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

Abstract

X 1822-371 is an eclipsing binary system with a period close to 5.57 hr and an orbital period derivative ${\dot{P}}_{\mathrm{orb}}$ of 1.42(3)$\times 10^{-10}$ s s${}^{-1}$. The extremely high value of its ${\dot{P}}_{\mathrm{orb}}$ 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 $7.4\times 10^{-12}$ Hz s${}^{-1}$. Assuming a luminosity at the Eddington limit, a neutron star magnetic field strength of $B=8\times 10^{10}$ G is estimated. However, a direct measure of $B$ could be obtained observing a CRSF in the energy spectrum. Analysis of \textit{XMM-Newton} data suggested the presence of a cyclotron line at 0.73 keV, with an estimated magnetic field strength of $B=(8.8\pm 0.3)\times 10^{10}$ G. 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 \textit{NuSTAR} spectrum to investigate the presence of a cyclotron absorption line and the complex continuum emission spectrum. 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. We confirm the presence of a cyclotron line at 0.66 keV inferring a NS magnetic field of $B=(7.9\pm 0.5)\times 10^{10}$ G and suggesting that the Comptonized component originates in the accretion columns.

16 pages, 16 figures, Accepte for publication in A&A on December 28, 2023