Stellar Tidal Disruption Events And Quasar Absorption Line Variability

Over the past ten years, with the successful operation of large digital sky surveys such as SDSS, a large volume of photometric and spectroscopic data have been accu-mulated. The massive dataset makes it possible to identify rare outburst events such as stellar tidal disruption events(TDE). Also multi-epoch spectroscopic observations of quasars are available for systematic study of quasar absorption lines. This thesis mainly presents two works based on massive SDSS data and follow-up spectroscopic observation:spectral evolution of TDE candidates and quasar absorption line variabil-ity. We also briefly descibe two pilot researches:optical variability of quasars and super-luminous supernovae(SLSN) search based on PTF survey.Firstly, we present results of follow-up optical spectroscopic observations of seven rare, extreme coronal line emitting galaxies reported by Wang et al. (2012) with Multi-Mirror Telescope (MMT). Large variations in coronal lines are found in four objects, making them strong candidates of tidal disruption events (TDE). For the four TDE can-didates, ail the coronal lines with ionization status higher than [Fe Ⅶ] disappear within 5-9 years. The [Fe Ⅶ] faded by a factor of about five in one object (J0952+2143) within 4 years, whereas emerged in other two without them previously. A strong in-crement in the [O Ⅲ] flux is observed, shifting the line ratios towards the loci of ac-tive galactic nucleus on the BPT diagrams. Surprisingly, we detect a non-canonical [OⅢ]5007/[OⅢ]4959-2 in two objects, indicating a large column density of O2+ and thus probably optical thick gas. This also requires a very large ionization pa-rameter and relatively soft ionizing spectral energy distribution (e.g. blackbody with T<5×104 K). Our observations can be explained as echoing of a strong ultraviolet to soft X-ray flare caused by tidal disruption events, on molecular clouds in the inner parsecs of the galactic nuclei. Re-analyzing the SDSS spectra reveals double-peaked or strongly blue-shouldered broad lines in three of the objects, which disappeared in the MMT spectra in two objects, and faded by a factor of ten in 8 years in the remaining ob-ject with a decrease in both the line width and centroid offset. We interpret these broad lines as arising from decelerating biconical outflows. Our results demonstrate that the signatures of echoing can persist for as long as ten years, and can be used to probe the gas environment in the quiescent galactic nuclei.Secondly, we present a qualitative analysis of the variability of quasar intrinsic absorption lines using the large multi-epoch spectroscopic dataset of the Sloan Digi-tal Sky Survey. We confirm that variations of absorption lines are highly coordinated among different components of the same ion or the same absorption component of dif- ferent ions for C IV, Si IV and N V Furthermore, we show that the equivalent widths of the lines decrease or increase statistically when the continuum brightens or dims. This is further support by the synchronized variations of emission and absorption line equivalent width, when the well established intrinsic Baldwin effect for emission lines is taken into account. We find that the emergence of an absorption component is usually accompanying with dimming of the continuum and the disappearance of an absorption line component with brightening of the continuum. This suggests that the emergence or disappearance of a C TV absorption component is only the extreme case, when the ionic column density is very sensitive to continuum variations or the continuum variability amplitude is larger. These results support the idea that absorption line variability is driven mainly by the change of gas ionization in response to the continuum variations, that the line-absorbing gas is highly ionized, and in some extreme cases, too highly ionized to be detected in UV absorption lines.Finally, we briefly descibe two pilot researches based on PTF survey. We match the SDSS DR7 spectroscopic quasar catalog with PTF survey and get-34000 quasars’ light curves with a baseline of 5 years and relatively high cadence on short time scale of 5-10 days. This sample could be used for quasar variability study and provides useful reference for further quasar monitoring surveys. We also select 5 high galactic latitude fields with more than 500 images for SLSN search. Using image stacking method we identify 2 strong candidates of intermediate redshift SLSNs. Follow-up observation shows that one is an AGN at redshift～0.63 and the other is a low redshift galaxy. Our method provides a benchmark for searching this kind objects in the future surveys.