Caltech-NRAO Stripe 82 Survey (CNSS). III: The First Radio-discovered Tidal Disruption Event, CNSS J0019+00

Abstract

We present the discovery of a nuclear transient with the Caltech-NRAO Stripe 82 Survey (CNSS), a dedicated radio transient survey carried out with the Karl G. Jansky Very Large Array (VLA). This transient, CNSS J001947.3+003527, exhibited a turn-on over a timescale of $\lesssim$1 yr, increasing in flux density at 3 GHz from $<0.14$ mJy in 2014 February to $4.4\pm 0.1$ mJy in 2015 March, reaching a peak luminosity of $5\times 10^{28}{\text{erg s}}^{-1}{\text{Hz}}^{-1}$ around 2015 October. The association of CNSS J0019+00 with the nucleus (Gaia and our VLBI positions are consistent to within 1 pc) of a nearby S0 Seyfert galaxy at 77 Mpc, together with the radio spectral evolution, implies that this transient is most likely a tidal disruption event (TDE). Our equipartition analysis indicates the presence of a $\sim$15,000 km s${}^{-1}$ outflow, having energy $\sim$10${}^{49}$ erg. We derive the radial density profile for the circumnuclear material in the host galaxy to be proportional to $R^{-2.5}$. All of these properties suggest resemblance with radio-detected thermal TDEs like ASASSN-14li and XMMSL1 J0740-85. No significant X-ray or optical emission is detected from CNSS J0019+00, although this may simply be due to the thermal emission being weak during our late-time follow-up observations. From the CNSS survey we have obtained the first unbiased measurement of the rate of radio TDEs, $R(>500\hat{I}¼\mathrm{Jy})$ of about $2\times 10^{-3}$ deg${}^{-2}$, or equivalently a volumetric rate of about 10 Gpc${}^{-3}$ yr${}^{-1}$. This rate implies that all-sky radio surveys such as the VLA Sky Survey and those planned with ASKAP, will find many tens of radio TDEs over the next few years.

18 pages, 7 figures, ApJ accepted