Unification of strongly magnetized neutron stars with regard to X‑ray emission from hot spots

Strongly magnetized isolated neutron stars (NSs) are categorized into two families according, mainly, to their magnetic field strength. The one with a higher magnetic field of 1014–1015 Gauss is called "magnetar," and the other is the X‑ray isolated neutron star (XINS) with 1013 Gauss. Both magnetars and XINSs show thermal emission in X‑rays, whose spectra are different. The soft X‑ray spectrum (below 10 keV) of a magnetar is reproduced with a two‑temperature blackbody (2BB), whereas that of an XINS shows only a single‑temperature blackbody (1BB), and its temperature is even lower than that of magnetars. On the basis of the magnetic field and temperature, it is often speculated that XINSs may be old and cooled magnetars. However, no other strong observational evidence has yet been reported to support the speculation. Here, we report that all the seven known XINSs show high‑temperature emission, which should have a similar origin to that of magnetars. Analyzing all the XMM‑Newton data of the XINSs with the highest statistics ever achieved, we find that their X‑ray spectra are all reproduced with a 2BB model, similar to magnetars, as opposed to the traditional 1BB model. Their emission radii and temperature ratios are also similar to those of magnetars except for two XINSs, which show significantly smaller radii than the others. The remarkable similarity in the X‑ray spectra between XINSs and magnetars suggests that the origins of their emitting regions are also the same. The lower temperature in XINSs can be explained if XINSs are older than magnetars. Therefore, this result is another observational indication that supports the standard hypothesis of classification of highly magnetized NSs. This article is based on our paper Yoneyama et al. (2019; accepted to PASJ).