The fast transient AT 2023clx in the nearby LINER galaxy NGC 3799 as a tidal disruption of a very low-mass star

We present an extensive analysis of the optical and UV properties of AT2023clx, the closest TDE to date, that occurred in the nucleus of the interacting LINER galaxy, NGC3799 (z=0.01107). After correcting for the host reddening (E(B-V) = 0.179 mag), we find its peak absolute g-band magnitude to be -18.03{+/-}0.07 mag, and its peak bolometric luminosity to be L=(1.57{+/-}0.19)x10^43 erg/s. AT2023clx displays several distinctive features: first, it rose to peak within 10.4{+/-}2.5 days, making it the fastest rising TDE to date. Our SMBH mass estimate of M_BH ~ 10^6 Msol rules out the possibility of an intermediate-mass BH as the reason for the fast rise. Dense spectral follow-up reveals a blue continuum that cools slowly and broad Balmer and HeII lines as well as weak HeI 5876,6678 emission features that are typically seen in TDEs. The early, broad (width ~ 15000 km/s) profile of Ha matches theoretical expectations from an optically thick outflow. A flat Balmer decrement (~ 1.58) suggests that the lines are collisionally excited rather than being produced via photoionisation, in contrast to typical active galactic nuclei. A second distinctive feature, seen for the first time in TDE spectra, is a sharp, narrow emission peak at a rest wavelength of ~ 6353 A. This feature is clearly visible up to 10d post-peak; we attribute it to clumpy material preceding the bulk outflow, which manifests as a high-velocity component of Ha (-9584 km/s). Its third distinctive feature is the rapid cooling during the first ~ 20 days after peak, reflected as a break in the temperature evolution. Combining these findings, we propose a scenario for AT2023clx involving the disruption of a very low-mass star (<=0.1 Msol) with an outflow launched in our line of sight and with disruption properties that led to efficient circularisation and prompt accretion disc formation, observed through a low-density photosphere.
Comment: Accepted for publication in Astronomy and Astrophysics (in production; official acceptance date: 28/06/2024)