Kavli Affiliate: Erin Kara
| First 5 Authors: Joheen Chakraborty, Peter Kosec, Erin Kara, Giovanni Miniutti, Riccardo Arcodia
| Summary:
Quasi-Periodic Eruptions (QPEs) are recurring bursts of soft X-ray emission
from supermassive black holes (SMBHs), which a growing class of models explains
via extreme mass-ratio inspirals (EMRIs). QPEs exhibit blackbody-like emission
with significant temperature evolution, but the minimal information content of
their almost pure-thermal spectra has limited physical constraints. Here we
study the recently discovered QPEs in ZTF19acnskyy (“Ansky”), which show
absorption-like features evolving dramatically within eruptions and correlating
strongly with continuum temperature and luminosity, further probing the
conditions underlying the emission surface. The absorption features are
well-described by dense ionized plasma of column density $N_{rm H}gtrsim
10^{21}$ cm$^{-2}$, blueshift $0.06lesssim v/c lesssim 0.4$, and either
collisional or photoionization equilibrium. With high-resolution spectra, we
also detect ionized blueshifted emission lines suggesting a nitrogen
over-abundance of $21.7^{+18.5}_{-11.0}times$ solar. We interpret our results
with orbiter-disk collisions in an EMRI system, in which each impact drives a
shock that locally heats the disk and expels X-ray emitting debris undergoing
radiation pressure-driven homologous expansion. We explore an analytical toy
model that links the rapid change in absorption lines to the evolution of the
ionization parameter and the photosphere radius, and suggest that $sim
10^{-3}M_odot$ ejected per eruption with expansion velocities up to $v_{rm
max}sim 0.15c$, can reproduce the absorption features. With these assumptions,
we show a P Cygni profile in a spherical expansion geometry qualitatively
matches the observed line profiles. Our work takes a first step towards
extending existing physical models for QPEs to address their implications for
spectral line formation.
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