A strong magnetic field around the supermassive black hole at the centre of the Galaxy (1308.3147v1)

Abstract: The centre of our Milky Way harbours the closest candidate for a supermassive black hole. The source is thought to be powered by radiatively inefficient accretion of gas from its environment. This form of accretion is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which it can be fed. However, the magnetization of the gas, a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of the accretion, remove angular momentum from the infalling gas, expel matter through relativistic jets and lead to the observed synchrotron emission. Here we report multi-frequency measurements with several radio telescopes of a newly discovered pulsar close to the Galactic Centre and show that its unusually large Faraday rotation indicates a dynamically relevant magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission from the black hole, from radio to X-rays.

• The paper identifies an exceptionally high Faraday rotation measure near Sgr A*, evidencing the presence of a strong magnetic field around the supermassive black hole.
• The study employs multi-frequency radio observations to estimate a magnetic field lower bound of 50 μG while accounting for turbulence and inversion effects.
• The results highlight significant implications for accretion dynamics and open opportunities to test general relativity in extreme gravitational environments.

A Strong Magnetic Field Around the Supermassive Black Hole at the Center of the Galaxy

This paper examines the presence of a strong magnetic field surrounding the supermassive black hole located at the center of the Milky Way, Sagittarius A* (Sgr A*). The paper utilizes multi-frequency radio observations of a recently discovered pulsar situated in close proximity to this black hole to better understand the magnetization within this region. The discovery of this pulsar, PSR J1745-2900, and the measurement of its high Faraday rotation provide new insights into the magnetic environment near Sgr A*.

Key Findings and Methodology

The research draws attention to several critical findings and executes a detailed observational strategy:

• Discovery: The pulsar PSR J1745-2900 was detected following a significant X-ray flare near Sgr A*. The pulsar exhibits properties consistent with being a magnetar—highly magnetized neutron stars that produce enormous magnetic fields.
• Faraday Rotation Measurement: Utilizing linearly polarized radio waves transmitted through a magnetized medium, the paper identifies an unprecedentedly high rotation measure (RM) of $$(-6.696 \pm 0.005) \times 10^4 \; \text{rad m}^{-2}$$. This RM is second only to that observed directly from Sgr A*, reinforcing the presence of a strong magnetic field in the vicinity.
• Magnetic Field Estimation: The authors approximate the magnetic field using asymptotic calculations of RM and the path-integrated dispersion measure (DM). They estimate a lower bound of $B \ge 50 \, \upmu \text{G}$, though they acknowledge the actual field could be much stronger due to potential inversions or turbulence.
• Theoretical Implications: The paper considers the involvement of such magnetic fields in accretion processes and the dynamics around Sgr A*. The potential for ordered magnetic fields to contribute to synchrotron emissions and relativistic jets is also explored.

Theoretical and Practical Implications

The discovery has broader implications for both theoretical modeling of galactic nuclei accretion processes and for practical astrophysical observations:

• Dynamics of Accretion Flows: Strong magnetic fields, as evidenced by this paper, significantly alter the dynamics of accretion flows. The removal of angular momentum, driven by magnetic fields, is a crucial element of these processes and may account for the synchronous radio to X-ray emissions from Sgr A*.
• Potential for Testing General Relativistic Effects: A pulsar in such close orbit to Sgr A* provides a unique opportunity to paper the effects of extreme gravity and potentially test predictions of general relativity in high-field environments.
• Opportunity for Further Pulsar Surveys: Given the extreme conditions and significant magnetization quantified by the authors, there is a compelling case for searching for additional pulsars that could be masked by dispersion and scattering effects. Such discoveries would enhance the mapping of accretion regions and help to understand the intricate structure of space-time around supermassive black holes.

Speculation on Future Developments

The paper opens pathways for continued exploration in several avenues:

• Enhanced Telescopic Capabilities: Future advancements in radio telescope sensitivity and resolution will enable more precise measurements of RM and DM, allowing for a more detailed mapping of fields surrounding black holes.
• Comprehensive Accretion Modeling: With empirical backing of strong magnetic fields near Sgr A*, future theoretical models can incorporate these complex magnetic interactions more effectively, leading to a more holistic portrayal of galactic center environments.
• Gravitational Physics Tests: Continued monitoring of PSR J1745-2900, in conjunction with its orbital dynamics, offers sensitivity to possible deviations from the predictions of general relativity, an exciting prospect for physicists and astronomers alike.

In conclusion, the identification of a robust magnetic field within the central parsecs of our galaxy contributes significantly to our understanding of both magnetic field distribution and accretion processes surrounding supermassive black holes. These findings encourage ongoing observational efforts and theoretical modeling to elucidate the astrophysical phenomena governing the dynamic environments of galactic centers.