AT 2017gbl: a dust obscured TDE candidate in a luminous infrared galaxy

Abstract

We present the discovery with Keck of the extremely infrared (IR) luminous transient AT 2017gbl, coincident with the Northern nucleus of the luminous infrared galaxy (LIRG) IRAS 23436+5257. Our extensive multi-wavelength follow-up spans ∼900 days, including photometry and spectroscopy in the optical and IR, and (very long baseline interferometry) radio and X-ray observations. Radiative transfer modelling of the host galaxy spectral energy distribution and long-term pre-outburst variability in the mid-IR indicate the presence of a hitherto undetected dust obscured active galactic nucleus (AGN). The optical and near-IR spectra show broad ∼2000 km s−1 hydrogen, He i and O i emission features that decrease in flux over time. Radio imaging shows a fast evolving compact source of synchrotron emission spatially coincident with AT 2017gbl. We infer a lower limit for the radiated energy of 7.3 × 1050 erg from the IR photometry. An extremely energetic supernova would satisfy this budget, but is ruled out by the radio counterpart evolution. Instead, we propose AT 2017gbl is related to an accretion event by the central supermassive black hole, where the spectral signatures originate in the AGN broad line region and the IR photometry is consistent with re-radiation by polar dust. Given the fast evolution of AT 2017gbl, we deem a tidal disruption event (TDE) of a star a more plausible scenario than a dramatic change in the AGN accretion rate. This makes AT 2017gbl the third TDE candidate to be hosted by a LIRG, in contrast to the so far considered TDE population discovered at optical wavelengths and hosted preferably by post-starburst galaxies.

Accretion disc cooling and narrow absorption lines in the tidal disruption event AT 2019dsg

“…In contrast, ASASSN-14li (e.g. Bright et al (2018)) and AT2017gbl (Kool et al 2020), showed their 5.0 GHz emission peaking around 150 d. At the end of our followup, the 5.0 GHz emission from AT 2019dsg has still not reached its peak. We stress that a comparison of phases across different TDEs is not straightforward, as the reference time is defined differently in different works (e.g.…”

Reverberation in Tidal Disruption Events: Dust Echoes, Coronal Emission Lines, Multi-wavelength Cross-correlations, and QPOs

“…A radio detection rate of 75% is much higher than the fraction of AGN or TDEs that are considered to be "radio-loud", see the Radio Chapter (Alexander et al 2020) of this journal. Based on this high detection rate and similarities with other radio-detected IR flares (Mattila et al 2018;Kool et al 2020), one could conclude that IR-selection yields a subclass of TDEs that are often accompanied by powerful jets, with total energies of ∼ 10 52 erg. Such powerful jetted TDEs have been observed before (see Bloom et al 2011;Zauderer et al 2011, and the Gamma-ray Chapter of this book), although the estimated volumetric rate of these events, ∼ 1 Gpc −1 yr −1 or ∼ 10 −7 galaxy −1 yr −1 (van Velzen et al 2018), is three orders of magnitude lower than typical rate of WISE-selected flares from galaxies with SDSS spectra (∼ 10 −4 galaxy −1 yr −1 ; Wang et al 2018).…”

“…The transient nature of the IR at emission was confirmed later via WISE observations at 12 µm (the WISE W3 band), obtained 1.9 year after the Spitzer spectrum, showing that the flux at this wavelength has decreased by a factor of 1.8 (Dou et al 2016). (Mattila et al 2018), 12 (Kool et al 2020), 13 14 (Komossa et al 2009), 15 (Dou et al 2016) Infrared photometric data of optically discovered TDEs are often obtained from the Wide-field Infrared Space Explorer (WISE; Wright et al 2010) which has been systematically scanning the whole sky in 3.4 and 4.6 µm since 2010. The primary (cryogenic, 4 band) and post-cryogenic WISE surveys ran from Jan 2010 to Feb 2011.…”

“…This approach only works if the observed IR emission is dominated by a single shell that emits at the sublimation temperature. If the dust echo contains emission from larger scales (e.g., because the galaxy is devoid of dust at R < R sub ), a measurement of the dust temperature is also needed to estimate L bol (e.g., Kool et al 2020).…”

“…Better spectroscopic coverage is one area of improvement for future near-IR follow-up observations. And with well-sampled IR light curves, we can obtain a better handle of the geometry of the dust, allowing us to differentiate between a torus model (Dou et al 2016;) and a polar or spherical dust geometry (van Velzen et al 2016;Mattila et al 2018;Kool et al 2020).…”