The high-Eddington NLS1 Ark 564 has the coolest corona
  Ark 564 is an archetypal Narrow line Seyfert 1 that has been well observed in
soft X-rays from 0.3-10 keV, revealing a steep spectrum, strong soft excess,
iron K emission line and dramatic variability on the order of hours. Because of
its very steep spectrum, observations of the source above 10 keV have been
sparse. We report here on the first NuSTAR observation of Ark 564. The source
was observed for 200 ks with NuSTAR, 50 ks of which were concurrent with Suzaku
observations. NuSTAR and Suzaku observed a dramatic flare, in which the hard
emission is clearly delayed with respect to the soft emission, consistent with
previous detections of a low-frequency hard lag found in XMM-Newton data. The
NuSTAR spectrum is well described by a low-temperature Comptonisation continuum
(with an electron temperature of 15 +/- 2 keV), which irradiates a highly
ionised disc. No further relativistic broadening or ionized absorption is
required. These spectral results show that Ark 564 has one of the lowest
temperature coronae observed by NuSTAR to date. We discuss possible reasons for
low-temperature coronae in high-Eddington sources.
