NuSTAR Detection Of A Cyclotron Line In The Supergiant Fast X-ray Transient IGR J17544-2619

Varun Bhalerao, Patrizia Romano, John Tomsick, Lorenzo Natalucci, David M. Smith, Eric Bellm, Steven E. Boggs, Deepto Chakrabarty, Finn E. Christensen, William W. Craig, Felix Fuerst, Charles J. Hailey, Fiona A. Harrison, Roman A. Krivonos, Ting-Ni Lu, Kristin Madsen, Daniel Stern, George Younes, William Zhang

Published 2014-07-01, updated 2014-12-23Version 2

We present NuSTAR spectral and timing studies of the Supergiant Fast X-ray Transient (SFXT) IGR J17544-2619. The spectrum is well-described by a ~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 a SFXT. The inferred magnetic field strength, B = (1.45 +/- 0.03) * 10^12 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.