Spectral features of tidal disruption candidates and alternative origins for such transient flares

Graham

et al. 2021

ApJ

The Zwicky Transient Facility (ZTF) has discovered five new events belonging to an emerging class of AGN undergoing smooth flares with large amplitudes and rapid rise times. This sample consists of several transients that were initially classified as supernovae with narrow spectral lines. However, upon closer inspection, all of the host galaxies display resolved Balmer lines characteristic of a narrow-line Seyfert 1 (NLSy1) galaxy. The transient events are long-lived, over 400 days on average. We report UV and X-ray follow-up of the flares and observe persistent UV-bright emission, with two of the five transients detected with luminous X-ray emission, ruling out a supernova interpretation. We compare the properties of this sample to previously reported flaring NLSy1 galaxies, and find that they fall into three spectroscopic categories: Transients with 1) Balmer line profiles and Fe II complexes typical of NLSy1s, 2) strong He II profiles, and 3) He II profiles including Bowen fluorescence features. The latter are members of the growing class of AGN flares attributed to enhanced accretion reported by Trakhtenbrot et al. (2019). We consider physical interpretations in the context of related transients from the literature. For example, two of the sources show high amplitude rebrightening in the optical, ruling out a simple tidal disruption event scenario for those transients. We conclude that three of the sample belong to the Trakhtenbrot et al. ( 2019) class, and two are TDEs in NLSy1s. We also aim to understand why NLSy1s are preferentially the sites of such rapid enhanced flaring activity.

Section: A Preponderance Of Rapid Optical Transients Inmentioning

confidence: 99%

A Family Tree of Optical Transients from Narrow-line Seyfert 1 Galaxies

Frederick

Graham

et al. 2021

ApJ

“…For example, CSS100217:102913+404220 displayed a high state (M V = −22.7 at 45 days post-peak) accompanied by broad Hα and was interpreted either as a Type IIn SN (Drake et al 2011) or TDE (Saxton et al 2018) near the nucleus (∼150 pc) of a NLS1. It eventually faded back to its original level after one year.…”

Section: Is Ztf18aajupnt/at2018dyk a Tde Or Agn Activity?mentioning

confidence: 99%

A New Class of Changing-look LINERs

Frederick

Graham

et al. 2019

ApJ

100

We report the discovery of six active galactic nuclei (AGN) caught "turning on" during the first nine months of the Zwicky Transient Facility (ZTF) survey. The host galaxies were classified as LINERs by weak narrow forbidden line emission in their archival SDSS spectra, and detected by ZTF as nuclear transients. In five of the cases, we found via follow-up spectroscopy that they had transformed into broad-line AGN, reminiscent of the changing-look LINER iPTF16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up HST UV and groundbased optical spectra revealed the transformation into a narrow-line Seyfert 1 (NLS1) with strong [Fe VII, X, XIV] and He II λ4686 coronal lines. Swift monitoring observations of this source reveal bright UV emission that tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks ∼60 days later. Spitzer follow-up observations also detect a luminous mid-infrared flare implying a large covering fraction of dust. Archival light curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset of the optical nuclear flaring from a prolonged quiescent state. Here we present the systematic selection and follow-up of this new class of changing-look LINERs, compare their properties to previously reported changing-look Seyfert galaxies, and conclude that they are a unique class of transients well-suited to test the uncertain physical processes associated with the LINER accretion state.

“…It is noticed that nebular arguments may not be valid for TDE candidates despite being frequently used in the literature. Bogdanović et al (2004), and has been recently studied with CLOUDY caclulations (Gaskell & Rojas Lobos 2014;Strubbe & Murray 2015;Saxton et al 2016) and full radiative transfer calculations (Roth et al 2016). …”

Section: Helium-to-hydrogen Ratiomentioning

confidence: 99%

Revisiting Optical Tidal Disruption Events with iPTF16axa

Hung

Blagorodnova

et al. 2017

ApJ

177

174

We report the discovery by the intermediate Palomar Transient Factory (iPTF) of a candidate tidal disruption event (TDE) iPTF16axa at z=0.108 and present its broadband photometric and spectroscopic evolution from threemonths of follow-up observations with ground-based telescopes and Swift. The light curve is well fitted with a t −5/3 decay, and we constrain the rise time to peak to be <49 rest-frame days after disruption, which is roughly consistent with the fallback timescale expected for the ∼5×106 M e black hole inferred from the stellar velocity dispersion of the host galaxy. The UV and optical spectral energy distribution is well described by a constant blackbody temperature of T∼3×10 4 K over the monitoring period, with an observed peak luminosity of 1.1×10 44 erg s −1. The optical spectra are characterized by a strong blue continuum and broad He II and Hα lines, which are characteristic of TDEs. We compare the photometric and spectroscopic signatures of iPTF16axa with 11 TDE candidates in the literature with well-sampled optical light curves. Based on a single-temperature fit to the optical and near-UV photometry, most of these TDE candidates have peak luminosities confined between log(L [erg s 4 K during their power-law declines, implying blackbody radii on the order of 10 times the tidal disruption radius, that decrease monotonically with time. For TDE candidates with hydrogen and helium emission, the high helium-to-hydrogen ratios suggest that the emission arises from high-density gas, where nebular arguments break down. We find no correlation between the peak luminosity and the black hole mass, contrary to the expectations for TDEs to have M M BH 1 2 µ -.