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

Abstract

Aims. To estimate the compactness of the thermally emitting isolated neutron star 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 data set 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 as developed by Gregory & Loredo (1992). A number of complex models (single and double peaked) 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 -approximated with a mixed von Mises distribution -, and its parameters were estimated by using the Expected Maximization method. Spin phase-resolved spectra were extracted, and a number of highly magnetized atmosphere models of an INS were used to fit simultaneously, the results were verified via an MCMC approach. Results. The phase-folded light curves in different energy bands with high S/N ratio show a high complexity and variations depending on time and energy. They can be parameterized with a mixed von Mises distribution, i.e. with double-peaked light curve profile showing a dependence of the estimated parameters (mean directions, concentrations, and proportion) upon the energy band, indicating that radiation emerges from at least two emitting areas. Conclusions. The genuine spin-period of the isolated neutron star RX J0720−3125 derived as more likely is twice of that reported in the literature (16.78s instead of 8.39s). The gravitational redshift of RX J0720.4−3125 was determined to z = 0.205 +0.006 −0.003 and the compactness was estimated to (M/M )/(R/km) = 0.105 ± 0.002.

A deep XMM-Newton look on the thermally emitting isolated neutron star RX J1605.3+3249

“…Using this model, Hambaryan et al (2011) derived the temperatures of the X-ray emitting areas and the magnetic field intensity at the poles; moreover, they could constrain the compactness of the neutron star and the gravitational redshift on the surface, suggesting a very stiff equation of state. Similar conclusions were reached for the M7 RX J0720.4-3125 (Hambaryan et al 2017). These results are only marginally compatible with the most favoured range of the true radius of a 1.5 M neutron star, 10 − 11.5 km, from the analysis of Özel & Freire (2016).…”

XMM-Newton observations of PSR J0726−2612, a radio-loud XDINS

“…We have marked in Fig. 10 the values of the angles ξ and χ estimated for RX J1308.6+2127 and RX J0720.4−3125 by Hambaryan et al (2011) and Hambaryan et al (2017). They imply that these two pulsars are nearly orthogonal rotators (ξ ≈ 90 • ) seen with a large impact parameter η = |χ − ξ| ≈ 45 • .…”

The Masses of Isolated Neutron Stars Inferred from the Gravitational Redshift Measurements

“…Fortunately, the X-ray spin phase-resolved spectroscopic study of some bright thermally emitting INSs and the fit of highly magnetized atmospheric models allow the community to estimate their compactness. Based on the multiepoch observations conduced by XMM-Newton, the gravitational redshifts of RBS 1223, RX J0720.4-3125, and RX J1856.5-3754, three members of the socalled "The Magnificent Seven," have been determined to be 0.16 +0.03 −0.02 , 0.205 +0.006 −0.003 , and 0.22 +0.06 −0.12 (Hambaryan et al 2014(Hambaryan et al , 2017. Applying the approach described in Sec.2.3, the masses of these three sources are evaluated.…”