Variable oxygen emission from the accretion disk of Mrk 110
  Six XMM-Newton observations of the bright narrow line Seyfert 1, Mrk 110,
from 2004-2020, are presented. The analysis of the grating spectra from the
Reflection Grating Spectrometer (RGS) reveals a broad component of the He-like
Oxygen (OVII) line, with a full width at half maximum (FWHM) of $15900\pm1800$
km s$^{-1}$ measured in the mean spectrum. The broad OVII line in all six
observations can be modelled with a face-on accretion disk profile, where from
these profiles the inner radius of the line emission is inferred to lie between
about 20-100 gravitational radii from the black hole. The derived inclination
angle, of about 10 degrees, is consistent with studies of the optical Broad
Line Region in Mrk 110. The line also appears variable and for the first time,
a significant correlation is measured between the OVII flux and the continuum
flux from both the RGS and EPIC-pn data. Thus the line responds to the
continuum, being brightest when the continuum flux is highest, similar to the
reported behaviour of the optical HeII line. The density of the line emitting
gas is estimated to be $n_{\rm e}\sim10^{14}$ cm$^{-3}$, consistent with an
origin in the accretion disk.
