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 ∼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 ∼4.7 keV, with hints for two possible harmonics at ∼10.4 and ∼15.8 keV. The position of the cyclotron fundamental allows an estimate for the pulsar magnetic field of (5.27 ± 0.06) × 10¹¹ 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 (≳100 R_g) 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 50-115 R_g. From these estimates, we derive the magnetospheric radius for disc accretion to be ∼0.2 times the classical Alfvén radius for radial accretion.