Discovery and follow-up of ASASSN-23bd (AT 2023clx): The lowest redshift and luminosity optically-selected tidal disruption event

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 ± 0.4) × 1042 erg s−1, ASASSN-23bd is the lowest-redshift and least-luminous TDE discovered to date. Spectroscopically, ASASSN-23bd shows Hα and He i emission throughout its spectral time series, there are no coronal lines in its NIR spectrum, 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$^{+1}_{-1}$, ∼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 ∼106 M⊙, which is on the lower end of supermassive black hole masses. ASASSN-23bd is a dim X-ray source, with an upper limit of L0.3 − 10 keV < 1.0 × 1040 erg s−1 from stacking all Swift observations prior to MJD 60061, but with soft (∼0.1 keV) thermal emission with a luminosity of L0.3 − 2 keV ∼ 4 × 1039 erg s−1 in 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 ΔL40 ≈ −0.7 dex make ASASSN-23bd one of the dimmest TDEs to date and a member of the growing ‘Low Luminosity and Fast’ class of TDEs.