Slow-moving magnetar a new breed?

Abstract

Magnetars are observationally rare, highly magnetic neutron stars, potentially arising from core-collapse supernovae. However, other origins have been suggested: stellar mergers or an induced collapse within a progenitor binary system. Determining the formation mechanism of magnetars is an ongoing pursuit, one that is often complicated by dense natal environments. Ashley Chrimes and colleagues have measured the proper motion of Galactic magnetar SGR 0501+4516, finding that its velocity is too low to have been created in a standard core-collapse supernova.

SGR 0501+4516 was discovered in 2008, and its relatively low extinction and low crowding allowed the detection and monitoring of a near-infrared counterpart for 12 subsequent years, with HST and ground-based telescopes. The tangential velocity of the magnetar was determined to be 51 ± 14 km s⁻¹, comparable to the slowest ~10% of pulsars. Such a low velocity, along with the direction of motion, rules out an origin within the nearby supernova remnant HB9. There also do not appear to be any nearby young star-forming regions or clusters in the direction opposite to that of the magnetar’s travel, even if SGR 0501+4516 were an order of magnitude older than it appears to be. This lack also disfavours SGR 0501+4516’s origin in a supernova with a low ejecta mass (for instance, that produced by a stripped star), in which case the remnant would be undetectable.