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Double tidal disruption events or repeating partial tidal disruption events in AT 2020vdq

Zhang XueGuang

TL;DR

The study tackles whether AT 2020vdq’s two re-brightening flares arise from repeating partial TDEs or from two separate TDEs in a binary system. It adopts MOSFIT/TDEFIT to fit two TDE events, allowing different ($\beta$, $T_{vs}$, etc.) parameters and deriving the disrupted-star masses, $M_{*,1}$ and $M_{*,2}$, alongside a SMBH mass of $M_{BH}\approx5.4\times10^{5}\,M_\odot$. The best-fit results yield a substantial mass difference between the two disrupted stars (about $2.02\,M_\odot$ vs $0.36\,M_\odot$, or $1.50\,M_\odot$ vs $0.58\,M_\odot$ under a common-$\beta$ assumption), which is inconsistent with the expectation for repeating pTDEs where the remnant mass remains close to the original. This favors a double TDE scenario from a binary star system, where two distinct stars are tidally disrupted by the same SMBH. The authors propose future observations to test the repeating-pTDE hypothesis (e.g., a predicted next flare around 2026) and emphasize that the mass-comparison method offers a robust discriminant between the two scenarios.

Abstract

AT 2020vdq has been known as a candidate of repeating partial tidal disruption events (pTDEs), due to its two flares with a time interval of $\sim$1000 days. Here, a simplified method is proposed to test such repeating pTDEs scenario considering a main-sequence star tidally disrupted twice. For the two flares in AT 2020vdq if related to the repeating pTDEs scenario, theoretical TDE model determined stellar mass of the original star disrupted for the first flare should be not very different from the mass of the star (to trace the reminder of the original star) disrupted for the second flare, because a partial TDE with impact parameter $β$ smaller than 1 can lead to most of (probable higher than 90\%) the stellar mass also bound to the reminder of the original star. After considering theoretical TDE model applied to describe the two flares in AT 2020vdq, the model determined stellar masses are about 2${\rm M_\odot}$ and $0.36{\rm M_\odot}$ for the stars disrupted in the first flare and the second flare. The large mass difference cannot be reasonably expected by the repeating pTDEs with $β$ around 0.6 in AT 2020vdq. The results in this manuscript indicate that the repeating pTDEs scenario is not preferred at current stage in AT 2020vdq, but the probable double TDEs for two individual stars tidally disrupted should be currently recommended.

Double tidal disruption events or repeating partial tidal disruption events in AT 2020vdq

TL;DR

The study tackles whether AT 2020vdq’s two re-brightening flares arise from repeating partial TDEs or from two separate TDEs in a binary system. It adopts MOSFIT/TDEFIT to fit two TDE events, allowing different (, , etc.) parameters and deriving the disrupted-star masses, and , alongside a SMBH mass of . The best-fit results yield a substantial mass difference between the two disrupted stars (about vs , or vs under a common- assumption), which is inconsistent with the expectation for repeating pTDEs where the remnant mass remains close to the original. This favors a double TDE scenario from a binary star system, where two distinct stars are tidally disrupted by the same SMBH. The authors propose future observations to test the repeating-pTDE hypothesis (e.g., a predicted next flare around 2026) and emphasize that the mass-comparison method offers a robust discriminant between the two scenarios.

Abstract

AT 2020vdq has been known as a candidate of repeating partial tidal disruption events (pTDEs), due to its two flares with a time interval of 1000 days. Here, a simplified method is proposed to test such repeating pTDEs scenario considering a main-sequence star tidally disrupted twice. For the two flares in AT 2020vdq if related to the repeating pTDEs scenario, theoretical TDE model determined stellar mass of the original star disrupted for the first flare should be not very different from the mass of the star (to trace the reminder of the original star) disrupted for the second flare, because a partial TDE with impact parameter smaller than 1 can lead to most of (probable higher than 90\%) the stellar mass also bound to the reminder of the original star. After considering theoretical TDE model applied to describe the two flares in AT 2020vdq, the model determined stellar masses are about 2 and for the stars disrupted in the first flare and the second flare. The large mass difference cannot be reasonably expected by the repeating pTDEs with around 0.6 in AT 2020vdq. The results in this manuscript indicate that the repeating pTDEs scenario is not preferred at current stage in AT 2020vdq, but the probable double TDEs for two individual stars tidally disrupted should be currently recommended.

Paper Structure

This paper contains 3 sections, 4 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Main results and necessary discussions
  3. Summary and Conclusions

Figures (4)

  • Figure 1: Left panels show the best descriptions to the ZTF $gri$-band light curves with different $\beta$ applied to the two flares. In left panels, panel (a1), (c1) and (e1) show the ZTF $gri$-band light curves (open circles plus error bars in blue) of AT 2020vdq in observer frame, and the corresponding best descriptions (solid red line) by the proposed TDEs model. Panel (b1), (d1) and (f1) show the corresponding residuals calculated by the light curves minus the best descriptions, with horizontal solid red line as residuals to be zero. Right panels show the best descriptions to the ZTF $gri$-band light curves with the same $\beta$ applied to the two flares. In each right panel, symbols and line styles have the same meanings as those in the left panel.
  • Figure 2: The posterior distributions of the model parameters. In each panel, the vertical solid red line marks the position of the accepted value of the model parameter, the horizontal solid red line marks the full width at half maximum which has been accepted to determine the lower and upper boundaries of the corresponding $1\sigma$ uncertainties of the model parameter.
  • Figure 3: Time dependent evolution of the physical accretion rates in AT 2020vdq. Solid line in blue and in red show the corresponding results for the first TDE and the second TDE, respectively.
  • Figure 4: The color between ZTF g-band and r-band, after subtractions of host galaxy contributions in AT 2020vdq. Horizontal red lines show the color equal to zero and the corresponding 1RMS standard deviations.