Young isolated neutron stars from the Gould Belt

Abstract

Abstract. The origin of the local population of young, cooling neutron stars is investigated with a population synthesis model taking into account the contribution of neutron stars born in the Gould Belt, in addition to those originating in the Galactic disk. We estimate their emission in the soft X-ray band as a function of distance and age and construct the Log N-Log S distribution. It is shown that the inclusion of neutron stars from the Gould Belt provides a good fit to the observed Log NLog S distribution. As the Sun is situated inside the Gould Belt, one can naturally explain the comparative local overabundance of massive progenitors and can remove the difficulty of the deficit of relatively bright ( > ∼ 0.1 ROSAT PSPC cts s −1 ) cooling neutron stars previously reported from models where only neutron stars from the Galactic disk were accounted for.Key words. stars: neutron -stars: evolution -stars: statistics -X-rays: stars IntroductionObservations of isolated neutron stars (INSs) are important for gaining deeper insight on their structure and thermal evolution and ultimately might prove decisive in unveiling the physical properties of matter at ultra-high densities. Up to a decade ago, the only known INSs were active radio pulsars, with the addition of the γ-ray pulsar Geminga. Despite the fact that X-ray emission from some radio pulsars was already detected by Einstein, it was in the 90's that ROSAT, thanks to its sensitivity in the 0.1−2 keV band, gave a clearer picture of the faint X-ray emission produced by the cooling surface of the closest INSs. ROSAT, supplemented by more recent observations by Chandra and XMM-Newton, revealed a variety of types of behavior in the X-ray emission from INSs and its relation with radio activity.In particular, a substantial contribution of ROSAT to this field has been the discovery of a group of seven radio- Since the seven RINSs have remarkably similar observed properties, it is quite natural to assume that they belong to the same class, i.e. all of them are close-by, cooling NSs. This, however, poses a major problem. A useful and standard way to study a population of sources is to compute the Log N-Log S distribution starting from a model, and then compare it with observations. This has been done for RINSs by Neuhäuser & Trümper (1999) and Popov et al. (2000b). The main conclusion of the latter investigation is that the typical spatial density of radio pulsars in our Galaxy is too low to explain RINSs. In other words, the assumption that RINSs and ordinary radio pulsars derive from the same parent population underpredicts the number of observed "coolers", i.e. NSs which are hot and close enough for their thermal emission to be detected in X-rays.An obvious solution is to invoke a local (both in space and time) overabundance of NSs with respect to those originating in the Galactic disk and seen now as radio pulsars. The main goal of this paper is to investigate the possible origin of these objects. Here we suggest that the likely birthplace for many of the INSs in th...

The Gould Belt

“…At present, 7 neutron stars are observed in the region of the Gould belt [77,78] and they could definitely be the residues of such supernovae.…”

The Magnificent Seven in the dusty prairie

“…The main motivation for us to take into account better absorption treatment is an improved population synthesis model based on that of Popov et al (2000Popov et al ( , 2003 and Popov et al (2005c). A detailed discussion of the improvements is beyond the scope of this article and will be provided elsewhere (Popov, Posselt, in preparation).…”

“…Therefore, we summarize here only the new developments and concentrate on illustrating the importance of the interstellar absorption. Popov et al (2003) for Gould Belt radii of RGB = 500 pc and RGB = 300 pc respectively. The only difference to the solid curve is the progenitor distribution.…”

“…The results of taking into account the new progenitor distribution and additionnally the new absorption models are shown by the dashed-dotted line for the refined analytical ISM model and the dot-dot-dot-dashed line for the extinction model based on Hakkila et al (1997). Furthermore the measurement points of young NSs with thermal X-ray emission are plotted as in Popov et al (2003). For discussion see text.…”

“…While Popov et al (2003Popov et al ( , 2005c modeled the neutron star progenitor distribution as coming from infinitely thin disks, either from the galactic plane or the Gould Belt, the new initial distribution is more realistic. Up to the Hipparcos limit of 400 pc (ESA 1997) the known B2-O8 stars are considered individually as well as their affiliation to an OB association (de Zeeuw et al 1999) resulting in birth properties depending on the age of the OB-association.…”