Scientists have taken a giant leap forward in understanding black holes, moving beyond capturing their image to deciphering their complex physics. In a groundbreaking study, researchers from the Shanghai Astronomical Observatory (SHAO) and international collaborators have harnessed the power of dual-frequency imaging to probe the plasma surrounding the black hole at the center of galaxy M87.
The team combined data from the Event Horizon Telescope and the Global Millimeter Very Long Baseline Interferometry (VLBI) Array, capturing images at 1.3 mm and 3.5 mm frequencies. This innovative approach yielded a spatially resolved spectral-index map, revealing how the spectral index fluctuates with distance from the black hole. By analyzing these fluctuations, the scientists were able to determine the physical conditions of the plasma and the processes generating the observed radiation.
The findings, published in The Astrophysical Journal Letters, paint a fascinating picture. The radiation properties surrounding the black hole exhibit a systematic variation with distance, as evidenced by the spectral index's spatial distribution. In the innermost region, the spectral index is positive and increases slightly with radius, indicating significant synchrotron self-absorption. As we move farther from the black hole, the spectral index decreases and transitions from positive to negative values, signifying a shift towards a more optically thin emission regime.
What's truly remarkable is that this transition occurs at a distance of approximately 30 μas from the black hole, aligning precisely with the radius of the ring-like structure observed at 3.5 mm. This discovery suggests that the ring-like structure isn't merely an emission feature but is intimately linked to the physical state of the plasma near the event horizon.
Dr. ZHAO Shanshan, the study's first author, emphasizes the significance of this achievement. By obtaining the first spatially resolved spectral-index distribution of the M87 black hole, they can quantitatively characterize the radiation properties across the region surrounding the black hole. This enables them to directly explore how plasma properties vary on horizon scales, offering new insights into accretion flows and jet formation.
Looking ahead, the researchers anticipate that continued advancements in millimeter VLBI will unlock observations at even more frequencies, with heightened sensitivity and time-resolved imaging capabilities. These improvements will provide an unprecedented wealth of information about black hole accretion, jet formation, and radiation processes in strong gravitational fields, further deepening our understanding of the extreme environments surrounding black holes.
Dr. LU Rusen, the corresponding author, highlights the potential of multi-frequency horizon-scale imaging. By disentangling the effects of plasma physics from gravitational signatures in black hole images, this approach will enable more precise studies of black hole accretion, jet formation, and strong-field gravity.
This groundbreaking study, funded by various Chinese institutions, marks a significant milestone in our quest to unravel the mysteries of black holes. It opens up new avenues for research, inviting further exploration and discovery in the captivating realm of astrophysics.
