# Tag Info

41

There was a mention of Sagittarius A* during the Q+A portion of the press conference; the team indicated that they hope to produce an image sometime in the future (although they were careful to make no promises, and they're not assuming they'll be successful). That said, I'm not wholly surprised that we ended up seeing M87, rather than Sgr A*, for a couple ...

28

We have reasonably good measurements of the mass of Sagittarius A*, thanks to measurements of the movements of stars like S0-2 over several decades. It's been well-established that the mass of the central object is $M\approx4\times10^6M_{\odot}$; this alone is fairly good evidence for a supermassive black hole, and we can constrain the size of the object ...

23

The orbital elements are on wikipedia: $$e=0.884\ a=0.125'',\ i=134^\circ,\, \Omega=228^\circ$$ (At an assumed distance of 8kpc, $0.125'' = 1000au$) It is the inclination that means that the black hole is not at the focus of the projected ellipse. Imagine a circular orbit, with a central body. If viewed from a distant but high inclination, the orbit is ...

21

I've found an explanation in Dutch here by Heino Falcke, one of the EHT founders. Translation: Hard to photograph It was easiest to take a picture of M87. "It is very difficult to photograph the black hole in our Milky Way, because the material around it moves very fast: the vortex rotates around its axis in 20 minutes. Compare it to a toddler who has ...

13

I'm not sure what the focus is on Sgr A*? Only the stars that are very close to the Galactic center can be said to be "orbiting Sgr A*", the rest of the stars in the Galaxy orbit in the the entire Galactic potential, to which Sgr A* is a minor contributor. The stars near Sgr A* have a wide variety of orbital eccentricities. This is shown clearly in ...

13

I expect that all the itelescope.net instruments work at visible wavelengths. Therefore you have no chance at all to image the stars around Sgr A*, since it is behind about 25-30 magnitudes of optical extinction, caused by dust between us and the Galactic centre. The published images you have seen were taken by large telescopes working with adaptive optics ...

12

Question: How are these images obtained? Later in the video the narrator says they took the images using ESO's VLT. 03:40 [Narrator] 14.​ Making these measurements pushed the power of ESO’s Very Large Telescope to the limits. (Source: ESO transscript) Over the whole observation period multiple telescopes and imaging instruments were used. Early ...

7

We don't know the answer to this (or if someone does, I'd really like to hear that answer). Our galaxy's Super-Massive Black Hole (SMBH) is an unusually quiet one, with little to no accretion disk. If one indeed exists, and it almost assuredly exists, even if it is tenuous and small, we have not yet seen it. Around 2013, there was a gas cloud designated G2 ...

6

To put it simply, you can't. Both cases are explained in these two papers: Binary: https://arxiv.org/pdf/1410.1884.pdf Just a Cloud: https://arxiv.org/pdf/1112.3264.pdf In both cases you have to assume some kind of model to account for the detected fluxes in different near-infrared bands. In the "Just-a-cloud" case you would need a cloud with 3 solar ...

5

Observations of stars near Sgr A* are done in the near-infrared, usually with adaptive optics. Most of these are done by two groups: Andrea Ghez's group at UCLA, using the Keck Telescopes in Hawaii, and Reinhard Genzel's group at MPE (Max-Planck-Institute for Extraterrestrial Physics), using the European Southern Observatory's Very Large Telescope (VLT) in ...

5

The orbit of star S2 is completely determined by the astrometric observations. i.e. One has the orbital period, the angular scale of the orbit and the inclination of the orbital plane. With this information alone, the distance and central mass are degenerate. If you know the distance you get the mass and vice versa. However, here the mass is not needed. ...

5

From Genzel et al. (2010), here's part of Fig. 7.7.1: This is part of the spectral energy distribution of Sagittarius A*, a flot of $\nu$ (frequency) vs. $\nu L_{\nu}$ (frequency times luminosity). For comparison, visible light is in wavelengths from $\sim4\times10^{14}\text{ Hz}$ to $\sim8\times10^{14}\text{ Hz}$, which happens to be around the bottom of ...

4

Black holes (even SMBs) are actually very small objects on an astronomical scale and in practical terms probably very little would happen. There would be some perturbations of stellar orbits within the formation they were in, but unless there was a fairly close approach very little would happen and on the scale of the entire formation the changes would ...

4

Today, April 10th 2019, there was a press conference where finally an image of M87 was released: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more ...

3

I'm sure someone will write a longer/better answer, so I'll keep this short and just mention that when a black hole passes between the observer an a star, we usually say that it becomes gravitationally lensed rather than simply occulted. This can potentially provide much richer data than an occultation but requires more complex analysis. Optical/Infrared ...

3

The Milky Way's central supermassive black hole (SMBH) is feeding, albeit at a very low level. Radio emission from the accretion disk (and/or weak jets) is responsible for the long-lived "Sgr A*" radio source. Here is a paper from 2000 (Falcke et al.) arguing that VLBI (as used by the Event Horizon Telescope) should be able to image the "black ...

3

They know there's molecular gas because they observed emission from the CO molecule in two of the previously identified (atomic-hydrogen-emitting) gas clouds. From the paper: ... we targeted two objects (hereafter, MW-C1 and MW-C2), highlighted by red boxes in Fig. 1, in the 12CO(2 → 1) emission line at 230.538 GHz with the 12-m Atacama Pathfinder ...

2

It's very similar to the question noted in the comments which has a nice mathematical answer. But some comments in your question are incorrect and worth addressing. As far as I can see, the escaping star can at most end up with the kinetic energy of the other star on top of its own, accelerating it by a factor of 2–√ if the stars have the same mass. ...

1

Not much will be visible in optical wavelength as the region will be too bright or nothing will be visible. You can observe in X-ray practically as black holes emit synchrotron radiation and that is visible in x-ray region pretty good. So you can write a proposal to CXO and make them understand your need to observe them Sag A* and they will allot you time. ...

1

After some research, I think this answers my question: The orientation of the disk seems not to be known with certainty, however, Meyer et al. (2007) gives estimates with $3\sigma$ confidence that "the position angle is ∼60◦−108◦ (east of north) in combination with a large inclination angle", meaning that the disk is seen more or less edge on. This 2009 ...

1

This neewpaper article appears to reflect new information from the project. The observations, by the Event Horizon Telescope, are expected to be unveiled in March . . . The team is in the final phase of reviewing data that was gathered in 2017 . . . Nothing there about the 2018 data.

1

Nature of the variation in signal strength From the paper Unprecedented variability of Sgr A* in NIR mentioned in Bit Chaser's comment, Do et al. record enormous variability in the strength of emissions from SGR A* in the NIR (Near InfraRed) band typically described at about 215 THz to 400 THz. Here is a figure from their paper: Here we see enormous ...

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