HOW TO DETECT A BLACK HOLE Effects on matter/light outside the horizon –gravitational attraction of other bodies –“dark star” with mass distinguish from normal star, white dwarf, neutron star Accretion (swallowing) of gas –gas heated by compression/turbulence in strong gravity field X-rays –but need a source of gas accretion from interstellar matter insignificant mass transfer in binaries to the rescue

CAN WE IMAGE BLACK HOLES?

HUBBLE: read 1 mile –Optical/UV telescope in space –Falls short by 100,000 VLBA: read newspaper in Philly –Transcontinental radio telescope –Falls short by 1,000 MAXIM: Read newspaper on moon –X-ray interferometer in space –Can do it! Ready for launch (?) …NOT YET, BUT SOMEDAY…?

MAXIM = Micro- arcsecond X-ray Imaging Mission

HOW TO DETECT BLACK HOLES 1. Mass of “compact “ companion in close binary system (stellar remnants only) X-ray binary (artist’s impression)

HOW TO DETECT BLACK HOLES 2. Orbital motion of stars or gas clouds (supermassive holes) M87 disk

HOW TO DETECT BLACK HOLES 3. Random motions of stars in galaxy’s nucleus (supermassive holes) Globular cluster M3 (similar appearance to a galactic nucleus)

Gas almost never falls directly into a black hole Too much “swirl” (angular momentum) …

…makes it more like a whirlpool Gas almost never falls directly into a black hole

ACCRETION DISK Like a flattened whirlpool Gas must give up angular momentum to go down the drain VISCOSITY (~FRICTION)

ACCRETION DISKS ALLOW US to PROBE the HORIZON GRAVITY MOTION HEAT RADIATION (X-rays, UV…) Energy flows from one form to another... matter swirling inward friction

ENERGY FLOW IN ACCRETION DISK GRAVITATIONAL POTENTIAL ENERGY KINETIC ENERGY HEAT RADIATION Energy flows from one form to another... falling matter compression/turbulence particle collisions, etc.

EVOLUTION OF CLOSE BINARIES “Algol Paradox” and its resolution Roche lobe = “sphere” of influence –actually teardrop shaped Matter flows across Lagrange point Too much angular momentum ACCRETION DISK

ALGOLS CAN EVOLVE INTO X-RAY BINARIES Crucial that mass ratio flips –otherwise stars can fly apart Compact star either NS or BH –depends on mass of precursor Two modes of mass transfer –stellar wind: star smaller than Roche lobe –“Roche lobe overflow”: star swells to fill Roche lobe

BINARY MASS FUNCTION depends on... Orbit period: easy Doppler shift of normal star: easy Mass of normal star: hard Orbit inclination: hard Log Mass (solar units)

NEUTRON STAR VS. BLACK HOLE: …how to tell BH if: –mass (reliable) –distinctive spectrum (unreliable ????) NS if: –pulsing (X-ray pulsar) –evidence of nuclear explosions on surface (X-ray burster)

X-ray pulsar (accretion) X-ray burster (thermonuclear)