Time-varying double-peaked emission lines following the sudden ignition of the dormant galactic nucleus AT2017bcc
arxiv(2023)
摘要
We present a pan-chromatic study of AT2017bcc, a nuclear transient that was
discovered in 2017 within the skymap of a reported burst-like gravitational
wave candidate, G274296. It was initially classified as a superluminous
supernova, and then reclassified as a candidate tidal disruption event. Its
optical light curve has since shown ongoing variability with a structure
function consistent with that of an active galactic nucleus, however earlier
data shows no variability for at least 10 years prior to the outburst in 2017.
The spectrum shows complex profiles in the broad Balmer lines: a central
component with a broad blue wing, and a boxy component with time-variable blue
and red shoulders. The Hα emission profile is well modelled using a
circular accretion disc component, and a blue-shifted double Gaussian which may
indicate a partially obscured outflow. Weak narrow lines, together with the
previously flat light curve, suggest that this object represents a dormant
galactic nucleus which has recently been re-activated. Our time-series
modelling of the Balmer lines suggests that this is connected to a disturbance
in the disc morphology, and we speculate this could involve a sudden violent
event such as a tidal disruption event involving the central supermassive black
hole, though this cannot be confirmed, and given an estimated black hole mass
of ≳10^7-10^8 M_⊙ instabilities in an existing disc may be more
likely. Although we find that the redshifts of AT2017bcc (z=0.13) and G274296
(z>0.42) are inconsistent, this event adds to the growing diversity of both
nuclear transients and multi-messenger contaminants.
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