An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz (2006.02454v2)

Abstract: At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to date, with a luminosity intermediate between the bulk of the population and iPTF16fnl. Its proximity allowed a very early detection and triggering of multiwavelength and spectroscopic follow-up well before maximum light. The velocity dispersion of the host galaxy and fits to the TDE light curve indicate a black hole mass $\approx 10^6$ M$\odot$, disrupting a star of $\approx 1$ M$\odot$. Comprehensive UV, optical and X-ray data shows that the early optical emission is dominated by an outflow, with a luminosity evolution $L \propto t^2$, consistent with a photosphere expanding at constant velocity ($\gtrsim 2000$ km s$^{-1}$), and a line-forming region producing initially blueshifted H and He II profiles with $v=3000-10000$ km s$^{-1}$. The fastest optical ejecta approach the velocity inferred from radio detections (modelled in a forthcoming companion paper from K.~D.~Alexander et al.), thus the same outflow may be responsible for both the fast optical rise and the radio emission -- the first time this connection has been observed in a TDE. The light curve rise begins $29 \pm 2$ days before maximum light, peaking when the photosphere reaches the radius where optical photons can escape. The photosphere then undergoes a sudden transition, first cooling at constant radius then contracting at constant temperature. At the same time, the blueshifts disappear from the spectrum and Bowen fluorescence lines (N III) become prominent, implying a source of far-UV photons, while the X-ray light curve peaks at $\approx 10^{41}$ erg s$^{-1}$. Assuming that these X-rays are from prompt accretion, the size and mass of the outflow are consistent with the reprocessing layer needed to explain the large optical to X-ray ratio in this and other optical TDEs, possibly favouring accretion-powered over collision-powered outflow models.

Insights into the Optical Rise of Tidal Disruption Event AT2019qiz Powered by an Outflow

The research paper presents a meticulous examination of AT2019qiz, a nearby tidal disruption event (TDE) marked by its proximity at 66 Mpc, facilitating comprehensive multi-wavelength monitoring. This paper showcases a pivotal observation in the field of TDEs: the optical rise in AT2019qiz is primarily driven by an outflow rather than direct accretion or stream collisions. This interpretation is rooted in a combination of photometric, spectroscopic, and X-ray data.

Highlights and Numerical Results

• Proximity and Early Detection: AT2019qiz's distance of 66 Mpc allowed its early detection and sustained multi-wavelength follow-up. This comprehensive dataset offers valuable insights into the nature of TDEs.
• Black Hole and Stellar Masses: The velocity dispersion of the host galaxy implies a black hole mass of approximately $10^6 M_\odot$, interacting with a disrupted star of about $1 M_\odot$.
• Outflow Dynamics: The optical emission exhibits a luminosity evolution proportional to $L \propto t^2$, indicative of a photosphere expanding at a constant velocity of $\gtrsim 2000$ km/s. The line-forming region produces blueshifted profiles of H and He II at velocities between 3000–10000 km/s.
• Radio and Optical Connection: This paper establishes a novel connection between fast optical rise and radio emission in TDEs, suggesting a shared outflow origin.
• Optical and X-ray Relationships: The photosphere's evolution transitions from expansion to contraction, accompanied by significant spectral changes, including the suppression of blueshifts and the appearance of Bowen fluorescence lines. This correlates to an X-ray peak luminosity of $\approx 10^{41}$ erg/s, favoring an accretion-powered model over a collision-powered outflow model.

Practical and Theoretical Implications

This research advances our understanding of the diverse mechanisms that power TDEs. The identification of an outflow-driven optical rise redefines the initial conditions following a tidal disruption, emphasizing the role of photoreprocessing layers in shaping the observed emission. This suggests that the spectrum and light curve of TDEs are shaped by the dynamics of ejected material rather than solely by accretion processes.

The implications are significant for future spectroscopic and photometric surveys aiming to discern outflow properties in TDEs. This understanding aids in refining models for predicting TDE emission across wavelengths, optimizing strategies for identifying TDEs in distant galaxies.

Future Developments

The paper paves the way for investigating the interplay between outflows and accretion phenomena in TDEs, particularly in delineating their contributions to observed emissions. Upcoming observations with enhanced resolution in the radio and optical regimes will be key in verifying the proposed models for TDE outflows. Furthermore, theoretical developments in simulating these events will benefit from incorporating robust models of outflow dynamics and their interactions with surrounding environments.

In conclusion, the paper of AT2019qiz offers a compelling narrative of outflow-driven emission in the optical domain of TDEs, challenging existing paradigms and steering future research towards a comprehensive understanding of these cataclysmic events.