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arXiv 1005.0627gr-qcastro-ph.CO
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准圆双星合并的最大黑洞自旋

Maximum black-hole spin from quasi-circular binary mergers

Michael Kesden, Guglielmo Lockhart, E. Sterl Phinney

摘要:

质量为M的黑洞的自旋角动量S必须低于克尔极限chi = S/M^2 < 1,但天体物理学上的黑洞能否达到这个极限自旋取决于它们的吸积历史。薄圆盘的气体吸积限制了黑洞的自旋,使其x_gas < 0.9980 +- 0.0002,因为来自这个圆盘的具有逆行角动量的电磁辐射优先被黑洞吸收。等质量双黑洞合并的数值相对论模拟外推到最大初始自旋表明,这些合并产生的最大自旋chi_eq < 0.95。在这里,我们表明,对于较小的质量比q = m/M << 1,从较大的黑洞的角动量的超辐射提取强加了一个基本的限制chi_lim < 0.9979 +- 0.0001的最终黑洞自旋,即使在测试粒子限制q -> 0的二元黑洞合并。chi_gas和chi_lim的值几乎相等,这意味着对超大质量黑洞自旋的测量无法区分由气体吸积形成的黑洞和由致密恒星残骸盘的引力螺旋形成的黑洞。我们还展示了超辐射散射如何改变来自外推测试粒子合并有限质量比的模型预测的质量和自旋。

英文摘要:

Black holes of mass M must have a spin angular momentum S below the Kerr limit chi = S/M^2 < 1, but whether astrophysical black holes can attain this limiting spin depends on their accretion history. Gas accretion from a thin disk limits the black-hole spin to chi_gas < 0.9980 +- 0.0002, as electromagnetic radiation from this disk with retrograde angular momentum is preferentially absorbed by the black hole. Extrapolation of numerical-relativity simulations of equal-mass binary black-hole mergers to maximum initial spins suggests these mergers yield a maximum spin chi_eq < 0.95. Here we show that for smaller mass ratios q = m/M << 1, the superradiant extraction of angular momentum from the larger black hole imposes a fundamental limit chi_lim < 0.9979 +- 0.0001 on the final black-hole spin even in the test-particle limit q -> 0 of binary black-hole mergers. The nearly equal values of chi_gas and chi_lim imply that measurement of supermassive black-hole spins cannot distinguish a black hole built by gas accretion from one assembled by the gravitational inspiral of a disk of compact stellar remnants. We also show how superradiant scattering alters the mass and spin predicted by models derived from extrapolating test-particle mergers to finite mass ratios.

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