NGC 3147 is so far the most convincing case of a "true" Seyfert 2 galaxy,
i.e. a source genuinely lacking the Broad Line Regions. We obtained a Suzaku
observation with the double aim to study in more detail the iron line complex,
and to check the Compton-thick hypothesis for the lack of observed optical
broad lines.
The Suzaku XIS and HXD/PIN spectra of the source were analysed in detail. The
line complex is composed of at least two unresolved lines, one at about 6.45
keV and the other one at about 7 keV, most likely identified with Fe XVII/XIX,
the former, and Fe XXVI, the latter. The high-ionization line can originate
either in a photoionized matter or in an optically thin thermal plasma. In the
latter case, an unusually high temperature is implied. In the photoionized
model case, the large equivalent width can be explained either by an extreme
iron overabundance or by assuming that the source is Compton-thick. In the
Compton-thick hypothesis, however, the emission above 2 keV is mostly due to a
highly ionized reflector, contrary to what is usually found in Compton-thick
Seyfert 2s, where reflection from low ionized matter dominates. Moreover, the
source flux varied between the XMM-Newton and the Suzaku observations, taken
3.5 years apart, confirming previous findings and indicating that the size of
the emitting region must be smaller than a parsec.
The hard X-ray spectrum is also inconclusive on the Compton-thick hypothesis.
Weighting the various arguments, a "true" Seyfert 2 nature of NGC 3147 seems to
be still the most likely explanation, even if the "highly ionized reflector"
Compton-thick hypothesis cannot at present be formally rejected.Comment: 6 pages, accepted for publication in Astronomy & Astrophysic
