Gamma-Ray Bursts

1. Gamma-Ray Bursts • Discovered by DoD satellites searching for signs of atmospheric nuclear testing • irregular bursts of gamma-ray radiation lasting from milliseconds to seconds • originally thought to be more energetic versions of X-ray bursts • now thought to be due to colliding neutron stars or black holes • Compton Gamma-Ray Observer (CGRO) satellite is the prime telescope for observing bursts • BATSE instrument on CGRO has measured the positions of thousands of bursts since its launch • about one per day detected • no apparent concentration about the position of the galactic plane • no clustering • no repeating (maybe) • smoothness of the sky distribution of bursts (isotropy) hints that most bursts must be at very far distances (> 1 Gpc) • to appear as bright as they do at these distances requires the source to be extremely bright • 1053 ergs needed, making bursts the most powerful explosions in the Universe

2. Gamma-Ray Bursts (cont.) • millisecond time variations imply the maximum size of the emitting regions • d = speed of light X time = 3 x 1010 X 0.001 = 3 x 107 cm or 300 km !!!! • Leading explanation is the merger of two neutron stars or black holes in a binary system • gravitational radiation is the key • any time a mass is accelerated, a gravitational wave is emitted • can carry away angular momentum and energy • can cause orbits to decay • observed in a neutron star binary system • can be observed with special observatories • use interference of light from lasers to detect passage of gravitational waves • LIGO (Laser Interferometric Gravitational -Wave Observatory) is being built in two states (WA and LA) and may or may not (probabkly the latter) see anything (but is costing you the taxpayer 250 million dollars)

3. Millisecond Pulsars • Some pulsars have periods in the millisecond range • imagine an object containing 1.5 times the mass of the Sun about 10 miles across spinning around 1000 times per second ! • it approaches the maximum speed where centripetal force can equal gravity for a neutron star • Some are found in our Galaxy, but a surprising number are found in globular clusters • neutron stars are thought to slow down after birth in Type II supernovae • objects should have been made to spin faster through the accretion of matter • requires about 100 million years for the process to work • dense stellar environment of globular clusters increases the likelihood of a chance encounter with another star

4. Black Holes • Like white dwarfs, neutron stars also have a maximum limit to their mass based on degeneracy pressure • can only squeeze two neutrons together to within a certain distance • white dwarf limit known is the Chandrasekhar limit = 1.4 Msol • neutron star limit ~ 3 Msol • if no other force is present to support matter against gravity, an unstoppable collapse forming a black hole occurs • collapse causes gravity to increase via the inverse square dependency of gravity • gravity is so strong that the everyday theory of gravity (Newtonian) doesn’t apply....Einsteins General Theory of Relativity must be used • Since a black hole is defined by some mass collapsing into an infinitesimally small volumne, we use the event horizon to describe the “size” of a black hole • balance the energy of a particle in motion at the speed of light with gravity at some radius

5. Black Holes (cont.) • this is the limit at which light cannot escape the gravity of a black hole • it is also known as the Schwarzschild radius • some values Earth 1 cm Jupiter 3 m Sun 3 km 3 Msol 9 km 106 Msol 3 x 106km • this is the region beyond which no information (via light) can ever be known • General Relativity describes how material behaves in and around black holes • the most important concept is that mass warps space in its vicinity • orbits of planets around stars are really like billiard balls moving along the surface of a cardboard tube • around black holes, space is extremely deformed so that it folds over onto itself at the event horizon • light also must follow similar space trajectories in warped space • detected in an experiment by Einstein where the position of a star moved as it passed close to the limb of the Sun • lengthening of space near a black hole causes a gravitational redshift since the space over which the original wavelength was emitted is being stretched out

6. Black Holes (cont.) • stretching of space causes strong tidal forces on matter falling into black holes • caused by the difference in gravity across an object • nothing can possibly survive intact through the event horizon • heating caused by the tidal disruption should be visible as UV and X-rays • this is one way of finding black holes • gravity also affects time • since information is carried from point A to point B by light, an observer watching a clock fall into a black hole would see it slow down as it falls deeper into the black hole • eventually the clock would appear to be frozen in space and time • actually it would have passed through the event horizon • if someone were with the clock, he would notice no change in his/her clock, but see the same effect on the clock on the spaceship • this is called time dilation

7. Black Holes (cont.) • One can observe the presence of black holes in a number of ways • look for the influece of strong gravity on background stars and galaxies • very similar to Einstein experiment with the Sun • light “bends” around black hole or other gravitating bodies • can lead to a brightening of a star • can lead to a gravitational arc of a background galaxy • used by the MACHO project to find the nature of dark matter • black holes in binary systems • usually hidden by the brightness of the companion star • use the evolutionary status of the companion star and the orbital properties to infer a mass for the black hole • Cygnus X-1 • discovered from the emission of X-ray bursts • companion star is a ZAMS B star • orbital period of 5.6 days and other stellar properties of the B star put a limit of 35 Msol on the total mass for the system, leaving a limit of 10 Msol for the mass of the black hole • A0620-00 • same type of analysis as Cygnus X-1 • much better data • a range of 3.8 - 6 Msol for the mass of the black hole

8. Black Holes (cont.) • use light variability from burst emissions • apply the speed of light X time argument like before with neutron stars • Cygnus X-1 has millisecond oscillations => a 300 km size to the emitting region • use the presence of an accretion disk or rotating objects • rare to find one • take Doppler shift measurements froom different positions along the disk to find the rotation speed • apply centripetal force - gravitational force balance to find the mass contained within the disk • M87 • Hubble observations show large rotating disk at the center of the galaxy • disk rotation infers a mass of about 2 x 109 Msol contained within a region a few pc across • our Galaxy • motions of stars and gas around the center of our Galaxy called Sgr A* • a mass of a few million Msol contained within the central 1 pc of the Galaxy