Neutron Stars and Black Holes Chapter 14

The preceding chapters have traced the story of stars from their birth as clouds of gas in the interstellar medium to their final collapse. This chapter finishes the story by discussing the kinds of objects that remain after a massive star dies. How strange and wonderful that we humans can talk about places in the universe where gravity is so strong it bends space, slows time, and curves light back on itself! To carry on these discussions, astronomers have learned to use the language of relativity. Throughout this chapter, remember that our generalized discussions are made possible by astronomers studying general relativity in all its mathematical sophistication. That is, our understanding rests on a rich foundation of theory. This chapter ends the story of individual stars. The next three chapters, however, extend that story to include the giant communities in which stars live— the galaxies. Guidepost

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 Outline

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 Outline (continued)

Neutron Stars The central core will collapse into a compact object of ~ a few M sun. A supernova explosion of a M > 8 M sun star blows away its outer layers.

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 + e  Neutron Star

Properties of Neutron Stars Typical size: R ~ 10 km Mass: M ~ 1.4 – 3 M sun Density:  ~ 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 => Collapsing stellar core spins up to periods of ~ a few milliseconds. Angular momentum conservation => Rapidly pulsed (optical and radio) emission from some objects interpreted as spin period of neutron stars Magnetic fields are amplified up to B ~ 10 9 – G. (up to times the average magnetic field of the sun)

Pulsars / Neutron Stars Wien’s displacement law,  max = 3,000,000 nm / T[K] gives a maximum wavelength of max = 3 nm, which corresponds to X-rays. Cas A in X-rays Neutron star surface has a temperature of ~ 1 million K.

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)

Images of Pulsars and Other Neutron Stars The vela Pulsar moving through interstellar space The Crab nebula and pulsar

The Crab Pulsar Remnant of a supernova observed in A.D Pulsar wind + jets

The Crab Pulsar (2) Visual image X-ray image

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.

Neutron Stars in Binary Systems: X-ray Binaries Example: Her X-1 2 M sun (F-type) star Neutron star Accretion disk material heats to several million K => X-ray emission Star eclipses neutron star and accretion disk periodically Orbital period = 1.7 days

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!

Escape Velocity Velocity needed to escape Earth’s gravity from the surface: v esc ≈ 11.6 km/s. v esc Now, gravitational force decreases with distance (~ 1/d 2 ) => Starting out high above the surface => lower escape velocity. v esc If you could compress Earth to a smaller radius => higher escape velocity from the surface.

The Schwarzschild Radius => There is a limiting radius where the escape velocity reaches the speed of light, c: V esc = c R s = 2GM____ c2c2 R s is called the Schwarzschild Radius. G = Universal const. of gravity M = Mass

Schwarzschild Radius and Event Horizon No object can travel faster than the speed of light  We have no way of finding out what’s happening inside the Schwarzschild radius. => nothing (not even light) can escape from inside the Schwarzschild radius  “Event horizon”

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

“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) Time dilation Event Horizon Clocks starting at 12:00 at each point. After 3 hours (for an observer far away from the BH): Clocks closer to the BH run more slowly. Time dilation becomes infinite at the event horizon.

General Relativity Effects Near Black Holes (3) Gravitational Red Shift Event Horizon All wavelengths of emissions from near the event horizon are stretched (red shifted).  Frequencies are lowered.

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)

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

The X-Ray Burster 4U 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 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 Accretion disks around black holes

Radio Jet Signatures The radio jets of the Galactic black- hole candidate GRS

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 Later discovered with X-ray and optical afterglows lasting several hours – a few days GRB of May 10, 1999: 1 day after the GRB 2 days after the GRB Many have now been associated with host galaxies at large (cosmological) distances. Probably related to the deaths of very massive (> 25 M sun ) stars.

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 New Terms