UV and Optical Signatures of Late-time Disk Instabilities in Tidal Disruption Events

Abstract

Tidal disruption events (TDEs) are unique probes of evolving accretion in supermassive black holes. Recent models of TDE disks show that they undergo brief thermal instabilities with temporal super-Eddington accretion at late times, which has been suggested as a possibility to explain the ubiquitous late radio emergence in TDEs. We model the ultraviolet (UV) and optical signatures of such disk instabilities, expected from the accretion power being reprocessed by the optically-thick outflow following super-Eddington accretion. Our model predicts brief UV-bright transients lasting for days, with luminosities of $10^{42}$-$10^{43}$ erg s$^{-1}$ in near-UV and $10^{41}$-$10^{42}$ erg s$^{-1}$ in optical for a typical TDE by a $10^6~M_\odot$ black hole. These could be detectable by near-future surveys such as ULTRASAT, Vera C. Rubin Observatory and Argus Array, for TDEs of redshifts out to $\approx 0.1$. We further conduct a search for these transients in existing nearby TDEs using data from the Zwicky Transient Facility, placing upper limits on the flare rate for each TDE of $1$-$2$ yr$^{-1}$ dependent on the outflow mass. In the era of future surveys, combined UV/optical and radio monitoring would be an important test to the disk instability phenomena, as well as its explanation for the late-time radio emission in TDEs.

Figures (8)

• Figure 1: Light curve predictions for parameters: $M_{\rm BH}=10^6M_\odot$, $r_d=300R_g$, $M_{\rm fl}=0.02~M_\odot$, and $t_{\rm fl}=2$ days. Top panels show the bolometric luminosity and emission temperature, and the bottom right panel shows the AB magnitudes in the near-UV and optical under a greybody approximation. Bottom left panel shows the evolution of various radii defined in Section \ref{['sec:temperature']}.
• Figure 2: Emission properties in the near-UV (250 nm). Left and right panels respectively show the peak magnitude and duration within 1 mag from peak. Top and bottom panels are respectively for $M_{\rm BH}=10^6, 10^7~M_\odot$. Vertical dashed lines show the radius $r_d\approx 5r_T$ inspired from the model of Piro25, where $r_T$ is the tidal radius (see main text).
• Figure 3: Same as Figure \ref{['fig:param_study_optical']}, but for the optical (600 nm).
• Figure 4: Redshift and epoch of last ZTF detection (relative to discovery), for the 28 TDE samples analyzed in this work. Stars show three TDEs used for constraints on the flare rate, AT 2019qiz, AT 2019azh, and AT 2021ehb (see main text).
• Figure 5: The two highest-significance candidates found in our analysis, from AT2019azh and AT2021nwa. Left panels show the full light curve, while the right panels show that at $\pm 100$ days around the candidate. Shaded regions show the temporal regions used in our search for the flux comparison.