lpetrich
Contributor
[1807.09409] Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole
[1904.05721] A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
While we await the Event Horizon Telescope consortium's planned release of results on our Galaxy's central black hole, Sgr A*, I wish to note what may be the next best thing: measurements of the orbit of a star that orbits very close to it. The star S2 is the second fasted orbiting object for Sgr A*, and it has been observed over the last 27 years. It orbits with a period of 16 years, and it has done nearly two orbits over that time.
Its mean distance is about 1000 astronomical units (AU's - the average Earth-Sun distance), and its orbit eccentricity is about 0.885. This means that its distance ranges between 120 AU and 1900 AU. So its closest approach is about at Voyager 1's current distance from the Sun.
Its mean orbital velocity is 1900 km/s, and it ranges between 470 km/s and 7700 km/s, about 0.026 c.
The first paper was on detecting the gravitational redshift of that star's light, and it reports success. That effect is combined with the transverse Doppler shift from the star's motion, and that combined effect is about 0.00066, equivalent to a velocity of 200 km/s. The authors report agreement with GR of 0.90 +- 0.09(stat) +- 0.15(sys)
The second paper was about doing a "dynamical parallax", finding S2's distance by finding (orbit linear size from orbital velocity) / (orbit angular size from direct observation). Their authors report 8178 +- 13(stat.) +- 22(sys.) parsecs, or nearly 27,000 light years. Don't expect missions to there anytime soon. S2's redshift factor they find to be 1.04 +- 0.05.
Let us say that you were orbiting
S2 (star) or S0-2. What would you see? Sgr A*'s mass is about 4.15 million solar masses, giving an event-horizon radius of 0.082 AU, and a shadow diameter of 0.425 AU. That is about 12 minutes of arc, large enough to be observed without a telescope (the Sun and the Moon are about 32 minutes of arc across).
If one orbits Sgr A* at the Earth's distance, one would have an orbit period of 4.3 hours, and Sgr A*'s shadow would be about 24 degrees across, very easy to see. One would travel at 61,000 km/s or 0.20 c.
With the Earth's orbit period, 1 year, one would orbit at 160 AU.
The fastest orbiting star for Sgr A* is S0–102, with a period of 11.5 years. Its closest approach is 260 AU, not as close as S2's. Though on average closer, its orbital eccentricity is less, about 0.68. It has been observed over its entire orbit, and it should next pass through its pericenter in 2020. S2 last did so in 2018.
Another effect that could become detectable is the orbit precession due to GR's departure from Newtonian gravity. This is best-known as some observed extra precession in the orbit of planet Mercury, and it has also been been observed in binary pulsars.
[1904.05721] A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
While we await the Event Horizon Telescope consortium's planned release of results on our Galaxy's central black hole, Sgr A*, I wish to note what may be the next best thing: measurements of the orbit of a star that orbits very close to it. The star S2 is the second fasted orbiting object for Sgr A*, and it has been observed over the last 27 years. It orbits with a period of 16 years, and it has done nearly two orbits over that time.
Its mean distance is about 1000 astronomical units (AU's - the average Earth-Sun distance), and its orbit eccentricity is about 0.885. This means that its distance ranges between 120 AU and 1900 AU. So its closest approach is about at Voyager 1's current distance from the Sun.
Its mean orbital velocity is 1900 km/s, and it ranges between 470 km/s and 7700 km/s, about 0.026 c.
The first paper was on detecting the gravitational redshift of that star's light, and it reports success. That effect is combined with the transverse Doppler shift from the star's motion, and that combined effect is about 0.00066, equivalent to a velocity of 200 km/s. The authors report agreement with GR of 0.90 +- 0.09(stat) +- 0.15(sys)
The second paper was about doing a "dynamical parallax", finding S2's distance by finding (orbit linear size from orbital velocity) / (orbit angular size from direct observation). Their authors report 8178 +- 13(stat.) +- 22(sys.) parsecs, or nearly 27,000 light years. Don't expect missions to there anytime soon. S2's redshift factor they find to be 1.04 +- 0.05.
Let us say that you were orbiting
S2 (star) or S0-2. What would you see? Sgr A*'s mass is about 4.15 million solar masses, giving an event-horizon radius of 0.082 AU, and a shadow diameter of 0.425 AU. That is about 12 minutes of arc, large enough to be observed without a telescope (the Sun and the Moon are about 32 minutes of arc across).If one orbits Sgr A* at the Earth's distance, one would have an orbit period of 4.3 hours, and Sgr A*'s shadow would be about 24 degrees across, very easy to see. One would travel at 61,000 km/s or 0.20 c.
With the Earth's orbit period, 1 year, one would orbit at 160 AU.
The fastest orbiting star for Sgr A* is
S0–102, with a period of 11.5 years. Its closest approach is 260 AU, not as close as S2's. Though on average closer, its orbital eccentricity is less, about 0.68. It has been observed over its entire orbit, and it should next pass through its pericenter in 2020. S2 last did so in 2018.Another effect that could become detectable is the orbit precession due to GR's departure from Newtonian gravity. This is best-known as some observed extra precession in the orbit of planet Mercury, and it has also been been observed in binary pulsars.