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New Swift monitoring observations of the variable, radio-quiet quasar, PDS456, are presented. A bright X-ray flare was captured in September 2018, theflux increasing by a factor of 4 and with a doubling time-scale of 2 days. Fromthe light crossing argument, the coronal size is inferred to be about 30gravitational radii for a black hole mass of $10^{9} {\rm M}_{\odot}$ and thetotal flare energy exceeds $10^{51}$ erg. A hardening of the X-ray emissionaccompanied 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, thelack of any optical or UV variability being consistent with a constantaccretion rate. Simultaneous XMM-Newton and NuSTAR observations were performed,$1-3$ days after the flare peak and during the decline phase. These caught PDS456 in a bright, bare state, where no disc wind absorption features areapparent. The hard X-ray spectrum shows a high energy roll-over, with ane-folding energy of $E_{\rm fold}=51^{+11}_{-8}$ keV. The deduced coronaltemperature, of $kT=13$ keV, is one of the coolest measured in any AGN and PDS456 lies well below the predicted pair annihilation line in X-ray corona. Thespectral variability, becoming softer when fainter following the flare, isconsistent with models of cooling X-ray coronae. Alternatively, an increase ina non-thermal component could contribute towards the hard X-ray flare spectrum.

The flaring X-ray corona in the quasar PDS 456