Mystery Object Revealed to be "Black Widow" Pulsar

May 5, 2015

A KIPAC professor and graduate student have used savvy astronomical detective work to piece together the identity of a previously enigmatic gamma-ray source. The object is a black widow pulsar which is destroying its companion star.

Optical brightness (top 2 panels) and inferred velocity (bottom panel) for the stellar companion to the black widow pulsar. The light blue data points are from the Stanford student telescope.

The Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope, which was built at SLAC and in which KIPAC is the leading science institution, has seen almost 2000 gamma-ray sources, from the distant reaches of the Universe to our own Sun. While most of these objects are identified - meaning which class of source they are is known - there are a few which have defied classification. One such unidentified object was known by the unimpressive name of J2339-0533, but it was impressive in its ability to keep a secret.
 
J2339-0533 was suspected to be a pulsar, the phenomenon where a rapidly spinning and highly magnetized neutron star gives rise to jets of particles and radiation that sweep the sky like a lighthouse. However Fermi-LAT observations could not reveal any periodic pulsations in the gamma-ray output, and radio telescope observations failed to do the same in radio waves.  It was seen, however, as a source in X-ray light as observed by the Chandra X-ray Observatory.  When optical data from the Lulin telescope in Taiwan and the Tenagra telescope in Arizona revealed a small star to be very near the position of the X-ray object, KIPAC professor Roger Romani, along with graduate student Michael Shaw, set out to prove that the gamma-ray object and the star were a unique kind of pulsar system.
 
Romani and Shaw took careful optical measurements of the light from the companion star with the 9.2 meter diameter Hobby Eberly Telescope in Texas, the 3.6 meter WIYN telescope in Arizona, and even the 0.61 meter telescope at the Stanford Student Observatory in the hills above the Stanford campus. The data revealed that the companion star oscillates markedly in brightness and apparent temperature with a distinct period.
 
These dramatic results point to only one conclusion, that J2339-0533 is a so-called "black widow" pulsar - its powerful wind of radiation and charged particles is literally evaporating the companion star. The side of the star facing the pulsar glows brightly from the intense heat generated by the punishing wind, with a surface temperature that is twice that of the cold side facing away from the pulsar. As the star and pulsar orbit each other, we alternately see the hot side of the star and the cold side, giving rise to the observed periodic oscillations in brightness and temperature. The unfortunate star being slowly destroyed by the pulsar now has less than 1/10th the mass of our Sun.
 
The pulsar and star orbit each other approximately once every 4.5 hours.  The period of the pulsar's rotation is likely on the order of milliseconds, indicating that it is "recycled", meaning it has been spun-up by the interaction with the orbiting companion star.  J2339-0533 is one of only a few black widow systems known. If the pulsations in gamma rays can eventually be detected with the accumulation of more Fermi-LAT data, it will enable an unusually precise determination of the mass of the neutron star. The identification and characterization of this black widow pulsar has turned out to be the latest in a long line of exciting Fermi results.
 
This work is described in a paper to appear in Astrophysical Journal Letters and available from astro-ph at arXiv:1111.3074.
 
Science Contact:
Roger Romani
KIPAC
email: rwr@astro.stanford.edu