We present 21-cm H i observations from the Southern Galactic Plane Survey of the field around the anomalous X-ray pulsar 1E 1048.1–5937 , a source whose X-ray properties imply that it is a highly magnetized neutron star ( a “ magnetar ” ) .
These data reveal an expanding hydrogen shell , GSH 288.3–0.5–28 , centered on 1E 1048.1–5937 , with a diameter of 35 \times 23 pc ( for a distance of 2.7 kpc ) and an expansion velocity of \approx 7.5 km s ^ { -1 } .
We interpret GSH 288.3–0.5–28 as a wind bubble blown by a 30–40 M _ { \odot } star , but no such central star can be readily identified .
We suggest that GSH 288.3–0.5–28 is the wind bubble blown by the massive progenitor of 1E 1048.1–5937 , and consequently propose that magnetars originate from more massive progenitors than do radio pulsars .
This may be evidence that the initial spin period of a neutron star is correlated with the mass of its progenitor , and implies that the magnetar birth rate is only a small fraction of that for radio pulsars .