Mid-infrared Flare of TDE Candidate PS16dtm: Dust Echo and Implications for the Spectral Evolution

Ning Jiang,Huiyuan Wang,Tinggui Wang,Ting Xiao,Liming Dou,Amy Mainzer,Chenwei Yang,Lin Yan,Roc Cutri

doi:10.3847/1538-4357/aa93f5

Abstract

Abstract PS16dtm was classified as a candidate tidal disruption event in a dwarf Seyfert 1 galaxy with a low-mass black hole ( ) and has presented various intriguing photometric and spectra characteristics. Using the archival Wide-field Infrared Survey Explorer and the newly released NEOWISE data, we found that PS16dtm is experiencing a mid-infrared (MIR) flare that started ∼11 days before the first optical detection. Interpreting the MIR flare as a dust echo requires close pre-existing dust with a high covering factor and suggests that the optical flare may have brightened slowly for some time before it became bright detectable from the ground. More evidence is given at the later epochs. At the peak of the optical light curve, the new inner radius of the dust torus has grown to a much larger size (i.e., a factor of seven of the initial radius) due to the strong radiation field. At ∼150 days after the first optical detection, the dust temperature has dropped well below the sublimation temperature. Other peculiar spectral features shown by PS16dtm are the transient, prominent Fe ii emission lines and outflows indicated by broad absorption lines detected during the optical flare. Our model explains the enhanced Fe ii emission from iron that is newly released from the evaporated dust. The observed broad absorption line outflow could be explained by accelerated gas in the dust torus due to the radiation pressure.

Highlights

When a star passes within the tidal radius of a supermassive black hole (SMBH), it will be tidally disrupted and partially accreted by the central black hole, producing luminous flares in the soft X-ray, ultraviolet (UV), and optical wavelengths
Previous detections of MIR flares in tidal disruption event (TDE) are all attributed to dust echoes in the view of the basic fact that the MIR flares lag behind the optical flares (Dou et al 2016, 2017; Jiang et al 2016; van Velzen et al 2016)
We found that the MIR flare was detected 11 days earlier than its optical discovery by ASASSN, which is even earlier than Pan-STARRS

Summary

Introduction

When a star passes within the tidal radius of a supermassive black hole (SMBH), it will be tidally disrupted and partially accreted by the central black hole, producing luminous flares in the soft X-ray, ultraviolet (UV), and optical wavelengths. The flare usually lasts a few months to years (decaying as t-5 3) and releases a total energy of 1053 erg for a solar-type star (see Rees 1988; Phinney 1989) This phenomenon is known as a tidal disruption event (TDE). Blanchard et al (2017) have claimed that they have discovered a TDE candidate PS16dtm in SDSS J015804.75-005221.8 (hereafter J0158 for short), which is a Seyfert 1 galaxy with a low-mass black hole (~106 M , Greene & Ho 2007; Xiao et al 2011) This event shows several distinctive characteristics revealed by multiwavelength photometric and spectroscopic follow-up monitorings.

Mid-infrared and Optical Light Curves

The Early Detection of the MIR Flare

The Rapid Emergence of Fe II Lines

Broad Absorption Line Outflow

Summary