Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event

Abstract

We report the discovery of non-stellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05c) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr (z = 0.071). In the HST UV spectra, absorption of high-and low-ionization lines are also present at this velocity, making AT2018zr resemble a low-ionization broad absorption line (LoBAL) QSO. We conclude that these transient absorption features are more likely to arise in fast outflows produced by the TDE than absorbed by the unbound debris. In accordance with the outflow picture, we are able to reproduce the flat-topped Hα emission in a spherically expanding medium, without invoking the typical prescription of an elliptical disk. We also report the appearance of narrow (∼1000 km s −1 ) N III λ4640Å, He II λ4686, Hα, and Hβ emission in the late-time optical spectra of AT2018zr, which may be a result of UV continuum hardening at late time as observed by Swift. Including AT2018zr, we find a high association rate (3 out of 4) of BALs in the UV spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and may be less sensitive to viewing angle effects compared to QSO outflows.

Accretion disc winds in tidal disruption events: ultraviolet spectral lines as orientation indicators

“…For example, the likely presence of P λ1118 3 in iPTF15af suggests a column density of N H > 10 23 cm −2 (Blagorodnova et al 2019). A similar constraint has been obtained for AT2018zr from the detection of BAL signatures in Balmer and metastable Helium lines (Hung et al 2019). From a modelling perspective, Roth & Kasen (2018) have shown that the detailed line profile shapes seen in TDEs can be understood if these features are formed in an optically thick outflow.…”

“…Compton heating and inverse Compton cooling are also included, together with Auger ionization and dielectronic recombination (see Higginbottom et al 2013). (Brown et al 2017b;Blagorodnova et al 2017); and AT2018zr (Hung et al 2019), observed at similar post-flare epochs. Also included is a composite LoBALQSO spectrum (Brotherton et al 2001) and a rest-frame UV spectrum of the Nitrogen rich QSO SDSS J 164148.19 +223225.22 (Batra & Baldwin 2014).…”

“…iPTF15af and AT2018zr) show a striking similarity to those of BALQSOs. However, the UV spectra of TDE are unique in that the Mg λ2796, 2804 and C ] λ1909 lines, commonly seen in QSO spectra, are either weak or missing entirely (Cenko et al 2016;Brown et al 2017a;Hung et al 2019). Conversely, TDEs also tend to display strong N ] λ1750 emission, similar to what is seen in the rare Nitrogen-rich QSOs (Osmer 1980;Bentz & Osmer 2004;Batra & Baldwin 2014;Kochanek 2016).…”

“…BALs have been now detected in three out of four UV spectra of TDEs. As pointed out by Hung et al (2019), this suggests that the appearance of these outflows is less sensitive to viewing angle than that of the outflows in QSOs, where BALs are seen in only 20 per cent of sources (e.g. Knigge et al 2008;Allen et al 2010).…”

“…These features are seen, for example, in hot stars, accreting white dwarfs (WDs) and QSOs. Recently, blue-shifted BALs have been found in the UV spectra of iPTF15af (Blagorodnova et al 2019), iPTF16fnl (Blagorodnova et al 2017) and AT2018zr (Hung et al 2019) at around ∆t ≈ 50 days post flare. In another TDE, ASASSN14li (Cenko et al 2016), X-ray observations have provided evidence for more highly ionized outflowing gas Kara et al 2018).…”