Chandra Full-Field Image  This image shows the entire field-of-view for the Chandra observations of 47 Tucanae. There are over 100 X-ray sources in this field, over ten times the amount detected by previous X-ray satellites. (Credit: NASA /CfA /J.Grindlay et al.) Scale: Image is 2 arcmin per side.

The identification of 16 millisecond pulsars in one X-ray image, with a common distance, age, and environment, has allowed detailed comparison of their X-ray properties with their other properties, and with other millisecond pulsars in different environments. In other words, it opens a new window for studying this large sample of unusual objects.

Yes, very much a new discovery, which has stimulated a lot of work both on explaining their X-ray properties, and looking for similar objects in other globular clusters.

Millisecond pulsars are identified by radio pulses emitted nearly 1,000 times per second. Radio pulsars are neutron stars, compact remnants of dead stars, which emit beams of radiation from their magnetic poles, which we see as radio pulses expressing the spin frequency of the neutron star. Millisecond pulsars were formed billions of years ago (generally), and slowed down until they no longer emitted radio pulses. These neutron stars then entered a close embrace with another star, allowing the neutron star to suck gas from the outer layers of the other star. As this gas fell onto the neutron star, it spun up the neutron star until it reached a spin frequency approaching 1,000 Hz, whereupon the neutron star was able to start emitting radio pulses again. Our observation identifies X-ray radiation from the hot magnetic poles of 16 millisecond pulsars in one star cluster, and provides the largest sample to date of such detections. Our analysis allows constraints upon the physical processes near the neutron star, the history of these millisecond pulsars, and the total number of millisecond pulsars in the cluster not yet detected in the radio.

I am a graduate student, working for Prof. Josh Grindlay on these X-ray observations and analyses. As I applied to graduate schools in astronomy, I inquired what projects possible advisors might have available. Of the projects I was offered, this work on X-ray observations of 47 Tuc was the most interesting and challenging.

Craig Henke
Harvard-Smithsonian Center for Astrophysics
Cambridge, MA, USA