Kavli Affiliate: Dheeraj R. Pasham
| First 5 Authors: Yanan Wang, Ranieri D. Baldi, Santiago del Palacio, Muryel Guolo, Xiaolong Yang
| Summary:
AT 2019avd is a nuclear transient detected from infrared to soft X-rays,
though its nature is yet unclear. The source has shown two consecutive flaring
episodes in the optical and the infrared bands and its second flare was covered
by X-ray monitoring programs. During this flare, the UVOT/Swift photometries
revealed two plateaus: one observed after the peak and the other one appeared
~240 days later. Meanwhile, our NICER and XRT/Swift campaigns show two declines
in the X-ray emission, one during the first optical plateau and one 70-90 days
after the optical/UV decline. The evidence suggests that the optical/UV could
not have been primarily originated from X-ray reprocessing. Furthermore, we
detected a timelag of ~16-34 days between the optical and UV emission, which
indicates the optical likely comes from UV reprocessing by a gas at a distance
of 0.01-0.03 pc. We also report the first VLA and VLBA detection of this source
at different frequencies and different stages of the second flare. The
information obtained in the radio band – namely a steep and a late-time
inverted radio spectrum, a high brightness temperature and a radio-loud state
at late times – together with the multiwavelength properties of AT 2019avd
suggests the launching and evolution of outflows such as disc winds or jets. In
conclusion, we propose that after the ignition of black hole activity in the
first flare, a super-Eddington flaring accretion disc formed and settled to a
sub-Eddington state by the end of the second flare, associated with a compact
radio outflow.
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