Friday, March 27, 2015

Wide Pair of Neutron Stars

A pulsar is basically a rotating neutron star that emits a bipolar beam of electromagnetic radiation. This beam of electromagnetic radiation is only visible when directed at the observer. As the neutron star rotates, the beam periodically sweeps pass the observer, resulting in a pulsed appearance of the emission. Hence the term “pulsar” denotes such a neutron star. Of all the pulsars known to date, ~10 percent are in binary systems with white dwarf, neutron star or main sequence star companions.

PSR J1930-1852 is a pulsar in a double neutron star system (DNS). Both neutron stars orbit each other every 45 days, making this system the widest known pair of neutron stars. The rest of the known DNS systems are much more tightly bound and have orbital periods of a few hours to a few days. PSR J1930-1852 itself spins at a rate of over 300 times per minute. Although this may seem fast, it is actually extremely slow for a pulsar. The combined mass of PSR J1930-1852 and its companion neutron star is 2.6 times the Sun’s mass. PSR J1930-1852 is estimated to contain less than 1.3 times the Sun’s mass while its companion is estimated to contain more than 1.3 times the Sun’s mass.

Neutron stars form from the collapsed cores of massive stars in supernova explosions. DNS systems are rare because the formation of such a binary system requires it to survive two supernova explosions. PSR J1930-1852 is likely to have formed before its companion star went supernova. During that time, it accreted material from its companion star and spun-up. This process of accretion and spin-up is known as recycling. However, for PSR J1930-1852, the recycling process was shorter than average in duration and/or inefficient. The companion star went supernova before PSR J1930-1852 spun-up sufficiently.

Swiggum et al. (2015), “PSR J1930-1852: a pulsar in the widest known orbit around another neutron star”, arXiv:1503.06276 [astro-ph.HE]