AGN, GAMMA RAYS, & BLACK HOLE EVAPORATION
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
4/6/20 AGN, GAMMA RAYS, & BLACK HOLE EVAPORATION HOMEWORK #7 AVAILABLE ON WEBWORK – DUE WEDNESDAY, 4/15, AT 11:59PM EDT Astronomy 102 | Spring 2020 April 7, 2020 3 3 AGN, GAMMA RAYS, & BLACK HOLE EVAPORATION Accretion disks in AGN Gamma-ray bursts & black hole formation Hawking’s “area increase” theorem Entropy and the area of the event horizon: the thermodynamics of black holes Quantum-mechanical vacuum fluctuations and the emission of light by black hole: “Hawking radiation” Evaporation of black holes Exotic matter Virtual particles near a black hole’s horizon (S.W. Hawking) Astronomy 102 | Spring 2020 April 7, 2020 4 4 1
4/6/20 TYPES OF AGN Radio galaxy Seyfert galaxy • Elliptical galaxy with active nucleus at • Spiral galaxy with active nucleus at center center • Zero, one, or two jets • Two jets, visible in the radio Quasar Blazar • Galaxy with active nucleus at center • Galaxy with active nucleus at center • One jet, visible in the radio (line of sight is • Extremely bright, no jets (line of sight is close to jet axis) along the jet axis) Astronomy 102 | Spring 2020 April 7, 2020 5 5 A B C D FIND THE ACTIVE GALAXY Which of these is a visible-light picture of a radio galaxy? Astronomy 102 | Spring 2020 April 7, 2020 6 6 2
4/6/20 A B C D FIND THE ACTIVE GALAXY Which of these is a radio image of a radio galaxy? Astronomy 102 | Spring 2020 April 7, 2020 7 7 A B C D FIND THE ACTIVE GALAXY Which of these is a visible-light picture of a Seyfert galaxy? Astronomy 102 | Spring 2020 April 7, 2020 8 8 3
4/6/20 MATTER FALLING INTO AGN BLACK HOLES: LARGE ACCRETION DISKS The disk-shaped collection of matter surrounding the black hole in an AGN arises rather naturally from the influence of the black hole on stars and other material in the galactic center. • Stars in a galaxy perpetually interact with each other’s gravity as well as the gravity of the galaxy at large. • These interactions – long-range collisions – usually result in transfers of energy and momentum between stars. Two stars, originally in similar orbits and undergoing such a collision, will usually find themselves pushed to different orbits: one going to a smaller- circumference orbit and the other going to a larger orbit. Astronomy 102 | Spring 2020 April 7, 2020 9 9 MATTER FALLING INTO AGN BLACK HOLES: LARGE ACCRETION DISKS Thus, some stars are pushed to the very center of the galaxy after a number of these encounters. What happens if there is a black hole there? • The star begins to fall in, but the spin of its orbital motion and the tidal forces that tend to rip the star apart keep this from happening all at once. • Stellar material spreads out into a rotating, flat distribution around the black hole: the beginnings of an accretion disk. Astronomy 102 | Spring 2020 April 7, 2020 10 10 4
4/6/20 AGN ACCRETION DISK FORMATION Eventually, the tidally-disrupted material from many stellar encounters settles down into a flattened disk. Collisions among particles in the disk cause material to lose its spin and become accreted by the black hole. The disk is thus gradually consumed. Black hole horizon Perspective view Accretion disk Can contain − ⊙ and extend hundreds of ly from the black hole Rotation of disk Astronomy 102 | Spring 2020 April 7, 2020 11 11 OPERATION OF AGN ACCRETION DISKS • Recall that for non-spinning black holes, orbits with circumference less than 3 $ are unstable, and no orbits exist with circumference less than 1.5 $ . Within this volume, the disk structure breaks down and material tends to stream in toward the horizon. • A large amount of power, mostly in the form of X-rays and -rays, is emitted by the infalling material. Pressure exerted by this light slows down the rate at which accretion takes place. • Much of this high-energy light is absorbed by the disk which heats up and re-radiates the energy as longer wavelength light. • Heated disk = compact central object seen in radio images of radio galaxies and quasars. Astronomy 102 | Spring 2020 April 7, 2020 12 12 5
4/6/20 OPERATION OF AGN ACCRETION DISKS • Some of the particles absorbing the highest-energy light are accelerated to speeds approaching that of light. If their velocity takes them into the disk, they just collide with the disk material and lose their energy to heat. If their velocity takes them perpendicular to the disk, they may escape (Blandford & Rees 1975). • High-speed particles escaping perpendicular to the disk = jets seen in radio and visible images of radio galaxies and quasars. Their high speeds (approaching ) explain the one-sidedness and “faster than light” motion of quasar jets. • Several other possibilities exist for jet acceleration. Astronomy 102 | Spring 2020 April 7, 2020 13 13 STRUCTURE OF AN AGN ACCRETION DISK Jet Innermost stable orbit Ingoing: matter, Outgoing: X and g rays, heating disk and accelerating being accreted jets Accretion disk (cross-section view) Astronomy 102 | Spring 2020 April 7, 2020 14 14 6
4/6/20 DISK AT THE CENTER OF RADIO GALAXY NGC 4261 Jaffe et al. (1996) Astronomy 102 | Spring 2020 April 7, 2020 15 15 GAMMA-RAY BURSTS • The -ray bursts (below) come from locations spread randomly and uniformly all over the sky. This is very different from non-burst - ray sources (above). • Bright sources of -rays: neutron stars or black holes? • There is no tendency for the -ray bursts to cluster in the Milky Way, which would be the case if their sources were stellar remnants. Astronomy 102 | Spring 2020 April 7, 2020 17 17 7
4/6/20 ANALYSIS OF THE GAMMA-RAY BURST DISTRIBUTION Obviously, -ray bursts are not numerously distributed throughout our galaxy as the stars are. What other explanations are there? • Very nearby objects that are evenly distributed on the sky, like the very nearest stars, or the cloud of comets surrounding the Solar system. • But how would these objects emit -rays? • Very distant objects. Distant galaxies and galaxy clusters are evenly distributed on the sky. • But if the -ray bursts are that far away, their energy outputs are (problematically) enormous. Astronomy 102 | Spring 2020 April 7, 2020 18 18 -RAY BURSTS ORIGINATE IN DISTANT GALAXIES Green crosses indicate where the bursts were seen; all lie within distant galaxies (Andy Fruchter, STScI) Astronomy 102 | Spring 2020 April 7, 2020 19 19 8
4/6/20 TWO TYPES OF -RAY BURSTS The -ray bursts come in two major varieties: long (> 2 s, typically 30 s) and short (typically 0.01-0.1 s). • The short bursts are also less luminous and emit a larger fraction of their - rays at the highest energies observed than the long bursts. Histogram of BATSE -ray bursts Astronomy 102 | Spring 2020 April 7, 2020 20 20 LONG BURSTS OCCUR IN STAR-FORMATION REGIONS Their afterglows resemble supernovae. Images of the -ray burst of 23 January 1999, taken with the STIS instrument on the HST 16, 59, and 380 days after the outburst (Andy Fruchter, STScI). It faded at the same pace supernovae do. Astronomy 102 | Spring 2020 April 7, 2020 21 21 9
4/6/20 SHORT BURSTS DO NOT The NASA Swift satellite, launched in 2004, was built with visible-UV (UVOT) and X-ray (XRT) telescopes along with a -ray detector (BAT) that can sort more finely by burst duration. • Swift has found and localized many more GRBs than was previously possible, including examples at the greatest distances at which galaxies have been detected. • Swift also localized short bursts for the first time and showed that they do not resemble supernovae. Visible afterglow of the short -ray burst of 7/9/2005, pictured by HST 6, 10, 19, and 35 days after Swift discovered it. It fades too fast to be a supernova. (Derek Fox, Penn State U.) Astronomy 102 | Spring 2020 April 7, 2020 22 22 EXTRAGALACTIC ORIGIN: -RAY BURSTS ARE EXTREMELY ENERGETIC The spectrum of the galaxy in which the -ray burst of January 23, 1999 lives indicates that its distance is 39 billion light years. • At that distance, the -ray burst amounted to an energy of × erg in -rays alone, if it emitted its energy uniformly in all directions. • For scale: that is equivalent to a mass of 3×10() erg = = 3.3×10++ g = 1.7 ⊙ * suddenly (in a span of about 40 seconds) converted completely into -rays. Astronomy 102 | Spring 2020 April 7, 2020 23 23 10
4/6/20 -RAY ENERGY IS PROBABLY LESS THAN THAT… • The most efficient way to produce lots of -rays quickly is in shock waves within exploding relativistic jets. • Assuming, therefore, an explosion violent enough to be relativistic, and noting that relativistic objects tend to emit light mostly in the direction they are going… • As is the case with quasar / radio galaxy radio jets. • …the same brightness could be produced with a smaller energy: smaller by a factor of about ,! 1− , where is the outward speed of the explosion. -! • That reduces the energy requirement to about erg. Still a huge amount of energy, considering the short time and the fact that it is all -rays. Astronomy 102 | Spring 2020 April 7, 2020 24 24 WHAT ARE THE SOURCES OF -RAY BURSTS? 10(* erg is an awful lot of energy to release in less than one minute, and -rays are an extreme form for the energy to assume. • 10 ordinary supernovae account for this much emitted light, but it would take months and would do so at much longer wavelengths. • The entire Milky Way emits this much light in about 10 years, but it also does so with much longer wavelength light. So, it is difficult to imagine how to do this with normal stellar processes. But, now you know about more powerful tools. How about: • Accretion of a very massive compact object by a black hole (compact, so the accretion does not take very long) and radiation of much of its mass energy? • Formation of a rapidly-spinning black hole, and driving a relativistic expansion with its ergosphere? (Requires strong magnetic fields threading the ergosphere.) Astronomy 102 | Spring 2020 April 7, 2020 25 25 11
4/6/20 LEADING CONTENDERS FOR THE SOURCES OF -RAY BURSTS • Binary neutron stars, coalescing to form a black hole? Observationally confirmed as a source of short bursts. • Neutron star – black hole binary, with the neutron star captured by the black hole? • Hypernova (a.k.a. Collapsar): collapse of a very massive (50 − 120 ⊙ ) star to form a black hole, accompanied by a supernova-like explosion? • Currently-favored model for long bursts • Works even better with collapse that follows soon after a binary-star merger Maybe all three mechanisms are represented among the sources. All involve black hole formation or growth. All three naturally produce rapidly spinning black holes. All should be rare processes. Astronomy 102 | Spring 2020 April 7, 2020 26 26 GW170817 – A NS-NS MERGER • On August 17, 2017, LIGO and VIRGO detected a gravitational wave signal, the intensity and frequency of which matched the theoretical values for a NS-NS merger. • 1.7 seconds after the GW reached us, Fermi and INTEGRAL detected a short GRB coming from the same region in the sky. • Follow-up observations were made with numerous telescopes over the following hours, days, and weeks across the EM spectrum. These observations were consistent with the debris ejected from a NS-NS merger (a fast-moving, rapidly- Artist’s impression of a NS-NS merger (Dana Berry, NASA) cooling cloud of neutron-rich material). Astronomy 102 | Spring 2020 April 7, 2020 27 27 12
4/6/20 “Pair production” 150 ⊙ supernova (no BH produced) FATES OF THE MOST “Quiet” BH 120 ⊙ MASSIVE STARS formation CXO/CfA/NASA 100 ⊙ Hypernova/ GRB/SN II 50 ⊙ 20 ⊙ Ordinary SN II 10 ⊙ Astronomy 102 | Spring 2020 April 7, 2020 28 28 BLACK HOLE FORMATION? SN 2006gy, in NGC 1260 (238 Mly away) was the most luminous supernova every observed (100 times more luminous than the naked- eye-visible SN 1987a, a Type II SN). Swift detected no -ray burst, but a relatively weak X-ray emission is seen in the afterglow. Is SN 2006gy most likely to be A. An ordinary (I or II) supernova B. A NS-NS or NS-BH merger C. BH formation from stellar collapse (plus hypernova) D. An extraordinary supernova, but not linked to BH formation Astronomy 102 | Spring 2020 April 7, 2020 29 29 13
4/6/20 BLACK HOLE FORMATION? On March 19, 2008, a simultaneous burst was seen by Swift at , X, and UV wavelengths, and by the Pi of the Sky camera at visible wavelengths. The brightest part of the burst – bright enough to have been seen with the naked eye – was about 30 seconds long. Which of the following events is this most likely to be? A. An ordinary (I or II) supernova B. A NS-NS or NS-BH merger C. BH formation from stellar collapse (plus hypernova) D. An extraordinary supernova, but not linked to BH formation Astronomy 102 | Spring 2020 April 7, 2020 30 30 THE HORIZON AREA THEOREM In 1970, Stephen Hawking used general relativity to prove a useful rule called the horizon area theorem: The total horizon area in an enclosed system containing black holes never decreases. It can only increase or stay the same. Increases in total horizon area come from growth of black holes by collapse or accretion of “normal” matter, and by the coalescence of black holes. Astronomy 102 | Spring 2020 April 7, 2020 31 31 14
You can also read
