Letter to the Editor

Alive and kicking: A new QPE phase in GSN 069 revealing a quiescent luminosity threshold for QPEs

¹ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, ESAC campus, 28692 Villanueva de la Cañada, Madrid, Spain
e-mail: gminiutti@cab.inta-csic.es
² MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³ Telespazio-Vega UK for ESA, Operations Department, European Space Astronomy Centre (ESAC), Villanueva de la Cañada, 28692 Madrid, Spain
⁴ Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

Received: 14 April 2023
Accepted: 10 May 2023

Abstract

X-ray quasi-periodic eruptions (QPEs) are intense repeating soft X-ray bursts from the nuclei of nearby galaxies. Their physical origin is still largely unconstrained, and several theoretical models have been proposed ranging from disc instabilities to impacts between an orbiting companion and the existing accretion disc around the primary, or episodic mass transfer at pericentre in an extreme mass-ratio binary. We present here results from a recent XMM-Newton observation of GSN 069, the galactic nucleus where QPEs were first discovered. After about two years of absence, QPEs have reappeared in GSN 069, and we detect two consecutive QPEs separated by a much shorter recurrence time than ever before. Moreover, their intensites and peak temperatures are remarkably different, a novel addition to the QPE phenomenology. We study the QPE spectral properties from all XMM-Newton observations assuming QPEs to either represent an additional emission component superimposed on that from the disc, or the transient evolution of the disc emission itself. In the former scenario, QPEs are consistent with black-body emission from a region that expands by a factor of 2–3 during the individual QPE evolution with radius ≃5 − 10 × 10¹⁰ cm at QPE peak. In the alternative non-additive scenario, QPEs originate from a region with an area ∼6 − 30 times smaller than the quiescent state X-ray emission, with the smallest regions corresponding to the hottest and most luminous eruptions. The QPE reappearance reveals that eruptions are only present below a quiescent luminosity threshold corresponding to an Eddington ratio λ_thresh ≃ 0.4 ± 0.2 for a 10⁶ M_⊙ black hole. The disappearance of QPEs above λ_thresh is most likely driven by the ratio of QPE to quiescence temperatures, kT_QPE/kT_quiesc, approaching unity at high quiescent luminosity, making QPE detection challenging, if not impossible, above threshold. We briefly discuss some of the consequences of our results on the proposed models for the QPE physical origin.