Confirmation of the presence of a CRSF in the NICER spectrum of X 1822-371

Abstract

Aims. X 1822-371 is an eclipsing binary system with a period close to 5.57 h and an orbital period derivative Porb of 1.42(3) × 10-10 s s-1. The extremely high value of its Porb is compatible with a super-Eddington mass transfer rate from the companion star and, consequently, an intrinsic luminosity at the Eddington limit. The source is also an X-ray pulsar, it shows a spin frequency of 1.69 Hz and is in a spin-up phase with a spin frequency derivative of 7.4 × 10-12 Hz s-1. Assuming a luminosity at the Eddington limit, a neutron star magnetic field strength of B=8 × 1010 G is estimated. However, a direct measure of B could be obtained observing a CRSF in the energy spectrum. Analysis of XMM-Newton data suggested the presence of a cyclotron line at 0.73 keV, with an estimated magnetic field strength of B=(8.8 ± 0.3)× 1010 G. Methods. Here we analyze the 0.3-50 keV broadband spectrum of X 1822-371 combining a 0.3-10 keV NICER spectrum and a 4.5-50 keV NuSTAR spectrum to investigate the presence of a cyclotron absorption line and the complex continuum emission spectrum. Results. The NICER spectrum confirms the presence of a cyclotron line at 0.66 keV. The continuum emission is modeled with a Comptonized component, a thermal component associated with the presence of an accretion disk truncated at the magnetospheric radius of 105 km and a reflection component from the disk blurred by relativistic effects. Conclusions. We confirm the presence of a cyclotron line at 0.66 keV inferring a NS magnetic field of B=(7.9 ± 0.5)× 1010 G and suggesting that the Comptonized component originates in the accretion columns.