Long-Term Ultraviolet Spectroscopy of a Tidal Disruption Event at only 90 Mpc

**Revised 2016 Oct 19** Per the STIS team lead recommendation, we have changed from ACCUM mode to TIME-TAG mode to allow for time domain analysis. For each epoch and grating, we have swapped single 3-orbit ACCUM exposures for triple 1-orbit TIME-TAG exposures. Total instrumental count rates are expected to be background-dominated. We have selected BUFFER TIME 1500s for FUV epochs and 1000s for NUV epochs. Acquisition exposures should be unaltered from our previous submission. Please double-check that this new setup is sensible and correctly constructed. ---- Since this is the second cycle of a two-cycle program, many things are the same, including the target. We are observing epochs 3 and 4 (Cycle 24) of a 4-epoch program, and the science observations of epochs 3 and 4 are essentially copied from epoch 2 (Cycle 23). The major differences are the acquisition time and checkbox (to adjust for decay of the transient, and acquisition data from the previous cycle). Also, of course, the viewing windows are different. Visits are broken up by instrument, but FUV and NUV epochs should be scheduled in pairs. 1195898992 ----- Attempting Submission 1 (Fri Jul 08 21:03:44 GMT 2016) ----- ----- Attempting Submission 2 (Wed Oct 19 21:15:32 GMT 2016) ----- Long-Term Ultraviolet Spectroscopy of a Tidal Disruption Event at only 90 Mpc We propose multi-epoch ultraviolet spectroscopy of ASASSN-14li, a stellar tidal disruption event (TDE) at ~90 Mpc. Such very nearby stellar TDEs have been thus far only observed about once per decade and provides an exceptional opportunity to study broad emission lines which describe the abundances and accretion flow of the stellar debris in one of the most important physical regimes for understanding basic TDE behavior. We also request brief XMM observations to constrain the high-energy spectral evolution on similar timescales. These observations will build upon surprising new results, and will provide an important foundation for follow-up of more ambiguous TDE candidates subsequently identified by LSST and WFIRST at higher redshifts. APT Revised 2016 10 20: per the recommendation of the STIS team lead, we have switched from ACCUM mode to TIME-TAG mode. We have replaced single 3-orbit ACCUM exposures with triple 1-orbit TIME-TAG exposures per visit. We have selected BUFFER TIME of 1500s for FUV and 1000s for NUV. We expect the instrumental total count rate to be background-dominated. --- This is the first cycle of a two-cycle program to spectroscopically monitor the decay of transient ASASSN-14li at the nucleus of galaxy PGC 043234/LEDA 43234. This transient appears to be due to the tidal disruption of a star by a supermassive black hole, and has been observed as a TOO in Cycle 22 by Cenko et al. To first order, we expect the host-subtracted transient spectrum to look like a quasar in the UV with some significant differences in terms of broad line strength. Cycle 23 data (epochs 1 and 2 of this program) show a similar spectral shape, but many broad emission lines fainter or disappeared. For this program, we will be observing two UV STIS spectroscopic epochs in cycle 24 (epochs 3 and 4). We are expecting joint time from a recently-successful Chandra program (#17700613) as well, which will improve temporal coverage of the transient, as well as 2x10 ks observations from XMM-Newton joint time in 2017. We hope to coordinate Chandra or XMM-Newton observations as close to the HST epochs as possible, but given the narrow available observing windows for early HST cycle 24, the X-ray observatories may have better relative scheduling flexibility. For epochs 3 and 4 (both in Cycle 24), we are (as was previously done for epochs 1 and 2 in Cycle 23) acquiring on the nucleus of PGC 043234 which is also the site of the target transient. Since the galaxy has good elliptical symmetry and the transient is well-centered (to within ~0.02 arcsec, vs 0.2 arcsec slit width for all spectroscopic observations), we are acquiring for a diffuse object. Since we are interested in weighting the acquisition on the UV-bright transient, we are using the CLEAR filter (F25ND3 was used by Cenko et al, but we expect exposure times for F25ND3 to be prohibitive by Cycle 23). In Cycle 23 we requested 12 seconds per acquisition to be conservative. Acquisition and saturation were not problems for the observations as-executed. For Cycle 24, we assume continuing flux decay, and base our acquisition exposure time on our Cycle 23 acquisition images. Assuming UV flux from the STIS spectrum is due to a ~50,000 K blackbody, we infer (from the STIS ETC) the brightness of a 3x3 box covering the host nucleus. In a worst-case scenario, the transient will be negligible relative to the host galaxy. With a 35 second acquisition exposure, we reach SNR ~54 in a 3x3 box. From the Cycle 23 acquisition image, even with significant decay of the UV transient a 5-pixel checkbox should be sufficient for acquisition using the galactic nucleus. Saturation will not be an issue even at current brightness. Epochs 3 and 4 consists of 6 orbits each. Each epoch is broken up into two visits for ease of scheduling. We can tolerate 5-day separation, though a smaller interval is preferred to avoid short-timescale object variability. We plan 3 NUV G230L exposures with the 52x0.2 arcsec slit in 3 orbits for the first visit, and 3 FUV G140L exposures with the 52x0.2 arcsec slit in 3 orbits for the second visit. For epoch 3 we have designated a window in Nov. 2016 - Jan. 2017. For epoch 4 we have designated a window in May - Aug. 2017. AGN/QUASARS 12 false true false false false 13 false SUPPORTED GALAXY ACCRETION DISK BLR NUCLEUS NO Coordinates determined from Keck AO imaging matched to SDSS. PGC-043234 LEDA-43234 J2000 J2000 STIS data from the most recent cycle had NUV~17.7, and is expected to decay by ~1 magnitude or more in this time STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 1476905322021 Timing link suitability includes constraints ([Group (01 02) Within 5.0 days]) The scheduling data was generated by CASM 24.2. 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