* Astronomy

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RE: Sagittarius A*

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Milky Way's Black Hole Shows Signs of Increased Chatter

Three orbiting X-ray telescopes have been monitoring the supermassive black hole at the center of the Milky Way galaxy for the last decade and a half to observe its behaviour, as explained in our latest press release. This long monitoring campaign has revealed some new changes in the patterns of this 4-million-solar-mass black hole known as Sagittarius A* (Sgr A*).
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Title: The reflection of two past outbursts of Sagittarius A* observed by Chandra during the last decade
Author: Maïca Clavel (1,2), R. Terrier (1), A. Goldwurm (1,2), M. R. Morris (3), G. Ponti (4), S. Soldi (1), G. Trap (1,2) ((1) APC - Paris, (2) CEA - Saclay, (3) UCLA, (4) MPE - Garching)

The supermassive black hole at the Galactic center, Sagittarius A*, has experienced periods of higher activity in the past. The reflection of these past outbursts is observed in the molecular material surrounding the black hole but reconstructing its precise lightcurve is difficult since the distribution of the clouds along the line of sight is poorly constrained. Using Chandra high-resolution data collected from 1999 to 2011 we studied both the 6.4 keV and the 4-8 keV emission of the region located between Sgr A* and the Radio Arc, characterising its variations down to 15" angular scale and 1-year time scale. The emission from the molecular clouds in the region varies significantly, showing either a 2-year peaked emission or 10-year linear variations. This is the first time that such fast variations are measured. Based on the cloud parameters, we conclude that these two behaviours are likely due to two distinct past outbursts of Sgr A* during which its luminosity rose to at least 10^39 erg/s.

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NASA's Chandra Observatory Catches Giant Black Hole Rejecting Material

Astronomers using NASA's Chandra X-ray Observatory have taken a major step in explaining why material around the giant black hole at the center of the Milky Way Galaxy is extraordinarily faint in X-rays. This discovery holds important implications for understanding black holes.
New Chandra images of Sagittarius A* (Sgr A*), which is located about 26,000 light-years from Earth, indicate that less than 1 percent of the gas initially within Sgr A*'s gravitational grasp ever reaches the point of no return, also called the event horizon. Instead, much of the gas is ejected before it gets near the event horizon and has a chance to brighten, leading to feeble X-ray emissions.
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Magnetar found at giant black hole

Its position near Sgr A* makes it a precious find. The magnetar's regular radio pulses could be used to measure the warping of space-time near the monster black hole and to test predictions of Einstein's general theory of relativity.
The magnetar's accidental discovery is a by-product of astronomers' excitement about the arrival of the gas cloud, dubbed G2. The cloud, which is about three times the mass of Earth, was first spotted near Sgr A* in 2012 (and was later found in 2002 data). Its arrival would deliver insight into how objects accrete into the swirling disk of material around a black hole, as well as offering the first chance for astronomers to measure the time that it takes for objects to be captured and swallowed up.
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Herschel finds hot gas on the menu for Milky Ways black hole

ESA's Herschel space observatory has made detailed observations of surprisingly hot molecular gas that may be orbiting or falling towards the supermassive black hole lurking at the centre of our Milky Way galaxy.
Our local black hole is located in a region known as Sagittarius A* - Sgr A* - after a nearby radio source. It has a mass about four million times that of our Sun and lies around 26 000 light-years away from the Solar System.
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Title: Method for detecting a boson star at Sgr A* through gravitational lensing
Authors: Amitai Y. Bin-Nun

Observations of the Sgr A* region in the galactic center confirm the presence of a large amount of matter in a small volume, leading to the consensus that a black hole exists there. However, dynamical observations cannot rule out the presence of a boson star, a compact object made up of scalar particles, as both objects are far more compact than the scale of current observational constraints. While a boson star in the galactic center is disfavoured for a number of theoretical considerations, we outline the first test that can directly observe a boson star. We accomplish this by studying the strong gravitational lensing of S stars resulting from the assumption of a boson star in the Galactic Center. Boson stars have an extended mass distribution and are transparent to electromagnetic radiation, giving rise to a radial caustic curve. We calculate the brightness of images formed by stars crossing these radial caustics and show that a boson star would give rise to much brighter images than a black hole with a similar mass and that those images would be easily bright enough to be detected with upcoming instruments.

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Title: Deviation of Stellar Orbits from Test Particle Trajectories Around Sgr A* Due to Tides and Winds
Authors: Dimitrios Psaltis (Arizona), Gongjie Li (Harvard), Abraham Loeb (Harvard)

Monitoring the orbits of stars around Sgr A* offers the possibility of detecting the precession of their orbital planes due to frame dragging, of measuring the spin and quadrupole moment of the black hole, and of testing the no-hair theorem. Here we investigate whether the deviations of stellar orbits from test-particle trajectories due to wind mass loss and tidal dissipation of the orbital energy compromise such measurements. We find that the effects of stellar winds are, in general, negligible. On the other hand, for the most eccentric orbits (e>0.96) for which an optical interferometer, such as GRAVITY, will detect orbital plane precession due to frame dragging, the tidal dissipation of orbital energy occurs at timescales comparable to the timescale of precession due to the quadrupole moment of the black hole. As a result, this non-conservative effect is a potential source of systematic uncertainty in testing the no-hair theorem with stellar orbits.

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The Diner at the Center of the Galaxy

Spoiler

The Milky Way's supermassive black hole is generally a picky eater, but NASA's NuSTAR spacecraft recently caught it in the act of having a snack

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Title: X-ray Echo from the Sagittarius C Complex and 500-year Activity History of Sagittarius A*
Authors: Syukyo Gando Ryu, Masayoshi Nobukawa, Shinya Nakashima, Takeshi Go Tsuru, Katsuji Koyama, Hideki Uchiyama

This paper presents the Suzaku results obtained for the Sagittarius (Sgr) C region using the concept of X-ray reflection nebulae (XRNe) as the echo of past flares from the super massive black hole, Sgr A*. The Sgr C complex is composed of several molecular clouds proximately located in projected distance. The X-ray spectra of Sgr C were analysed on the basis of a view that XRNe are located inside the Galactic center plasma X-ray emission with an oval distribution around Sgr A*. We found that the XRNe are largely separated in the line-of-sight position, and are associated with molecular clouds in different velocity ranges detected by radio observations. We also applied the same analysis to the Sgr B XRNe and completed a long-term light curve for Sgr A* occurring in the past. As a new finding, we determined that Sgr A* was experiencing periods of high luminosity already 500 years ago, which is longer than the previously reported value. Our results are consistent with a scenario that Sgr A* was continuously active with sporadic flux variabilities of Lx = 1-3 x 10^39 erg s^-1 in the past 50 to 500 years. The average past luminosity was approximately 4-6 orders of magnitude higher than that presently observed. In addition, two short-term flares of 5-10 years are found. Thus, the past X-ray flare should not be a single short-term flare, but can be interpreted as multiple flares superposed on a long-term high state.

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