We report the results from archival XMM-Newton and INTEGRAL observations of the Supergiant Fast X-ray Transient ( SFXT ) IGR J18483 - 0311 in quiescence .
The 18–60 keV hard X-ray behaviour of the source is presented here for the first time , it is characterized by a spectral shape ( \Gamma \sim 2.5 ) similar to that during outburst activity and the lowest measured luminosity level is \sim 10 ^ { 34 } erg s ^ { -1 } .
The 0.5–10 keV luminosity state , measured by XMM-Newton during the apastron passage , is about one order of magnitude lower and it is reasonably fitted by an absorbed black body model yielding parameters consistent with previous measurements .
In addition , we find evidence ( \sim 3.5 \sigma significance ) of an emission-like feature at \sim 3.3 keV in the quiescent 0.5–10 keV source spectrum .
The absence of any known or found systematic effects , which could artificially introduce the observed feature , give us confidence about its non-instrumental nature .
We show that its physical explanation in terms of atomic emission line appears unlikely and conversely we attempt to ascribe it to an electron cyclotron emission line which would imply a neutron star magnetic field of the order of \sim 3 \times 10 ^ { 11 } G. Importantly , such direct estimation is in very good agreement with that independently inferred by us in the framework of accretion from a spherically symmetric stellar wind .
If firmly confirmed by future longer X-ray observations , this would be the first detection ever of a cyclotron feature in the X-ray spectrum of a SFXT , with important implications on theoretical models .