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Supergiant fast X-ray transients are wind-fed binaries hosting neutron staraccretors, which display a peculiar variability in the X-ray domain. Differentmodels have been proposed to explain this variability and the strength of thecompact object magnetic field is generally considered a key parameter todiscriminate among possible scenarios. We present here the analysis of twosimultaneous observational campaigns carried out with Swift and NuSTARtargeting the supergiant fast X-ray transient sources AX J1841.0-0536 and SAXJ1818.6-1703. A detailed spectral analysis is presented for both sources, withthe main goal of hunting for cyclotron resonant scattering features that canprovide a direct measurement of the neutron star magnetic field intensity. AXJ1841.0-0536 was caught during the observational campaign at a relatively lowflux. The source broad-band spectrum was featureless and could be welldescribed by using a combination of a hot blackbody and a power-law componentwith no measurable cut-off energy. In the case of SAX J1818.6-1703, thebroad-band spectrum presented a relatively complex curvature which could bedescribed by an absorbed cut-off power-law (including both a cut-off and afolding energy) and featured a prominent edge at $\sim$7 keV, compatible withbeing associated to the presence of a "screen" of neutral material partlyobscuring the X-ray source. The fit to the broad-band spectrum also requiredthe addition of a moderately broad ($\sim$1.6 keV) feature centered at $\sim$14keV. If interpreted as a cyclotron resonant scattering feature, our resultswould indicate for SAX J1818.6-1703 a relatively low magnetized neutron star($\sim$1.2$\times$10$^{12}$ G).

NuSTAR and Swift observations of two supergiant fast X-ray transients: AX J1841.0-0536 and SAX J1818.6-1703