2020A&A...638A..71S

We report on the results of a NuSTAR observation of the supergiant fast X-ray transient pulsar IGR J11215-5952 during the peak of its outburst in June 2017. IGR J11215-5952 is the only SFXT undergoing strictly periodic outbursts (every 165 days). NuSTAR caught several X-ray flares, spanning a dynamic range of 100, and detected X-ray pulsations at 187.0s, which is consistent with previous measurements. The spectrum from the whole observation is well described by an absorbed power law (with a photon index of 1.4), which is modified, above ∼7keV, by a cutoff with an e-folding energy of ∼24keV. A weak emission line is present at 6.4keV, consistent with K_α emission from cold iron in the supergiant wind. The time-averaged flux is ∼1.5x10^–10erg/cm²/s (3-78keV, corrected for the absorption), translating into an average luminosity of about 9x10³⁵erg/s (1-100keV, assuming a distance of 6.5kpc). The NuSTAR observation allowed us to perform the most sensitive search for cyclotron resonant scattering features in the hard X-ray spectrum, resulting in no significant detection in any of the different spectral extractions adopted (time-averaged, temporally selected, spin-phase-resolved and intensity-selected spectra). The pulse profile showed an evolution with both the energy (3-12keV energy range compared with 12-78keV band) and the X-ray flux: a double-peaked profile was evident at higher fluxes (and in both energy bands), while a single-peaked, sinusoidal profile was present at the lowest intensity state achieved within the NuSTAR observations (in both energy bands). The intensity-selected analysis allowed us to observe an anti-correlation of the pulsed fraction with the X-ray luminosity. The pulse profile evolution can be explained by X-ray photon scattering in the accreting matter above magnetic poles of a neutron star at the quasi-spherical settling accretion stage.