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We report on the results of a NuSTAR observation of the Supergiant Fast X-rayTransient pulsar IGRJ11215-5952 during the peak of its outburst in June 2017.IGRJ11215-5952 is the only SFXT undergoing strictly periodic outbursts, every165 days. NuSTAR caught several X-ray flares, spanning a dynamic range of 100,and detected X-ray pulsations at 187.0 s, consistent with previousmeasurements. The spectrum from the whole observation is well described by anabsorbed power-law (with a photon index of 1.4) modified, above 7 keV, by acutoff with an e-folding energy of 24 keV. A weak emission line is present at6.4 keV, consistent with Kalpha emission from cold iron in the supergiant wind.The time-averaged flux is 1.5E-10 erg/cm2/s (3-78 keV, corrected for theabsorption), translating into an average luminosity of about 9E35 erg/s (1-100keV, assuming a distance of 6.5 kpc). The NuSTAR observation allowed us toperform the most sensitive search for cyclotron resonant scattering features inthe hard X-ray spectrum, resulting in no significant detection in any of thedifferent spectral extractions adopted (time-averaged, temporally-selected,spin-phase-resolved and intensity-selected spectra). The pulse profile showedan evolution with both the energy (3-12 keV energy range compared with 12-78keV band) and the X-ray flux: a double peaked profile was evident at higherfluxes (and in both energy bands), while a single peaked, sinusoidal profilewas present at the lowest intensity state achieved within the NuSTARobservations (in both energy bands). The intensity-selected analysis allowed usto observe an anti-correlation of the pulsed fraction with the X-rayluminosity. The pulse profile evolution can be explained by X-ray photonscattering in the accreting matter above magnetic poles of a neutron star atthe quasi-spherical settling accretion stage.

NuSTAR observation of the Supergiant Fast X-ray Transient IGR J11215-5952 during its 2017 outburst