Abstract
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 (kT ~ 85 eV) that was ≳90 times brighter in the 0.2−2 keV band than a previous 3σ upper flux detection limit (with no archival X-ray detection at this position). The ZTF optical light curve in the ~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 ([Fe X] 6375 Å, [Fe XIV] 5303 Å), along with persistent broad Balmer emission lines (FWHM ~ 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.
Highlights
Accreting supermassive black holes (SMBHs) have long been known to exhibit large amplitude flaring behaviour (e.g. Tohline & Osterbrock 1976; Antonucci & Cohen 1983; Penston & Pérez 1984; Shappee et al 2014; Storchi-Bergmann et al.2017; Frederick et al 2019), whereby multi-epoch observations of galaxy nuclei, over year-long timescales, have revealed drastic changes in their luminosity
This paper presents an overview of a set of multi-wavelength observations of an exceptional nuclear transient, AT 2019avd, whose main observed features are as follows: 1. eROSITA detected an ultra-soft X-ray brightening ( 90 times brighter than a previous 3σ upper flux limit) from a previously X-ray-inactive galaxy (Sect. 2). 2
AT 2019avd shows a net brightening in the 0.2−2 keV band by a factor of at least 600 relative to the 3σ upper detection limit derived from an XMM-Newton pointing in 2015
Summary
Accreting supermassive black holes (SMBHs) have long been known to exhibit large amplitude flaring behaviour (e.g. Tohline & Osterbrock 1976; Antonucci & Cohen 1983; Penston & Pérez 1984; Shappee et al 2014; Storchi-Bergmann et al.2017; Frederick et al 2019), whereby multi-epoch observations of galaxy nuclei, over year-long timescales, have revealed drastic changes in their luminosity. Whilst the sample of ignition events in galactic nuclei was previously limited to only a few objects, the advance of widefield, high-cadence surveys over the last decade has facilitated the discovery of an increasing number of extreme state changes. This has resulted in tighter constraints on the timescales of flaring events for these systems. No X-ray source has previously been detected at the location of AT 2019avd Using both the Upper Limit Server and webPIMMS5, and assuming an absorbed black-body spectral model with kT = 80 eV, and Galactic neutral hydrogen column density
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