爱因斯坦探针在活动星系核研究领域的科学机遇

Einstein Probe's scientific opportunities in the field of active galactic nuclei

活动星系核(AGN)是指那些具有强烈电磁辐射的星系核心,其巨大能量是由中心超大质量黑洞(SMBH)吸积周围物质,将其释放的引力能转化为热能、辐射能而产生的.各种时标的多波段光变是AGN的典型观测特征,其中尤以X射线光变最为引人注意,因为其光变剧烈、时标短、携带着AGN吸积盘最内区的丰富物理信息,长期以来一直被用作研究星系中心SMBH的探针.但是,目前对AGN的X射线光变还没有很根本的认识,对其来源和产生机制仍不甚清楚.爱因斯坦探针(Einstein Probe,EP)卫星具有空前的软X射线巡天能力,其grasp(即探测器有效面积与视场的乘积)比起之前同类卫星提高了一到两个量级,这使得EP能在从百秒到年的采样频率上对全天数百个亮AGN进行监测,获得前所未有的大样本、宽采样频率覆盖的光变数据.我们拟据此重点开展以下4个方面的研究:对大样本亮AGN的软X射线功率谱进行完整测量;系统监测和研究AGN中罕见的大幅度X射线光变和耀发现象;研究大样本AGN的长期光谱变化以及开展其爆发监测;开展AGN/类星体巡天.这些数据将促进对AGN X射线辐射的剧烈光变与爆发现象及其产生机制,SMBH的吸积盘、喷流及冕区的物理条件、结构、动力学与辐射过程,以及AGN的宇宙学演化等科学问题的进一步认识.另外,鉴于EP能够探索一个全新的发现空间(大天区、长时标软X射线的系统监测),将有可能发现前所未见的黑洞吸积现象——宇宙的复杂多样性总是超乎人类的想象.

Active galactic nuclei （AGNs） are accreting supermassive black holes （SMBHs） that reside at the centers of galaxies and emit excessive electromagnetic radiation, whose ultimate energy source is the release of the gravitational potential energy of the accreted matter in the form of thermal and radiation energy. Multi-timescale and multi-band variability is a characteristic observational feature of AGNs. In particular, X-ray variability is most worth noting because of being intense and rapid, as well as carrying rich physical information of the innermost region of the accretion disk. Therefore, AGN X-ray variability has long been used as a probe for studying SMBHs. However, at present, we still lack a very fundamental understanding of AGN X-ray variability, whose origin and mechanisms remain largely unclear. The Einstein Probe （EP） satellite has an unprecedented soft X-ray survey capability, with a grasp （i.e., the product of detector effective area and field of view） being one to two orders of magnitude larger than previous similar satellites. EP is able to monitor hundreds of bright AGNs in the whole sky with sampling intervals ranging from 100 s to years, and will thereby build an exceptional AGN variability database. Accordingly, we propose to carry out AGN investigations mainly in the following four aspects： comprehensive measurement of the soft X-ray power spectral density for a large sample of bright AGNs; systematic monitoring and study of the rare large-amplitude AGN X-ray variability and flaring phenomenon; examination of long-term spectral variability for a large sample of AGNs and monitoring of their outbursts; and AGN/ quasar survey. These unprecedented data will facilitate our further understanding of many scientific questions, such as the intense AGN X-ray variability and flaring phenomenon and its mechanisms, the physical conditions, structures, dynamics, and radiation processes of the AGN accretion disk, jet, and corona, as well as the AGN cosmological evolution. Furthermore, given EP＇s capability of exploring a brand-new discovery space （large sky-area, long-timescale, and systematic soft X-ray monitoring）, we are likely to discover some black hole accretion phenomena never seen before --the complexity and diversity of the Universe are always beyond human imagination.