Time-resolved Hubble Space Telescope UV observations of an X-ray quasi-periodic eruption source

Kavli Affiliate: Dheeraj R. Pasham

| First 5 Authors: Thomas Wevers, Muryel Guolo, Sean Lockwood, Andrew Mummery, Dheeraj R. Pasham

| Summary:

X-ray quasi-periodic eruptions (QPEs) are a novel mode of variability in
nearby galactic nuclei whose origin remains unknown. Their multi-wavelength
properties are poorly constrained, as studies have focused almost entirely on
the X-ray band. Here we report on time-resolved, coordinated Hubble Space
Telescope far ultraviolet and XMM-Newton X-ray observations of the shortest
period X-ray QPE source currently known, eRO-QPE2. We detect a bright UV point
source ($L_{rm FUV} approx {rm few} times 10^{41}$ erg s$^{-1}$) that does
not show statistically significant variability between the X-ray eruption and
quiescent phases. This emission is unlikely to be powered by a young stellar
population in a nuclear stellar cluster. The X-ray-to-UV spectral energy
distribution can be described by a compact accretion disk ($R_{rm out} =
343^{+202}_{-138} R_{rm g}$). Such compact disks are incompatible with
typical disks in active galactic nuclei, but form naturally following the tidal
disruption of a star. Our results rule out models (for eRO-QPE2) invoking i) a
classic AGN accretion disk and ii) no accretion disk at all. For orbiter
models, the expected radius derived from the timing properties would naturally
lead to disk-orbiter interactions for both quasi-spherical and eccentric
trajectories. We infer a black hole mass of log$_{10}(M_{rm BH}) = 5.9 pm
0.3$ M$_{odot}$ and Eddington ratio of 0.13$^{+0.18}_{-0.07}$; in combination
with the compact outer radius this is inconsistent with existing disk
instability models. After accounting for the quiescent disk emission, we
constrain the ratio of X-ray to FUV luminosity of the eruption component to be
$L_{rm X} / L_{rm FUV} > 16-85$ (depending on the intrinsic extinction).

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