CSS161010: a luminous, fast blue optical transient with broad blueshifted hydrogen lines

TL;DR

This paper reports on CSS161010, a luminous, fast blue optical transient with unique spectral features, likely caused by a star being tidally disrupted by an intermediate-mass black hole in a dwarf galaxy.

Contribution

It presents detailed multi-wavelength observations of CSS161010, revealing unprecedented spectral features and proposing a novel origin scenario involving tidal disruption by an intermediate-mass black hole.

Findings

CSS161010 reached an absolute peak magnitude of -20.66 in 3.8 days.

Spectra show broad, blueshifted hydrogen emission lines up to 10% of light speed.

The transient's properties suggest a tidal disruption event in a dwarf galaxy.

Abstract

We present ultraviolet, optical and near-infrared photometric and optical spectroscopic observations of the luminous, fast blue optical transient (LFBOT), CSS161010:045834-081803 (CSS161010). The transient was found in a low-redshift (z=0.033) dwarf galaxy. The light curves of CSS161010 are characterized by an extremely fast evolution and blue colours. The V-band light curve shows that CSS161010 reaches an absolute peak of M$_{V}^{max}=-20.66\pm0.06$ mag in 3.8 days from the start of the outburst. After maximum, CSS161010 follows a power-law decline $\propto t^{-2.8\pm0.1}$ in all optical bands. These photometric properties are comparable to those of well-observed LFBOTs such as AT 2018cow, AT 2020mrf and AT 2020xnd. However, unlike these objects, the spectra of CSS161010 show a remarkable transformation from a blue and featureless continuum to spectra dominated by very broad, entirely blueshifted hydrogen emission lines of velocities of up to 10% of the speed of light. The persistent blueshifted emission and the lack of any emission at the rest wavelength of CSS161010 are unique features not seen in any transient before CSS161010. The combined observational properties of CSS161010 and its M$_{*}\sim10^{8}$ M$_\odot$ dwarf galaxy host favour the tidal disruption of a star by an intermediate-mass black hole as its origin.