0 Assigned ID: 624 ----- Attempting Submission 1 (Tue Jan 08 10:21:01 GMT 2019) ----- Splitted observations into two visits. 1574219588 ----- Attempting Submission 1 (Thu Jan 17 14:08:28 GMT 2019) ----- ----- Attempting Submission 2 (Wed Jan 23 17:25:53 GMT 2019) ----- ----- Attempting Submission 3 (Fri Jan 25 22:12:58 GMT 2019) ----- Searching for Outflow Signatures in the Tidal Disruption Event ZTF18abxftqm Wide-field optical transient surveys have discovered about a dozen of tidal disruption events (TDEs), which belong to a rare class of transient phenomena that occur when an unlucky star is tidally disrupted by a supermassive black hole (SMBH). In recent years, two emission mechanisms (circularization v.s. reprocessing) have been put forward to explain the excess in the UV and optical emission in TDEs, which is unexpected according to the classical TDE framework. UV spectroscopy is critical to disentangling these scenarios, as most strong atomic features fall in this bandpass, and emission and absorption line diagnostics can shed light on the physical state (velocity, density, composition) of the emitting gas (in analogy with quasars). However, despite its important diagnostic capabilities, UV spectroscopy has only been obtained for 5 TDEs so far, each of which have yielded important insights on the presence of an outflow and/or an intriguing nitrogen-rich chemical composition. Here we propose to obtain an epoch of FUV and NUV HST/STIS spectroscopic observations of the most recent TDE discovered by ZTF: ZTF18abxftqm (AT2018hco) at z=0.088. The slow UV evolution of ZTF18abxftqm confirmed by Swift observations, provides an excellent opportunity to get insights of the physical conditions in a TDE with high S/N UV spectroscopy. The proposed observations will allow us to test whether outflow is ubiquitous amongst TDEs, and if so, whether their velocities are consistent with current models. We request a single epoch (that could be split into multiple visits, if necessary) of UV spectroscopy of the TDE ZTF18abxftqm, over the wavelength range from 1200 - 3000A. HST, with its sensitive UV spectrographs and exquisite angular resolution, is the only facility that can conduct such observations (the source is much too faint for the UV grism on Swift). We take the most recent UVW2 (~1928 A) magnitude measured by the Swift-UVOT, 18.3 (AB) on 28 Dec 2018, and the inferred blackbody temperature from the UV+optical photometry (T~20,000 K) to estimate the expected SNR in dierent instrumental configurations. We find we can achieve the largest SNR in the near-UV using STIS with the NUV-MAMA detector and the G230L (1570-3180A) grating. Assuming an orbital visibility of 53 minutes and including overheads for guide-star acquisition (6 min for the first orbit, 4 min for the subsequent orbits), target acquisition (8 min for an extended source, no peak-up required for 0.2" slit width), and wavelength calibration (4 min per orbit), we find that we can obtain 35 min of exposure in the first orbit (43 in subsequent orbits). According to the exposure time calculator, we would expect a SNR per resolution element > 5 for the range from 1650 to 3150A for the continuum emission for a single orbit. This should allow us to easily detect strong narrow (Wr > 1A) absorption lines, as well as emission lines as were observed from, e.g., ASASSN-14li and iPTF15af. In the far UV, we have compared the throughput possible with both COS (G140L grating) and STIS (G140L grating) and find that they are roughly comparable. To enable isolation of the transient emission from the underlying host (via the greater spatial resolution), we have chosen STIS with the FUV-MAMA detector. Using similar overheads as above, with a two orbits we find a SNR > 5 over the range from 1150 to 1700A for the continuum. To summarize, in total we request a single epoch of STIS spectroscopy of ZTF18abxftqm, for a total of 3 orbits (2 FUV + 1 NUV). Massive Black Holes And Their Host Galaxies 0 false SUPPORTED GALAXY ACCRETION DISK NUCLEUS WIND J2000 J2000 UVW2 = 18.3 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 POS-TARG DITHER 0.4165 LINE false false 3 90.0 STIS-ALONG-SLIT 1 false POS-TARG DITHER 0.4165 LINE false false 5 90.0 STIS-ALONG-SLIT 2 false The scheduling data was generated by CASM 26.2.2. CASM was run in Phase II mode. /Users/tiarahung/Documents/work/AT2018hco/OP-cache/PhaseII_4-v01.xml (Tic-Visit :proposal-id 15679 :visit-id "01" :proposal-version -1 :trans-version "20181130" :date "25-Jan-2019 14:06:04.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 383152 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15679 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15679 :visit-id "01" :orbit-number 1 :pcs-time 241 :si-vis-utilization 2890 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3157 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15679 :visit-id "01" :orbit-number 2 :pcs-time 256 :si-vis-utilization 2871 :al-overhead-at-end 0 :internal-dur-at-end 450 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3157 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 26.2.2. CASM was run in Phase II mode. /Users/tiarahung/Documents/work/AT2018hco/OP-cache/PhaseII_4-v02.xml (Tic-Visit :proposal-id 15679 :visit-id "02" :proposal-version -1 :trans-version "20181130" :date "25-Jan-2019 14:07:04.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 226576 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15679 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15679 :visit-id "02" :orbit-number 1 :pcs-time 241 :si-vis-utilization 2878 :al-overhead-at-end 0 :internal-dur-at-end 450 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3157 :pcs-gap 0 ) true 1 0