Netherlands Research School for Astronomy (NOVA)
ASTRON Netherlands Institute for Radio Astronomy
Dwingeloo, 14 August 2013
Published by the editorial team, 14 August 2013
A team of astronomers, including Heino Falcke (Radboud University Nijmegen/ ASTRON) and Adam Deller (ASTRON), has discovered radio pulses from a neutron star practically next door to the supermassive black hole which resides at the centre of the Milky Way. Radio ‘pulsars' are rapidly spinning neutron stars, ubiquitous in the rest of the Milky Way but until now perplexingly unseen in the Galactic Centre region. By studying the pulsar emission, the team was able to show that the matter being gobbled by the supermassive black hole is pervaded by a magnetic field strong enough to regulate the black hole's feeding habits and to explain its radio and X-ray glow. The results will be published in Nature on 14 August (R.P. Eatough et al.).
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 act as extremely precise cosmic clocks, and a pulsar near Sgr A* could be used to measure the properties of space and time in strong gravitational fields, and to see if Einstein's theory of General Relativity could hold up to the strictest tests.
The young ultramagnetic pulsar PSR J1745-2900 was discovered when the Swift satellite observed a strong X-ray flash originating very close to the centre of the Milky Way - likely less than 1 light year from Sgr A* - and the subsequent observations showing a rotation period of 3.76 seconds by NASA's NuSTAR telescope. With the 100m-telescope in Effelsberg near Bonn, Germany, the team discovered radio pulses from the same region with the same period. Additional observations were made in parallel and thereafter with the Jodrell Bank, Nancay and Very Large Array radio telescopes worldwide, while other groups studied PSR J1745-2900 using the ATCA, Parkes and Green Bank telescopes; the ATCA results appear in this week's journal of MNRAS (Shannon & Johnston).
Sgr A* is slowly swallowing the hot, ionized gas which surrounds it - a process called accretion. The accreted gas is also threaded by magnetic fields, which are dragged along with the gas and interact with the accretion process in a complicated fashion, regulating the amount of material accreted and potentially launching powerful plasma jets. Until now, the strength of these fields was very uncertain, hampering efforts to understand the accretion process.
The radio pulses from PSR J1745-2900 are strongly polarized; much of the emitted radiation oscillates in a preferred plane. However, as the radiation traverses the magnetized material surrounding Sgr A*, the ‘Faraday effect' changes the plane of polarization in a manner dependent on the wavelength of the radiation and the strength of the magnetic field. By observing PSR J1745-2900, the team were able to characterize the strength of the magnetic field in the immediate vicinity of Sgr A*. ‘It is amazing how much information we can extract from this single object', said Deller.
Astronomers predict that there should be thousands of pulsars around the centre of the Milky Way. Despite that, PSR J1745-2900 is the first pulsar discovered there. ‘Astronomers have searched for decades for a pulsar around the central black hole in our galaxy, without success. This discovery is an enormous breakthrough, but it remains a mystery why it has taken so long to find a pulsar there', says Falcke. This pulsar is too magnetically active and just a little too far away from the black hole to measure the subtle effects of Einstein's General Relativity theory with great accuracy. However, with old pulsars, that are closer to the black hole and have a less variable rotation period, the theory can be tested. ‘If there is a young pulsar, there should also be many older ones; we just have to find them', agrees M. Kramer, director at the Max Planck Institute in Bonn which operates the Effelsberg telescope.
Additional high angular resolution follow-up observations of PSR J1745-2900 are now being undertaken to map its orbit around the super massive black hole. From this, scientists can determine the origin of the pulsar and, potentially, refine the estimate of the mass of the black hole.
Artist's impression of PSR J1745-2900, a pulsar with a very high magnetic field ('magnetar') in direct vicinity of the central source of our Galaxy, a supermassive black hole of approximately 4 million times the mass of our sun. Measurements of the pulsar imply that a strong magnetic field exists in the vicinity around the black hole. Credit: MPIfR/Ralph Eatough.