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. The one with a higher magnetic field of $10^{14}$ - $10^{15}$ G is called magnetar, characterized with repeated short bursts, and the other is X-ray isolated neutron star (XINS) with $10^{13}$ G. Both magnetars and XINSs show thermal emission in X-rays, but it has been considered that the thermal spectrum of magnetars is 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 the seven known XINSs show a high-energy component in addition to the 1BB model. 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. 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 their origins of the emission are also the same. The lower temperature in XINSs can be explained if XINSs are older than magnetars. Therefore, these results are the observational indication that supports the standard hypothesis on the classification of highly-magnetized NSs.

References in corpus (6)

• The Magnificent Seven: Magnetic fields and surface temperature distributions
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• Magnetars as persistent hard X-ray sources: INTEGRAL discovery of a hard tail in SGR 1900+14
• Maximum gravitational-wave energy emissible in magnetar flares
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Cited by in corpus (8)

• Discovery of a radio emitting neutron star with an ultra-long spin period of 76 seconds
• Evidence for an abundant old population of Galactic ultra long period magnetars and implications for fast radio bursts
• Cooling Theory Faced with Old Warm Neutron Stars: Role of Non-Equilibrium Processes with Proton and Neutron Gaps
• Thermal and non-thermal X-ray emission from the rotation-powered radio/$\gamma$-ray pulsar PSR J1740+1000
• XMM-Newton observations of PSR J0726-2612, a radio-loud XDINS
• Two decades of X-ray observations of the isolated neutron star RX J1856.5-3754: detection of thermal and non-thermal hard X-rays and refined spin-down measurement
• Low X-ray Efficiency of a Young High-B Pulsar PSR J1208-6238 Observed with Chandra
• X-ray nondetection of PSR J0250+5854