GRO J1744-28: an intermediate B-field pulsar in a low-mass X-ray binary

The bursting pulsar, GRO J1744-28, went again in outburst after similar to 18 yr 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 3 d later, thus covering the 1.3-70.0 keV 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 15 keV, 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 similar to 4.7 keV, with hints for two possible harmonics at similar to 10.4 and similar to 15.8 keV. The position of the cyclotron fundamental allows an estimate for the pulsar magnetic field of (5.27 +/- 0.06) x 10(11) G, 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 (greater than or similar to 100 R-g) and for the inclination angle (18 degrees-48 degrees). 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 50-115 R-g. From these estimates, we derive the magnetospheric radius for disc accretion to be similar to 0.2 times the classical Alfven radius for radial accretion.