An Ultraviolet Spectrum of the Tidal Disruption Flare ASASSN-14li

TL;DR

This paper presents the first rest-frame UV spectrum of a tidal disruption flare, revealing unique emission and absorption features that shed light on the physical conditions and outflows in TDFs.

Contribution

It provides the first UV spectrum of a TDF, offering detailed spectral features and insights into the emission processes and outflows, advancing understanding of TDFs' physical nature.

Findings

UV continuum fit by a blackbody at 3.5 x 10^4 K

Detection of narrow absorption and broad emission lines with unique abundance patterns

Evidence of outflows with velocities of 250-400 km/s

Abstract

We present a Hubble Space Telescope STIS spectrum of ASASSN-14li, the first rest-frame UV spectrum of a tidal disruption flare (TDF). The underlying continuum is well fit by a blackbody with $T_{\mathrm{UV}} = 3.5 \times 10^{4}$ K, an order of magnitude smaller than the temperature inferred from X-ray spectra (and significantly more precise than previous efforts based on optical and near-UV photometry). Super-imposed on this blue continuum, we detect three classes of features: narrow absorption from the Milky Way (probably a high-velocity cloud), and narrow absorption and broad (FWHM $\approx 2000$-8000 km s$^{-1}$) emission lines at/near the systemic host velocity. The absorption lines are blueshifted with respect to the emission lines by $\Delta v = -(250$-400) km s$^{-1}$. Due both to this velocity offset and the lack of common low-ionization features (Mg II, Fe II), we argue these arise from the same absorbing material responsible for the low-velocity outflow discovered at X-ray wavelengths. The broad nuclear emission lines display a remarkable abundance pattern: N III], N IV], He II are quite prominent, while the common quasar emission lines of C III] and Mg II are weak or entirely absent. Detailed modeling of this spectrum will help elucidate fundamental questions regarding the nature of the emission process(es) at work in TDFs, while future UV spectroscopy of ASASSN-14li would help to confirm (or refute) the previously proposed connection between TDFs and "N-rich" quasars.