Searching for substellar companions of young isolated neutron stars^*

¹ Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergäßchen 2-3, 07745 Jena, Germany e-mail: bposselt@cfa.harvard.edu
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312 85741 Garching, Germany

Received: 7 May 2008
Accepted: 27 October 2008

Abstract

Context. Only two planetary systems orbiting old ms-pulsars have been discovered. Young radio pulsars and radio-quiet neutron stars cannot be analysed by the usually-applied radio-pulse-timing technique. However, finding substellar companions orbiting these neutron stars would be of significant importance: the companion may have had an exotic formation, its observation may also enable us to study neutron-star physics.

Aims. We investigate the closest young neutron stars to Earth to search for orbiting substellar companions.

Methods. Young, thus warm substellar companions are visible in the Near infrared, in which the neutron star itself is much fainter. Four young neutron stars are at sufficient speed to enable a common proper-motion search for substellar companions within few years.

Results. For Geminga, RX J0720.4-3125, RX J1856.6-3754, and PSR J1932+1059 we found no comoving companion of masses as low as 12, 15, 11, and 42 Jupiter masses, respectively, for assumed ages of 1, 1, 1, and 3.1 Myr, and distances of 250, 361, 167, and 361 pc, respectively. Near infrared limits are presented for these four and five additional neutron stars for which we have observations for only one epoch.

Conclusions. We conclude that young, isolated neutron stars rarely have brown-dwarf companions.