September 26, 2013
A Pulsar With A Split Personality
Lee Rannals for redOrbit.com - Your Universe Online
[ Watch the Video: Millisecond Pulsar with Magnetic Field Structure ]
The team used NASA's Swift and Chandra X-ray Observatory to find a pulsar that readily shifts back and forth between two mutually exclusive styles of pulsed emission, including one in X-rays and another in radio. Scientists say this discovery represents an intermediate phase in the life of these powerful objects.
"This transitional object took us decades to find, and it provides us with a unique opportunity to observe a pulsar's intense magnetic field in action," said Sergio Campana, an astronomer at Brera Observatory in Merate, Italy, and a co-author of the paper.
Pulsars are magnetized neutron stars that emit regular pulses of light. Millisecond pulsars are incredibly dense and powerful magnetic fields with extreme rotation. The fastest known pulsar whirls around at 43,000 revolutions per minute. Astronomers believe pulsars are able to achieve speeds like this because they reside in binary systems with normal stars.
During part of a pulsar's life, gas flows from a normal star and falls onto the neutron star, helping to emit X-rays in the process. The pulsar's magnetic field directs the gas onto its magnetic poles, producing hot spots that rotate with the neutron star and giving rise to X-ray pulses.
Over time, the gas stream gradually speeds up the pulsar's rotation, and after a billion years the gas declines and eventually stops. When this happens, it puts an end to X-ray pulses powered by the accretion of gas. However, thanks to its increased spin and intense magnetic field, the neutron star can continue operating as a radio pulsar.
The European Space Agency's (ESA) International Gamma-Ray Astrophysics Laboratory detected a flash of X-rays from a previously unknown source back in March this year. The next day, astronomers used Swift's X-ray Telescope (XRT) to observe pulsar IGR J18245-2452 once more. This enabled them to place the source at the core of the globular star cluster M28, which is located about 18,000 light-years away toward the constellation Sagittarius.
"Swift provided the first accurate, subarcminute localization of the X-ray burst, which allowed for the additional discovery of the pulsar's radio waves by the Australia Telescope Compact Array (ATCA)," said Jamie Kennea, a Swift team member at Penn State.
NASA said a week after the discovery, astronomers found that the neutron star was spinning once every 3.9 milliseconds, or at about 15,000 revolutions per minute. They determined that the pulsar was joined by a small companion star less than one-fifth the mass of our sun. After a few weeks, astronomers detected clear evidence of rotation-powered signals at radio wavelengths.
According to the findings, the rise and fall of fast streaming gas onto the pulsar from a normal companion star is what helps to flip the switch between X-rays and radio.
"At high mass flow rates, the gas squeezes the magnetic field and is able to reach the surface to produce X-ray emission. At the same time, the dense cloud of ionized gas surrounding the pulsar quenches the radio signals, effectively blocking them from our view," said Alessandro Papitto of the Institute of Space Sciences in Barcelona, Spain.