Host galaxy properties of quasi-periodically erupting X-ray sources
  Quasi-periodic X-ray eruptions (QPEs) are a recently discovered phenomenon,
the nature of which remains unclear. Based on their discovery in active
galactic nuclei (AGN), explanations related to an AGN accretion disk, or
potentially stellar tidal disruption event (TDE), were put forward. Following
the report of QPEs in apparently passive galaxies, alternatives including
highly unequal mass compact object binaries have been proposed to explain their
properties. We perform a systematic study of the five known QPE host galaxies
with the aim of providing new insights into their nature. We analyse new and
archival medium resolution optical spectroscopy of the QPE hosts. We measure
emission (and absorption) line fluxes, their ratios and equivalent widths
(EWs), to locate the QPE hosts on diagnostic diagrams. We also measure the
velocity dispersion of the stellar absorption lines to estimate their black
hole masses. All QPE host galaxies show emission lines in their optical
spectra. Based on their ratios and EWs, we find evidence for the presence of an
active galactic nucleus in all sources, including those previously reported as
passive. We measure velocity dispersions between 36 and 90 km/s, implying the
presence of low mass (10^5-6.7 solar masses) black holes, consistent with
literature findings. Finally, we find a significant over-representation (2/5
sources, or a factor of 13 +13 -10.5) of quiescent, Balmer strong (post
starburst) galaxies among QPE hosts. The presence of a narrow line region
consistent with an AGN in all QPE host galaxies implies that a pre-existing
accretion flow likely plays an integral part to the QPE phenomenon. The strong
over-representation of quiescent Balmer strong galaxies among QPE hosts can be
naturally explained in both the TDE and interacting extreme mass ratio inspiral
hypotheses.
