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XMM-Newton and INTEGRAL study of the SFXT IGR J18483-0311 in quiescence: hint of a cyclotron emission feature?

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 ($Γ$ about 2.5) similar to that during outburst activity and the lowest measured luminosity level is about 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 (about 3.5 sigma significance) of an emission-like feature at about 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 about 3x10^11 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.