UR: Towards Solving the Puzzle of Stable, Thin Disks at High Luminosities

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Jessica Anderson

College of Charleston

This guest post was written by Jessica Anderson. Jessica Anderson is in her second year at the College of Charleston majoring in Astrophysics and minoring in Mathematics after transferring from Wake Forest University. She presented this research at the 237th virtual meeting of the American Astronomical Society with her research professor Dr. P. Chris Fragile and postdoc Bhupendra Mishra.

For years, there has been a discrepancy between theory and observation in the properties of low mass X-ray binary systems (LMXRB) in the soft X-ray state. LMXRBs are systems in which a star of the same mass or less than that of the sun accretes onto a black hole, forming an accretion disk. We see these systems in the soft state, because the soft X-ray emission originates from the accretion disk itself. It would be seen in the hard states typically if there are any outbursts of the system. There is a model to explain the disk geometry and evolution of LMXRB systems called the Shakura-Sunyaev thin disk model, which in our case surrounds a 6.62 solar-mass Schwarzschild black hole. In thin Shakura-Sunyaev black hole accretion disks, there is expected to be a radiation-pressure dominated, and consequently thermally unstable, inner region. Heating and cooling both depend on midplane temperature, however, they depend on the midplane temperature at different exponential rates. This is where the instability arises from, and leads to a thermal runaway, and the expansion or collapse of the disk. When the disk is unstable, as it is proposed to be, it would be seen in the hard-X-ray state. However, observers see the exact opposite — a very stable disk with low rms variability, corresponding to the soft-X-ray state.

Thermal stability can be restored if radiation pressure is replaced by some other supporting mechanism. Strong magnetic fields have been proposed as the key missing supporting mechanism. To test this idea, we simulate three different initially weak magnetic field configurations: 1) a zero-net-flux, multiple-loop configuration; 2) a net-flux, vertical-field configuration; 3) and a zero-net-flux, radial-field configuration. All three cases were chosen from previous research in the field, however, our research group has only tested the multiple-loop configuration previously.

The multiple-loop case is used as a control as it has been demonstrated to be unstable. The second case does not show evidence of thermal stability. Instead, we see a hot puffy inner region connected to a thin cooler outer region, which looks like the truncated disk geometry, often associated with the luminous hard state. If confirmed, this would be the first time a truncated disk has spontaneously arisen from a thin disk simulation! Not much is known about LMXRB systems in the hard-X-ray state, so understanding if this truncated shape is the true disk geometry, and the vertical magnetic field supports it is a big step in the field. Even more exciting, the third configuration that we used appears to be thermally stable! Heating and cooling remain about equal throughout the simulation: the magnetic pressure dominates radiation pressure, as expected, and the height and temperatures remain relatively constant. We still need to run the simulations further to verify these results, but we are excited to possibly show the first global simulation of a stable thin Shakura-Sunyaev disk, and maybe even the first truncated disk!

Illustration of proposed disk shape evolution and how this affects the X-ray luminosity and X-ray hardness
Figure 1. An illustration of the proposed evolution of a disk from the truncated shape on the hard-X-ray spectrum to a thin disk on the soft-X-ray spectrum including the vertical field configuration (Begelman & Armitage 2014).
Animation 1. Animation of the net-flux, vertical field simulation. The left panel shows a false color-image of the gas density, while the right panel shows a heat image of the magnetization. This movie covers a period from 0–20,000 \frac{GM}{c^2}.
Animation 2. Animation of the zero-net-flux, radial-field simulation. The panels are the same as animation 1. This movie covers the period from 0–11,500 \frac{GM}{c^2}.

Astrobite edited by: Sabina Sagynbayeva

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