NuSTAR observation of the supergiant fast X-ray transient IGR J11215−5952 during its 2017 outburst

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via A. Corti 12, 20133 Milano, Italy
e-mail: lara.sidoli@inaf.it
² Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, 13 Universitetskij Pr., 119234 Moscow, Russia
³ Kazan Federal University, Kremlevskaya 18, 420008 Kazan, Russia
⁴ Scuola Universitaria Superiore IUSS Pavia, Palazzo del Broletto, Piazza della Vittoria 15, 27100 Pavia, Italy
⁵ INFN, Sezione di Pavia, Via A. Bassi 6, 27100 Pavia, Italy
⁶ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, Area della Ricerca del CNR, Via Gobetti 101, 40129 Bologna, Italy
⁷ INAF, Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Roma, Italy

Received: 3 April 2020
Accepted: 17 April 2020

Abstract

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 187.0 s, 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 ∼7 keV, by a cutoff with an e-folding energy of ∼24 keV. A weak emission line is present at 6.4 keV, consistent with K_α emission from cold iron in the supergiant wind. The time-averaged flux is ∼1.5 × 10⁻¹⁰ erg cm⁻² s⁻¹ (3−78 keV, corrected for the absorption), translating into an average luminosity of about 9 × 10³⁵ erg s⁻¹ (1–100 keV, assuming a distance of 6.5 kpc). 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 (3−12 keV energy range compared with 12−78 keV 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.