[Paper Review] Repeating tidal disruptions in GSN 069: Long-term evolution and constraints on quasi-periodic eruptions' models

G. Miniutti, M. Giustini+5 more|arXiv (Cornell University)|Jul 15, 2022

Geophysics and Gravity Measurements4 citations

TL;DR

This study analyzes 12 years of X-ray data from GSN 069, revealing that its quasi-periodic eruptions (QPEs) are transient phenomena lasting at least 1.05 years, with oscillating intensity and recurrence times. The QPEs are driven by thermal emission from a compact, expanding region, and the long-term evolution indicates two repeating tidal disruption events (TDEs) approximately 9 years apart, with precursor flares suggesting disc reformation after each TDE.

ABSTRACT

GSN 069 is the first galactic nucleus where quasi-periodic eruptions (QPEs) have been identified. These are high-amplitude, soft X-ray bursts recurring every ~9 hr, lasting ~1 hr, and during which the X-ray count rate increases by up to two orders of magnitude with respect to an otherwise stable quiescent level. The X-ray spectral properties and the long-term evolution of GSN 069 in the first few years are consistent with a long-lived tidal disruption event (TDE). Here we derive the properties of QPEs and of the long-term X-ray evolution in GSN 069 over the past 12 yr by studying timing and spectral X-ray data from 11 XMM-Newton, one Chandra, and 34 Swift observations on timescales ranging from minutes to years. QPEs in GSN 069 are a transient phenomenon with a lifetime > 1.05 yr. The QPE intensity and recurrence time oscillate and allow for alternating strong-weak QPEs and long-short recurrence times to be defined. In observations with QPEs, the quiescent level exhibits a quasi-periodic oscillation with a period equal to the average separation between consecutive QPEs. The QPE spectral evolution is consistent with thermal emission from a very compact region that heats up quickly and subsequently cools down via X-ray emission while expanding by a factor of ~3 in radius. The long-term evolution of the quiescent level is characterised by two repeating TDEs ~9 yr apart. We detect a precursor X-ray flare prior to the second TDE that may be associated with the circularisation phase during disc formation. A similar precursor flare is tentatively detected just before the first TDE. Future X-ray observations of GSN 069 promise that the QPE origin and the relation between QPEs and repeating TDEs in this galactic nucleus will be constrained, with consequences for the other sources where QPEs have been identified. [abridged]

Motivation & Objective

• To understand the long-term X-ray evolution of GSN 069 over 12 years since its initial detection in 2010.
• To characterize the properties of quasi-periodic eruptions (QPEs) in GSN 069, including their recurrence, duration, and spectral evolution.
• To investigate the connection between QPEs and tidal disruption events (TDEs), particularly the origin of repeating TDEs.
• To constrain theoretical models of QPEs by analyzing timing, spectral, and long-term variability patterns.
• To determine whether QPEs are driven by an orbiting companion or internal disc instabilities in the accretion flow.

Proposed method

• Analyzed 11 XMM-Newton, 1 Chandra, and 34 Swift X-ray observations of GSN 069 across timescales from minutes to years.
• Performed timing analysis to measure QPE recurrence times and intensity variations, identifying oscillations between strong-weak QPEs and long-short recurrence periods.
• Conducted spectral fitting of QPEs to model thermal emission, deriving temperature evolution and source size expansion (~3× radius increase).
• Tracked the quiescent X-ray level over time, detecting a quasi-periodic oscillation at the average QPE recurrence period.
• Compared long-term flux decay with TDE fallback timescales to infer the presence of two TDEs separated by ~9 years.
• Identified precursor flares before each TDE, suggesting disc reformation or circularization processes.

Experimental results

Research questions

• RQ1 What is the long-term evolution of the quiescent X-ray level in GSN 069 over 12 years?
• RQ2 How do the recurrence time and intensity of QPEs vary over time, and what does this imply about their origin?
• RQ3 Are the QPEs linked to tidal disruption events, and if so, how many TDEs have occurred and what is their timescale?
• RQ4 What causes the precursor flares observed before each TDE, and how do they relate to disc formation or circularization?
• RQ5 Can the QPE phenomenon be explained by an orbiting companion or by internal disc instabilities such as tearing?

Key findings

• QPEs in GSN 069 are transient, with a minimum lifetime of ≥1.05 years, and exhibit oscillating intensity and recurrence times.
• The quiescent X-ray level shows a quasi-periodic oscillation with a period equal to the average separation between consecutive QPEs.
• QPE spectral evolution is consistent with thermal emission from a compact region that heats rapidly and cools via X-ray emission while expanding by a factor of ~3 in radius.
• The long-term X-ray evolution reveals two repeating TDEs occurring approximately 9 years apart, with a precursor flare detected before the second TDE.
• A tentative precursor flare is also observed before the first TDE, suggesting a possible link to disc circularization or reformation.
• The similarity in TDE timescales and QPE properties supports a scenario where partial disruptions of a star on a ~9-year orbit may drive the QPEs, or alternatively, a surviving orbiter or disc tearing mechanism may be responsible.