NuSTAR discovery of a cyclotron absorption line in the transient X-ray pulsar 2S 1553-542

• Published in: arXiv.org 2015-12
• Main Authors: Tsygankov, Sergey S, Lutovinov, Alexander A, Krivonos, Roman A, Molkov, Sergey V, Jenke, Peter J, Finger, Mark H, Poutanen, Juri
• Format: Article
• Language: English
• Subjects: Absorption; Accretion disks; Blackbody; Broadband; Cyclotron resonance; Field strength; Neutron stars; Neutrons; Observatories; Pulsars; Resonance scattering
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1512.00851

We report 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. Properties of the source at high energies (>30 keV) are studied for the first time and the sky position had been essentially improved. The source broadband spectrum has a quite complicated shape and can be reasonably described by a composite model with two continuum components - a black body emission with the temperature about 1 keV at low energies and a power law with an exponential cutoff at high energies. Additionally an absorption feature at \(\sim23.5\) 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 \(B\sim3\times10^{12}\) G. Based on the Fermi/GBM data the orbital parameters of the system were substantially improved, that allowed us to determine the spin period of the neutron star P = 9.27880(3) s and a local spin-up \(\dot P \simeq -7.5\times10^{-10}\) s s\(^{-1}\) due to the mass accretion during the NuSTAR observations. Assuming accretion from the disk and using standard torque models we have estimated the distance to the system \(d=20\pm4\) kpc.