Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event. (arXiv:1903.05637v1 [astro-ph.HE])

Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event. (arXiv:1903.05637v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hung_T/0/1/0/all/0/1">T. Hung</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cenko_S/0/1/0/all/0/1">S. B. Cenko</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Roth_N/0/1/0/all/0/1">Nathaniel Roth</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gezari_S/0/1/0/all/0/1">S. Gezari</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Veilleux_S/0/1/0/all/0/1">S. Veilleux</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Velzen_S/0/1/0/all/0/1">Sjoert Van Velzen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gaskell_C/0/1/0/all/0/1">C. Martin Gaskell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Foley_R/0/1/0/all/0/1">Ryan J. Foley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blagorodnova_N/0/1/0/all/0/1">N. Blagorodnova</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yan_L/0/1/0/all/0/1">Lin Yan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Graham_M/0/1/0/all/0/1">M. J. Graham</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brown_J/0/1/0/all/0/1">J. S. Brown</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Siebert_M/0/1/0/all/0/1">M. R. Siebert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Frederick_S/0/1/0/all/0/1">Sara Frederick</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ward_C/0/1/0/all/0/1">Charlotte Ward</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gatkine_P/0/1/0/all/0/1">Pradip Gatkine</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gal_yam_A/0/1/0/all/0/1">Avishay Gal-yam</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yang_Y/0/1/0/all/0/1">Yi Yang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schulze_S/0/1/0/all/0/1">S. Schulze</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dimitriadis_G/0/1/0/all/0/1">G. Dimitriadis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kupfer_T/0/1/0/all/0/1">Thomas Kupfer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shupe_D/0/1/0/all/0/1">David L. Shupe</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rusholme_B/0/1/0/all/0/1">Ben Rusholme</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Masci_F/0/1/0/all/0/1">Frank J. Masci</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Riddle_R/0/1/0/all/0/1">Reed Riddle</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Soumagnac_M/0/1/0/all/0/1">Maayane T. Soumagnac</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Roestel_J/0/1/0/all/0/1">J. Van Roestel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dekany_R/0/1/0/all/0/1">Richard Dekany</a>

We report the discovery of non-stellar hydrogen Balmer and metastable helium
absorption lines accompanying a transient, high-velocity (0.05$c$) broad
absorption line (BAL) system in the optical spectra of the tidal disruption
event (TDE) AT2018zr ($z=0.071$). In the HST UV spectra, absorption of high-
and low-ionization lines are also present at this velocity, making AT2018zr
resemble a low-ionization broad absorption line (LoBAL) QSO. We conclude that
these transient absorption features are more likely to arise in fast outflows
produced by the TDE than absorbed by the unbound debris. In accordance with the
outflow picture, we are able to reproduce the flat-topped H$alpha$ emission in
a spherically expanding medium, without invoking the typical prescription of an
elliptical disk. We also report the appearance of narrow
($sim$1000~km~s$^{-1}$) NIII$lambda$4640, HeII$lambda4686$, H$alpha$, and
H$beta$, emission in the late-time optical spectra of AT2018zr, which may be a
result of UV continuum hardening at late time as observed by Swift. Including
AT2018zr, we find a high association rate (3 out of 4) of BALs in the UV
spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and
may be less sensitive to viewing angle effects compared to QSO outflows.

We report the discovery of non-stellar hydrogen Balmer and metastable helium
absorption lines accompanying a transient, high-velocity (0.05$c$) broad
absorption line (BAL) system in the optical spectra of the tidal disruption
event (TDE) AT2018zr ($z=0.071$). In the HST UV spectra, absorption of high-
and low-ionization lines are also present at this velocity, making AT2018zr
resemble a low-ionization broad absorption line (LoBAL) QSO. We conclude that
these transient absorption features are more likely to arise in fast outflows
produced by the TDE than absorbed by the unbound debris. In accordance with the
outflow picture, we are able to reproduce the flat-topped H$alpha$ emission in
a spherically expanding medium, without invoking the typical prescription of an
elliptical disk. We also report the appearance of narrow
($sim$1000~km~s$^{-1}$) NIII$lambda$4640, HeII$lambda4686$, H$alpha$, and
H$beta$, emission in the late-time optical spectra of AT2018zr, which may be a
result of UV continuum hardening at late time as observed by Swift. Including
AT2018zr, we find a high association rate (3 out of 4) of BALs in the UV
spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and
may be less sensitive to viewing angle effects compared to QSO outflows.

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