X-ray Quasi-Periodic Eruptions from two previously quiescent galaxies

TL;DR

This paper reports the discovery of X-ray Quasi-Periodic Eruptions in two previously inactive galaxies, suggesting a new mechanism involving orbiting compact objects, which could link to extreme mass ratio inspirals and multi-messenger astrophysics.

Contribution

It presents the first systematic search revealing QPEs in galaxies without active nuclei, challenging existing models and proposing a new orbital-driven mechanism involving a secondary compact object.

Findings

QPEs detected in two new galaxies without signs of active black holes

QPE properties are inconsistent with current accretion disk instability models

QPEs may be caused by orbiting compact objects, indicating a new physical process

Abstract

Quasi-Periodic Eruptions (QPEs) are extreme high-amplitude bursts of X-ray radiation recurring every few hours and originating near the central supermassive black holes in galactic nuclei. It is currently unknown what triggers these events, how long they last and how they are connected to the physical properties of the inner accretion flows. Previously, only two such sources were known, found either serendipitously or in archival data, with emission lines in their optical spectra classifying their nuclei as hosting an actively accreting supermassive black hole. Here we present the detection of QPEs in two further galaxies, obtained with a blind and systematic search over half of the X-ray sky. The optical spectra of these galaxies show no signature of black hole activity, indicating that a pre-existing accretion flow typical of active nuclei is not required to trigger these events. Indeed, the periods, amplitudes and profiles of the newly discovered QPEs are inconsistent with current models that invoke radiation-pressure driven accretion disk instabilities. Instead, QPEs might be driven by an orbiting compact object. Furthermore, their observed properties require the mass of the secondary object to be much smaller than the main body and future X-ray observations may constrain possible changes in the period due to orbital evolution. This scenario could make QPEs a viable candidate for the electromagnetic counterparts of the so-called extreme mass ratio inspirals, with considerable implications for multi-messenger astrophysics and cosmology.