Infrared Echo Of Stellar Tidal Disruption Events And X-ray Radiation Of AGNs

Recent development in the multi-λ time-domain surveys has led to many excit-ing discoveries of transient astrophysical phenomena. Tidal disruption of a star by the central supermassive black hole is one of such phenomena which occurs with very low probability. With the WISE data, we discover the infrared echo from five tidal disrup-tion events (TDEs). The discovery opens a new window to find new TDE candidate and to explore the environment of dormant black holes. Furthermore, we also study the X-ray properties of active galactic nuclei (AGN). In this thesis, I will firstly present a brief review of the studies of TDEs and AGNs, and then introduce the details of our investigations. Finally, I show the prospect of our related studies.Infrared Echo from Tidal Disruption Event:The sporadic accretion following the tidal disruption of a star by a supermassive black hole leads to a bright UV and soft X-ray flare in the galactic nucleus. The gas and dust surrounding the black hole responses to such a flare with an echo in emission lines and infrared emission. Firstly, for the first time, we detect the variations of the TDE ASASSN-14li in mid-infrared bands with the recently released ALLWISE and NEOWISE-R data. In comparison with the quiescent state, the infrared flux is brightened by 0.12 and 0.16 magnitude in the W1 and W2 bands at 36 days after the optical discovery (or～110 days after the date of the peak disruption). The flux excess is still detectable～170 more days later. Assuming that the flare-like infrared emission is from the dust around the black hole, its blackbody temperature is estimated to be～2100 K, slightly higher than the dust sublimation tem-perature, indicating that the dust is likely located close to the dust sublimation radius. With the equilibrium between the heating and radiation of the dust, bolometric luminos-ity was estimated to be～1043-1045 ergs-1, the uncertainty is dominated by the size of dust grains. The estimated bolometric luminosity is comparable with the observed peak luminosity from other wavebands. For the first time, our result confirmed the detection of infrared emission from the dust echo of TDE.Secondly, we also detect the long fading mid-IR emission lasting up to 14 years after the flare in four TDE candidates with transient coronal lines, using the WISE public data release. We estimate that the reprocessed mid-IR luminosities are in the range between 4 × 1042 to 2 × 1043 erg s-1 and dust temperature in the range of 570-800K when WISE first detected these sources 3-5 years after the flare. Both luminosity and dust temperature decreases with time. We interpret the mid-IR emission as the infrared echo of the tidal disruption flare. We estimate the UV luminosity at the peak flare to be one to thirty times of 1044 erg s-1 and warm dust masses in the range of 0.05 to 1.3M; within a few parsecs. Our results suggest that mid-infrared echo is a general signature of TDE in the gas-rich environment.X-ray Radiation of AGN:We present an analysis of the spectrum and variability of the bright reddened narrow line Seyfert 1 galaxy Was 61 using 90 ks archival XMM-Newton data. The X-ray spectrum in 0.2-10keV can be characterized by an absorbed power-law plus soft excess and an Fe Kcα emission line. The power-law spectral index remains constant during the flux variation. The absorbing material is mildly ionized, with a column density of 3.2 × 1021 cm-2, and does not appear to vary during the period of the X-ray observation. If the same material causes the optical reddening, it must be located outside the narrow line region with a dust-to-gas ratio similar to the average Galactic value. We detect significant variations of the Fe Kα line during the observa-tional period. A broad Fe Kα line at(?)6.7 keV with a width of～0.6 keV is detected in the low flux segment of the first 40 ks exposure, and is absent in the spectra of other seg-ments; a narrow Fe Kα emission line～6.4keV with a width of～0.1 keV is observed in the subsequent 20 ks segment, which has a count rate of 35% higher and is in the next day. We believe this is due to the change in geometry and kinematics of the X-ray emitting corona. The temperature and flux of soft X-ray excess appear to correlate with the flux of the hard power-law component. Comptonization of disc photons by a warm and optically thick inner disk is preferred to interpret the soft excess, rather than the ionized reflection.In addition, as a major partner, I also involve in the exploratory Chandra obser-vation of the ultraluminous quasar SDSS J010013.02+280225.8 at redshift 6.30. This quasar is X-ray bright, with inferred X-ray-to-optical flux ratio αox= - 1.22+0.07 0.05, higher than the values found in other quasars of comparable ultraviolet luminosity. The prop-erties inferred from this exploratory observation indicate that this ultraluminous quasar might be growing with super-Eddington accretion.