The Magni ent Seven: Nearby Isolated Neutron Stars with strong Magneti Fields

Estimates from the current pulsar birth rate and the number of supernovae to account for the heavy-element abundance suggest that in total 10 to 10 isolated neutron stars exist in our Galaxy. Only a small fraction is detectable as young neutron stars due to their emission of thermal X-rays (for ∼10 years) or pulsed radio emission (for ∼10 years). Proposals that an old ”recycled” population of isolated neutron stars re-heated by accretion from the interstellar medium (ISM) should be detectable by ROSAT, triggered several projects to search for such objects in the ROSAT all-sky survey data. Over the last decade seven very soft X-ray sources with particular characteristics were discovered in ROSAT data. Extreme X-ray to optical flux ratios and low absorption column densities strongly suggest that these objects are nearby isolated neutron stars (see reviews by Treves et al. 2000; Motch 2001; Haberl 2004). Using HST, parallax measurements yield a distance of 117±12 pc for RXJ1856.5−3754 (Walter & Lattimer 2002). The detection of relatively high proper motion (PM) for the three brightest stars makes accretion from the ISM highly ineffective and favours the picture of cooling neutron stars with an age of ∼10–10 years to power the X-rays. Tracing back the apparent trajectories suggests that the brightest of the ROSAT-discovered isolated neutron stars were born in the Sco OB2 complex which is the closest OB association (see e.g. Motch et al. 2005, and references therein). The X-ray spectra of the “magnificent seven”, as they are sometimes called in the literature, are thermal and blackbody-like without a hard power-law tail as it is often observed in other isolated neutron stars (e.g. Pavlov et al. 2002). Typical observed blackbody temperatures kT are in the range of 40−110 eV (see Table 1). From five stars X-ray pulsations were detected with pulse periods between 3 s and 12 s and pulsed fractions between 4% and 18% (Fig. 1). However, for the X-ray brightest star RX J1856.5−3754 no pulsations with a stringent upper limit on periodic variation of 1.3% (2σ confidence level in the 0.02 1000 s range) were found (Burwitz et al. 2003). A surprising discovery was that the X-ray spectrum and the pulsed fraction observed from RXJ0720.4−3125 changes on a time-scale of years which may be caused by precession of the neutron star (de Vries et al. 2004).