The radio detection and accretion properties of the peculiar nuclear transient AT 2019avd

Abstract

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.

Rapid Dimming Followed by a State Transition: A Study of the Highly Variable Nuclear Transient AT 2019avd over 1000+ Days

“…In jetted TDEs, Zauderer et al (2013) argue that the rapid decline in X-rays of Swift 1644 + 57 corresponds to the closure of the relativistic jet, which is most likely a consequence of the decrease in the mass accretion rate below the critical value. Regarding AT 2019avd, although there are only four radio detections spanning roughly 3 yr, Wang et al (2023) were able to fit the multi-epoch radio SED with the self-absorbed synchrotron model developed by Granot & Sari (2002), and the result suggests that the radio flux increases by 50% during the flaring episode (from Day -106 to 348). The latest VLASS detection of AT 2019avd on Day 846 reveals a further increase in flux density at 3 GHz to 2.8 mJy.…”

“…The ultrasoft X-ray spectrum and optical spectral lines of AT 2019avd are consistent with a TDE, but the two consecutive optical flares are atypical of TDEs. In addition, Wang et al (2023) report the radio detection of this transient with the Very Large Array and Very Long Baseline Array (VLBA), suggesting the possible ejection of a compact radio outflow (jet or wind) when the sources moved to a lowluminosity state.…”

“…Figure 1(d) shows the optical and UV photometries derived from the g band of ZTF and the UVW1 band of Swift with symbols of dots and stars. These data are taken from Wang et al (2023) and have been corrected for the Galactic extinction and contribution from the host galaxy. Figure 1(e) shows the X-ray-to-optical/UV luminosity ratio: the dots represent the ratios of luminosity inferred from NICER and ZTF, and the stars represent the ratios of luminosity inferred from XRT and UVOT.…”

“…If the inner region of the accretion disk evaporates into a thick ADAF as the mass accretion rate decreases, this could result in a gradually hardening X-ray spectrum. To interpret the fast formation of the accretion disk in AT 2019avd, Wang et al (2023) propose a slim disk with a large height-to-radius ratio to reduce the viscous timescale. If the late-time X-ray emission in AT 2019avd could be attributed to an inefficiently radiated ADAF due to a decrease in the mass accretion rate, a slim disk to ADAF evolution is required.…”

Dissonance in Harmony: The UV/Optical Periodic Outbursts of ASASSN-14ko Exhibit Repeated Bumps and Rebrightenings

“…Rebrightenings are a common phenomenon in TDEs when the sources are monitored for a long enough duration (Jiang et al 2019;Yao et al 2023). They can be caused by the circularization of debris and delayed accretion following the initiation disruption (Chen et al 2022;Wang et al 2023). Other potential explanations for rebrightenings include partial TDE Malyali et al 2023;Wevers et al 2023) and SMBHB systems (Liu et al 2014;Shu et al 2020).…”