A CONTINUUM OF H- TO He-RICH TIDAL DISRUPTION CANDIDATES WITH A PREFERENCE FOR E+A GALAXIES

Abstract

We present the results of a Palomar Transient Factory (PTF) archival search for blue transients that lie in the magnitude range between "normal" core-collapse and superluminous supernovae (i.e., with −21 M R (peak) −19). Of the six events found after excluding all interacting Type IIn and Ia-CSM supernovae, three (PTF09ge, 09axc, and 09djl) are coincident with the centers of their hosts, one (10iam) is offset from the center, and a precise offset cannot be determined for two (10nuj and 11glr). All the central events have similar rise times to the He-rich tidal disruption candidate PS1-10jh, and the event with the best-sampled light curve also has similar colors and power-law decay. Spectroscopically, PTF09ge is He-rich, while PTF09axc and 09djl display broad hydrogen features around peak magnitude. All three central events are in low star formation hosts, two of which are E+A galaxies. Our spectrum of the host of PS1-10jh displays similar properties. PTF10iam, the one offset event, is different photometrically and spectroscopically from the central events, and its host displays a higher star formation rate. Finding no obvious evidence for ongoing galactic nuclei activity or recent star formation, we conclude that the three central transients likely arise from the tidal disruption of a star by a supermassive black hole. We compare the spectra of these events to tidal disruption candidates from the literature and find that all of these objects can be unified on a continuous scale of spectral properties. The accumulated evidence of this expanded sample strongly supports a tidal disruption origin for this class of nuclear transients.

The X-Ray Through Optical Fluxes and Line Strengths of Tidal Disruption Events

“…Occasionally, associated optical emission is also detected. These events have been interpreted as due to a relativistic jet generated via BH accretion; such "jetted" TDEs appear to be observationally rare compared to the soft X-ray and UV/optical flares (Arcavi et al 2014;van Velzen & Farrar 2014), in line with theoretical expectations (De Colle et al 2012).…”

“…In general, TDE envelopes can produce a range of a H equivalent widths and hydrogen-to-helium line ratios, depending on the gas configuration and bolometric luminosity. Such a variation might help to explain the diversity of hydrogen-to-helium line ratios seen in observed TDE candidates (Arcavi et al 2014).…”

“…These events typically rise to a peak (observer frame) R-band luminosity of ∼2×10 43 erg s −1 on a timescale of ∼months (Arcavi et al 2014), with a late time fall consistent with t −5/3 . Intriguingly, PTF10iya, Swift J2058.4, ASASSN14li, and ASASSN-15oi have been simultaneously observed in both optical and X-ray (Cenko et al 2012a(Cenko et al , 2012b(Cenko et al , 2016Miller et al 2015;Holoien et al 2016aHoloien et al , 2016b.…”

“…(2) Why do the optical spectra show strong lines of helium, but little or no hydrogen line emission (Gezari et al 2012;Arcavi et al 2014)?…”

“…We show that escaping line photons are produced only in the outer layers of the envelope, such that the H to He line ratio is suppressed and will vary with the envelope extent. Indeed, some optical TDE candidates do show detectable, but varying, Hα emission (Arcavi et al 2014;Holoien et al 2016b).…”