Universal detection of high-temperature emission in X-ray isolated neutron stars

Abstract

Strongly magnetized isolated neutron stars (NSs) are categorized into two families, according mainly to their magnetic field strength. Those with a higher magnetic field of 10¹⁴-10¹⁵ G are called magnetars, characterized by repeated short bursts, and the others are X-ray isolated neutron stars (XINSs) with 10¹³ G. Both magnetars and XINSs show thermal emission in X-rays, but it has been considered that the thermal spectrum of magnetars can be reproduced with a two-temperature blackbody (2BB), while that of XINSs shows only a single-temperature blackbody (1BB) and the temperature is 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. Here we report that all seven known XINSs show a high-energy component in addition to the 1BB model. Analyzing all the XMM-Newton data for 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. 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 emissions are the same. The lower temperature in XINSs can be explained if XINSs are older than magnetars. Therefore, these results are an observational indication that supports the standard hypothesis on the classification of highly magnetized NSs.