The NuSTAR view of the true type 2 Seyfert NGC 3147

Abstract

We present the first NuSTAR observation of a 'true' type 2 Seyfert galaxy. The 3-40 keV X-ray spectrum of NGC 3147 is characterized by a simple power-law, with a standard Γ ∼ 1.7 and an iron emission line, with no need for any further component up to ∼40 keV. These spectral properties, together with significant variability on time-scales as short as weeks (as shown in a 2014 Swift monitoring campaign), strongly support an unobscured line of sight for this source. An alternative scenario in terms of a Compton-thick source is strongly disfavoured, requiring an exceptional geometrical configuration, whereas a large fraction of the solid angle to the source is filled by a highly ionized gas, whose reprocessed emission would dominate the observed luminosity. Moreover, in this scenario, the implied intrinsic X-ray luminosity of the source would be much larger than the value predicted by other luminosity proxies, like the [O III]λ5007 emission-line extinction-corrected luminosity. Therefore, we confirm with high confidence that NGC 3147 is a true type 2 Seyfert galaxy, intrinsically characterized by the absence of a broad-line region.