Black Hole Event Horizon A Black Hole has a truly unique property: it possesses an Event Horizon A “virtual” surface One-way membrane --- matter/energy can fall in, but nothing gets out, not even light Event Horizon Singularity

A Black Hole is Amazingly Simple Mass: M Spin: a* (J=a*GM2/c) Charge: Q Even though it is a macroscopic object, a BH is as simple as an elementary particle!

Can We Test These Features? Confirm that astrophysical BHs actually do have Event Horizons Measure the two fundamental parameters of BHs Mass (relatively easy) Spin (much harder) Ramesh Narayan, Jeff McClintock

Do Black Hole Candidates Have Event Horizons? Are Black Hole Candidates really BHs? Can we find independent evidence that our BH candidates actually possess Event Horizons

X-ray Binaries Image credit: Robert Hynes (2002)

Narayan, Garcia & McClintock (1997) The Idea Find BH X-ray Binaries in which the accretion flow radiates very little Objects with surfaces (NSs) will then be much brighter than objects with event horizons (BHs) Look for a huge difference in luminosity Narayan, Garcia & McClintock (1997)

Large difference in luminosity between BHs and NSs is dramatically verified (Narayan, Garcia & McClintock; Garcia et al. 2001; McClintock et al. 2003; …) Black Holes are really black!!  Event Horizon

Black Hole Spin The material from which a BH is formed almost always has angular momentum Also, accretion adds angular momentum So we expect astrophysical BHs to be spinning: J = a*GM2/c, 0  a*  1 Can we measure a*?

The Basic Idea Measure radius of hole by estimating area of the bright inner disk (Zhang et al. 1997; Shafee et al. 2006; McClintock et al. 2006)

Black Hole Parameters X-ray Binary Mass (M) a* 4U1543-47 7.4—11.4 0.75—0.85 LMC X-3 5.9—9.2 < 0.3 GRO J1655-40 6.0—6.6 0.65—0.75 GRS 1915+105 10--18 0.98—1