Sagittarius A*, the super massive black hole at the center of the Milky Way galaxy, has long been the subject of many astrophysical studies. Recent measurements in the near-infrared and mid-infrared spectral regions have provided exciting new insights into the flares observed within the accretion disk of the black hole.
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Black hole research is vital because it sheds light on the foundational characteristics of the universe, such as how gravity, space, and time behave1. They serve as harsh testing grounds for theoretical physics, like general relativity as proposed by Einstein. Particularly in environments where the gravitational pull is so strong that light cannot escape, black holes offer a special setting for testing and improving the theoretical framework of gravity and the structure of spacetime.
Supermassive black holes may have an impact on the birth and evolution of galaxies surrounding them, including the Milky Way, and they are also important in the evolution of galaxies. Since black holes may be the remnants of massive stars from the early universe, studying them provides insight into the universe's formation and structure.
Despite their significance, black holes are difficult to observe directly since they don't emit light, necessitating the use of advanced tools to examine their impact on nearby matters.
Recent Observations of Sagittarius A*
With a mass of 4.3 million times that of the sun, the supermassive black hole Sagittarius A*
(Sgr A*) in the heart of the Milky Way galaxy offers an unmatched testbed to observe up close how materials get radiated, accreted, and expelled by a black hole2.
Young, mass-losing stars with yearly to decadal orbital periods just within a few arcseconds of the black hole provide the winds that power the accretion disk of Sgr A*.
Sgr A*’s accretion disk has been observed to be continuously flaring. Some flares are dazzlingly bright eruptions occurring every day, while others are feeble flickers that last only a few seconds. Flickers that spike for months at a time are even fainter. The amount of activity takes place throughout a broad time span, ranging from brief bursts to extended periods.
The X-ray, near-infrared (NIR), and radio-to-millimeter regions of the electromagnetic spectrum have long been used to study the time-varying emission from the accretion flow of Sgr A*. New insights have been gained from recent JWST observations in the NIR and mid-infrared (MIR) regions, facilitated by the high sensitivity and angular resolution of the NIRCam and MIRI instruments.
Significance of Infrared Observations
Over the course of seven days, NIR photometric monitoring data of Sgr A* were acquired using the JWST NIRCam instrument3. The spectral evolution of the infrared (IR) flare has been effectively characterized through simultaneous measurements at two IR wavelengths across seven epochs. The wavelengths measured were at 2.1 and 4.8 μm. This has provided information about the process of particle acceleration and how the highest-energy particles subsequently cool, and has allowed tracking the evolution of the accelerated electron spectrum. Estimates of the electron density and magnetic field strength of hot patches in the accretion flow have been made possible by the extra information obtained from detecting a flaring event at two wavelengths.
The primary gap between the NIR and millimeter regimes has been the observation of MIR flares, which has been a critical missing observational connection. The temporal stability of ground-based MIR observations was insufficient to identify the fluctuating flux of Sgr A*.
New observations were conducted using the MIRI instrument on the JWST4. The findings imply that synchrotron emission by a cooling population of electrons is the source of Sgr A*'s MIR emission. Magnetized turbulence and magnetic reconnection may work together to accelerate the particles. Furthermore, the findings suggest that nonthermal mechanisms may be responsible for the varied emission that occurs right after high-energy eruptions.
Discovery of a Binary Star System Near Sgr A*
Based on data gathered recently by the Very Large Telescope (VLT) of the European Southern Observatory (ESO), scientists have discovered a double orbiting Sgr A*5.
The binary star system, now named D9, the closest yet found to Sgr A*, presents a surprising challenge to previous assumptions about black hole environments. Estimated to be a mere 2.7 million years old, its existence suggests that supermassive black holes may be less destructive than previously thought. This discovery provides new insights into stellar survival within intense gravitational fields and raises the possibility of finding planets orbiting stars near Sgr A*. The fact that these two stars have managed to maintain their binary orbit, despite the immense gravitational forces that are predicted to merge them within a million years, indicates a less hostile environment than anticipated, potentially opening the door to the discovery of planets in such extreme locations.
Future Outlook
The precise origins and nature of objects orbiting Sgr A*, particularly those formed in its immediate vicinity, remain a mystery. While current NIR and MIR data have yielded valuable insights, scientists are eager for more extensive, multi-wavelength observations to reduce noise, enhance signal detection, and reveal finer details.
Future advancements, such as the METIS instrument on the upcoming ESO Extremely Large Telescope and the GRAVITY+ upgrade to the VLT Interferometer, promise to deliver more crucial observations. Coupled with the capabilities of the James Webb Space Telescope and advanced ground-based facilities, comprehensive studies of the Galactic center can reveal properties of known objects and potentially uncover new young stellar systems and binary stars.
References and Further Reading
- Tognetti, L. (29 May 2024) "Black Holes: Why study them? What makes them so fascinating?" Universe Today. Available online at: https://www.universetoday.com/articles/black-holes-why-study-them-what-makes-them-so-fascinating#:~:text=in%20your%20browser.-,Dr.,massive%20processing%20power%20and%20memory.%E2%80%9D
- Northwestern University (18 February 2025) "Flickers and flares: JWST reveals Milky Way's central black hole constantly bubbles with light.". Phys.org. Available online at: https://phys.org/news/2025-02-flickers-flares-jwst-reveals-milky.html#:~:text=Double%20vision,spiraling%20around%20magnetic%20field%20lines.
- Yusef-Zadeh, F., H. Bushouse, R. G. Arendt, M. Wardle, J. M. Michail, and C. J. Chandler. "Nonstop Variability of Sgr A* Using JWST at 2.1 and 4.8 μm Wavelengths: Evidence for Distinct Populations of Faint and Bright Variable Emission." The Astrophysical Journal Letters 980, no. 2 (2025): L35.
- von Fellenberg, Sebastiano D., Tamojeet Roychowdhury, Joseph M. Michail, Zach Sumners, Grace Sanger-Johnson, Giovanni G. Fazio, Daryl Haggard et al. "First mid-infrared detection and modeling of a flare from Sgr A." The Astrophysical Journal Letters 979, no. 1 (2025): L20.
- ESO (17 December 2024) "First-ever binary star found near our galaxy's supermassive black hole." Phys.org. Available online at: https://phys.org/news/2024-12-binary-star-galaxy-supermassive-black.html#:~:text=An%20international%20team%20of%20researchers,of%20a%20supermassive%20black%20hole.
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