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

Abstract

The bursting pulsar, GRO J1744-28, went again in outburst after $\sim$18 years of quiescence in mid-January 2014. 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 days 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 broadband 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 $\sim$ 4.7 keV, with hints for two possible harmonics at 10.4 keV and 15.8 keV. The position of the cyclotron fundamental allows an estimate for the pulsar magnetic field of (5.27 $\pm$ 0.06) $\times$ 10${}^{11}$ G, if the feature is produced at its surface. From the dynamical and relativistic smearing of the disk reflected component, we obtain a lower limit estimate for the truncated accretion disk inner radius, ($\gtrsim$ 100 R${}_g$), and for the inclination angle (18${}^{\circ }$-48${}^{\circ }$). We also detect the presence of a softer thermal component, that we associate with the emission from an accretion disk truncated at a distance from the pulsar of 50-115 R${}_g$. From these estimates, we derive the magneto-spheric radius for disk accretion to be $\sim$ 0.2 times the classical Alfvén radius for radial accretion.

Accepted for publication in MNRAS

References in corpus (6)

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• XMM-Newton detects a relativistically broadened iron line in the spectrum of the ms X-ray pulsar SAX J1808.4-3658
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• Discovery of the 5 keV cyclotron line followed by three harmonics in SwiftJ1626.6-5156
• The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1
• Observations of GRO J1744-28 in quiescence with XMM-Newton
• X-ray flashes from the low-mass X-ray binary IGR J17407-2808
• Recurrent Instability in LMXB Accretion Disks: How Strange is GRS 1915+105?