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Bach’s Flare: Brightening the Galactic Core

Looking at the central black holes in galaxies other than our own has forced a question: What’s going at Milky Way galactic central? We know there is a black hole there, and a big one, weighing in at about four million solar masses. But the Milky Way’s black hole, called Sagittarius A* (pronounced ‘A-star’) seems quiet compared to what we see in other galaxies, emitting but a trace of the radiation they are pushing into the cosmos. A new study from a Japanese team proposes an answer. Three hundred years ago, Sgr A* put out a huge flare, making it a million times brighter than today. Today’s quiet black hole may simply be the slumbering aftermath of what must have been a frenetic round of activity.

“We have wondered why the Milky Way’s black hole appears to be a slumbering giant,” says team leader Tatsuya Inui of Kyoto University in Japan. “But now we realize that the black hole was far more active in the past. Perhaps it’s just resting after a major outburst.”

X-ray panels of Sagittarius A*

Figuring out this possibility involved measuring the brightening of clouds of gas near the black hole, so-called ‘light echoes’ that resulted from the clouds being bathed in X-ray pulses from just outside Sgr A*. The pulses would have been the result of gases spiraling in toward the black hole, being heated to millions of degrees, and emitting X-rays. The observed brightening involved the large cloud called Sagittarius B2, which is about 300 light years from the black hole, its iron atoms emitting X-rays of their own as a result of a flare that went off, in relative terms, when Bach was still writing fugues.

Image: Four X-ray satellites imaged a small region in the gas cloud Sagittarius B2, and saw pockets brighten and fade over the course of nearly 12 years. These light echoes are caused by varying X-ray output from our galaxy’s central black hole. Credits: ASCA and Suzaku: JAXA; Chandra: NASA/CXC; XMM-Newton: ESA.

The actual event, of course, occurred long before that. The Milky Way’s center is 26,000 light years from Earth; we’re looking at ancient history, but able to make sense out of the sequence of events to parse out at least a bit of Sagittarius A*’s activity. This particular flare lit up the Sagittarius B2 cloud in a region ten light years across, with results varying widely over a five year period. The work follows on an earlier study of Sagittarius A* that tracked an X-ray burst some fifty years ago. The recently discovered flare was ten times stronger than that event.

Working in these wavelengths involved using archival data over an eleven year period from the major X-ray observatories — Japan’s Suzaku and ASCA X-ray satellites, NASA’s Chandra X-ray Observatory, and the European Space Agency’s XMM-Newton X-ray Observatory. The study is Inui et al., “Time Variability of the Neutral Iron Lines from the Sgr B2 Region and its Implication of a Past Outburst of Sgr A*,” slated to appear in Publications of the Astronomical Society of Japan (abstract).

Addendum: A light echo preserving an enormous X-ray flare has been observed in the galaxy SDSSJ0952+2143, as per this news release from the Max Planck Institute for Extraterrestrial Physics. The flare seems to have been produced when a single star was disrupted by a supermassive black hole. In this study, light echoes are used both to study the disruption of the star and also to map the galactic nucleus. “Reverberation-mapping of light echoes opens up new possibilities to study galaxies,” says the Planck Institute’s Stefanie Komossa, leader of the team on this work. Abstract of their paper here.

Comments on this entry are closed.

  • andy April 17, 2008, 9:37

    As I understand it, gamma-ray bursters are basically beamed events – anyone significantly off the axis of a fairly narrow beam sees a type Ib/c supernova, which wouldn’t be able to sterilise habitable planets over nearly so large a range. This does not strike me as a particularly good way to prevent the emergence of complex life in a galaxy (most of the time the GRB beam would be aimed out of the galactic plane), despite the current fashionability of this explanation.

    Perhaps events like the galactic core flaring would be a better mechanism to prevent the emergence of complex life in the earlier history of the galaxy: particularly considering that the way the metallicity evolves outwards with time means that the first terrestrial planets would likely be closer to the core, and thus receive worse effects from activity in that region.

  • ljk May 4, 2008, 23:26

    In situ formation of the massive stars around SgrA*

    Authors: M. Mapelli (1), T. Hayfield (1,2), L. Mayer (1,2), J. Wadsley (3) ((1) University of Zürich, (2) ETH Zürich, (3) McMaster University)

    (Submitted on 2 May 2008)

    Abstract: The formation of the massive young stars surrounding SgrA* is still an open question. In this paper, we simulate the infall of an isothermal, turbulent molecular cloud towards the Galactic Centre (GC). As it spirals towards the GC, the molecular cloud forms a small and dense disc around SgrA*. Efficient star formation (SF) is expected to take place in such a dense disc. We model SF by means of sink particles. At ~6×10^5 yr, ~6000 solar masses of stars have formed, and are confined within a thin disc with inner and outer radius of 0.06 and 0.5 pc, respectively. Thus, this preliminary study shows that the infall of a molecular cloud is a viable scenario for the formation of massive stars around SgrA*. Further studies with more realistic radiation physics and SF will be required to better constrain this intriguing scenario.

    Comments: 6 pages, 4 figures, resubmitted to MNRAS Letters after addressing referee report

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.0185v1 [astro-ph]

    Submission history

    From: Michela Mapelli [view email]

    [v1] Fri, 2 May 2008 08:05:27 GMT (1749kb)


  • ljk May 22, 2008, 7:49

    On the Formation of Compact Stellar Disks Around Sgr A*

    Authors: Mark Wardle (Macquarie University), Farhad Yusef-Zadeh (Northwestern University)

    (Submitted on 21 May 2008)

    Abstract: The recent identification of one or two sub-parsec disks of young, massive stars orbiting the ~4e6 solar mass black hole Sgr A* has prompted an “in-situ” scenario for star formation in disks of gas formed from a cloud captured from the Galactic center environment. To date there has been no explanation given for the low angular momentum of the disks relative to clouds passing close to the center.

    Here we show that the partial accretion of extended Galactic center clouds, such as the 50 km/s giant molecular cloud, that temporarily engulf Sgr A* during their passage through the central region of the Galaxy provide a natural explanation for the angular momentum and surface density of the the observed stellar disks. The captured cloud material is gravitationally unstable and forms stars as it circularizes, potentially explaining the large eccentricity and range of inclinations of the observed stellar orbits. The application of this idea to the formation of the circumnuclear ring is also discussed.

    Comments: 11 pages, 2 figs, submitted to ApJ Letters

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.3274v1 [astro-ph]

    Submission history

    From: Mark Wardle [view email]

    [v1] Wed, 21 May 2008 13:14:23 GMT (18kb)


  • ljk December 9, 2008, 23:42

    Unprecedented 16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole (ESO 46/08)‏

    From: ESO education and Public Outreach Department (information@eso.org)

    Sent: Tue 12/09/08 6:01 PM

    To: esonews@eso.org

    Dear all, In a 16-year long study, using several of ESO’s flagship telescopes, a team of German astronomers has produced the most detailed view ever of the surroundings of the monster lurking at our Galaxy’s heart — a supermassive black hole.

    The research has unravelled the hidden secrets of this tumultuous region by mapping the orbits of almost 30 stars, a five-fold increase over previous studies. One of the stars has now completed a full orbit around the black hole. Read more in ESO 46/08


    Kind Regards, The ESO education and Public Outreach Department

    10 December 2008

  • ljk June 24, 2009, 15:59

    Observing a Black Hole Event Horizon: (Sub)Millimeter VLBI of Sgr A*

    Authors: Vincent L. Fish, Sheperd S. Doeleman (MIT Haystack Observatory)

    (Submitted on 22 Jun 2009)

    Abstract: Very strong evidence suggests that Sagittarius A*, a compact radio source at the center of the Milky Way, marks the position of a super massive black hole. The proximity of Sgr A* in combination with its mass makes its apparent event horizon the largest of any black hole candidate in the universe and presents us with a unique opportunity to observe strong-field GR effects.

    Recent millimeter very long baseline interferometric observations of Sgr A* have demonstrated the existence of structures on scales comparable to the Schwarzschild radius. These observations already provide strong evidence in support of the existence of an event horizon. (Sub)Millimeter VLBI observations in the near future will combine the angular resolution necessary to identify the overall morphology of quiescent emission, such as an accretion disk or outflow, with a fine enough time resolution to detect possible periodicity in the variable component of emission.

    In the next few years, it may be possible to identify the spin of the black hole in Sgr A*, either by detecting the periodic signature of hot spots at the innermost stable circular orbit or parameter estimation in models of the quiescent emission.

    Longer term, a (sub)millimeter VLBI “Event Horizon Telescope” will be able to produce images of the Galactic center emission to the see the silhouette predicted by general relativistic lensing.

    These techniques are also applicable to the black hole in M87, where black hole spin may be key to understanding the jet-launching region.

    Comments: 5 pages, 3 figures, submitted to Proceedings IAU Symposium No. 261 “Relativity in Fundamental Astronomy: Dynamics, Reference Frames, and Data Analysis”

    Subjects: Galaxy Astrophysics (astro-ph.GA)

    Cite as: arXiv:0906.4040v1 [astro-ph.GA]

    Submission history

    From: Vincent L. Fish [view email]

    [v1] Mon, 22 Jun 2009 15:39:36 GMT (593kb)