Radiative efficiency, variability and Bondi accretion on to massive black holes: the transition from radio AGN to quasars in brightest cluster galaxies
Radiative efficiency, variability and Bondi accretion on to massive black holes: the transition from radio AGN to quasars in brightest cluster galaxies
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DOI：
10.1093/mnras/stt490
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发表时间：
2012-11
影响因子：
4.8
通讯作者：
H. Russell;B. McNamara;A. Edge;M. Hogan;R. Main;A. Vantyghem;U. Waterloo;P. Institute;Cfa;Durham
中科院分区：
文献类型：
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作者：
H. Russell;B. McNamara;A. Edge;M. Hogan;R. Main;A. Vantyghem;U. Waterloo;P. Institute;Cfa;Durham
我们研究了57个最亮的星系团星系中未分辨的核X射线源，以研究核X射线发射与超大质量黑洞吸积之间的关系。我们样本中的大多数星系团在周围的热大气中嵌入了显著的X射线空穴，我们用它来估计过去数百年来超大质量黑洞的平均喷流功率和平均吸积率。我们发现，大约有一半的样本具有可检测到的核X射线发射。核X射线光度与利用X射线空穴确定的平均吸积率相关，这与核X射线发射跟踪正在进行的吸积的假设是一致的。这些结果表明，在最近一次∼107年经历过活动星系核爆发的系统中，至少有一半的时间是“开启”的。随着平均吸积率的增加，相对于机械喷注功率，核X射线源变得更加明亮。我们发现，当平均吸积率超过爱丁顿速率的几个百分点，或超过∼1045ergs−1的功率输出时，核辐射超过喷注功率，在那里活动星系核明显转变为类星体。三个天体(A2052、九头蛇A、M84)的核X射线辐射在6个月至10年的时间尺度上变化2-10倍。如果这一水平的可变性是一种普遍现象，它可以解释平均吸积率和核X射线光度之间关系中的大部分散射。我们在可变物体中没有发现光谱能量分布作为光度的函数的显著变化。核X射线光度与喷流、平流主导的吸积流或两者的组合的辐射是一致的，尽管其他来源也是可能的。我们还考虑了一个长期存在的问题，即喷流是由冷的环核气体的吸积提供动力，还是由近球形的热Kev气体流入提供动力。对于我们样本中的13个邻近系统的子集，我们重新检查了喷流功率和邦迪吸积率之间的关系。结果表明，由于邦迪半径处空穴体积和气体密度的不确定性，邦迪吸积和喷流功率之间存在趋势的证据较弱。我们认为，冷气体补充可能是这些天体中吸积能量的一个可能来源；然而，我们不能排除邦迪吸积作用，它可能在低功率喷气式飞机中发挥重要作用。
We examine unresolved nuclear X-ray sources in 57 brightest cluster galaxies to study the relationship between nuclear X-ray emission and accretion on to supermassive black holes. The majority of the clusters in our sample have prominent X-ray cavities embedded in the surrounding hot atmospheres, which we use to estimate mean jet power and average accretion rate on to the supermassive black holes over the past several hundred Myr. We find that roughly half of the sample have detectable nuclear X-ray emission. The nuclear X-ray luminosity is correlated with average accretion rate determined using X-ray cavities, which is consistent with the hypothesis that nuclear X-ray emission traces ongoing accretion. The results imply that jets in systems that have experienced recent active galactic nucleus (AGN) outbursts, in the last ∼107 yr, are ‘on’ at least half of the time. Nuclear X-ray sources become more luminous with respect to the mechanical jet power as the mean accretion rate rises. We show that nuclear radiation exceeds the jet power when the mean accretion rate rises above a few per cent of the Eddington rate, or a power output of ∼1045ergs−1, where the AGN apparently transitions to a quasar. The nuclear X-ray emission from three objects (A2052, Hydra A, M84) varies by factors of 2–10 on time-scales of 6 months to 10 years. If variability at this level is a common phenomenon, it can account for much of the scatter in the relationship between mean accretion rate and nuclear X-ray luminosity. We find no significant change in the spectral energy distribution as a function of luminosity in the variable objects. The nuclear X-ray luminosity is consistent with emission from either a jet, an advection-dominated accretion flow, or a combination of the two, although other origins are possible. We also consider the longstanding problem of whether jets are powered by the accretion of cold circumnuclear gas or nearly spherical inflows of hot keV gas. For a subset of 13 nearby systems in our sample, we re-examine the relationship between the jet power and the Bondi accretion rate. The results indicate weaker evidence for a trend between Bondi accretion and jet power, due to uncertainties in the cavity volumes and gas densities at the Bondi radius. We suggest that cold gas fuelling could be a likely source of accretion power in these objects; however, we cannot rule out Bondi accretion, which could play a significant role in low-power jets.