An Unusual Transient in the Extremely Metal-Poor Galaxy SDSS J094332.35+332657.6 (Leoncino Dwarf)

TL;DR

This paper reports the serendipitous discovery of a transient event in the ultra-faint, low-metallicity galaxy Leoncino Dwarf, exploring various astrophysical scenarios and ruling out many, to understand the nature of the observed phenomena.

Contribution

It presents the first detailed analysis of a transient in Leoncino Dwarf, considering and discarding multiple potential origins, and proposing a rare astrophysical event as the likely cause.

Findings

Transient source observed in Leoncino Dwarf.

Multiple scenarios considered and ruled out.

A rare astrophysical event is the probable cause.

Abstract

We have serendipitously discovered that Leoncino Dwarf, an ultra-faint, low-metallicity record-holder dwarf galaxy, may have hosted a transient source, and possibly exhibited a change in morphology, a shift in the center of brightness, and peak variability of the main (host) source in images taken approximately 40 yr apart; it is highly likely that these phenomena are related. Scenarios involving a Solar System object, a stellar cluster, dust enshrouding, and accretion variability have been considered, and discarded, as the origin of the transient. Although a combination of time-varying strong and weak lensing effects, induced by an intermediate mass black hole (10$^4$ - 5 $\times$ 10$^{5}$ M$_{\odot}$) moving within the Milky Way halo (0.1 -- 4 kpc), can conceivably explain all of the observed variable galaxy properties, it is statistically highly unlikely according to current theoretical predictions, and, therefore, also discarded. A cataclysmic event such as a supernova/hypernova could have occurred, as long as the event was observed towards the later/late-stage descent of the light curve, but this scenario fails to explain the absence of a post-explosion source and/or host HII region in recent optical images. An episode related to the giant eruption of a luminous blue variable star, a stellar merger or a nova, observed at, or near, peak magnitude may explain the transient source and possibly the change in morphology/center of brightness, but can not justify the main source peak variability, unless stellar variability is evoked.