October 25, 2012
Centaurus Gamma-Ray Pulsar Is Accompanied By ‘Black Widow’
Lee Rannals for redOrbit.com — Your Universe Online
Scientists at the Max Planck Institute have discovered a millisecond pulsar accompanied by an unusual sub-stellar partner known as a "black widow."The team has solved the mystery and identified the culprit of a source of strong gamma-rays found in the constellation of Centaurus back in 1994.
Millisecond gamma-ray pulsar PSR J1311-3430 was found through a new data analysis method developed by the scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI).
To find the gamma-ray pulsar, astronomers must know its properties to a very high precision, including its position, spin frequency and how the latter changes over time.
If the pulsar is in a binary system, the analysis problem is even more complicated, and at least three additional orbital parameters have to be determined as well.
Astronomers have already observed the companion of PSR J1311-3430 with optical telescopes, so they were able to partially constrain the orbital parameters and confine the position of the pulsar.
“We developed a particularly efficient method to search the data from NASA´s Fermi satellite for gamma-ray millisecond pulsars, including those in binary systems. Only this method enabled us to probe the wide parameter ranges,” Holger Pletsch, lead author of the article published in Science, said in a statement.
The new analysis method enabled the scientists to conduct a "blind search" for gamma-ray millisecond pulsars for the first time.
“Our search used data collected by the gamma-ray satellite over a total of four years. Very soon after we started running the analysis, a clear signal showed up in the results. What we saw was very exciting,” Pletsch said in the statement.
The pulsar spins 390 times per second, emitting gamma-ray photons into space in the form of a beam, similar to a lighthouse. In about one in a million revolutions of the pulsar, a single photon reaches the detector on board Fermi.
The gamma-ray signal also reveals much about the companion to the astronomers. It helped reveal that the orbital motion of the binary system modulates the photon arrival times.
“The companion object is small and unusually dense,” AEI Director Bruce Allen said in a statement. “It is at least eight times as massive as the planet Jupiter, but has at most 60 percent the planet's radius.”
The team was able to use this information to calculate the density of the companion, which turned out to be unusually high.
“At present, the remaining stellar core, which presumably consists mainly of helium, is heated by the radiation from the pulsar and literally evaporated,” Holger Pletsch said in the statement.
Astronomers call this pulsar a "black widow," which is an analogy with a species of spider that kills the smaller male after mating.
PSR J1311-4330 may one day completely vaporize its companion and from then on travel through space alone.
"Our discovery is not only a first, it also sets several new records,” Bruce Allen said in the statement.
The binary system currently has the shortest known orbital period of all pulsars in binary systems, making a perfectly circular trajectory in about 93 minutes.
The team had an opportunity to look at older observations with the Green Bank radio telescope in West Virginia, but they were unable to find the fast pulsar there.
“Apparently, the cloud of vaporized material from the companion absorbs most of the radio wave emissions from the pulsar and possibly makes it invisible to radio telescopes,” Lucas Guillemot from the Max Planck Institute for Radio Astronomy in Bonn, co-author of the publication, said in the statement.
The scientists hope to use further observations at higher radio frequencies to accurately determine the object's distance from Earth.