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 $(5.4\pm0.4)\times10^{42}$ 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.

• The paper reports the discovery and follow-up of ASASSN-23bd, the lowest redshift and least luminous tidal disruption event, broadening the empirical framework of TDEs.
• Methodologies include multi-wavelength monitoring that revealed a rapid (<15 days) linear light curve rise and distinct spectroscopic features such as prominent Hα emission.
• Implications of the study challenge existing TDE models by highlighting how low-luminosity events in LINER galaxies provide new insights into accretion disk formation and emission mechanisms.

Analysis of ASASSN-23bd: A Low-Redshift, Low-Luminosity Tidal Disruption Event

The paper "Discovery and Follow-up of ASASSN-23bd (AT~2023clx)" presents an in-depth study of ASASSN-23bd, identified as the lowest redshift and least luminous tidal disruption event (TDE) discovered to date. Occurring in the galaxy NGC 3799, ASASSN-23bd provides critical insights into the nature and properties of low-luminosity TDEs. The significance of this study lies in the observation of a TDE in a nearby galaxy classified as a LINER without significant AGN activity, reinforcing the diversity and complexity of TDE phenomena in varying galactic environments.

Observational Highlights

Key parameters of ASASSN-23bd include its redshift of $z=0.01107$ and a modest peak UV/optical luminosity of $(5.4\pm0.4)\times10^{42}$ erg s$^{-1}$. Spectroscopic analysis reveals prominent H$\alpha$ emission, with nitrogen lines in the UV spectrum but lacking the carbon lines typical of AGN emissions. X-ray observations noted dimness at $L_{0.3-10\,\mathrm{keV}} < 1.0\times10^{40}$ erg s$^{-1}$, consistent with its classification as an LLAGN.

The early light curve, captured by ASAS-SN and other surveys, showcases a rapid ($t < 15$ days) linear rise that contrasts with the more gradual increases seen in some other TDEs, suggesting potential variability in stellar disruption mechanisms.

The Black Hole Mass and TDE Classification

The study's modeling efforts indicate a black hole mass of approximately $10^6 M_\odot$, which is smaller than those typically associated with "normal" TDEs. ASASSN-23bd falls into the recently recognized class of Low Luminosity and Fast (LLaF) TDEs, exemplifying a rapid, linear rise but with significantly reduced peak luminosity.

The peak-decline relationship for ASASSN-23bd, characterized by $\Delta L_{40}\approx-0.7$, places it within the spectrum of other known LLaF events, further refining the criteria and understanding of this subclass of TDEs.

Implications and Future Directions

ASASSN-23bd broadens the spectrum of known TDE behavior by contributing data from an atypical low-luminosity event. The consistent blackbody temperature and rapid decline of the light curve extend the diversity of TDE characteristics, emphasizing the need for continued surveillance of low-redshift galaxies for further low-luminance discoveries.

From a theoretical perspective, ASASSN-23bd challenges existing models of TDE emission and accretion disk formation. The potential independence of rise behavior (linear versus quadratic) in relation to AGN activity in the host galaxy calls for new models examining how the pre-existing nuclear environment influences the TDE's development and observability.

The presented research advances the theoretical framework needed for understanding the physical and observational coupling between TDEs and their host environments, particularly in low-luminosity contexts. Future telescopes and sky surveys, like the Vera C. Rubin Observatory, can provide deeper insights through their ability to capture faint and fast transients, expanding our comprehension of these cosmic phenomena.