X-ray eruptions every 22 days from the nucleus of a nearby galaxy

TL;DR

This paper reports the discovery of a galaxy nucleus exhibiting 22-day recurring X-ray eruptions, bridging the gap between known quasi-periodic X-ray eruptions and longer-term nuclear transients, with unique features and potential extreme mass ratio inspirals.

Contribution

It presents the first detailed multi-wavelength study of Swift J0230+28, revealing a new intermediate recurrence timescale and distinct eruption characteristics, suggesting new models for nuclear transient phenomena.

Findings

Recurrence time of ~22 days, intermediate between known sources.

Eruptions show distinct shapes and temperature evolution.

Transient radio emission associated with X-ray flares.

Abstract

Galactic nuclei showing recurrent phases of activity and quiescence have recently been discovered, with recurrence times as short as a few hours to a day -- known as quasi-periodic X-ray eruption (QPE) sources -- to as long as hundreds to a thousand days for repeating nuclear transients (RNTs). Here we present a multi-wavelength overview of Swift J023017.0+283603 (hereafter Swift J0230+28), a source that exhibits repeating and quasi-periodic X-ray flares from the nucleus of a previously unremarkable galaxy at $\sim$ 165 Mpc, with a recurrence time of approximately 22 days, an intermediary timescale between known RNTs and QPE sources. The source also shows transient radio emission, likely associated with the X-ray emission. Such recurrent soft X-ray eruptions, with no accompanying UV/optical emission, are strikingly similar to QPE sources. However, in addition to having a recurrence time that is $\sim 25$ times longer than the longest-known QPE source, Swift J0230+28's eruptions exhibit somewhat distinct shapes and temperature evolution than the known QPE sources. Scenarios involving extreme mass ratio inspirals are favored over disk instability models. The source reveals an unexplored timescale for repeating extragalactic transients and highlights the need for a wide-field, time-domain X-ray mission to explore the parameter space of recurring X-ray transients.