論文の公開元へA study on the relation for the high magnetic field isolated neutron star population with X-ray observation
概要
We present a study on the relation for the high magnetic field isolated neutron star population with X-ray observation.
Neutron stars (NSs) are extremely compact, fast-spinning remnants of core- collapse supernova explosions, which occurs when the life of a massive star with masses about more than 9M⊙ ends. Typical NSs have the dipole magnetic field Bd of 1012 G, the rotation period P ranging milliseconds and sec, the mass M ∼ 1.4M⊙, the radius R ∼ 10 km, and the density ρ ∼ 1014 g/cm3, respectively. And, the NS is supported by nuclear power mainly composed of neutrons. These states cannot be reproduced in the laboratories on the Earth so that NSs are one of the most important objects in order to verify the physics in the extreme state.
Today, we recognise that the high magnetic field isolated NSs whose dipole magnetic field is Bd > 1013 G are divided into the three major populations. Each population has its own characteristics. Their characteristics are most evident in the X-ray bands. The first population is the rotation-powered pulsars (RPPs). RPPs are the standard NS population and are usually discovered in the radio bands or/and the gamma-ray bands. They are thought to be shined with its rota- tion energy. The X-ray luminosity Lx is about 1/1000 of the rotation luminosity Lrot empirically. The second population is the high magnetic pulsars, called mag- netars for short. Magnetars are characterised by the frequent bursting activity and by the high X-ray luminosity that is larger than the rotation luminosity. It is thought to be shine with the magnetic energy rather than the rotation energy. The third population is the X-ray isolated neutron stars, abbreviated XINSs or XDINSs. XINSs shine with the thermal energy which is likely to be fueled by the magnetic field. XINSs are observationally characterized by the thermal emission in the soft X-ray bands and by the high X-ray luminosity larger than the rota- tion luminosity. What makes this distinctive manifestation of the neutron stars, namely RPPs, magnetars and XINSs, are not understood so far.
The variety of the supernova explosion is one of the candidates to solve this mys- tery. However, according to Keane & Kramer (2008), the Galactic core-collapse supernova rate (CCSN) is smaller than the total birthrate of each population derived by the observation. Thus, they suggest that some of PSRs, XINSs and possibly also magnetars may be different evolutionary stages of the same objects. Recently, XINSs are thought to be evolved from magnetars. Since magnetars and XINSs shine with the magnetic energy due to the strong dipole magnetic field larger than that of RPPs, the dipole magnetic field strength may determine the distinctive manifestation of them. However, the discovery of “weak field magne-tar” whose dipole field Bd ∼ 6.1×1012 G is the comparable magnetic field strength as compare with RPPs (Esposito et al. 2010; Rea et al. 2010, 2013) indicates that the strong dipole field is not always essential. Theoretically, the toroidal field is proposed to play an essential role in the magnetar activity.
The great importance objects for understanding the magnetic field formation and its evolution scenario are the strong magnetic field rotation powered pulsars (High-B RPPs), which have a strong dipole magnetic field about 1013−14 G in RPPs. Since some of High-B RPPs exhibit the magnetar-like and the XINS-like features, they may be the key to understand the relation for the high magnetic field isolated NS population. However, the survey for such objects with the relatively small spin-down luminosity is not complete. Toward a more complete survey, in this paper, we carry out a systematic survey for the X-ray counterparts using the archival data and the subsidiary observations with the X-ray Telescope (XRT; Burrows et al. 2005) onboard the Neil Gehrels Swift Observatory (Gehrels et al. 2004). We aim to get a hint linking these three populations and hopefully to understand what physical parameters make the different characteristics seen in the population.
We analyse 21 out of the 27 high-B PSRs that are in the ATNF pulsar catalogue but have not been reported or have no effective upper-limits in the X-ray bands, where 6 objects are newly observed by us, and 15 objects are taken from the archival data. As a result, we have new 3σ upper-limits for all the 21 objects. Since the upper-limits are tight, we conclude that we do not find any magnetar-like high- B PSRs such as PSR J1819−1458. The probability of the high X-ray efficiency in the high-B PSRs is obtained to be 11% − 29%. Also, combining the previous observations, we discuss which parameter causes magnetar-like properties. It may be suggested that the magnetar-like properties appear only when Bd ≳ 1013.5 G for the radio pulsar population. This is true even if the radio-quiet high-B RPPs are included.
We revisit the relation for the high and low temperature ratio that is obtained by the double blackbody model fitting for the three populations as shown by Yoneyama et al. (2019). As a result, we found that ordinary RPPs also follow the relation. We also perform the verification about the relationship for the tempera- ture of the double blackbody components and its luminosities. The temperature and the luminosity relation suggests that XINSs, high-B RPPs and RPPs have local hot spots such as polar cap heating, unlike magnetars.
