut descending down towards the event horizon of the BH will be stretched vertically (tidal effects) and squeezed laterally. General Relativity Effects Near Black Holes (2) Clocks starting at 12:00 at each point. After 3 hours (for an observer far away from the BH): Time dilation Clocks closer to the BH run more slowly. Event Horizon Time dilation becomes infinite at the event horizon. 10
General Relativity Effects Near Black Holes (3) Gravitational Red Shift All wavelengths of emissions from near the event horizon are stretched (red shifted). Frequencies are lowered. Event Horizon Observing Black Holes No light can escape a black hole => Black holes can not be observed directly. If an invisible compact object is part of a binary, we can estimate its mass from the orbital period and radial velocity. Mass > 3 M sun => Black hole! End States of Stars (SLIDESHOW MODE ONLY) 11
Candidates for Black Hole Compact object with > 3 M sun must be a black hole! Compact Objects with Disks and Jets Black holes and neutron stars can be part of a binary system. => Strong X-ray source! Matter gets pulled off from the companion star, forming an accretion disk. Heats up to a few million K. X-Ray Bursters Several bursting X-ray sources have been observed: Rapid outburst followed by gradual decay Repeated outbursts: The longer the interval, the stronger the burst 12
The X-Ray Burster 4U 1820-30 In the cluster NGC 6624 Optical Ultraviolet Black-Hole vs. Neutron-Star Binaries Black Holes: Accreted matter disappears beyond the event horizon without a trace. Neutron Stars: Accreted matter produces an X-ray flash as it impacts on the neutron star surface. Black Hole X-Ray Binaries Accretion disks around black holes Strong X-ray sources Rapidly, erratically variable (with flickering on time scales of less than a second) Sometimes: Quasi-periodic oscillations (QPOs) Sometimes: Radio-emitting jets 13
Radio Jet Signatures The radio jets of the Galactic blackhole candidate GRS 1915+105 Model of the X-Ray Binary SS 433 Optical spectrum shows spectral lines from material in the jet. Two sets of lines: one blue-shifted, one red-shifted Line systems shift back and forth across each other due to jet precession Gamma-Ray Bursts (GRBs) Short (~ a few s), bright bursts of gamma-rays GRB of May 10, 1999: 1 day after the GRB 2 days after the GRB Later discovered with X-ray and optical afterglows lasting several hours a few days Many have now been associated with host galaxies at large (cosmological) distances. Probably related to the deaths of very massive (> 25 M sun ) stars. 14
New Terms neutron star pulsar lighthouse model pulsar wind glitch magnetar gravitational radiation millisecond pulsar singularity black hole event horizon Schwarzschild radius (R S ) Kerr black hole ergosphere time dilation gravitational red shift X-ray burster quasi-periodic oscillations (QPOs) gamma-ray burster soft gamma-ray repeater (SGR) hypernova collapsar 15