GROJ1744−28: an intermediate B-field pulsar in a low-mass X-ray binary

D'Aì, A. ; Di Salvo, T. ; Iaria, R. ; García, J. A. ; Sanna, A. ; Pintore, F. ; Riggio, A. ; Burderi, L. ; Bozzo, E. ; Dauser, T. ; Matranga, M. ; Galiano, C. G. ; Robba, N. R.

In: Monthly Notices of the Royal Astronomical Society, 2015, vol. 449, no. 4, p. 4288-4303

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    The bursting pulsar, GROJ1744−28, went again in outburst after ∼18yr of quiescence in 2014 mid-January. We studied the broad-band, persistent, X-ray spectrum using X-ray data from a XMM-Newton observation, performed almost at the peak of the outburst, and from a close INTEGRAL observation, performed 3d later, thus covering the 1.3-70.0keV band. The spectrum shows a complex continuum shape that cannot be modelled with standard high-mass X-ray pulsar models, nor by two-components models. We observe broad-band and peaked residuals from 4 to 15keV, and we propose a self-consistent interpretation of these residuals, assuming they are produced by cyclotron absorption features and by a moderately smeared, highly ionized, reflection component. We identify the cyclotron fundamental at ∼4.7keV, with hints for two possible harmonics at ∼10.4 and ∼15.8keV. The position of the cyclotron fundamental allows an estimate for the pulsar magnetic field of (5.27±0.06)×1011G, if the feature is produced at its surface. From the dynamical and relativistic smearing of the disc reflected component, we obtain a lower limit estimate for the truncated accretion disc inner radius (≳100Rg) and for the inclination angle (18°-48°). We also detect the presence of a softer thermal component that we associate with the emission from an accretion disc truncated at a distance from the pulsar of 50-115Rg. From these estimates, we derive the magnetospheric radius for disc accretion to be ∼0.2times the classical Alfvén radius for radial accretion