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.