6 November 2001Astronomy 102, Fall Image: the NASA Compton Gamma-Ray Observatory (GRO), shortly after deployment in 1991 by the crew of the space shuttle Atlantis (NASA/Marshall Space Flight Center). Today in Astronomy 102: black hole observations, v.4  Active galaxies (conclusion): accretion disks around the black holes in active galactic nuclei, their formation and workings.  Gamma-ray bursters: a longtime mystery, and the greatest of all natural disasters, now seen as black-hole formation.

6 November 2001Astronomy 102, Fall 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 others’ 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 one going to a larger orbit.

6 November 2001Astronomy 102, Fall Matter falling into AGN black holes: large accretion disks (continued) 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.

6 November 2001Astronomy 102, Fall How accretion disks form: tidal disruption of stars Star View from high above, along orbit’s axis. Black hole Simulation by P. Quinn and G. Sussman, 1985

6 November 2001Astronomy 102, Fall How accretion disks form (continued) Perspective view Black hole horizon Rotation of disk Accretion disk 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.

6 November 2001Astronomy 102, Fall Operation of accretion disks  Recall that for non-spinning black holes, orbits with circumference less than 3 C S are unstable, and no orbits exist with circumference less than 1.5 C S. 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.

6 November 2001Astronomy 102, Fall Operation of accretion disks (continued)  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 disk material and lose their energy to heat. If their velocity takes them perpendicular to the disk, they may escape (Blandford and 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 c) explain the one-sideness and “faster than light” motion of quasar jets. Several other possibilities exist for jet acceleration; see Thorne’s figure 9.7 (pg. 349).

6 November 2001Astronomy 102, Fall Structure of an accretion disk Jet Accretion disk (cross- section view) Ingoing: matter, being accreted Outgoing: X and  rays, heating disk and accelerating jets Not drawn to scale! Innermost stable orbit Horizon

6 November 2001Astronomy 102, Fall ,000 light years Direct observation of an accretion disk in an AGN: radio galaxy NGC light years Left: radio (red) and visible (white) view with ground-based telescopes. Right: visible light image by the Hubble Space Telescope (NASA/STScI).

6 November 2001Astronomy 102, Fall Direct observation of an accretion disk in an AGN: Seyfert galaxy NGC 4258 The same things happen around the central black holes in Seyfert galaxies as happen in radio galaxies and quasars. But Seyfert galaxies are spirals, and have a lot more interstellar gas and dust than the elliptical hosts of RGs and quasars, which can stop the jets. In the center of NGC 4258, Moran et al. (1995) detected a molecular disk about 1 ly in diameter, rotating at 1000 km/sec near the outer edge. This implies a black hole mass of 3.6  10 7 M . (Radio and visible-light images: NRAO)

6 November 2001Astronomy 102, Fall Seyfert galaxy NGC 4258 (continued) A jet is seen emerging perpendicular to the disk, but is entrained by interstellar material in NGC 4258 and appears as an extra set of spiral features. (Artwork and images by Inoue, Kagaya, Greenhill and De Pree.)

6 November 2001Astronomy 102, Fall Mid-lecture Break. Exam #2 is on Thusday, 2-3:15 PM; Homework #5 is due just after midnight tonight.  Remember, there’s a practice exam, with solutions, on the AST 102 Web site.  There’s even going to be a review session: 6 PM, tomorrow evening, right here in Hubbell Auditorium. The Chandra X-ray Observatory, launched in 1999 (CfA/NASA).

6 November 2001Astronomy 102, Fall Gamma-ray bursters In the mid 1960s, after ratification of the Nuclear Test-Ban Treaty, the US and USSR each put up satellites with X- and  - ray detectors to monitor the other’s compliance with the treaty.  Immediately these instruments detected many brief, bright bursts of  rays, similar to the expectations for above-ground nuclear detonations. Naturally, this worried all concerned, even though the bursts were not correlated with seismic events.  The satellites could not determine very well the direction from which the  rays came, so it took a while to determine that they actually came from outer space rather than Earth. (Even then, the data remained top secret.)

6 November 2001Astronomy 102, Fall Typical gamma-ray burst (CGRO/NASA)

6 November 2001Astronomy 102, Fall Gamma-ray bursters (continued) Soon it became possible to measure the directions of the  rays well enough to show that the bursts came from locations spread randomly and uniformly all over the sky. This is very different from non-burst  ray sources.  Bright sources of  rays: neutron stars or black holes?  The nearest stars also appear to be randomly and uniformly spread all over the sky. Are the  bursters just remnants of nearby dead stars?

6 November 2001Astronomy 102, Fall Gamma-ray bursters (continued) Still, it was not possible to measure the position of any of the  -ray bursters precisely enough to observe them at any other wavelength.  One can’t really make  ray telescopes with which this could be done.  rays do not reflect or refract significantly.  The original determinations of  -ray burster locations on the sky were made by triangulation among several satellite detectors, using the different arrival times of the burst at each satellite.  Typical bursts only last about 30 or 40 seconds, and it takes hours to notify ground-based observers that a burst has occurred. So years passed without any explanation of their nature.

6 November 2001Astronomy 102, Fall The BATSE experiment on the Compton Gamma- Ray observatory In 1991, the NASA Compton Gamma-Ray Observatory was deployed. It included the Burst and Transient Source Experiment (BATSE), designed to detect more and fainter  ray bursts, and measure their locations more precisely, than was possible hitherto.  The expectation was that the distribution of fainter  ray bursters would look more like the Milky Way. Just like stars: the brighter nearby ones are evenly distributed in the sky, but the more distant, fainter ones comprise the Milky Way.  The expectation was not borne out, though -- the  ray bursters still looked uniform on the sky, even at faint levels.

6 November 2001Astronomy 102, Fall A BATSE gamma-ray burster sky map. This is a map of the whole sky, displayed so that the Milky Way lies along the equator. The positions of 1776 BATSE detections are plotted. Note that there is no tendency for the  -ray bursters to cluster in the Milky Way. Image: Michael Briggs and the BATSE team, NASA MSFC.

6 November 2001Astronomy 102, Fall Analysis of the BATSE results Obviously  -ray bursters are not numerously distributed throughout our galaxy, as 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 bursters are that far away, their luminosities are (problematically) enormous.

6 November 2001Astronomy 102, Fall The BeppoSAX satellite In 1996 the Italian and Dutch space agencies launched BeppoSAX, a satellite observatory designed (in part) to detect X rays from some  ray bursters.  X-ray telescopes can be made, though with difficulty. (As you know, X-rays are good at passing unhindered through matter, too, so they’re hard to reflect or refract.)  The hope was that for each  ray burst they could find a corresponding, bursting or fading, X-ray source and measure its position.  It worked. About 1 out of every 20  ray bursters found by BATSE is also detected and localized by BeppoSAX, and the position is made available to observers on the ground within hours.

6 November 2001Astronomy 102, Fall Results of visible-light observations of BeppoSAX positions:  ray bursters live in distant galaxies Image of the  ray burst of 28 February 1997, taken with the STIS instrument on the Hubble Space Telescope on 5 September 1997 (Andy Fruchter, STScI/NASA).

6 November 2001Astronomy 102, Fall  ray bursters live in distant galaxies (continued) Image of the  ray burst of 8 May 1997, taken with the STIS instrument on the Hubble Space Telescope in early June (upper) and early August (lower) 1998, by Andy Fruchter, STScI/NASA). Note that as the bright star-like object fades, a faint, fuzzy galaxy remains.

6 November 2001Astronomy 102, Fall  ray bursters live in distant galaxies (continued) Recent image of the  ray burst of 14 December 1997, taken with the STIS instrument on the Hubble Space Telescope (Steve Odewahn, Caltech). The spectrum of this galaxy indicates that its distance is 12 billion light years. At that distance, the  ray burst amounted to an energy of 3x10 53 erg in  rays alone, if it emitted its energy in all directions.

6 November 2001Astronomy 102, Fall So what are  ray bursters? 3x10 53 erg is an awful lot of energy to emit in 40 seconds.  100 supernovae account for this much energy, but take months and do so at longer wavelengths.  The entire Milky Way emits this much light in 250 years, but does so with much longer wavelength light. Note that mc 2 for a 2 M  object (the heaviest neutron star) is Accretion of a neutron star by a black hole at 10% efficiency of conversion of rest energy to light would do it. Formation of a black hole much more massive than this -- say, by merger of two neutron stars – would also do.

6 November 2001Astronomy 102, Fall Some of the possibilities for  ray bursters  Binary neutron stars, coalescing to form a black hole? Involves formation of a hot accretion disk, and burst composed of  rays and relativistic, massive particles (cosmic rays).  Neutron star-black hole binary, with the neutron star captured by the black hole? Also seems to work, theoretically.  Hypernova : collapse of a M  star to form a black hole, accompanied by a supernova-like explosion? Currently favored model, despite problems getting the collapsing star to do the right sort of “bounce.” Maybe all three mechanisms are represented among the bursters. All involve black hole formation or growth.

6 November 2001Astronomy 102, Fall  ray bursters: the greatest of all natural disasters A  ray burst like that on 14 December 1997 would destroy all life within several thousand light years of the burster. At 3000 ly:   rays would ionize Earth’s atmosphere; the gas would recombine to form nitric oxides, which in turn would eliminate the ozone layer.  If the  rays are followed by a month-long blast of cosmic rays (as models predict), everything within 200 m of the surface would receive a lethal dose of radiation. Sky and Telescope, February 1998