Unraveling the Nature of the Nuclear Transient AT2020adpi

Transient events associated with supermassive black holes provide rare opportunities to study accretion and the environments of supermassive black holes. We present a multiwavelength study of AT2020adpi (ZTF20acvfraq), a luminous optical/UV transient in the nucleus of the galaxy WISEA J231853.77-103505.6 (z=0.26) that exhibits the properties of an ambiguous nuclear transient. Near peak, its spectral energy distribution is well described by a power law ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>λ</mml:mi> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>λ</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msup> <mml:mi>λ</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mi>α</mml:mi> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>α</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.44</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:math> ), with a maximum <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>g</mml:mi> </mml:math> -band luminosity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mn>3.6</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.6</mml:mn> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>44</mml:mn> </mml:msup> </mml:mrow> </mml:math> erg s, which is consistent with luminous AGN flares. We detect a strong mid-infrared flare ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msubsup> <mml:mi>L</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> <mml:mi mathvariant="normal">a</mml:mi> <mml:mi mathvariant="normal">k</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> <mml:mi mathvariant="normal">I</mml:mi> <mml:mi mathvariant="normal">R</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mn>2.3</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.05</mml:mn> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>44</mml:mn> </mml:msup> </mml:mrow> </mml:math> erg s) delayed by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mo>∼</mml:mo> </mml:math> 210 rest-frame days, indicating a hot dust echo from material at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mo>∼</mml:mo> </mml:math> 0.2 pc. The optical and near-infrared spectra show broad H, He I, [OIII] lines, as well as narrow Fe II, and prominent Mg II, which is a combination not typical of TDEs. Taken together, these features suggest AT2020adpi is an ambiguous nuclear transient, where an accretion episode was triggered by stellar disruption of an accretion disk or instabilities within an active nucleus. This source demonstrates the need for careful multiwavelength analysis to distinguish between extreme AGN variability and TDEs.