AstroNote 2020-105

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2020-05-17 14:33:45
Type: Object/s-Discovery/Classification
AT 2019avd shows strong Bowen fluorescence emission lines
Authors: Benny Trakhtenbrot (TAU), Iair Arcavi (TAU), Claudio Ricci (UDP), Jamie Burke (UCSB/LCO)
Keywords: AGN, Spectroscopy
AT 2019avd was recently reported as a new soft X-ray transient by eROSITA (ATel #13712). Our optical spectrum from 2020 May 10 (LCO/FLOYDS-S) shows AGN-like emission lines, specifically a broad Halpha line (similar to NLSy1s), and exceptionally strong HeII4686 and NII4640 emission lines, driven by Bowen fluorescence. AT 2019avd therefore resembles the recently identified class of nuclear transients with such emission features (e.g., AT 2017bgt). These Bowen fluorescence lines were not present in a 2019 March 15 spectrum (Gezari et al., ATel #13717). We are pursuing additional spectroscopic and multi-wavelength observations.

AT 2019avd (also known as ZTF19aaiqmgl) was originally identified as an optical transient in the nucleus of a galaxy at z=0.0296, by the ZTF, on 2019 Feb. 9. After about 10 months of decline, it had another, slower re-brightening, reaching a secondary peak during April 2020. On April 28, eROSITA detected a new, ultra-soft X-ray source associated with the galaxy nucleus (ATel #13712), with a factor 90x (at least) increase in X-ray emission, compared to an XMM upper-limit from 2015.

We obtained an optical spectrum of AT 2019avd using the Las Cumbres Observatory 2m telescope at Siding Spring (Australia) during the night of 2020 May 10. The spectrum, publicly available on the TNS, shows a combination of AGN-like emission lines (specifically, NLSy1-like Halpha) as well as a very strong, broad and double-peaked emission feature around 4680A (rest-frame), consistent with the expected location of the HeII4686 and NII4640 emission lines. These features are much stronger than seen in AGN, with F(NIII)~F(HeII) > 0.5*F(Hbeta), and are likely driven by intense UV flares which excite the relevant Bowen fluorescence (BF) cascades. Our new optical spectrum of AT 2019avd therefore shares the key characteristics of the recently identified class of transients from accreting SMBHs, with the best-studied example being AT 2017bgt (Trakhtenbrot+19, Nature Astronomy, 3, 242). We note that Gezari et al. (ATel #13717) report that an earlier spectrum of AT 2019avd, obtained on 2019 Mar 15, shows no such BF features. Since similar (transient) BF features have also been reported in a number TDE candidates (Leloudas+19, ApJ 887, 218; Gezari+19, ATel #13127) and other nuclear transients (Neustadt+20, MNRAS 494, 2538), the nature of AT 2019avd remains unclear.

We are pursuing additional spectroscopic and multi-wavelength observations of AT 2019avd.

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