Do Central Compact Objects have Carbon Atmospheres?

Abstract

Only three of the dozen central compact objects (CCOs) in supernova remnants (SNRs) show thermal X-ray pulsations due to nonuniform surface temperature (hot spots). The absence of X-ray pulsations from several unpulsed CCOs has motivated suggestions that they have uniform-temperature carbon atmospheres (UTCAs), which adequately fit their spectra with appropriate neutron star (NS) surface areas. This is in contrast to the two-temperature blackbody or hydrogen atmospheres that also fit well. Here we investigate the applicability of UTCAs to CCOs. We show the following: (i) The phase-averaged spectra of the three pulsed CCOs can also be fitted with a UTCA of the appropriate NS area, despite pulsed CCOs manifestly having nonuniform surface temperature. A good spectral fit is therefore not strong support for the UTCA model of unpulsed CCOs. (ii) An improved spectrum of one unpulsed CCO, previously analyzed with a UTCA, does not allow an acceptable fit. (iii) For two unpulsed CCOs, the UTCA does not allow a distance compatible with the SNR distance. These results imply that, in general, CCOs must have hot, localized regions on the NS surface. We derive new X-ray pulse modulation upper limits on the unpulsed CCOs, and constrain their hot spot sizes and locations. We develop an alternative model that accounts for both the pulsed and unpulsed CCOs: a range of angles between hot spot and rotation axes consistent with an exponential distribution with scale factor λ ∼ 20°. We discuss the physical mechanisms that could produce such small angles and small hot spots.

A Quantum Mechanical Treatment of Electron Broadening in Strong Magnetic Fields. II. Large Enhancements due to Exchange Interactions

“…Since the elemental composition of neutron star atmospheres is unknown, we explore a couple of different cases. Predictions for the composition of nonaccreting neutron star atmospheres range from light elements, such as hydrogen or helium (Ho et al 2003;Güver et al 2011), to mid-Z elements, such as carbon, oxygen, or neon (Mori & Hailey 2006;Ho & Heinke 2009;Alford & Halpern 2023), and Fe (Rajagopal et al 1997;Nättilä et al 2015). Therefore, for this study, we examine how magnetic fields affect electron collisions of both neutral hydrogen and highly ionized oxygen.…”

What Is the Nature of the HESS J1731-347 Compact Object?

“…relies on the fact that the object has a uniform-temperature carbon atmosphere and that the star is located at a distance of 2.5 kpc. Further studies are needed to reduce the uncertainties and understand the validity of the obtained results (Alford & Halpern 2023).…”

Emission lines due to ionizing radiation from a compact object in the remnant of Supernova 1987A

“…For the young CNS scenario, previous models have found an upper limit to the thermal luminosity of ≲4×1035 erg s−1 and a surface temperature of ~(1.5 to 3) × 10 6 K at 35 years after the explosion ( 26 , 30 , 31 ). Central compact objects, like the NS in Cas A, are thought to have carbon-dominated atmospheres ( 21 ), although alternative models producing a hard spectrum have been discussed ( 32 ). We adopted a carbon-dominated CNS spectrum ( 21 ) and set Lion=3×1035 erg s−1.…”