1. 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

2. 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!

3. 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

4. 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

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

6. 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)

7. 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

8. 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*?

9. 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)

10. 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 J
6.0—6.6
0.65—0.75
GRS
10--18
0.98—1