Observing a recently-discovered pulsar has enabled astronomers to measure the magnetic field radiating from a black hole at the center of our Galaxy, providing a new tool to study this mysterious region.
The Milky Way, like most galaxies, has at its center a supermassive black hole, which is some four million times more massive than the Sun. Black holes pull in material from their surroundings forming a swirling disk around itself. Such disks concentrate this matter as well as the magnetic fields associated with the matter to form a giant magnetic field. This field is thought to propel some of the matter outward along its poles in superfast “jets.”
Gas and dust obscure this region from visible-light observations, making it somewhat of an enigma. The magnetic field in the center of the region is an important component to understanding this phenomenon, as it regulates the flow of material into the black hole.
The first link came last April when satellites detected a flare of X-rays near the Milky Way’s center. Further observations determined that the X-rays were coming in regular, identically spaced pulses. The object, dubbed PSR J1745-2900, was concluded to be a magnetar, a highly-magnetized pulsar.
This is the closest found pulsar to the black hole. Analysis of its radio waves indicates the waves undergoing a dramatic twist as they travel from the pulsar to Earth. Called Faraday rotation, such a twist comes as the waves travel through charged gas within a magnetic field.
The charged gas is directly between the pulsar and Earth. By measuring the twist, the scientists were able to calculate the strength of the magnetic field the waves passed through.
Based on the intensity of other waves coming from a similar area, the measured strength of the magnetic field is about what astronomers expected. Closer to the black hole the magnetic field should become stronger. The measurements also indicate the field is relatively well-ordered, rather than turbulent.
The scientists will continue to watch the pulsar, expecting to detect changes as it orbits around the black hole, which will provide additional measurements of the magnetic-field strength as it passes through different gas clouds. The team also hopes to find more pulsars and use the same technique to make a detailed map of the magnetic field.