X-ray emission from magnetized neutron star atmospheres at low mass-accretion rates

I. Phase-averaged spectrum

¹ Dr. Karl Remeis-Observatory & ECAP, University of Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: ekaterina.sokolova-lapa@fau.de
² Sternberg Astronomical Institute, M. V. Lomonosov Moscow State University, Universitetskij pr., 13, Moscow 119992, Russia
³ Kazan Federal University, 420008 Kazan, Russia
⁴ European Space Astronomy Center (ESA/ESAC), Science Operations Department, Villanueva de la Cañada, 28691 Madrid, Spain
⁵ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁶ CRESST, Department of Physics, and Center for Space Science and Technology, UMBC, Baltimore, MD 21250, USA
⁷ NASA Goddard Space Flight Center, Astrophysics Science Division, Greenbelt, MD 20771, USA
⁸ Department of Physics & Astronomy, George Mason University, Fairfax, VA 22030-4444, USA
⁹ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
¹⁰ NASA Marshall Space Flight Center, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA
¹¹ Universities Space Research Association, Science and Technology Institute, 320 Sparkman Drive, Huntsville, AL 35805, USA

Received: 23 December 2020
Accepted: 13 April 2021

Abstract

Recent observations of X-ray pulsars at low luminosities allow, for the first time, the comparison of theoretical models of the emission from highly magnetized neutron star atmospheres at low mass-accretion rates (Ṁ ≲ 10¹⁵ g s⁻¹) with the broadband X-ray data. The purpose of this paper is to investigate spectral formation in the neutron star atmosphere at low Ṁ and to conduct a parameter study of the physical properties of the emitting region. We obtain the structure of the static atmosphere, assuming that Coulomb collisions are the dominant deceleration process. The upper part of the atmosphere is strongly heated by the braking plasma, reaching temperatures of 30–40 keV, while its denser isothermal interior is much cooler (∼2 keV). We numerically solve the polarized radiative transfer in the atmosphere with magnetic Compton scattering, free–free processes, and nonthermal cyclotron emission due to possible collisional excitations of electrons. The strongly polarized emitted spectrum has a double-hump shape that is observed in low-luminosity X-ray pulsars. A low-energy “thermal” component is dominated by extraordinary photons that can leave the atmosphere from deeper layers because of their long mean free path at soft energies. We find that a high-energy component is formed because of resonant Comptonization in the heated nonisothermal part of the atmosphere even in the absence of collisional excitations. However, these latter, if present, affect the ratio of the two components. A strong cyclotron line originates from the optically thin, uppermost zone. A fit of the model to NuSTAR and Swift/XRT observations of GX 304−1 provides an accurate description of the data with reasonable parameters. The model can thus reproduce the characteristic double-hump spectrum observed in low-luminosity X-ray pulsars and provides insights into spectral formation.