Millisecond Pulsars Can't Hide From Multiwavelength Detectives
Although the Fermi Gamma-ray Space Telescope has seen the gamma-rays from over 100 pulsars, many potential pulsars remain undiscovered because the pulsations are not obvious in the Fermi data. Combining radio observations with Fermi, scientists have now found five more rapidly rotating pulsars, including one particularly intriguing object, indicating that techniques will continue to be successful.
Gamma-ray (blue) and radio (red) emission as seen from the newly identified millisecond pulsar PSR J0101–6422. The x-axis shows the phase of the pulsar, where 1 represents one complete period.
Pulsars are nature's lighthouses, where the enormous magnetic field surrounding a rapidly rotating neutron star causes twin beams of particles and radiation to sweep through space like a celestial beacon. The resulting periodic beeps of emission can be seen by an observer in radio, x-ray, or gamma-ray light. Pulsars are extreme laboratories with environments which cannot be mimicked on Earth, in which equally extreme physics can be studied. Much like humans, pulsars start fast, sweeping their beacons several times per second, and slow down over time. However, in a twist not availble to humans, a pulsar can be 'recycled' by gaining angular momentum from accreting matter from a binary companion star, and be spun up to faster that it ever was, with a pulse period of only milliseconds.
The Fermi Gamma-ray Space Telescope's Large Area Telescope (LAT), which was built at SLAC and in which KIPAC is the lead science institution, has revolutionized the study of pulsars in gamma rays, the highest energy kind of light. Over 100 pulsars have already been seen in gamma rays by the Fermi-LAT. In some, the pulsations can be picked out directly from the LAT data, while other objects were already known to be pulsars from radio observations and with this knowledge their pulsations can be found in the gamma-ray data. But there is another way to find pulsars with Fermi, which is to follow up Fermi-LAT gamma-ray sources with radio observations to see if pulsations can be detected. The later technique was recently employed by a team led by KIPAC postdoctoral researcher Matthew Kerr and Columbia University radio astronomer Fernando Camilo and including scientists from a number of other institutions, with very promising results.
The team observed 14 Fermi-LAT gamma-ray sources with pulsar-like emission spectra with the Parkes radio telescope, and ultimately were able to detect millisecond pulsations in five of them. The particular method of selecting gamma-ray sources based on their spectral properties thus proved successful, and with similar targeted radio observations at promising Fermi-LAT gamma-ray sources, a substantial fraction of the millisecond pulsars in our region of the galaxy, which may number in the dozens, may soon be known.
One of the new millisecond pulsars in particular, known by the catchy name of PSR J0101–6422, was followed up by 35 days of radio observations and provided a wealth of data, as gamma-ray pulsations could be detected in the Fermi data once the radio pulsation period and phase were known. The radio observations isolated the pulsar to be at a distance of 1750 light years, which is quite nearby on the scale of the Galaxy. PSR J0101–6422 has an unusual 'light curve' - the behavior of its emission over one pulse period - with two gamma-ray peaks sandwiching the bright radio peak in each period, which the team could not explain with standard geometric pulsar emission models. They propose that J0101–6422 may be a new "hybrid" class of pulsar with radio emission originating from both low and high altitudes above the neutron star, which would add it to the fascinating zoo of pulsars already known, and which data from the Fermi mission is adding to with pulsar-like regularity.
This work is described in part in a paper published in the Astrophysical Journal (ApJ, 2012, 748, 2).
Tidbit Author: Jack Singal