The nascent milliquasar VT J154843.06+220812.6: tidal disruption event or extreme accretion-state change?

Abstract

We present detailed multiwavelength follow up of a nuclear radio flare, VT J154843.06+220812.6, hereafter VT J1548. VT J1548 was selected as a $\sim 1$ mJy radio flare in 3 GHz observations from the VLA Sky Survey (VLASS). It is located in the nucleus of a low mass ($\log M_{\mathrm{BH}}/M_{\odot }\sim 6$) host galaxy with weak or no past AGN activity. VT J1548 is associated with a slow rising (multiple year), bright mid IR flare in the WISE survey, peaking at $\sim 10\%L_{edd.}$. No associated optical transient is detected, although we cannot rule out a short, early optical flare given the limited data available. Constant late time ($\sim 3$ years post-flare) X-ray emission is detected at $\sim 10^{42}$ erg s${}^{-1}$. The radio SED is consistent with synchrotron emission from an outflow incident on an asymmetric medium. A follow-up, optical spectrum shows transient, bright, high-ionization coronal line emission ($[\mathrm{Fe}\mathrm{X}]\lambda 6375,[\mathrm{Fe}\mathrm{XI}]\lambda 7894,[\mathrm{S}\mathrm{XII}]\lambda 7612$). Transient broad H$\alpha$ is also detected but without corresponding broad H$\beta$ emission, suggesting high nuclear extinction. We interpret this event as either a tidal disruption event or an extreme flare of an active galactic nucleus, in both cases obscured by a dusty torus. Although these individual properties have been observed in previous transients, the combination is unprecedented. This event highlights the importance of searches across all wave bands for assembling a sample of nuclear flares that spans the range of observable properties and possible triggers.

30 pages, 11 figures, 5 tables. Submitted to ApJ

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