Observations of young nearby isolated neutron stars

TYPEAstrophysics Seminar
Speaker:Prof. Ralph Neuhaeuser
Affiliation:Astrophysical Institute  and University Observatory, University Jena
Location:Lidow 620

We try to constrain the nuclear Equation-of-State (EoS) by observations of neutron stars in our galactic neighbourhood. There are seven thermally emitting solated neutron stars known from X-ray and optical observations, the so-called Magnificent Seven (M7), which are young (few Myrs), nearby (one to few hundred pc), and radio-quiet with blackbody-like X-ray spectra, so that we can observe their surfaces. As bright X-ray sources, we can determine their rotational (pulse) periods and their period derivatives from X-ray timing. >From XMM and/or Chandra X-ray spectra, we can determine their temperatures. With precise astrometric observations using the Hubble Space Telescope, we can determine their parallax (done for RXJ1856 and RXJ0720). >From flux, distance, and temperature, one can derive the radius. Then, from identifying atomic or cyclotron absorption lines in X-ray spectra and also from rotational phase-resolved spectroscopy, we can determine the compactness (mass/radius) and/or gravitational redshift. This is currenty being applied for one case (RBS1223). If also applied to RXJ1856 or RXJ0720, radius (from luminosity and temperature) and compactness (from X-ray data) will yield the mass and radius - for the first time for an isolated single neutron star without previous mass exchange. Recently, 60Fe was found in the Earth crust, which is believed to have formed in a recent nearby supernova, which may have had influence on the Earth cosmic ray flux, the climate, and possibly on the biosphere. We try to find the neutron star that should have been formed in that supernova, so that we can constrain time and distance of that supernova and even the mass of the progenitor star. Knowing the positions, proper motions, and distances of dozens of nearby neutron stars (within a few kpc), we can determine their past flight path and possible kinematic origin. For such calculations, we have to assume the otherwise unknown radial velocity (and to account for errors in the observables) through Monte-Carlo simulations. We can then find the stellar association, in which the neutron star may have been formed by a recent supernova, by tracing back its motion. If a neutron star seems to have flewn through a nearby young stellar association, where at least one supernova may have taken place given its current mass function, it may have formed there. We search for additional indications for such events, like run-away stars ejected in supernovae in binaries, supernova remnants, 26Al gamma-ray sources, etc. Once the birth place of a neutron star in a supernova is found, we would have determined the distance of the supernova and the age of the neutron star (flight time as kinematic age). If all stars in such an association have formed roughly at the same time, as assumed by star formation theories, we also know the life-time and, hence, mass of the supernova progenitor star. In this way, we try to find the neutron star, which was born in the nearby recent supernova, which may have ejected the 60Fe found in the Earth crust. We can then test and calibrate supernova ejecta models. Any identification of a known neutron star with its birth association (and/or run-away star) would be interesting also to compare kinematic ages with characteristic ages, to test neutron star cooling curves, to study the formation and re-heating of the Local Bubble by supernovae, etc. We compiled a list of all known massive stars within a few kpc, estimated their extinctions, temperatures, and luminosities from published photometry, then their masses, ages, and remaining life-times, so that we can predict the local supernova rate for the near future, which should be similar to the recent past - a prediction with high time and spatial resolution, so that it might be usefull for gravitational wave detections.