We perform a set of numerical experiments studying the interaction of Type I X-ray bursts with thin, Shakura-Sunyaev type accretion discs. Careful observations of X-ray spectra during such bursts have hinted at changes occurring in the inner regions of the disc. We now clearly demonstrate a number of key effects that take place simultaneously, including: evidence for weak, radiation-driven outflows along the surface of the disc; significant levels of Poynting-Robertson (PR) drag, leading to enhanced accretion; and prominent heating in the disc, which increases the height, while lowering the density and optical depth. The PR drag causes the inner edge of the disc to retreat from the neutron star surface toward larger radii and then recover on the timescale of the burst. We conclude that the rich interaction of an X-ray burst with the surrounding disc provides a novel way to study the physics of accretion onto compact objects. 1 arXiv:2001.01032v1 [astro-ph.HE] 4 Jan 2020Thermonuclear explosions on the surface of neutron stars, commonly known as Type I X-ray bursts, can be used to study the behavior of matter under extreme conditions 1-3 . Careful analysis of the burst spectrum and luminosity may even provide constraints on the neutron star equation of state 1, 4-6 , one of the most important unsolved problems in high-energy astrophysics. In addition, it has recently been recognized that X-ray bursts are a potentially powerful probe of accretion physics, as the intense release of radiative energy in the burst over a short timescale (seconds for Type I bursts; hours for superbursts) could significantly impact the structure of both the disc and corona 7-9 .Observational evidence for the interaction of bursts with the accretion disc indicates a range of behaviors that signal a strong dependence on the accretion flow geometry, even for sources in the same spectral state. For example, several studies of bursts in the low/hard state 10-12 detected both a simultaneous rise in soft X-rays (owing to the burst itself) and drop in hard X-rays (attributed to cooling of a geometrically thick corona). On the other hand, bursts from other sources in the low/hard state produced reflection features in the X-ray spectra [13][14][15][16] , indicating the presence of an optically thick inner disc.Recently, we presented the first numerical simulation of an accretion disc subject to the sudden, intense radiation field of an X-ray burst 17 . That simulation focused on a hot, geometricallythick disc, and found that strong Compton cooling of the accreting plasma by the burst photons caused the disc temperature to drop by three orders of magnitude, the height to be reduced by one order of magnitude, and the accretion rate to increase by a factor of a few. All of these changes −7/2 K ρ cgs cm 2 g −1 and κ a R = 1.6 × 10 21 T −7/2 K ρ cgs cm 2 g −1 34 , respectively, where T K is the ideal gas temperature of the fluid in Kelvin and ρ cgs is the density in g cm −3 . In this work, we assume the flux mean, κ a F , is the same as the Rosseland me...

We present and analyze a set of three-dimensional, global, general relativistic radiation magnetohydrodynamic simulations of thin, radiation-pressure-dominated accretion disks surrounding a nonrotating, stellar-mass black hole. The simulations are initialized using the Shakura–Sunyaev model with a mass accretion rate of M ̇ = 3 L Edd / c 2 (corresponding to L = 0.17L Edd). Our previous work demonstrated that such disks are thermally unstable when accretion is driven by an α-viscosity. In the present work, we test the hypothesis that strong magnetic fields can both drive accretion through magnetorotational instability and restore stability to such disks. We test four initial magnetic field configurations: (1) a zero-net-flux case with a single, radially extended set of magnetic field loops (dipole), (2) a zero-net-flux case with two radially extended sets of magnetic field loops of opposite polarity stacked vertically (quadrupole), (3) a zero-net-flux case with multiple radially concentric rings of alternating polarity (multiloop), and (4) a net-flux, vertical magnetic field configuration (vertical). In all cases, the fields are initially weak, with a gas-to-magnetic pressure ratio ≳100. Based on the results of these simulations, we find that the dipole and multiloop configurations remain thermally unstable like their α-viscosity counterpart, in our case collapsing vertically on the local thermal timescale and never fully recovering. The vertical case, on the other hand, stabilizes and remains so for the duration of our tests (many thermal timescales). The quadrupole case is intermediate, showing signs of both stability and instability. The key stabilizing factor is the ability of specific field configurations to build up and sustain strong, P mag ≳ 0.5P tot, toroidal fields near the midplane of the disk. We discuss the reasons why certain configurations are able to do this effectively and others are not. We then compare our stable simulations to the standard Shakura–Sunyaev disk.

We present and analyze a set of three-dimensional, global, general relativistic radiation magnetohydrodynamic simulations of thin, radiation-pressure-dominated accretion disks surrounding a nonrotating, stellar-mass black hole. The simulations are initialized using the Shakura-Sunyaev model with a mass accretion rate of Ṁ = 3L Edd /c 2 (corresponding to L = 0.17L Edd ). Our previous work demonstrated that such disks are thermally unstable when accretion is driven by an α-viscosity. In the present work, we test the hypothesis that strong magnetic fields can both drive accretion through the magneto-rotational instability and restore stability to such disks. We test four initial magnetic field configurations: 1) a zero-net-flux case with a single, radially extended set of magnetic field loops (dipole); 2) a zero-net-flux case with two radially extended sets of magnetic field loops of opposite polarity stacked vertically (quadrupole); 3) a zero-net-flux case with multiple radially concentric rings of alternating polarity (multi-loop); and 4) a net-flux, vertical magnetic field configuration (vertical). In all cases, the fields are initially weak, with the gas-to-magnetic pressure ratio 100. Based on the results of these simulations, we find that the dipole and multi-loop configurations remain thermally unstable like their α-viscosity counterpart, in our case collapsing vertically on the local thermal timescale and never fully recovering. The vertical case, on the other hand, stabilizes and remains so for the duration of our tests (many thermal timescales). The quadrupole case is intermediate, showing signs of both stability and instability. The key stabilizing factor is the ability of specific field configurations to build up and sustain strong, P mag 0.5P tot , toroidal fields near the midplane of the disk. We discuss the reasons why certain configurations are able to do this effectively and others are not. We then compare our stable simulations to the standard Shakura-Sunyaev disk.

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