Last April, observations made with the space telescope NuSTAR1 suggested strongly that the centre-most region of our galaxy is occupied by a magnetar, a pulsar with a thousand fold stronger magnetic field than usual. To follow up on this discover, the world’s largest radio-telescopes, including the Paris Observatory’s Nançay facility, turned their attention in that direction and uncovered in effect a radio pulsar with a period of 3,76 seconds. Observations made at Nançay at verious frequencies led to the determination of two important characteristics of the pulsar : its dispersion measure2 and the Faraday effect3.
Artist’s view of the Galactic centre. Credits : Bill Saxton, NRAO/AUI/NSF. The centre of our galaxy, which houses a black hole of several million solar masses, has been studied for many years, in order to find new pulsars. Pulsars are what are left after the most massive stars explode : they are essentially made up of neutrons , and have a magnetic field which enables them to generate two radio beams which sweep through space periodically in a manner not unlike that of marine lighthouse beams : their periodicity is extremely stable. Pulsars can in this sense be thought of as "cosmic clocks". The determination of the arrival times of the signals emitted by the most stable pulsars is a means to characterize the properties of time and space in their immediate vicinity and thence to test Einstein’s General Theory of Relativity.
As is the case for many pulsars, this magnetar is extremely polarized : consequently, the Faraday rotation of the plane of the linear polarization can be measured, and thereby obtain an estimate of the magnetic field through which the radio signal has passed in the immediate neighborhood of the magnetar. This field, extrapolated to the edge of the black hole, turns out to be sufficiently strong to explain the observed radiation, ranging from radio to X-rays. A powerful and well ordered magnetic field could also hamper the fall of matter towards the black hole, which would explain the intermittent periods of inactivity.
At the Paris Observatory’s Nançay facility, pulsars have been studied for many years with the help of the large radio-telescope. Within the Observatoire des Science de l’Univers Centre (OSUC), the team from the Orleans Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E - Laboratory for the Physics and Chemistry of the Environment and of Space) is mainly involved in the regular measurement of the arrival times of signals from the most stable short period pulsars. These extremely accurate measurements are used in the search for the trace of gravitational waves emitted by the super-massive binary black holes which are in the centres of nearby galaxies.
Starting in May, French scientists have been using the Nançay radio-telescope to make frequent observations of the magnetar at two frequencies, different from those studied elsewhere. These observations have led to a better determination of its dispersion measure and of the Faraday effect. This regular monitoring will enable the changes in the rotation of the magnetar to be studied ; it will also show up the gravitational effects imposed by its proximity to the black hole. The data gathered will also be used in the search for other possible pulsars close to the black hole, in order to improve our understanding of our galactic centre.
Note
1- NASA’s NuStar satellite
2- The dispersion measure is the number of free electrons on the line of sight between us and the magnetar - this gives a rough idea of what the magnetar’s environment is like.
3- The Faraday rotation describes how light is affected by the presence of a magnetic field along its path.