Aug 16th: The Enigmatic and Surprising Galactic Center

By on August 16, 2019 in

Podcaster: Cosmic

Apogee Podcast

Title: The Apogee Podcast – The Enigmatic and Surprising Galactic Center

Twitter: @AstroAndMusic

Description:  In this Apogee Podcast, Cosmic discusses historical observations of the center of the Milky Way, and what current observations tell us about this fascinating and enigmatic object.


The Galactic Center

Variability of the compact radio source at the galactic center…253..108B

Radio Variability of Sagittarius A* – A 106 Day Cycle

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

Repeated X-ray flaring activity in Sagittarius A*

A Near-Infrared variability stufy of the Galactic black hole: a red noise source with no detected periodicity

Unprecedented variability of Sgr A* in NIR

Background Music: ‘Dreams Electric’ by Geographer

Bio: Cosmic (aka Matt Cheselka) is an independent research astronomer and space musician.

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Hello! This is Cosmic, and welcome to the Apogee podcast! In these podcasts, I chronicle a single astronomical reference thread from the past to the present. Many threads are possible — I’ve chosen just one. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.

The apogee for this podcast will take place tomorrow 17 August 2019, at 10:51 UTC. The lunar distance at that time will be 406,243 km, which is 765 km further away than last apogee on 21 July, and 134 km closer than the next apogee on 13 September.

If you have any topic suggestions for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT gmail DOT com.

By the 1970’s, it had been established that something very interesting and very inegmatic was at the center of our home galaxy, the Milky Way. In 1977,Jan Oort published a review of our understanding of the galactic center in the Annual Reviews of Astronomy and Astrophysics. In this paper, Oort discusses the mass distribution out to a radius of 1000 parsecs (3260 light years), expulsive phenomena including arms and streams of hydrogen and molecular gasses, the infrared core, and the possibility of a black hole at the exact center. Using RR Lyrae stars as distance indicators, he reports that we are some 8.7 kiloparsecs (28,362 light years) from the center. As best could be estimated at the time, the inner 0.4 parsecs (1.3 light years) contains a mass of between 1.5 and 5 million solar masses. This paper is incredibly well written and a “must read” for those of you interested in beginning an exploration of this region of the galaxy.

By 1982, the compact radio source at the galactic center (called Sagittarius A) had been located very precisely and observered extensively. It was reported by Brown and Lo in their Astrophysical Journal paper that this radio source was varying not only in intensity by 20-40% on multiple timescales from days to years, but also its apparent size had grown by about 15% (0.137 arcsec to 0.160 arcsec) over the course of two years, and that its spectral index (an indication of it’s color) varied from 0.55 to 0.08 over that same time period. While the authors don’t speculate on any reasons why these variations are taking place, they suggest that careful monitoring continue at all wavelengths.

Technology improved over the next two decades, accompanied by literally thousands of observations of the galactic center at wavelengths from X-ray to radio. In 2000, Zhao reported in the Astrophyscal Journal Letters that a 106-day cycle of radio variability at 1.3cm and 2cm had been observed using the Very Large Array (VLA). Since no actual object had been found emitting radio light to within 5 astronomical units of the galactic center (now called Sagittarius A-star), Zhao concludes that the likely cause of the variability has to do with instrinsic instability within an accretion disk surrounding a supermassive black hole. His best guess at this time is that of an advection dominated accretion flow where hot and dense bubbles are produced in the inner part of the disk, and then move to the outer part of the disk via convection where light from those bubbles are observed.

Rapid flaring in X-ray light was observed with the Chandra X-ray observatory in Earth orbit on 27 October 2000. This was reported in 2001 by Baganoff et al in the journal Nature. The entire observation lasts about 32,000 seconds (almost nine hours). Nothing seems to happen until about the 14,000 second mark, when a short 500 second event occurs followed by a 6,000 second period of enhanced emission. At 20,000 seconds, a large flare (or flares) is seen to last for about 10,000 seconds. At about 26,000 seconds, there’s a sudden 5x drop is brightness that lasts for about 600 seconds, followed by a partial recovery lasting about 1,200 seconds. The authors calculate a flaring-state emission that is about 45 times the quiescent-state luminosity. This is unlike other typical active galactic nuclei (AGN) that exhibit luminosity variations of 2 or 3 times quiescent-state ranging from minutes to years. So what does Baganoff et al think is going on here? They think that what they’re seeing is an effect called ‘synchrotron self-Comptonization’, where photons are initially produced at longer wavelengths (sub-millimeter in this case) and then ‘energized’ by relativistic electrons in a strong magnetic field, producing X-ray photons. The timescales for producing such photons in the models are consistent with the observations. So the bottom line is that these authors believe that it is mass accretion that’s producing the needed energy to create these variable X-ray emissions.

Two more X-ray flares were reported in a paper by Belanger et al in 2005. These flares happened 31 March 2004 and 31 August 2004 with luminocities about 40 times above the quiescent luminocity — similar to the

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by Planetary Science Institute. Audio post-production by Richard Drumm. Bandwidth donated by and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at This year we will celebrates the Year of Everyday Astronomers as we embrace Amateur Astronomer contributions and the importance of citizen science. Join us and share your story. Until tomorrow! Goodbye!

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