Examining the supermassive black hole in our galaxy
The supermassive black hole (SMBH) at the core of our galaxy, Sagittarius A*, is modest in size at only 4.15 million solar masses. The Event Horizon Telescope (EHT) recently released a dramatic submillimetre image of it, as seen illuminated by its glowing environment. Many galaxies have nuclear SMBHs a thousand times larger, for example the core of M87, which was imaged by the EHT in 2020. But SagA* is relatively close to us, only about twenty-five thousand light-years, and its proximity offers astronomers a unique opportunity to investigate the properties of SMBHs.
As gas and dust slowly fuse in the hot, disk-like environment of a black hole, they radiate across the electromagnetic spectrum. The episodic accretion and variable bursts of radiation provide clues to the nature of the accretion, the dimensions and locations of each event in the black hole’s complex environment (in or near the torus? in part of the wind?) and how the episodes might be. may be related to each other and to properties of the black hole, for example its spin. Each wavelength carries its own information, and one of the most important diagnostic tools is the time difference between flares at different wavelengths, tracing where in the eruption the different production mechanisms take place. Sag A* is so close that it has been tracked on radio wavelengths since its discovery in the 1950s; on average, Sgr A* takes on material at a very slow rate, a few hundredths of an Earth’s mass per year, but enough to produce variability and more dramatic eruptions.
CfA astronomers Steve Willner, Giovani Fazio, Mark Gurwell, Joe Hora and Howard Smith and their colleagues performed a timing analysis of coordinated, simultaneous near-infrared, X-ray and submillimeter observations of SagA* using the IRAC camera on Spitzer, the Chandra X-ray Observatory, the NuSTAR mission, ALMA and the GRAVITY instrument on the Very Large Telescope Interferometer; the campaign required complex mission planning and the reduction of multiple types of datasets. Flashing events were seen between July 17 and 26, 2019 (unfortunately, the SMA was shut down at that time due to protests on the mountain). The team notes that activity in 2019 appears to reflect an unusually high accretion rate. Although some events were observed to occur simultaneously, the submillimeter flare (ALMA) appeared about 20 minutes after the infrared and X-ray (Chandra) radiation.
The scientists consider three scenarios: the infrared and X-rays in these flares originated from charged particles spiraling in powerful magnetic fields; the infrared and submillimeters came from this first process, but the X-rays were produced when infrared photons collided with charged particles moving nearly at the speed of light; and finally that only the submillimetre radiation came from the first process and all other bands were produced by the second. Unfortunately, observations on the ground cannot be continuous, and as a result, the time of the peak of the submillimeter emission flame was not observed, making it difficult to establish a time lag between this and the X-rays that could indicate its origin in a different location or from a different process. Combining its results with previous variability studies, the team finds one consistent image in which the infrared and X-rays arise through the second process, followed by submillimeter emission from the first in an expanding, cooling magnetized plasma.
The research was published in The Astrophysical Journal†
Variable emission from the Milky Way’s supermassive black hole
H. Boyce et al, Multiwavelength Variability of Sagittarius A* in 2019 Jul, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac6104
Quote: Exploration of the supermassive black hole in our galaxy (2022, June 24) retrieved June 25, 2022 from https://phys.org/news/2022-06-supermassive-black-hole-galaxy.html
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