Most models of the low-frequency quasi-periodic oscillations (QPOs) in low-mass X-ray binaries (LMXBs) explain the dynamical properties of those QPOs. On the other hand, in recent years reverberation models that assume a lamp-post geometry have been successful in explaining the energy-dependent time lags of the broad-band noise component in stellar mass black holes and active galactic nuclei. We have recently shown that Comptonization can explain the spectral-timing properties of the kilo-hertz (kHz) QPOs observed in neutron star (NS) LMXBs. It is therefore worth exploring whether the same family of models would be as successful in explaining the low-frequency QPOs. In this work, we use a Comptonization model to study the frequency dependence of the phase lags of the type-C QPO in the BH LMXB GRS 1915+105. The phase lags of the QPO in GRS 1915+105 make a transition from hard to soft at a QPO frequency of around 1.8 Hz. Our model shows that at high QPO frequencies a large corona of ∼100–150 Rg covers most of the accretion disc and makes it $100{{\ \rm per\ cent}}$ feedback dominated, thus producing soft lags. As the observed QPO frequency decreases, the corona gradually shrinks down to around 3–17 Rg, and at 1.8 Hz feedback on to the disc becomes inefficient leading to hard lags. We discuss how changes in the accretion geometry affect the timing properties of the type-C QPO.

中文翻译：

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GRS 1915+105 的可变电晕

低质量 X 射线双星 (LMXB) 中的大多数低频准周期振荡 (QPO) 模型解释了这些 QPO 的动力学特性。另一方面，近年来，假设灯柱几何形状的混响模型已经成功地解释了恒星质量黑洞和活动星系核中宽带噪声分量与能量相关的时间滞后。我们最近表明，康普顿化可以解释在中子星 (NS) LMXB 中观察到的千赫 (kHz) QPO 的光谱时序特性。因此，值得探讨同一系列模型是否能成功解释低频 QPO。在这项工作中，我们使用 Comptonization 模型来研究 BH LMXB GRS 1915+105 中 C 型 QPO 相位滞后的频率依赖性。GRS 1915+105 中 QPO 的相位滞后在 QPO 频率约为 1.8 Hz 时从硬过渡到软。我们的模型表明，在高 QPO 频率下，~100-150 Rg 的大日冕覆盖了大部分吸积盘，并使其 $100{{\ \rm per\cent}}$ 反馈占主导地位，从而产生软滞后。随着观察到的 QPO 频率降低，电晕逐渐缩小到 3-17 Rg 左右，并且在 1.8 Hz 时，对圆盘的反馈变得低效，导致硬滞后。我们讨论吸积几何的变化如何影响 C 型 QPO 的时序特性。随着观察到的 QPO 频率降低，电晕逐渐缩小到 3-17 Rg 左右，并且在 1.8 Hz 时，对圆盘的反馈变得低效，导致硬滞后。我们讨论吸积几何的变化如何影响 C 型 QPO 的时序特性。随着观察到的 QPO 频率降低，电晕逐渐缩小到 3-17 Rg 左右，并且在 1.8 Hz 时，对圆盘的反馈变得低效，导致硬滞后。我们讨论吸积几何的变化如何影响 C 型 QPO 的时序特性。