• PDF / 1,540,884 Bytes
• 11 Pages / 595.28 x 793.7 pts Page_size
• 21 Downloads / 171 Views

DOWNLOAD

REPORT

---

. Article .

December 2020 Vol. 63 No. 12: 129512 https://doi.org/10.1007/s11433-020-1611-7

More than softer-when-brighter: The X-ray powerlaw spectral variability in NGC 4051 Yun-Jing Wu1,3* , Jun-Xian Wang1,2* , Zhen-Yi Cai1,2, Jia-Lai Kang1,2, Teng Liu4, and Zheng Cai3 1 CAS

Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China; 2 School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China; 3 Department of Astronomy, Tsinghua University, Beijing 100084, China; 4 Max-Planck-Institut f¨ ur Extraterrestrische Physik, Garching bei M¨unchen D-85748, Germany Received April 9, 2020; accepted August 13, 2020; published online October 16, 2020

The powerlaw X-ray spectra of active galactic nuclei at moderate to high accretion rates normally appear softer when they brighten, for which the underlying mechanisms are yet unclear. Utilizing XMM-Newton observations and excluding photons < 2 keV to avoid contamination from the soft excess, in this work we scrutinize the powerlaw spectral variability of NCG 4051 from two new aspects. We first find that a best-fit “softer-when-brighter” relation is statistically insufficient to explain the observed spectral variabilities, and intervals deviated from the empirical relation are clearly visible in the light curve of 2-4 keV/4-10 keV count rate ratio. The deviations are seen not only between but also within individual XMM-Newton exposures, consistent with random variations of the corona geometry or inner structure (with timescales as short as ∼1 ks), in addition to those behind the smooth “softer-when-brighter” trend. We further find the “softer-when-brighter” trend gradually weakens with the decreasing timescale (from ∼ 100 ks down to 0.5 ks). These findings indicate that the powerlaw spectral slope is not solely determined by its brightness. We propose a two-tier geometry, including flares/nano-flares on top of the inner disc and an embedding extended corona (heated by the flares, in analogy to solar corona) to explain the observations together with other observational clues in literature. Rapid spectral variabilities could be due to individual flares/nano-flares, while slow ones are driven by the variations in the global activity of inner disc region (akin to the variation of solar activity, but not the accretion rate) accompanied with heating/cooling and inflation/contraction of the extended corona. active galaxies, X-ray, Seyfert galaxies PACS number(s): Citation:

98.54.Cm, 98.70.Qy, 97.10.Gz

Y.-J. Wu, J.-X. Wang, Z.-Y. Cai, J.-L. Kang, T. Liu, and Z. Cai, More than softer-when-brighter: The X-ray powerlaw spectral variability in NGC 4051, Sci. China-Phys. Mech. Astron. 63, 129512 (2020), https://doi.org/10.1007/s11433-020-1611-7

1 Introduction The hard X-ray radiation of active galactic nuclei is believed to be produced through thermal Comptonization of low energy seed photons from the accretion disc by hot plasma surrounding the SMBH, pr