The first link to measuring the magnetic field near the supermassive black hole at the Milky Way's core came last April when NASA's Swift satellite detected a flare of X-rays from near the Milky Way's center. Observers soon determined that the X-rays were coming in regular pulses. Follow-on observations with radio telescopes, including ones in Germany, France, and the National Science Foundation's Karl G. Jansky Very Large Array (VLA), showed radio pulses identically spaced. The astronomers concluded the object, called PSR J1745-2900, is a magnetar, a highly-magnetized pulsar, or spinning neutron star.
Like most galaxies, our Milky Way harbors a supermassive black hole at its center, some 26,000 light-years from Earth. The Milky Way's central black hole is some four million times more massive than the Sun. Black holes, concentrations of mass so dense that not even light can escape them, can pull in material from their surroundings. That material usually forms a swirling disk around the black hole, with material falling from the outer portion of the disk inward until it is sucked into the black hole itself.
Such disks concentrate not only the matter pulled into them but also the magnetic fields associated with that matter, forming a giant, twisting magnetic field that is thought to propel some of the matter back outward along its poles in superfast "jets."
The region near the black hole is obscured from visible-light observations by gas and dust, and is an exotic, extreme environment still little-understood by astronomers. The magnetic field in the central portion of the region is an important component that affects other phenomena.
The charged gas, the astronomers said, is somewhere roughly 150 light-years from the black hole, directly between the pulsar and Earth. Measuring the twist in the waves caused by their passage through this gas allowed the scientists to calculate the strength of the magnetic field. The magnetic field is a crucial part of the black hole's environment, affecting the structure of the flow of material into the black hole, and even regulating that flow.
"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 measured strength of the magnetic field at the presumed distance of the gas cloud from the black hole is about what astronomers expected, based on the intensity of X-rays and radio waves coming from the area closest to the black hole. The measurements also indicate that the field is relatively well-ordered, instead of turbulent, the scientists said.
"The closer you get to the black hole and the disk surrounding it, the stronger the magnetic field should become," Demorest said. "Our measurement shows the field strength we would expect at the distance we believe that gas cloud is from the black hole," he added.
The discovery of a pulsar closely orbiting the candidate supermassive black hole at the centre of the Milky Way (called Sagittarius A*, or Sgr A** in short) has been one of the main aims of pulsar astronomers for the last 20 years. Pulsars, those extremely precise cosmic clocks, could be used to measure the properties of space and time around this object, and to see if Einstein's theory of General Relativity could hold up to the strictest tests.
Shortly after the announcement of a flaring X-ray source in the direction of the Galactic centre by NASA's Swift telescope, and the subsequent discovery of pulsations with a period of 3.76 seconds by NASA's NuSTAR telescope, a radio follow-up program was started at the Effelsberg radio observatory of the Max Planck Institute for Radio Astronomy (MPIfR).
"As soon as we heard about the discovery of regular pulsations with the NuSTAR telescope we pointed the Effelsberg 100-m dish in the direction of the Galactic centre", says Ralph Eatough from MPIfR's Fundamental Physics Research department, the 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 centre!"
Because this pulsar is so special, the research team spent a lot of effort to prove that it was a real object in deep space and not due to man-made radio interference created on Earth.
Additional observations were performed in parallel and subsequently with other radio telescopes around the world (Jodrell Bank, Very Large Array, Nançay). "We were too excited to sleep in between observations! We were calculating flux densities at 6am on Saturday morning and we could not believe that this magnetar had just turned on so bright." says Evan Keane from the Jodrell Bank Observatory. Other collaborations worked at different telescopes (Australia Telescope/ATCA, Parkes and Green Bank Telescope). A research paper on the ATCA results by Shannon & Johnston appears in this week's issue of the British journal MNRAS.
"The Effelsberg radio telescope was built such that it could observe the Galactic centre. And 40 years later it detects the first radio pulsar there", explains Heino Falcke, professor at Radboud Universiteit Nijmegen. "Sometimes we have to be patient. It was a laborious effort, but finally we succeeded."
The scientists plan to continue watching PSR J1745-2900, because they expect to detect changes as it moves in its orbital motion around the black hole. This will provide additional measurements of the magnetic-field strength in different gas clouds. Also, they expect -- and hope -- to find more pulsars that will allow them to use the same technique to make a detailed map of the magnetic field near the black hole.
The Daily Galaxy via nrao.edu
Image Credit: Image Credit: Ralph Eatough/MPIfR