A large age for the pulsar B1757-24 from an upper limit on its proper motion

The 'characteristic age' of a pulsar is usually considered to approximate its true age, but this assumption has led to some puzzling results, including the fact that many pulsars with small characteristic ages have no associated supernova remnants. The pulsar B1757-24 is located just outside the edge of a supernova remnant; the properties of the system indicate that the pulsar was born at the centre of the remnant with a substantial velocity, and that it has subsequently overtaken the expanding blast wave. With a characteristic age of 16,000 yr, the pulsar is expected to have a proper motion of 63-80 milliarcseconds (mas) per year. Here we report observations of the nebula surrounding the pulsar, which limit its proper motion to less than 25 mas yr(-1), implying a minimum age of 39,000 yr. A more detailed analysis argues that the true age may be as great as 170,000 yr, which is significantly larger than the characteristic age. We conclude from this result and other discrepancies associated with pulsars that characteristic ages greatly underestimate the true ages of pulsars.

The Radio Afterglow and the Host Galaxy of the X-Ray-rich GRB 981226.

We report the discovery of a radio transient VLA 232937.2-235553, coincident with the proposed X-ray afterglow for the gamma-ray burst GRB 981226. This gamma-ray burst (GRB) has the highest ratio of X-ray to gamma-ray fluence of all the GRBs detected by BeppoSAX so far, and yet no corresponding optical transient was detected. The radio light curve of VLA 232937.2-235553 is qualitatively similar to that of several other radio afterglows. At the subarcsecond position provided by the radio detection, optical imaging reveals an extended R=24.9 mag object, which we identify as the host galaxy of GRB 981226. Afterglow models that invoke a jetlike geometry for the outflow or that require an ambient medium with a radial density dependence, such as that produced by a wind from a massive star, are both consistent with the radio data. Furthermore, we show that the observed properties of the radio afterglow can explain the absence of an optical transient without the need for large extinction local to the GRB.