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PHYS 3380 - Astronomy I will not be here the week of Nov 30. Dr Russell Stoneback will be teaching the class PHYS 3380 - Astronomy Pulsar Periods Pulsar energy generated by rotation - as it blows away pulsar wind and blasts radiation outward, it slows down. So, over time, pulsars lose energy and angular momentum Pulsar rotation gradually slows down Oldest about 10 million years Glitches consequences of angular momentum transfer between a solid crust, which rotates at the measured pulsar periodicity, and a more rapidly rotating "loose' component of the neutron star interior. Possibly caused by “starquakes” or vortices in fluid (neutron) interior. PHYS 3380 - Astronomy The Crab Pulsar Pulsar wind + jets Remnant of a supernova observed in A.D. 1054 PHYS 3380 - Astronomy Pulsar Wind Combination of rapid rotating and strong magnetic field generate jets of matter and anti-matter moving away from the north and south poles and an intense wind flowing out in the equatorial direction carry 99.9% of energy released from slowing down of pulsar rotation rate. Chandra X-ray Image of Crab Nebula Inner X-ray ring thought to be shock wave marking boundary between surrounding nebula and the pulsar wind. Energetic electrons and positrons move outward from this ring to brighten the outer ring and produce an extended X-ray glow. Fingers, loops, and bays indicate that magnetic field of the nebula and filaments of cooler matter are controlling the motion of the electrons and positrons. The particles can move rapidly along the magnetic field and travel several light years before radiating away their energy - move much more slowly perpendicular to the magnetic field, and travel only a short distance before losing their energy. This effect can explain the long, thin, fingers and loops, as well as the sharp boundaries of the bays. The conspicuous dark bays on the lower right and left are likely due to the effects of a toroidal magnetic field - a relic of the progenitor star. PHYS 3380 - Astronomy Composite X-ray (Chandra - left) and visible (Hubble) movie PHYS 3380 - Astronomy Proper Motion of Neutron Stars Some neutron stars are moving rapidly through interstellar space - might be a result of anisotropies during the supernova explosion forming the neutron star Composite X-ray (red/white) and optical (green/blue) image of Black Widow Pulsar - shows elongated cloud, or cocoon, of high-energy particles flowing behind the rapidly rotating pulsar moving at a speed of almost a million kilometers per hour. Bow shock wave due to this motion optically visible - the greenish crescent shape. Pressure behind the bow shock creates a second shock wave that sweeps the cloud of high-energy particles back from the pulsar to form the cocoon. PHYS 3380 - Astronomy The vela Pulsar moving through interstellar space A recent change appears to be connected to the occurrence of a glitch rotation speed, which presumably released a burst of energy that was carried outward at near the speed of light by the pulsar wind. PHYS 3380 - Astronomy Magnetars Neutron stars with magnetic fields ~ 1000 times stronger than normal neutron stars - 21 currently known. Much more massive than regular neutron stars. Earthquake-like ruptures in the surface crust of Magnetars cause bursts of soft gamma-rays. Magnetars fizzle out in less than 100,000 years, rendering them all but undetectable - astronomers suspect that the Milky Way might be littered with dead magnetars. On 27 December, 2004, a burst of gamma rays arrived in our solar system from SGR 1806-20 (artist's conception). The burst was so powerful that it had effects on Earth's atmosphere, at a range of over 50,000 light years. PHYS 3380 - Astronomy Image of thin, glowing dust ring around a magnetar. So thin it's almost two-dimensional, and emits no radiation other than a faint, infrared glow. Probably formed after the magnetic star emitted a giant flare, spotted in 1998, which incinerated surrounding dust in all directions, leaving only the thin disk. The disk glows from the heat emitted by nearby massive stars, which the are probably relatives of the magnetar's forebearer. Researchers say they hope to nail the original mass of SGR 1900+14 by determining the masses of those relatives, and the resulting data could help them work out how heavy a star needs to be to become a magnetar rather than a neutron star. Collision of two magnetars may generate gravity waves - undeteced so for PHYS 3380 - Astronomy Binary Pulsars Some pulsars form binaries with other neutron stars (or black holes). Binary Pulsars 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. Similar to method of extrasolar planet discovery First one discovered in 2004 - only 20km across and have an orbital separation which is less than the size of the Sun. Already, four different effects have been measured consistent with Einstein's general theory of relativity. PHYS 3380 - Astronomy Neutron Stars in Binary Systems: X-ray Binaries Example: Her X-1 2 Msun (F-type) star Neutron star Orbital period = 1.7 days Accretion disk material heats to several million K => X-ray emission Star eclipses neutron star and accretion disk periodically PHYS 3380 - Astronomy 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. PHYS 3380 - Astronomy Black Holes Just like white dwarfs (Chandrasekhar limit: 1.4 M), there is a mass limit for neutron stars (neutron degeneracy): Neutron stars cannot exist with masses > 3 M We know of no mechanism to halt the collapse of a compact object with > 3 M. It will collapse into a single point – a singularity: => A Black Hole! PHYS 3380 - Astronomy Black Holes Black holes are completely collapsed objects - radius of the “star” becomes so small that the escape velocity approaches the speed of light Escape velocity for particle from an object of mass M and radius R 2GM v esc R If photons cannot escape, then vesc>c. Schwarzschild radius is 2GM M R RS 2 3 km c MSol 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 - the “event horizon” PHYS 3380 - Astronomy Size of black holes determined by mass. Example Schwarzschild radius for various masses given by: Object M (M) Rs Star 10 30 km Star 3 9 km Sun 1 3 km Earth 3x10-6 9 mm The event horizon is located at Rs - everything within the event horizon is lost. The event horizon hides the singularity from the outside Universe. If the entire mass of the Earth was confined to 9mm, it would be a black hole - can’t collapse spontaneously into black hole because mass < 3 M PHYS 3380 - Astronomy Black Holes in Supernova Remnants Some supernova remnants with no pulsar / neutron star in the center may contain black holes. PHYS 3380 - Astronomy “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) PHYS 3380 - Astronomy Types of Black Holes Schwarzschild - Non-rotating black hole -simplest black hole, in which the core does not rotate - only has a singularity and an event horizon Kerr - Rotating black hole -probably the most common form in nature, - rotates because the star from which it was formed was rotating. When the rotating star collapses, the core continues to rotate, and this carried over to the black hole (conservation of angular momentum). -Has an Ergosphere - egg-shaped region of distorted space around the event horizon -caused by the spinning of the black hole, which "drags" the space around it.) -Static limit - The boundary between the ergosphere and normal space PHYS 3380 - Astronomy Black Hole Gravity Well At a distance, the gravitational fields of a black hole and a star of the same mass are virtually identical. At small distances, the much deeper gravitational potential will become noticeable. PHYS 3380 - Astronomy 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 - friction would heat the astronaut to millions of degrees emitting x-rays and gamma rays. “Spaghettification” PHYS 3380 - Astronomy General Relativity Effects Near Black Holes Time dilation 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. Event Horizon PHYS 3380 - Astronomy General Relativity Effects Near Black Holes Gravitational Red Shift All wavelengths of emissions from near the event horizon are stretched (red shifted). Frequencies are lowered. Event Horizon PHYS 3380 - Astronomy Remember: General Theory of Relativity predicted gravity could bend space confirmed during a solar eclipse when a star's position was measured before, during and after the eclipse. An object with immense gravity (like a galaxy or black hole) between the Earth and a distant object could bend the light from the distant object into a focus, much like a lens can. Einstein ring -the deformation of the light from a source into a ring through gravitational lensing of the source's light by an object with an extremely large mass (such as another galaxy, or a black hole). PHYS 3380 - Astronomy Lensing by a Black Hole Animated simulation of gravitational lensing caused by a going past a background galaxy. - secondary image of the galaxy can be seen within the black hole Einstein ring on the opposite direction of that of the galaxy. - secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole. - surface brightness of the two images remain constant, but their angular size varies - produces an amplification of the galaxy luminosity as seen from a distant observer. The maximum amplification occurs when the background galaxy (or in the present case a bright part of it) is exactly behind the black hole. PHYS 3380 - Astronomy Brightening of MACHO-96-BL5 happened when a gravitational lens passed between it and the Earth. When Hubble looked at it, it saw two images of the object close together - indicated a gravitational lens effect - intervening object was unseen. - conclusion that a black hole had passed between Earth and the object. PHYS 3380 - Astronomy Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation. - applied quantum field theory in a static black hole background. - result - a black hole should emit particles in a perfect black body spectrum. - Hawking radiation. Virtual particle pairs constantly created near the horizon of the black hole, as they are everywhere - quantum fluctuations. Normally, they are created as a particleantiparticle pair and they quickly annihilate each other. But near the horizon of a black hole, it's possible for one to fall in before the annihilation can happen, in which case the other one escapes as Hawking radiation. - removes energy from black hole - evaporation Temperature of the emitted black body spectrum is proportional to the surface gravity of the black hole. - large black holes are very cold and emit very little radiation - black hole of 10 solar masses would have a Hawking temperature of several nanokelvin, much less than the 2.7K produced by the Cosmic Microwave Background. - micro black holes on the other hand could be quite bright producing high energy gamma rays.