邻近星系X射线核活动普查
A census of X-ray nuclear activity in nearby galaxies
摘要：
本文利用Chandra/ACIS系统研究了187个距离小于15 Mpc的邻近星系的X射线核活动。我们发现其中86个具有点状X射线核心，与吸积黑洞（BH）一致。我们认为，他们中的大多数是核BH，而不是X射线双星。具有X射线检测到的核BH的星系比例对于椭圆星系和早期型螺旋星系（E到Sb）较高（~ 60%），而对于晚期型螺旋星系（Sc到Sm）较低（~ 30%）。X射线核与大尺度棒没有优先关联;事实上，与类似哈勃类型的非棒星系或弱棒星系相比，强棒星系的核X射线源的检测分数和光度似乎略低。在0.3-8 keV波段，核源的累积光度分布在~2 x 10^{38} erg/s到~10^{42} erg/s之间，呈幂律分布，斜率约为-0.5。椭圆星系的爱丁顿比较低（L_{X} / L_{Edd} ~ 10^{-8}），而晚型螺旋星系的爱丁顿比较高（高达L_{X} / L_{Edd} ~ 10^{-4}），但在所有情况下，吸积速率都足够低，处于辐射无效区。本征NH一般较低，特别是对于发光度较低的光源：在光度<~ 10^{39} erg/s时似乎没有2型核BH。缺乏尘埃环或其他固有吸收源（例如，光学厚的盘风）可能与那些微弱的核黑洞周围缺乏标准的吸积盘直接相关。被遮蔽源的比例随着核BH光度的增加而增加：2/3的源在L_{X} > 10^{40} erg/s时，其拟合NH > 10^{22} cm^{-2}。这与在更明亮的活动星系核和类星体中观察到的遮蔽部分随光度增加而下降的趋势相反。
英文摘要：
We have studied the X-ray nuclear activity of 187 nearby (distance < 15 Mpc) galaxies observed with Chandra/ACIS. We found that 86 of them have a point-like X-ray core, consistent with an accreting black hole (BH). We argue that the majority of them are nuclear BHs, rather than X-ray binaries. The fraction of galaxies with an X-ray detected nuclear BH is higher (~60 per cent) for ellipticals and early-type spirals (E to Sb), and lower (~30 per cent) for late-type spirals (Sc to Sm). There is no preferential association of X-ray cores with a large-scale bar; in fact, strongly barred galaxies appear to have slightly lower detection fraction and luminosity for their nuclear X-ray sources, compared with non-barred or weakly barred galaxies of similar Hubble types. The cumulative luminosity distribution of the nuclear sources in the 0.3-8 keV band is a power-law with slope ~-0.5, from ~2 x 10^{38} erg/s to ~10^{42} erg/s. The Eddington ratio is lower for ellipticals (L_{X}/L_{Edd} ~ 10^{-8}) and higher for late-type spirals (up to L_{X}/L_{Edd} ~ 10^{-4}), but in all cases, the accretion rate is low enough to be in the radiatively-inefficient regime. The intrinsic NH is generally low, especially for the less luminous sources: there appear to be no Type-2 nuclear BHs at luminosities <~ 10^{39} erg/s. The lack of a dusty torus or of other sources of intrinsic absorption (e.g., an optically-thick disk wind) may be directly related to the lack of a standard accretion disk around those faint nuclear BHs. The fraction of obscured sources increases with the nuclear BH luminosity: 2/3 of the sources with L_{X} > 10^{40} erg/s have a fitted NH > 10^{22} cm^{-2}. This is contrary to the declining trend of the obscured fraction with increasing luminosities, observed in more luminous AGN and quasars.
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