The compactness of the isolated neutron star RX J0720.4−3125^⋆

¹ Astrophysikalisches Institut und Universitäts-Sternwarte, Universität Jena, Schillergäßchen 2-3, 07745 Jena, Germany
e-mail: vvh@astro.uni-jena.de
² Eberhard-Karls-Universität, Institut für Astronomie und Astrophysik, 72076 Tübingen, Germany
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany
⁴ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Gene Center of the LMU, Department of Biochemistry, Feodor-Lynen-Strasse 25, 81377 München, Germany

Received: 28 December 2016
Accepted: 24 February 2017

Abstract

Aims. To estimate the compactness of the thermally-emitting isolated neutron star (INS) RX J0720.4−3125, an X-ray spin-phase-resolved spectroscopic study is conducted. In addition, to identify the genuine spin-period, an X-ray timing analysis is performed.

Methods. The data from all observations of RX J0720.4−3125 conducted by XMM-Newton EPIC-pn with the same instrumental setup in 2000−2012 were reprocessed to form a homogenous dataset of solar barycenter corrected photon arrival times registered from RX J0720.4−3125. A Bayesian method for the search, detection, and estimation of the parameters of an unknown-shaped periodic signal was employed. A number of single- and double-peaked complex models of light curves from pulsating neutron stars were statistically analyzed. The distribution of phases for the registered photons was calculated by folding the arrival times with the derived spin-period and the resulting distribution of phases, which was approximated with a mixed von Mises distribution, and its parameters were estimated by using the expected maximization method. Spin-phase-resolved spectra were extracted, a number of highly magnetized atmosphere models of an INS were used to perform simultaneous fits, and the results were verified via an Markov chain Monte Carlo approach.

Results. The phase-folded light curves in different energy bands with high signal-to-noise ratio show high complexity and variations that depend on time and energy. They can be parameterized with a mixed von Mises distribution, meaning with a double-peaked light curve profile that shows a dependence of the estimated parameters, such as the mean directions, concentrations, and proportion upon the energy band, indicating that radiation emerges from at least two emitting areas.

Conclusions. We derive a most-likely genuine spin-period of the isolated neutron star RX J0720−3125 that is twice that reported in the literature, 16.78 s instead of 8.39 s. We determine the gravitational redshift of RX J0720.4−3125 to be z = 0.205_-0.003^+0.006 and estimate the compactness to be (M/M_⊙)/(R/ km) = 0.105 ± 0.002.