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

Abstract

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 $(5.4\pm 0.4)\times 10^{42}$ 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 $L_{0.3-10\mathrm{keV}}<1.0\times 10^{40}$ 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 $L_{0.3-2\mathrm{keV}}\sim 4\times 10^{39}$ 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.

18 pages, 14 figures, published in MNRAS

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