100,000-star cluster’s gamma rays come from a single energetic pulsar

Pulsars are rapidly rotating neutron stars, with magnetic fields that accelerate particles as they rotate. These particles then emit radiation as they lose energy, creating the bright periodic flashes that characterize a pulsar. These objects were first detected due to emissions in the radio end of the spectrum, but have since been spotted at frequencies as energetic as gamma rays. Now, researchers have spotted the most energetic one yet, a pulsar with gamma ray emissions that add up to a total of 2 x 10¹² Megatons of energy every second.

The observations were done with the Fermi space telescope, which specializes in high-energy wavelengths. While observing the globular cluster NGC 6624, Fermi was easily able to detect gamma rays, but the cluster contains 100,000 stars, and probably dozens of pulsars. Since it's difficult to get good spatial resolution at these energies, it was impossible to assign the gamma rays to any pulsar in particular.

So the authors focused on timing, and immediately noticed something dramatic: all of the cluster's emissions came in a single pulse. In the interval between pulses, the cluster was more or less silent at these wavelengths. By comparing the timing of these pulses with high-resolution radio observations, the authors were able to match them to a single source, named J1823−3021A. In other words, all of the cluster's pulsed emissions come from this single, high energy source.

And, as we noted above, it's exceptionally high energy, with a flux of 8 × 10³⁴ ergs every second. The authors were able to use that figure to calculate the strength of the body's magnetic field at the surface of the neutron star, and came up with a value of over 10⁹ Gauss. By comparison, the Earth's magnetic field is less than one Gauss.

Pulsars only reach the frequencies seen at J1823−3021A if they are spun up by matter sucked off a companion star. In this case, the spin up process had to have happened relatively recently, less than 25 million years ago (for astronomy, that's quite recent). That makes it both the youngest and most energetic pulsar we've yet detected.

Science, 2011. DOI: 10.1126/science.1207141  (About DOIs).

Listing image by Photograph by NASA