The Peculiar X-ray Transient Swift J0840.7-3516: an Unusual Low Mass X-ray Binary or a Tidal Disruption Event?

TL;DR

This paper analyzes the unusual X-ray transient Swift J0840.7-3516, presenting its complex temporal and spectral behavior, and explores potential classifications including a low mass X-ray binary or a tidal disruption event.

Contribution

It provides detailed observational analysis of Swift J0840.7-3516's X-ray properties and discusses its possible nature, highlighting its peculiar features not fitting known source classes.

Findings

Rapid flux increase and decay over days

Spectral evolution from hard to soft states

Detection of an absorption feature at 3-4 keV

Abstract

We report on the X-ray properties of the new transient Swift J0840.7$-$3516, discovered with Swift/BAT in 2020 February, using extensive data of Swift, MAXI, NICER, and NuSTAR. The source flux increased for $\sim 10^3$ s after the discovery, decayed rapidly over $\sim$ 5 orders of magnitude in 5 days, and then remained almost constant over 9 months. Large-amplitude short-term variations on time scales of 1--$10^4$ s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law shaped spectrum with a photon index of $\sim 1.0$ extending up to $\sim 30$ keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became $\sim 2$ at the end of the decay. A spectral absorption feature at 3--4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low mass X-ray binary with multiple extreme properties and a tidal disruption event by a supermassive black hole or a Galactic neutron star.