Late-time UV spectroscopy of Tidal Disruption Events with HST
Abstract
Once every 10^5 years, an accretion-powered flare may temporarily illuminate a supermassive black hole (SMBH) following a tidal disruption event (TDE), in which a star is shredded by tidal stresses close to the SMBH. TDEs provide a new window to study these otherwise dormant SMBHs and probe different accretion modes (super-Eddington to sub-Eddington) on a monthly timescale. Recent studies provide evidence for significant state changes in the late-time TDE emission properties (X-ray re-brightening, light curve flattening) that may be attributed to delayed disk formation or transitions in the accretion processes. These findings highlight our lack of understanding of how and when circularization occurs in TDEs. In light of this, we propose late-time follow-up UV spectroscopic observations of two TDEs that have been observed by HST in their early phases, in order to study the evolution of debris streams on the timescale of a year.
Since the UV band captures the strongest atomic transitions (N, C, Si) expected from TDEs, UV spectroscopy will allow us to measure these lines and infer the kinematics, density, and abundance in this late-time TDE environment. Past HST STIS observations of these two targets have identified cold, fast outflows (0.05c) initiated by super-Eddington accretion. One of our targets, AT2018zr, has undergone a remarkable UV spectral hardening that may be related to an increased contribution from the disk. With the proposed HST observations, we wish to ascertain whether such disk exists by searching for high-ionization emission lines similar to those seen in QSOs.- Publication:
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HST Proposal
- Pub Date:
- June 2019
- Bibcode:
- 2019hst..prop15899H