Fe Kα and Fe Kβ line detection in the NuSTAR spectrum of the ultra-bright Z-source Scorpius X-1

Low-mass  X-ray  binaries  hosting  a  low-magnetised  neutron  star,  which  accretes  matter  via  Roche-lobe  overflow,  are  generally  grouped  in  two  classes,  named Atoll  and  Z  sources  after  the  path  described  in  their  X-ray  colour-colour  diagrams. Scorpius X-1 is the brightest persistent low-mass X-ray binary known so far, and it isthe prototype of the Z sources.

We analysed the first NuSTAR observation of this source to study its spectral emission exploiting the high statistics data collected by this satellite. Examining the colour-colour diagram, the source was probably observed during the lower normal and flaring branches of its Z-track. We separated the data from the two branches in order to investigate the evolution of the source along the track.

We fitted the 3-60 keV NuSTAR spectra using the same models for both the branches. We adopted two description for the continuum: in the first case we used a blackbody and a thermal Comptonisation with seed photons originating in the accretiondisc;  in  the  second  one,  we  adopted  a  disc-blackbody  and  a  Comptonisation  with  a blackbody-shaped  spectrum  of  the  incoming  seed  photons.  A  power-law  fitting  the high energy emission above 20 keV was also required in both cases.

The two models provide the same physical scenario for the source in both the branches: a blackbody temperature between 0.8 and 1.5 keV, a disc-blackbody with temperature  between  0.4  and  0.6  keV,  and  an  optically  thick  Comptonising  corona with optical depth between 6 and 10 and temperature about 3 keV. Furthermore, two lines related to the Kα and Kβ transitions of the He-like Fe XXV ions were detected at 6.6 keV and 7.8 keV, respectively. A hard tail modelled by a power law with a photon index between 2 and 3 was also required for both the models.