# The tidal disruption event AT2017eqx: spectroscopic evolution from hydrogen rich to poor suggests an atmosphere and outflow

## TL;DR

This paper reports on the spectroscopic evolution of the TDE AT2017eqx, revealing a transition from hydrogen-rich to hydrogen-poor spectra, suggesting an atmosphere and outflow influenced by viewing angle and providing insights into TDE diversity.

## Contribution

It presents the first detailed observation of a TDE transitioning from hydrogen-rich to hydrogen-poor spectra, supporting models involving an atmosphere and outflow, and challenges assumptions about hydrogen absence indicating stellar stripping.

## Key findings

- Spectroscopic evolution shows disappearance of H I lines and blueshift of He II.
- Model suggests a solar-mass star disrupted by a ~2 million solar mass black hole.
- No evidence of a relativistic jet or strong X-ray emission at late times.

## Abstract

We present and analyse a new tidal disruption event (TDE), AT2017eqx at redshift z=0.1089, discovered by Pan-STARRS and ATLAS. The position of the transient is consistent with the nucleus of its host galaxy; it peaks at a luminosity of $L \approx 10^{44}$ erg s$^{-1}$; and the spectrum shows a persistent blackbody temperature $T \gtrsim 20,000$ K with broad H I and He II emission. The lines are initially centered at zero velocity, but by 100 days the H I lines disappear while the He II develops a blueshift of $\gtrsim 5,000$ km s$^{-1}$. Both the early- and late-time morphologies have been seen in other TDEs, but the complete transition between them is unprecedented. The evolution can be explained by combining an extended atmosphere, undergoing slow contraction, with a wind in the polar direction becoming visible at late times. Our observations confirm that a lack of hydrogen a TDE spectrum does not indicate a stripped star, while the proposed model implies that much of the diversity in TDEs may be due to the observer viewing angle. Modelling the light curve suggests AT2017eqx resulted from the complete disruption of a solar-mass star by a black hole of $\sim 10^{6.3} M_\odot$. The host is another quiescent, Balmer-strong galaxy, though fainter and less centrally concentrated than most TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500 days are among the deepest for a TDE at this phase.