AT 2019avd: A novel addition to the diverse population of nuclear transients. (arXiv:2101.08760v1 [astro-ph.HE])

AT 2019avd: A novel addition to the diverse population of nuclear transients. (arXiv:2101.08760v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Malyali_A/0/1/0/all/0/1">A. Malyali</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rau_A/0/1/0/all/0/1">A. Rau</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Merloni_A/0/1/0/all/0/1">A. Merloni</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nandra_K/0/1/0/all/0/1">K. Nandra</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Buchner_J/0/1/0/all/0/1">J. Buchner</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_Z/0/1/0/all/0/1">Z. Liu</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:+Sollerman_J/0/1/0/all/0/1">J. Sollerman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shappee_B/0/1/0/all/0/1">B. Shappee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Trakhtenbrot_B/0/1/0/all/0/1">B. Trakhtenbrot</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Arcavi_I/0/1/0/all/0/1">I. Arcavi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ricci_C/0/1/0/all/0/1">C. Ricci</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Velzen_S/0/1/0/all/0/1">S. van Velzen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Goobar_A/0/1/0/all/0/1">A. Goobar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Frederick_S/0/1/0/all/0/1">S. Frederick</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kawka_A/0/1/0/all/0/1">A. Kawka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tartaglia_L/0/1/0/all/0/1">L. Tartaglia</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Burke_J/0/1/0/all/0/1">J. Burke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hiramatsu_D/0/1/0/all/0/1">D. Hiramatsu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schramm_M/0/1/0/all/0/1">M. Schramm</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Boom_D/0/1/0/all/0/1">D. van der Boom</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Anderson_G/0/1/0/all/0/1">G. Anderson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Miller_Jones_J/0/1/0/all/0/1">J. C. A. Miller-Jones</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bellm_E/0/1/0/all/0/1">E. Bellm</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Drake_A/0/1/0/all/0/1">A. Drake</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Duev_D/0/1/0/all/0/1">D. Duev</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fremling_C/0/1/0/all/0/1">C. Fremling</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Graham_M/0/1/0/all/0/1">M. Graham</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Masci_F/0/1/0/all/0/1">F. Masci</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rusholme_B/0/1/0/all/0/1">B. Rusholme</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Soumagnac_M/0/1/0/all/0/1">M. Soumagnac</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Walters_R/0/1/0/all/0/1">R. Walters</a>

We report on SRG/eROSITA, ZTF, ASAS-SN, Las Cumbres, NEOWISE-R, and Swift
XRT/UVOT observations of the unique ongoing event AT 2019avd, located in the
nucleus of a previously inactive galaxy at $z=0.029$. eROSITA first observed AT
2019avd on 2020-04-28 during its first all sky survey, when it was detected as
an ultra-soft X-ray source ($kTsim 85$ eV) that was $gtrsim 90$ times
brighter in the $0.2-2$ keV band than a previous 3$sigma$ upper flux detection
limit (with no archival X-ray detection at this position). The ZTF optical
light curve in the $sim 450$ days preceding the eROSITA detection is double
peaked, and the eROSITA detection coincides with the rise of the second peak.
Follow-up optical spectroscopy shows the emergence of a Bowen fluorescence
feature and high-ionisation coronal lines ([ion{Fe}{X}] 6375 {AA},
[ion{Fe}{XIV}] 5303 {AA}), along with persistent broad Balmer emission lines
(FWHM$sim 1400$ km s$^{-1}$). Whilst the X-ray properties make AT 2019avd a
promising tidal disruption event (TDE) candidate, the optical properties are
atypical for optically selected TDEs. We discuss potential alternative origins
that could explain the observed properties of AT 2019avd, such as a stellar
binary TDE candidate, or a TDE involving a super massive black hole binary.

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