Properties of the GSN 069 Accretion Disk from a Joint X-Ray and UV Spectral Analysis: Stress-testing Quasi-periodic Eruption Models
Muryel Guolo, Andrew Mummery, T. Wevers, M. Nicholl, Suvi Gezari, Adam Ingram, Dheeraj R. Pasham
Abstract
Abstract We present an analysis of Hubble Space Telescope (HST) and XMM-Newton data of the tidal disruption event (TDE) candidate and quasi-periodic eruption (QPE) source GSN 069. Using ultraviolet (UV) and optical images at HST resolution, we show that GSN 069’s emission consists of a point source superimposed on a diffuse stellar component. The latter accounts for ≤5% of the UV emission in the inner 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 5 × 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 5 region, while the luminosity of the former cannot be attributed to stars. Analyzing the 2014/2018 HST UV spectra, we show that to leading order the intrinsic spectral shape is ν L ν ∝ ν 4/3 , with ∼10% far-UV flux variability between epochs. The contemporaneous X-ray and UV spectra can be modeled self-consistently in a thin disk framework. At observed epochs, the disk had an outer radius ( R out ) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">O</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:math> , showing both cooling and expansion over 4 yr. Incorporating relativistic effects via numerical ray tracing, we constrain the disk inclination angle ( i ) to be 30 ∘ ≲ i ≲ 65 ∘ and identify a narrow region of spin–inclination parameter space that describes the observations. These findings confirm that GSN 069 hosts a compact, viscously expanding accretion disk likely formed after a TDE. The implications for QPE models are (i) that no published disk-instability model can explain the disk’s stability in 2014 (no QPEs) and its instability in 2018 (QPEs present), and (ii) while the disk size in 2018 allows for orbiter–disk interactions to produce QPEs, in 2014 the disk was already sufficiently extended, yet no QPEs were present. These findings pose challenges to existing QPE models.