INTEGRAL and XMM–Newton observations of IGR J16418−4532: evidence of accretion regime transitions in a supergiant fast X-ray transient

Abstract

We report on combined INTEGRAL and XMM-Newton observations of the supergiant fast X-ray transient IGR J16418-4532. The observations targeted the X-ray eclipse region of IGR J16418-4532s orbit with continuous INTEGRAL observations across ~25% of orbital phase and two quasi-simultaneous XMM-Newton observations of length 20ks and 14ks, occurring during and just after the eclipse respectively. An enhanced INTEGRAL emission history is provided with 19 previously unreported outbursts identified in the archival 18-60 keV data set. The XMM-Newton eclipse observation showed prominent Fe-emission and a flux of 2.8*10^-13 erg cm^-2 s^-1 (0.5 - 10 keV). Through the comparison of the detected eclipse and post eclipse flux, the supergiant mass loss rate through the stellar wind was determined as \dot{M}_{w} = 2.3-3.8*10^-7 M_{\odot} yr^-1. The post eclipse XMM-Newton observation showed a dynamic flux evolution with signatures of the X-ray pulsation, a period of flaring activity, structured nH variations and the first ever detection of an X-ray intensity dip, or 'off-state', in a pulsating supergiant fast X-ray transient. Consideration is given to the origin of the X-ray dip and we conclude that the most applicable of the current theories of X-ray dip generation is that of a transition between Compton cooling dominated and radiative cooling dominated subsonic accretion regimes within the 'quasi-spherical' model of wind accretion. Under this interpretation, which requires additional confirmation, the neutron star in IGR J16418-4532 possesses a magnetic field of ~10^14 G, providing tentative observational evidence of a highly magnetised neutron star in a supergiant fast X-ray transient for the first time. The implications of these results on the nature of IGR J16418-4532 itself and the wider SFXT class are discussed.