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SCIENTIA SINICA Physica, Mechanica & Astronomica, Volume 48, Issue 3: 039508(2018) https://doi.org/10.1360/SSPMA2017-00251

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

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  • ReceivedSep 15, 2017
  • AcceptedNov 30, 2017
  • PublishedJan 31, 2018
PACS numbers

Abstract

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.


Funded by

中国科学院空间科学战略性先导科技专项(编号:)

国家自然科学基金(编号:)


Acknowledgment

向参与该活动星系核科学白皮书讨论的同事们表示感谢.


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  • Figure 1

    (Color online) (a) PSD comparison between the two AGN, NGC 4051 and MCG-6-30-15, and the BHXB Cygnus X-1 (data from RXTE and XMM-Newton) [9]; (b) correlation between the AGN PSD break timescale and the black hole mass [8].

  • Figure 2

    PSD of Akn 564. The crosses indicate the PSD derived using the short-timescale lightcurve, and the open circles indicate the PSD derived using the long-timescale lightcurve. There is an apparent low-frequency break around 8.7×10−7 Hz [13].

  • Figure 3

    Simulated EP lightcurve in the 0.5–4 keV band for an AGN at a flux level of 3.0×10−11 erg cm−2 s−1. The sampling time interval is 270 min (i.e., 3 EP orbital periods), and the total monitoring time span is half a year.

  • Figure 4

    PSD calculated from the simulated lightcurve shown in Figure 3. The folded line indicates the PSD model used to simulate the lightcurve. There is an apparent break around 10−6 Hz, which corresponds to a black hole mass of 1.2×107 solar masses, according to the correlation between the break frequency and the black hole mass.

  • Figure 5

    (Color online) (a) Sustained, short, intense soft X-ray outbursts from RX J1302+2747; (b) energy spectra of the source during its quiescent and flare states (data from Chandra and XMM-Newton) [23].

  • Figure 6

    (Color online) Long-term lightcurves of the ultra-soft X-ray-emission AGN XMM J1231+1106 (the dashed curve indicates the fitting result of a TDE model) (a) and GSN 069 (b) [25].

  • Figure 7

    Likely state transitions of NGC 7589: before 2001, it was in the low accretion state, being a low-luminosity AGN based on the upper limits on the luminosities; after 2001, it was in the high state (detected by XMM-Newton), being a normal-luminosity AGN, and was likely going into another low state [30].

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