NuSTAR detection of a cyclotron line in the supergiant fast X-ray transient IGR J17544-2619

Abstract

We present NuSTAR spectral and timing studies of the supergiant fast X-ray transient (SFXT) IGR J17544−2619. The spectrum is well described by an ~1 keV blackbody and a hard continuum component, as expected from an accreting X-ray pulsar. We detect a cyclotron line at 17 keV, confirming that the compact object in IGR J17544−2619 is indeed a neutron star. This is the first measurement of the magnetic field in an SFXT. The inferred magnetic field strength, B = (1.45 ± 0.03) × 10[superscript 12]G (1 + z) is typical of neutron stars in X-ray binaries, and rules out a magnetar nature for the compact object. We do not find any significant pulsations in the source on time-scales of 1–2000 s.