Structure and Instability of the Ionization Fronts around Moving Black Holes

Kazuyuki Sugimura; Massimo Ricotti

doi:10.1093/mnras/staa1394

paper

Structure and Instability of the Ionization Fronts around Moving Black Holes

arXiv:2003.05625 · doi:10.1093/mnras/staa1394

Abstract

In this paper we focus on understanding the physical processes that lead to stable or unstable ionization fronts (I-fronts) observed in simulations of moving black holes (BHs). The front instability may trigger bursts of gas accretion, rendering the BH significantly more luminous than at steady-state. We perform a series of idealized three dimensional radiation hydrodynamics simulations resolving the I-fronts around BHs of mass $M_{BH}$ and velocity $v_{\infty}$ accreting from a medium of density $n_{H}$ . The I-front, with radius $R_{I}$ , transitions from D-type to R-type as the BH velocity becomes larger than a critical value $v_{R} \sim 40 km/s$ . The D-type front is preceded by a bow-shock of thickness $Δ R_{I}$ that decreases as $v_{\infty}$ approaches $v_{R}$ . We find that both D-type and R-type fronts can be unstable given the following two conditions: i) for D-type fronts the shell thickness must be $Δ R_{I} / R_{I} < 0.05$ (i.e., $v_{\infty} ≳ 20 km/s$ .), while no similar restriction holds for R-type fronts; ii) the temperature jump across the I-front must be $T_{II} / T_{I} > 3$ . This second condition is satisfied if $T_{I} < 5000 K$ or if $n_{H} M_{BH} ≳ 1 0^{6} M_{⊙} c m^{- 3}$ . Due to X-ray pre-heating typically $T_{I} \sim 1 0^{4} K$ , unless the D-type shell is optically thick to X-rays, which also happens when $n_{H} M_{BH}$ is greater than a metallicity-dependent critical value. We thus conclude that I-fronts around BHs are unstable only for relatively massive BHs moving trough very dense molecular clouds. We briefly discuss the observational consequences of the X-ray luminosity bursts likely associated with this instability.

arXiv comment14 pages, 9 figures, published in MNRAS, movie available at https://www.youtube.com/watch?v=pj1dy-0AM0o

Structure and Instability of the Ionization Fronts around Moving Black Holes

Abstract

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