arXiv 2410.051662024-10-07astro-ph.HE

恒星-吸积盘碰撞产生准周期喷发现象的辐射输运模拟

Radiation Transport Simulations of Quasi-Periodic Eruptions from Star-Disk Collisions

Indrek Vurm, Itai Linial, Brian D. Metzger

备注：: 34 pages, 20 figures. Submitted to ApJ

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AI总结：

本研究通过球对称Monte Carlo辐射输运模拟，验证恒星-吸积盘碰撞解释X射线准周期喷发的合理性，给出不同参数下的辐射特征，并指出大半径恒星可能源于反复碰撞膨胀。

摘要：

围绕超大质量黑洞沿倾斜轨道运行的恒星与其吸积盘之间的周期性碰撞，为X射线“准周期喷发（quasi-periodic eruptions，QPEs）”提供了一种颇具前景的解释。恒星每次穿过吸积盘时，都会对其前方的气体产生冲击并使其压缩，这些受冲击气体随后会在吸积盘上方以类球云团的形式重新膨胀。\n我们开展了球对称Monte Carlo辐射输运模拟，追踪了辐射介导激波后方光子的产生过程、热电子引发的Compton化过程，以及辐射最终穿过膨胀碎片的逃逸过程。对于薄吸积盘而言，这类一维计算是近似合理的，因为半径为$R_{\rm \u2605}$的恒星穿过吸积盘的速度，快于受冲击气体绕恒星流动的速度。\n对于与QPE复发周期相符的恒星轨道所对应的典型碰撞速度$v_{\rm coll} \gtrsim 0.15 c$、吸积盘面密度$\Sigma\sim 10^{3}$ g cm$^{-2}$的情况，模拟预测的瞬变光变曲线峰值光度$\gtrsim 10^{42}$ erg s$^{-1}$，且呈现Compton化的类热（类Wien）光谱，其峰值能量$h\nu\sim 100$ eV，与QPE的观测性质大体一致。在这些条件下，气体与辐射处于非平衡状态，且由于激波后方光子产生效率较低，辐射温度高于黑体温度。\n另一方面，对于更高的吸积盘密度和/或更低的激波速度，QPE辐射可能反而对应激波突破后不久相对短暂的阶段，不过在这种情况下，大部分辐射会被热化，且出现在紫外波段而非X射线波段。无论哪种情形，要重现观测到的喷发性质（持续时间、光度、温度），都要求恒星具有较大的半径$R_{\rm \u2605} \gtrsim 10R_{\odot}$，这可能表明恒星大气因反复碰撞而发生了膨胀。

英文摘要：

Periodic collisions between a star on an inclined orbit around a supermassive black hole and its accretion disk offers a promising explanation for X-ray "quasi-periodic eruptions" (QPEs). Each passage through the disk shocks and compresses gas ahead of the star, which subsequently re-expands above the disk as a quasi-spherical cloud. We present spherically symmetric Monte Carlo radiation transport simulations which follow the production of photons behind the radiation-mediated shock, Comptonization by hot electrons, and the eventual escape of the radiation through the expanding debris. Such one-dimension calculations are approximately justified for thin disks, through which the star of radius $R_{\star}$ passes faster than the shocked gas can flow around the star. For collision speeds $v_{\rm coll} \gtrsim 0.15 c$ and disk surface densities $Σ\sim 10^{3}$ g cm$^{-2}$ characteristic of those encountered by stellar orbits consistent with QPE recurrence times, the predicted transient light curves exhibit peak luminosities $\gtrsim 10^{42}$ erg s$^{-1}$ and Comptonized quasi-thermal (Wien-like) spectra which peak at energies $hν\sim 100$ eV, broadly consistent with QPE properties. For these conditions, gas and radiation are out of equilibrium and the emission temperature is harder than the blackbody value due to inefficient photon production behind the shock. Alternatively, for higher disk densities and/or lower shock velocities, QPE emission could instead represent the comparatively brief phase shortly after shock break-out, though in this case the bulk of the radiation is thermalized and occurs in the ultraviolet instead of the X-ray band. In either scenario, reproducing the observed eruption properties (duration, luminosity, temperature) requires a large radius $R_{\star} \gtrsim 10R_{\odot}$, which may point to inflation of the star's atmosphere from repeated collisions.

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