A team of researchers from the Brera Astronomical Observatory in Italy and the University of Oxford in the UK have found a dead star moving away from its birth supernova, leaving a comet-like trail of radio emission. The star, named PSR J1914+1054g, is only the fourth known of its kind: a radio pulsar kicked at high velocity across space, for which astronomers were able to observe not only the pulsar but also the trail behind it known as a bow-shock nebula and the supernova remnant from which it was kicked. The team of scientists has named the nebula Mini Mouse.
The Death of a Massive Star
The death of a massive star is a violent affair. When the star runs out of fuel, the fusion providing the outward pressure that supports the star against the inward pressure of gravity suddenly drops off, and things get messy. The star explodes, releasing its contents everywhere, while the core collapses under gravity into an ultradense neutron star, which can be up to 2.16 times the mass of the Sun, packed into a sphere just 20 kilometers across.
These stellar remnants can be found sitting in the nebula created by their exploded remains in many cases. However, if the supernova explosion is lopsided, the uneven energy distribution can meet the neutron star across space at high speeds, zooming across the galaxy. To produce a nebula like the Mini Mouse, a special set of circumstances is required.
Pulsars and Bow Shock Nebula
First, the neutron star has to be a pulsar – one that is rotating at high speeds. A pulsar is named as such because it pulses like a cosmic lighthouse as beams of radiation shoot from its poles, guided and accelerated by powerful magnetic fields. The magnetic fields also accelerate charged particles into a furious wind that whips around the pulsar, sometimes interacting with the surrounding interstellar medium to generate a pulsar wind nebula.
If the pulsar receives a natal kick by an uneven supernova, a bow shock forms in the direction of travel, redirecting and channeling the pulsar wind behind the pulsar, like the tail of a comet. This is known as a pulsar bow-shock nebula, visible as a glow of light.
The researchers were using the MeerKAT radio telescope in South Africa to study a binary star named GRS 1915+105. Consisting of a black hole and a normal star, they noticed a smear of light streaking across their field of view looking startlingly similar to a pulsar bow-shock nebula discovered in 1987 called the Mouse.
A search through data collected by the ongoing FAST Galactic Plane Pulsar Snapshot (GPPS) survey revealed a newly discovered pulsar with a spin period of 138 milliseconds that appeared to be positioned at the front of this streak. Follow-up observations conducted by the team revealed that J1914 is placed exactly at the head of the nebula.
The MeerKAT radio data also revealed a faint circular shape far behind the pulsar and its tail, with the trajectory seeming to trace right back to its center. This, the researchers identified as the remnant of the supernova that birthed the pulsar J1914.
The length of the tail, the team found, is around 40 light-years, and the radius of the supernova remnant is about 43 light-years. They also determined that it’s been about 82,000 years since the pulsar was born (i.e., the supernova took place); tracing back towards the center of the nebula, that puts J1914’s velocity at a supersonic 320 to 360 kilometers per second (200 to 225 miles per second).
The discovery is significant because it will help astronomers better understand pulsars and their winds, supernova explosions, the local interstellar medium, high-speed particles, and wind-generated shocks. Moreover, it represents MeerKAT’s potential in uncovering these heretofore rare-to-find objects. The researchers believe that thanks to the detection of structures similar to the Mouse and Mini Mouse, MeerKAT will help unveil more young radio pulsars, which will add to the still small population of such objects, predicted to count thousands of members in our galaxy.