Black hole is 'burping out' a 'spaghettified' star it devoured years ago

[Admin]

A black hole is burping out the remnants of a star it ripped apart and ate years ago in a type of tidal disruption event that is like nothing astronomers have ever seen.

Black hole is 'burping out' a 'spaghettified' star it devoured years ago : Read more

 

[Unclear Engineer]

I guess the observation really can't tell us if this "burping out" somehow came from where it is not supposed to be able to come from - inside the event horizon. But, it seems to indicate some physics we don't understand, wherever it came from. It will be interesting to see what is learned from this and follow-on studies.

 

[billslugg]

As I understand the mechanism, it all occurs outside the event horizon. Infalling matter is highly conductive and orbits the black hole in the equatorial plane, outside the event horizon, acting as a dynamo and generating huge currents and magnetic fields. The field is dipolar, similar to the arrangement on Earth. The pressures are immense and the gas would love to squirt out somehow. Unfortunately charged particles cannot cross a magnetic field line easily and much prefer to move parallel to them. This leaves the north and south poles as the only two exits. Narrow beams form and are shot out at very high velocities.

I wonder why it took so long for the jets to appear.

 

[Unclear Engineer]

Bill, Yes I understand that theory about how the jets form. But, it seems that nobody understand how the material getting into the jets can be delayed by 3 years. And, there are also cases of single jets coming off just one pole. I can understand how the remains of a star can be on just one side of the disk and go to one pole. What I don't get is how that can take 3 years.

I am hoping that we can keep making radiotelescopes with wider and wider base lines and get improving details of what goes on around our own galaxy's black hole. It seems to be the best cosmological physics experiment we could have.

 

[billslugg]

I suppose the incoming star impinged high up on one hemisphere and was preferentially directed to that pole. But the delay is mystifying. They say it is moving at 50% the speed of light when most outflows from Tidal Disruption Events leave at 10% the speed of light. The three year delay is the longest they have ever seen.

Here is the paper, not behind a pay wall.
A Mildly Relativistic Outflow Launched Two Years after Disruption in Tidal Disruption Event AT2018hyz - IOPscience

 

[billslugg]

Per the above paper, possibilites for delayed outflow:

1) "One possibility is that the jet was weak or inactive at early times after the disruption and then suddenly became activated at ≈750 days. Such sudden activation could result from a state change in the SMBH accretion disk, such as a thin disk that transitioned to a hot accretion flow." Tchekhovskoy et al. 2014

2)An alternative explanation is that powering a relativistic jet via the Blandford–Znajek process requires a strong magnetic flux threading the black hole horizon. The original magnetic field of the disrupted star in a TDE is not expected to contain a strong enough magnetic field to power a relativistic jet (Giannios & Metzger 2011), which requires an alternative origin. The first possibility is that the magnetic flux could be generated through a dynamo by the accretion disk itself; Liska et al. (2020) found that it may take only ∼10 days to generate poloidal flux from the toroidal field through a dynamo effect once the disk is sufficiently thick, thereby connecting jet production to the disk getting thinner as the accretion rate drops.

3)"Alternatively, Tchekhovskoy et al. (2014) and Kelley et al. (2014) suggest that the required magnetic flux may originate from a preexisting AGN disk, which is "lassoed" in by the infalling fallback debris; because the matter falling back at later and later times in a TDE reaches larger and larger apocenter radii, depending on the radial profile of the magnetic flux in the preexisting AGN disk, this could delay the jet production."

4)Another possibility is that the delayed radio emission is due to the timescales for debris circularization and viscous accretion (Hayasaki & Jonker 2021).

5)It is also possible that instead the jet has been present for the entire duration of the TDE. However, due to the combination of the high density of the large cloud of circularizing TDE debris (e.g., Bonnerot et al. 2022) and the potential for jet precession (e.g., due to misalignment of the disk angular momentum relative to the black hole spin axis; e.g., Stone & Loeb 2012), the jet is initially choked. At later times, as the accretion rate and gas density surrounding the black hole drop, eventually the jet is able to propagate through the debris cloud and escape.

I will need some time to digest the above.