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A Quiet Day at the Galactic Core

The mammoth black hole Sagittarius A* isn’t the only interesting thing near the center of our galaxy. The European Space Agency’s Integral observatory, working with gamma rays, tracks about eighty high-energy sources in the area. About ten of those closest to the galaxy’s center had faded when Integral performed a series of observations last April. A mysterious force? Hardly. “All the sources are variable and it was just by accident or sheer luck that they had turned off during that observation,” says Erik Kuulkers of ESA’s Integral Science Operations Center.

Fair enough, and useful for astronomers, who were able to use the sudden quiet to look for still fainter sources, and to set limits on the brightness of the x-ray binaries involved. These consist of two stars orbiting each other, one a normal star, the other a collapsed object — a white dwarf, neutron star or black hole. The compressed star pulls off gaseous material from its companion, heating it to a million degrees Centigrade and causing it to emit x-rays and gamma rays. The brightness of the emission seems to depend on the amount of gas transferred.

Integral observation

Image: The center of the galaxy, as seen in a mosaic of exposures from Integral. The black hole Sagittarius A* is close to the source marked ‘1’ in the image. Due to the variability these sources possess on all time scales, the region never looks exactly the same. Surprisingly, the sources were ‘off’ around the time of the observation (including the normally bright well-known black-hole candidate and micro-quasar 1E 1740.7-2942), displaying an unusually ‘quiet’ galactic centre. Credit: ESA/ISDC.

What’s intriguing to Centauri Dreams is that dimming of sources near Sagittarius A* could lead to the detection of faint high-energy radiation associated with its immediate neighborhood, telling us more about this enigmatic object. A paper on recent Integral findings is Kuulkers et al., “The INTEGRAL Galactic bulge monitoring program: the first 1.5 years,” accepted by Astronomy & Astrophysics and available online.

Comments on this entry are closed.

  • Gregory Benford January 22, 2007, 15:19

    Magnetic structures ar galactic center are huge, gaudy advertisements for past energetic events — the neon signs in the Times Square of our galaxy. I’ve worked on explaining them for two decades, and it seems clear they really are like neon signs — big plasma discharges accelerating elecftrons to relativistic energies, so they light up in the magnetic fields that confine these 100-light-year long strands. Thjere are over 200 of them, and one, the Snake, has two kinks visible — just as one expects of current-carrying threads, that can twist like firehoses. These are the largest structures built anywhere, and can tell us much more.

  • ljk February 28, 2008, 7:12

    IRS 13N: a new comoving group of sources at the Galactic Center

    Authors: K. Muzic, R. Schoedel, A. Eckart, L. Meyer, A. Zensus

    (Submitted on 27 Feb 2008)

    Abstract: Context: The Galactic Center IRS 13E cluster is located ~3.2” from SgrA*. It is an extremely dense stellar association containing several Wolf-Rayet and O-type stars, at least four of which show a common velocity. Only half an arcsecond north from IRS 13E there is a complex of extremely red sources, so-called IRS 13N.

    Their nature is still unclear. Based on the analysis of their colors, there are two main possibilities: (1) dust embedded sources older than few Myr, or (2) extremely young objects with ages less than 1Myr.

    Aims: We present the first proper motion measurements of IRS 13N members, and additionally give proper motions of four of IRS 13E stars resolved in the L’-band.

    Methods: The L’-band (3.8 micron) observations have been carried out using the NACO adaptive optics system at the ESO VLT. Proper motions have been obtained by linear fitting of the stellar positions extracted by StarFinder as a function of time, weighted by positional uncertainties.

    Results: We show that six of seven resolved northern sources show a common proper motion, thus revealing a new comoving group of stars in the central half parsec of the Milky Way. The common proper motions of IRS 13E and IRS 13N clusters are significantly (greater than 5 sigma) different. We also performed a fitting of the positional data for those stars onto Keplerian orbits, assuming SgrA* as the center of the orbit. Our results favor the very young stars hypothesis.

    Comments: 7 pages, 2 figures; accepted by A&A

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.4004v1 [astro-ph]

    Submission history

    From: Koraljka Mu\v{z}i\’c [view email]

    [v1] Wed, 27 Feb 2008 12:21:12 GMT (242kb,D)


  • ljk March 12, 2008, 10:29

    Massive Star Formation in the Galactic Center

    Authors: D. F. Figer

    (Submitted on 11 Mar 2008)

    Abstract: The Galactic center is a hotbed of star formation activity, containing the most massive star formation site and three of the most massive young star clusters in the Galaxy. Given such a rich environment, it contains more stars with initial masses above 100 \Msun than anywhere else in the Galaxy.

    This review concerns the young stellar population in the Galactic center, as it relates to massive star formation in the region. The sample includes stars in the three massive stellar clusters, the population of younger stars in the present sites of star formation, the stars surrounding the central black hole, and the bulk of the stars in the field population.

    The fossil record in the Galactic center suggests that the recently formed massive stars there are present-day examples of similar populations that must have been formed through star formation episodes stretching back to the time period when the Galaxy was forming.

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.1619v1 [astro-ph]

    Submission history

    From: Christine Trombley [view email]

    [v1] Tue, 11 Mar 2008 16:08:59 GMT (718kb)


  • ljk March 13, 2008, 22:49

    Radial Velocities of Stars in the Galactic Center

    Authors: Qingfeng Zhu, Rolf P. Kudritzki, Donald F. Figer, Francisco Najarro, David Merritt

    (Submitted on 12 Mar 2008)

    Abstract: We present results from K band slit scan observations of a ~20”x20” region of the Galactic center (GC) in two separate epochs more than five years apart. The high resolution (R>=14,000) observations allow the most accurate radial velocity and acceleration measurements of the stars in the central parsec of the Galaxy. Detected stars can be divided into three groups based on the CO absorption band heads at ~2.2935 microns and the He I lines at ~2.0581 microns and ~2.112, 2.113 microns: cool, narrow-line hot and broad-line hot. The radial velocities of the cool, late-type stars have approximately a symmetrical distribution with its center at ~-7.8(+/-10.3) km/s and a standard deviation ~113.7(+/-10.3) km/s.

    Although our statistics are dominated by the brightest stars, we estimate a central black hole mass of 3.9(+/-1.1) million solar masses, consistent with current estimates from complete orbits of individual stars. Our surface density profile and the velocity dispersion of the late type stars support the existence of a low density region at the Galactic center suggested by earlier observations. Many hot, early-type stars show radial velocity changes higher than maximum values allowed by pure circular orbital motions around a central massive object, suggesting that the motions of these stars greatly deviate from circular orbital motions around the Galactic center.

    The correlation between the radial velocities of the early type He I stars and their declination offsets from Sagittarius A* suggests that a systematic rotation is present for the early-type population. No figure rotation around the Galactic center for the late type stars is supported by the new observations.

    Comments: 61 pages, 18 figures, 7 tables; accepted for publication in Astrophysical Journal

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.1826v1 [astro-ph]

    Submission history

    From: Qingfeng Zhu [view email]

    [v1] Wed, 12 Mar 2008 19:25:58 GMT (468kb)