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

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 the faint-and-fast event 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 ≈106 M⊙, disrupting a star of ≈1 M⊙. By analysing our comprehensive UV, optical, and X-ray data, we show that the early optical emission is dominated by an outflow, with a luminosity evolution L ∝ t2, consistent with a photosphere expanding at constant velocity (≳2000 km s−1), and a line-forming region producing initially blueshifted H and He ii profiles with v = 3000–10 000 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 ± 2 d 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 ≈1041 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.

Accretion disc cooling and narrow absorption lines in the tidal disruption event AT 2019dsg

“…Emission lines due to the Bowen fluorescence have previously been identified in other TDEs: iPTF15af (Blagorodnova et al 2019), AT2018dyb (Leloudas et al 2019), ASASSN-14li (Holoien et al 2016;Prieto et al 2016), iPTF16fnl (Onori et al 2019), and AT2019qiz (Nicholl et al 2020). iPTF15af and AT2018dyb were found in non-active galaxies, while the host galaxies of iPTF16fnl, ASASSN-14li, and AT2019qiz may harbour an AGN.…”

“…Unfortunately, the broad Hα and Hβ were not detected at the last two epochs when the blueshift of the He II line increases. The scenario of expanding material, where X-ray radiation is reprocessed and the broad optical lines are emitted, is similar to the one discussed in Nicholl et al (2020) for the TDE AT 2019qiz.…”

“…a decrease of the scale height of the disc that allows for a more direct view of the inner accretion disc. On the other hand, TDEs like AT 2018fyk (Wevers et al 2019b), AT 2018hyz (Hung et al 2020), or AT 2019qiz (Nicholl et al 2020) all showed evidence for prompt disc formation and X-ray emission with no brightening at later times (the X-ray data of AT 2019qiz cover only ∼100 d after the transient peak light).…”

Optical line spectra of tidal disruption events from reprocessing in optically thick outflows

“…Ho we v er, the y do not have Figure 10. Rest-frame synthetic optical spectra for two models (labelled in the legend) plotted on linear axes and against the optical spectra of ASASSN14-li bat t = 54 d (Holoien et al 2016 ) and AT2019qiz at t = 49 d (Nicholl et al 2020 ). The spectra for the objects have been corrected for foreground extinction assuming a Cardelli et al ( 1989 ) e xtinction curv e with R v = 3.1 for both TDEs and E ( B − V ) = 0.0225 and E ( B − V ) = 0.0939 for ASASSN14-li and AT2019qiz, respectively.…”

“…1 ). Additional evidence for po werful sub-relati vistic outflo ws in TDEs also comes from blueshifted broad emission lines (BELs; Arcavi et al 2014 ;Roth & Kasen 2018 ;Hung et al 2019 ;Nicholl et al 2020 ) and from radio observations (Alexander et al 2016(Alexander et al , 2017van Velzen et al 2016 ;Anderson et al 2020 ). Not all TDEs exhibit BALs at the same stage in their outburst e volution, ho we v er.…”