DOI

https://doi.org/10.48717/eq4b-5438

Project

ES104

Title

ES104

Abstract

The optical transient AT2017bgt is suggested to be a new type of flares from accreting supermassive black holes (SMBHs), characterized by a long-term emission of intensified accretion that remains constant in continuum and emission lines over a period of at least one year. Using the WISE survey data, we have identified the mid-IR outbursts likely associated with UV/optical ones, but found the post-peak mid-IR emission is declined to a quiescent level after a plateau period of about 500 days. By analyzing the archival VLASS, ASKAP as well as follow-up GMRT observations, we find unambiguous evidence of radio brightening and dramatic evolution in the radio spectral index from steep to rising shape within ~2 years. Such a spectral reversal is unlike anything seen so far associated with the transient accretion onto SMBHs. Interestingly, our recent VLBA observations detect two resolved emission components, which are however evolving rapidly within only half year, the nature of which is unknown. Here we request timely EVN+e-MERLIN observations at C-band, aiming to corroborate the resolved structure, and potentially detect other fainter structures, which are critical to tell whether it is associated with discrete jet ejections.