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Comment by pavel_lishin

7 hours ago

> The existence of two black holes in OJ287 was first suggested in 1982. Aimo Sillanpää, then a graduate student at the University of Turku, observed that the brightness of the quasar changed regularly over a 12-year cycle.

Damn, that's about the time it takes Jupiter to orbit the sun. That feels wildly close together for objects that mass 18 billion & 150 million times that of our own sun.

These black holes (according to a calculator I found online) have radii of 53 billion km and 400 million km, so I'm guessing they must be orbiting significantly further away, and significantly faster than Jupiter (which is ~800 million km away from the sun) - which makes sense, given the monstrous 18b figure. I wonder how far apart they are, but I don't really know how to easily calculate that right now.

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pavel_lishin

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hnuser123456  7 hours ago 

  Feature                  Primary Black Hole              Secondary Black Hole
  -----------------------  ------------------------------  ------------------------------
  Mass                     1.8 × 10^10 M                   1.5 × 10^8 M
  Schwarzschild Radius     356 AU                          3.0 AU
 
  --- Circular / Average Orbital Properties ---
  Orbital Period           12 years
  Semi-Major Axis          13,800 AU (~0.22 ly)
  Orbital Speed (avg)      282 km/s (0.094% c)             33,900 km/s (11.3% c)

  --- Elliptical Orbit (e ≈ 0.65) ---
  Pericenter Distance      4,830 AU                        (same)
  Orbital Speed (peri)     613 km/s (0.20% c)              73,600 km/s (24.5% c)

  Apocenter Distance       22,800 AU                       (same)
  Orbital Speed (apo)      130 km/s (0.043% c)             15,600 km/s (5.2% c)

So the "smaller" SMBH is punching through the larger one's disk at a significant fraction of c twice every 12 years. But it's losing energy to gravitational waves so quickly that they'll probably merge in around 10,000 years [1]

[1] https://archive.is/Ccy5M

kmm  7 hours ago

In Newtonian gravity, the relation between the orbital period T and the semimajor axis a of the orbital ellipse is a^3 / T^2 = GM / 4π^2, where M is the reduced mass of the system (in this case, with 99% of the mass being in one of the two black holes, it's simply the mass of the heavier one).

Plugging 12 years and 18e9 solar masses gives about 2e12 kilometers, or roughly a fifth of a lightyear. This also means the smaller black hole is zipping around the bigger one at around 6% of the speed of light, which is low enough that the Newtonian approximation is probably reasonable accurate (at least to give a rough idea of how large the distances must be).

ardel95  7 hours ago

Kepler’s laws should still provide a pretty good estimate, at least until black holes get much closer. I did a quick back of the envelope calculation, and looks like they’ll be roughly 14k astronomical units, or 0.22 light years apart.

hinkley  7 hours ago

How much time dilation do you get at those masses though?

I’m having more trouble visualizing how accretion disks would work for a binary black hole. Because the light is coming from the disks, not the black holes. So those are what are actually pulsing/girating.

  • ardel95  7 hours ago

    Unless I screwed up the math, they would be quarter of a light year apart. Plenty of space for each black hole to form its own accretion disk.

    • hinkley  2 hours ago

      Oh that chart is really awful then. It’s showing an accretion disk that’s half a light year in diameter at least.

  • pavel_lishin  7 hours ago

    Yeah, good point on that, too. I bet someone's written a simulator that I could run locally, but I've got a busy day ahead of me :(

    I thought that in this case, the light that they detected was coming from the jets coming from the poles, not the disk itself directly.