August 14, 2013
Highly Magnetized Pulsar Discovered Near Galactic Center
John P. Millis, PhD for redOrbit.com - Your Universe Online
At the center of the Milky Way lurks a supermassive black hole known as Sagittarius A*, with some four million times the mass of our Sun. More than any single object, it has the greatest impact on the formation, evolution and fate of our galaxy.
However, probing this massive object is difficult. Due to the nature of black holes, most of what we learn about their existence is derived by observing their influence on other nearby objects. For instance, measuring the motion of nearby stars can give us estimates about its location, proper motion and even mass. But there is still much more that we could learn.
For this reason, astronomers have been seeking a certain type of object known as a pulsar – a rapidly rotating stellar remnant that, itself, is incredibly dense. Because of their regular periods and strong magnetic fields, the existence of a nearby pulsar to the galactic center would reveal the strength and structure of the magnetic field in the gas field that is about to be accreted into the black hole.
“In order to understand the properties of Sgr A*, we need to comprehend the accretion of gas into the black hole," says Michael Kramer, director at the Max Planck Institute for Radio Astronomy (MPIfR) and head of its Fundamental Physics research department..
"However, up to now, the magnetization of the gas, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Our study changes that by using the discovered pulsar to probe the strength of the magnetic field at the start of this accretion flow of gas into the central object."
Now, an international team of astronomers has discovered just such an object. This particular source, known as PSR J1745-2900 is from a subclass of pulsars known as magnetars – similar to normal pulsars except that they have magnetic fields some 1,000 times stronger than their counterparts, making the fields 100 trillion times more powerful than Earth’s magnetic field.
Initially reported as X-ray pulsarion from the NuSTAR telescope, astronomers quickly turned to the Effelsberg 100-meter radio dish. “The Effelsberg radio telescope was built such that it could observe the galactic center. And 40 years later it detects the first radio pulsar there,” explains Heino Falcke, professor of astroparticle physics and radio astronomy at Radboud University Nijmegen. "Sometimes we have to be patient. It was a laborious effort, but finally we succeeded."
"As soon as we heard about the discovery of regular pulsation with the NuSTAR telescope we pointed the Effelsberg 100-m dish in the direction of the galactic center," says Ralph Eatough from MPIfR and lead author of the study. "On our first attempt the pulsar was not clearly visible, but some pulsars are stubborn and require a few observations to be detected. The second time we looked, the pulsar had become very active in the radio band and was very bright. I could hardly believe that we had finally detected a pulsar in the galactic center!"
Radio observatories around the world, including Jodrell Band and the Very Large Array, also contributed to confirm the results. One thing that struck the scientists was the brightness of the magnetar.
“The lucky alignment of this gas with a pulsar so close to the black hole has given us a valuable tool for understanding this difficult-to-observe environment,” said Paul Demorest, of the National Radio Astronomy Observatory. “The closer you get to the black hole and the disk surrounding it, the stronger the magnetic field should become. Our measurement shows the field strength we would expect at the distance we believe that gas cloud is from the black hole.”
Sadly, though, because this particular specimen spins rather slowly for a pulsar, and is more chaotic than normal, young pulsars, there is still more refinement that can be done. To this end, astronomers are now looking for other pulsars that may be lurking in the area - ideally ones that are even closer to the black hole than the half light-year orbital radius that J1745 follows. If found, closer pulsars could also allow astronomers to probe the warping of space-time caused by the black hole.
“Ideally we would like to find faster spinning pulsars even closer to Sgr A* allowing more accurate timing,” says Eatough. “The new pulsar has considerably raised our hopes of this possibility for the future.”