Discovery and Follow-up of ASASSN-23bd (AT 2023clx): The Lowest Redshift and Least Luminous Tidal Disruption Event To Date

We report the All-Sky Automated Survey for SuperNovae discovery of the tidal disruption event (TDE) ASASSN-23bd (AT 2023clx) in NGC 3799, a LINER galaxy with no evidence of strong AGN activity over the past decade. With a redshift of \(z = 0.01107\) and a peak UV/optical luminosity of \((5.4\pm0.4)\times10^{42}\) erg s\(^{-1}\), ASASSN-23bd is the lowest-redshift and least-luminous TDE discovered to date. Spectroscopically, ASASSN-23bd shows H\(\alpha\) and He I emission throughout its spectral time series, and the UV spectrum shows nitrogen lines without the strong carbon and magnesium lines typically seen for AGN. Fits to the rising ASAS-SN light curve show that ASASSN-23bd started to brighten on MJD 59988\(^{+1}_{-1}\), \(\sim\)9 days before discovery, with a nearly linear rise in flux, peaking in the \(g\) band on MJD \(60000^{+3}_{-3}\). Scaling relations and TDE light curve modelling find a black hole mass of \(\sim\)10\(^6\) \(M_\odot\), which is on the lower end of supermassive black hole masses. ASASSN-23bd is a dim X-ray source, with an upper limit of \(L_{0.3-10\,\mathrm{keV}} < 1.0\times10^{40}\) erg s\(^{-1}\) from stacking all \emph{Swift} observations prior to MJD 60061, but with soft (\(\sim 0.1\) keV) thermal emission with a luminosity of \(L_{0.3-2 \,\mathrm{keV}}\sim4\times10^{39}\) erg s\(^{-1}\) in \emph{XMM-Newton} observations on MJD 60095. The rapid \((t < 15\) days) light curve rise, low UV/optical luminosity, and a luminosity decline over 40 days of \(\Delta L_{40}\approx-0.7\) make ASASSN-23bd one of the dimmest TDEs to date and a member of the growing ``Low Luminosity and Fast'' class of TDEs.