1. “The First Spectroscopically Resolved Sup-pc Orbit of a Supermassive Binary Black Hole” E. Bon et al., accepted(?) to ApJ
Sarah Burke Spolaor

2. Argh!
Confusing and unscientific language makes it difficult to understand what they did, why, and their results.
“It is not that obvious, but one can see a peak…”
“We measured the width of the bump on the profiles”
“the pretty large value of eccentricity suggests…”
Entire sentence (putting it in context does not help): “gravitational wave detection campaigns.”
Few statements on statistical significance
(Minor) conflicting reports of numerical results
Etc.

3. NGC 4151 multi-wavelength overview

4. NGC 4151 multi-wavelength overview
Radio
X-ray
Optical

5. AGN Emission Lines Broad-line region clouds (orbiting BH)
Narrow-line region clouds (further out from central BH torus-accretion disk system)

6. Data span: 11 years regularly, 20 years total
Measured variability in Ha
Measured velocity changes in model components
CBC: “Core broad component” (fixed velocity)
VBC: “Very broad component”
Bump: Rogue component associated with ???
Unlabelled: Narrow emission line template

7. Periodicity: FOUND!
Variability in all components!

8. Possible interpretations of periodicity
Orbital period
 T depends on mass; ~ years
Precession period (from BH spin)
~ 105 yr
Accretion disk precession (from second BH’s orbit)
~ 250ish years?
“Self-warping” from radiation force
~ “consistent”
r16 = period/16 years; Rd = accretion disk radius, size ???; no explanation of other parameters

9. Possible interpretations of periodicity✓
Orbital period
 T depends on mass; ~ years
Precession period (from BH spin)
~ 105 yr
Accretion disk precession (from second BH’s orbit)
~ 250ish years?
“Self-warping” from radiation force
~ “consistent” ✗ ✗ ✓
r16 = period/16 years; Rd = accretion disk radius, size ???; no explanation of other parameters

10. “Actual binary” period
Velocity fit to drifting “bump” and “VBC” components
Solution:
e = 0.42
P = 5776 d = 15.8 y
a1, a2 sin(i) = 0.002, 0.008
m1,m2 sin3(i) = 3e7, 8.5e6 Msun
Discussion: variability comes from supersonic shocks (v>vsound when moving towards observer)
Danger: a1,a2 too small for bound BLR associations (Roche lobe overflow)?
VBC
Radial velocity
Bump
Continuum flux
Model component
fluxes

11. Bogdanovic et al. (2008):
Theoretical BLR particle modelling for Ha lines

12. Other interpretations
Disk warping
(Intrinsically) possible with or without binary
For binary: sinusoidal radial velocity variations not observed
Accretion disk emission
Explanation for Ha “bumps”
Elliptical, precessing disk episodes
Circular (accretion?) disk with spiral arm
“Tests need to be made to prove or disprove these”

13. Useful for a PPTA search?
Tgw ~ 108 years
Pgw ~ 8 years
z = 0.003; 13.8 Mpc away! (note: 88 Mpc, 1 Gpc)
havg < 1.6e-17 (ignoring eccentricity)
Probably not yet, but maybe with new techniques, ideally-positioned pulsars, a little boost from eccentricity, and a few more years of data!

14. VLBI follow-up? Resolved binary
0.008pc = 0.5 mas (way too small to resolve)
Jet modulation
15.8 years  possible periodicities at ~0.3 mas if jet flux is modulated by the binary (e.g. periodic accretion), and jet is travelling at c (also probably too small to detect)

15. Important secondary implications (if true)
BLR structures may remain intact to very small BH orbits.
Possible new method(s) to discover binary systems
Multi-component modelling to discover velocity shifts
Supersonic shock heating: multi-n variability
Specific shapes of hard X-ray spectrum
Seyfert-type switching: broad indication of binary activity?
Are many small-orbit binaries eccentric?

16. Open questions Periodicity seems to be genuine, but….
Reliability of LS periodogram for <= one period/sparse data?
Stability of clear BLR associations in small-orbit binaries?
What does the observed period relate to?
Orbital period: generally consistent.
Other explanations possible; yet to be explored
Are there more systems discoverable by this technique?