The flaring X-ray corona in the quasar PDS 456
  New Swift monitoring observations of the variable, radio-quiet quasar, PDS
456, are presented. A bright X-ray flare was captured in September 2018, the
flux increasing by a factor of 4 and with a doubling time-scale of 2 days. From
the light crossing argument, the coronal size is inferred to be about 30
gravitational radii for a black hole mass of $10^{9} {\rm M}_{\odot}$ and the
total flare energy exceeds $10^{51}$ erg. A hardening of the X-ray emission
accompanied the flare, with the photon index decreasing from $\Gamma=2.2$ to
$\Gamma=1.7$ and back again. The flare is produced in the X-ray corona, the
lack of any optical or UV variability being consistent with a constant
accretion rate. Simultaneous XMM-Newton and NuSTAR observations were performed,
$1-3$ days after the flare peak and during the decline phase. These caught PDS
456 in a bright, bare state, where no disc wind absorption features are
apparent. The hard X-ray spectrum shows a high energy roll-over, with an
e-folding energy of $E_{\rm fold}=51^{+11}_{-8}$ keV. The deduced coronal
temperature, of $kT=13$ keV, is one of the coolest measured in any AGN and PDS
456 lies well below the predicted pair annihilation line in X-ray corona. The
spectral variability, becoming softer when fainter following the flare, is
consistent with models of cooling X-ray coronae. Alternatively, an increase in
a non-thermal component could contribute towards the hard X-ray flare spectrum.
