Delayed and Fast-rising Radio Flares from an Optical and X-Ray-detected Tidal Disruption Event in the Center of a Dwarf Galaxy

Abstract

AT 2018cqh is a unique tidal disruption event (TDE) discovered in a dwarf galaxy. Both the light-curve fitting and galaxy scaling relationships suggest a central black hole mass in the range of 5.9 < logM BH/M ☉ < 6.4. The r-band peak luminosity is ∼ 1043 erg s−1, making AT 2018cqh relatively faint among known optical TDEs. A delayed X-ray brightening was found around 590 days after the optical discovery but shows an unusually long time rising to peak over at least 558 days, which could be coming from delayed accretion of a newly forming debris disk. We report the discovery of delayed radio flares around 1105 days since its discovery, characterized by an initial steep rise of ≳175 days, a flattening lasting about 544 days, and a phase with another steep rise. The rapid rise in radio flux coupled with the slow decay in the X-ray emission points to a delayed launching of outflow, perhaps due to a transition in the accretion state. However, known accretion models can hardly explain the origins of the secondary radio flare that is rising even more rapidly in comparison with the initial one. If confirmed, AT 2018cqh would be a rare faint TDE in a dwarf galaxy exhibiting optical, X-ray, and radio flares. We call for continued multifrequency radio observations to monitor its spectral and temporal evolution, which may help to reveal new physical processes that are not included in standard TDE models.