Mon. Not. R. Astron. Soc., 457, 258-266 (2016/March-3)
NuSTAR discovery of a cyclotron absorption line in the transient X-ray pulsar 2S 1553-542.
TSYGANKOV S.S., LUTOVINOV A.A., KRIVONOS R.A., MOLKOV S.V., JENKE P.J., FINGER M.H. and POUTANEN J.
Abstract (from CDS):
We report the results of a spectral and timing analysis of the poorly studied transient X-ray pulsar 2S 1553-542 using data collected with the NuSTAR and Chandra observatories and the Fermi/GBM instrument during an outburst in 2015. The properties of the source at high energies (>30 keV) are studied for the first time and the sky position has been essentially improved. The source broad-band spectrum has a quite complicated shape and can be reasonably described by a composite model with two continuum components - a blackbody emission with the temperature about 1 keV at low energies and a power law with an exponential cut-off at high energies. Additionally, an absorption feature at ∼23.5 keV is discovered both in phase-averaged and phase-resolved spectra and interpreted as the cyclotron resonance scattering feature corresponding to the magnetic field strength of the neutron star B ∼ 3 x 1012 G. Based on the Fermi/GBM data, the orbital parameters of the system were substantially improved, which allowed us to determine the spin period of the neutron star P = 9.27880(3) s and a local spin-up s /s due to the mass accretion during the NuSTAR observations. Assuming accretion from the disc and using standard torque models, we estimated the distance to the system as d = 20 ± 4 kpc.