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XMM-Newton observations of the supergiant fast X-ray transientIGR$~$J17544$-$2619 are reported and placed in the context of an analysis ofarchival INTEGRAL/IBIS data that provides a refined estimate of the orbitalperiod at 4.9272$\pm$0.0004 days. A complete outburst history across theINTEGRAL mission is reported. Although the new XMM-Newton observations (eachlasting $\sim$15 ks) targeted the peak flux in the phase-folded hard X-raylight curve of IGR$~$J17544$-$2619, no bright outbursts were observed, thesource spending the majority of the exposure at intermediate luminosities ofthe order of several 10$^{33}\,$erg$\,$s$^{-1}$ (0.5$\,-\,$10$\,$keV) anddisplaying only low level flickering activity. For the final portion of theexposure, the luminosity of IGR$~$J17544$-$2619 dropped to$\sim$4$\times$10$^{32}\,$erg$\,$s$^{-1}$ (0.5 - 10 keV), comparable with thelowest luminosities ever detected from this source, despite the observationsbeing taken near to periastron. We consider the possible orbital geometry ofIGR$~$J17544$-$2619 and the implications for the nature of the mass transferand accretion mechanisms for both IGR$~$J17544$-$2619 and the SFXT population.We conclude that accretion under the `quasi-spherical accretion' model providesa good description of the behaviour of IGR$~$J17544$-$2619, and suggest anadditional mechanism for generating outbursts based upon the mass accumulationrate in the hot shell (atmosphere) that forms around the NS under thequasi-spherical formulation. Hence we hope to aid in explaining the variedoutburst behaviours observed across the SFXT population with a consistentunderlying physical model.

New insights on accretion in Supergiant Fast X-ray Transients from XMM-Newton and INTEGRAL observations of IGR J17544$-$2619