The needle in the haystack: where to look for more isolated cooling neutron stars

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
² Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergäßchen 2-3, 07745 Jena, Germany
³ Observatoire Astronomique de Strasbourg, 11 rue de l' Universite, 67000 Strasbourg, France e-mail: bposselt@cfa.harvard.edu
⁴ Sternberg Astronomical Institute, Universitetski pr. 13, 119991 Moscow, Russia e-mail: polar@sai.msu.ru
⁵ University of Padua, Department of Physics, via Marzolo 8, 35131 Padova, Italy

Received: 7 August 2007
Accepted: 29 January 2008

Abstract

Context. Isolated, cooling neutron stars with thermal X-ray emission remain rarely-detected objects despite many searches investigating the ROSAT data.

Aims. We simulate the population of close-by young cooling neutron stars to explain the current observational results. Given the inhomogenity of the neutron star distribution on the sky, it is particularly interesting to identify promising sky regions with respect to ongoing and future searches.

Methods. Applying a population synthesis model, the inhomogenity of the progenitor distribution and the inhomogenity of the X-ray absorbing interstellar medium are considered for the first time. The total number of observable neutron stars is derived with respect to ROSAT count rates. In addition, we present sky maps of neutron star locations, and discuss age and distance distributions of the simulated neutron stars. Implications for future searches are discussed.

Results. With our advanced model, we can succesfully explain the observed distribution of close-by neutron stars. Cooling neutron stars will be most abundant in the directions of rich OB associations. We expect new candidates to be identified behind the Gould Belt, in particular in the Cygnus-Cepheus region. We expect them to be on average younger and hotter than the known population of isolated, cooling neutron stars. In addition, we propose to use data on runaway stars to search for more radio-quiet, cooling neutron stars.