A slow-spinning pulsar lights up its cradle

by Jay Gallagher |
Posted Apr 25, 2012

Composite image (x-rays in blue, optical in red & reen) of NGC602. The neutron star is part of the blue object to the right. (Copyright: X-ray: NASA/CXC/Univ.Potsdam/L. Oskinova et al & ESA/XMM-Newton; Optical: AURA/NOAO/CTIO/Univ.Potsdam/L. Oskinova et al)

X-ray image of the pulsar (Chandra X-ray observatory)

Near the edge of the Small Magellanic Cloud sits a supernova remnant that is remarkable in that it contains a B star that even more remarkably contains a slowly spinning neutron star.

To the naked eye, the Small Magellanic Cloud (SMC) resembles a small bit of the Milky Way that somehow became misplaced in the southern sky. In reality it is a small galaxy orbiting the Milky Way at a distance of 60,000 parsecs, in close company with its companion system, the Large Magellanic Cloud. The interactions between these two systems distorted the SMC and produced an appendage of stars and gas, the "SMC Wing" that extends towards the Large Cloud.

Professor Gallagher is part of an international team coordinated by Dr. Lida Oskinova from the Institute of Physics and Astronomy in Potsdam Germany that has been exploring populations of stars and the evolution of the SMC Wing. Until recently the Wing was largely ignored as it contains only a few sites of star formation and a smattering of younger stars seen against a low density sheet of older stars. It seemed a fairly ordinary but sparsely populated region of a galaxy, but interest has grown as astronomers began to explore processes that set the sizes of the stellar bodies of galaxies. An investigation of the Wing then offered the possibility of important insights into the nature of stellar populations and star formation in the low density boundaries of galaxies.

The small HII region, NGC 602, about the size of the Great Nebula in Orion, first attracted Professor Gallagher's interest in a Hubble Space Telescope Project led by European Space Agency Astronomers Drs. Antonella Nota and Elena Sabbi. Gallagher, working with Wisconsin graduate student Lou Nigra and Dr. Linda Smith of the Space Telescope Science Institute, undertook a study that suggested this region of star formation is the result of collisions between giant bubbles of interstellar gas, where the intersection collapsed to form stars.

This dynamic picture of the superficially inert tip of the SMC Wing led to discussions at international astronomy meetings in Flagstaff, Arizona and Bristol, Tennessee. Over beer and barbecue proposals were roughed out to study the energetics of the region by seeking X-ray emission from hot gas that might drive the interstellar bubbles. Dr. Oskinova took the lead and successful programs were carried out to obtain X-ray images with the Chandra and XMM space observatories in coordination with Professor You-Hua Chu's research
group at the University of Illinois. As is often the case in astronomy, things did not quite work out as planned.

Deep x-ray observations revealed little hot gas associated with the star formation in NGC 602, but detected a number of distant galaxies as well as a few stars, and there lay the first scientific surprise. One of the X-ray bright stars was shown by team member Dr Chris Evans from the Royal Observatory to be a Be star. Stars classified as spectral type "Be" are surrounded by disks of gas that they spin off their surfaces, and have long been of interest at Wisconsin, especially to our now emeritus Professors Cassinelli and Nordsieck. In this case we found a double coincidence: First the Be star has a companion neutron star that accretes gas from the Be star and produces X-rays. Second the Be star and its pulsar are located in the center of a glowing gas bubble produced by a supernova explosion. The size of the supernova remnant allows us to estimate that a star exploded and gave birth to the pulsar about 25,000 years ago, a result presented in our team research paper led by Edinburg graduate student Vincent Hénault-Brunet.

All fine. Pulsars are born in supernova explosions and are expected to be spinning fast. But here was the surprise: this object, SXP 1062, one of the younger pulsars, does not have a normal spin period of tens of seconds, but instead rotates in about 18 minutes. Why is it so slow? Was it born that way which would have theoretical implications, or is there some mechanism that can effectively stop a spinning pulsar in its tracks? The answer currently is unclear, but the observation carries an astrophysical implication that slowly spinning pulsars orbiting other Be stars, a relatively common situation, may be much younger than previously thought.

What next? It seems likely that the SMC Wing holds other surprises. Among these could be the presence of very massive stars in modest star clusters, an effect that would stress some theories of massive star formation. The SMC Wing is a case where what first looks like an astrophysical desert actually turns out to be a rich source of information about stars and star formation on the boundaries of galaxies.