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G C N E W S
* Newsflash *
- The Newsletter for Galactic Center Research -
gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews
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Vol. 29, No. 9 Jun 11, 2009
Recently submitted papers:
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1) Simultaneous H.E.S.S. and Chandra observations of Sgr A* during an
X-ray flare (Aharonian et al., A&A)
2) Influence of a stellar cusp on the dynamics of young stellar discs
and the origin of the S-stars in the Galactic Centre (Loeckmann et
al., MNRAS)
3) On the number of young stellar discs in the Galactic Centre
(Loeckmann & Baumgardt, MNRAS)
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Email : j.a.hinton@leeds.ac.uk
Title : Simultaneous H.E.S.S. and Chandra observations of Sgr A*
during an X-ray flare
Author(s): F. Aharonian and 154 other authors
Paper : A&A, 492, L25
Abstract:
The rapidly varying ( 10 minute timescale) non-thermal X-ray emission
observed from Sgr A* implies that particle acceleration is occuring
close to the event horizon of the supermassive black hole. The TeV
gamma -ray source HESS J1745-290 is coincident with Sgr A* and may be
closely related to its X-ray emission. Simultaneous X-ray and TeV
observations are required to elucidate the relationship between these
objects. We report on joint H.E.S.S./Chandra observations performed in
July 2005, during which an X-ray flare was detected. Despite a factor
of 9 increase in the X-ray flux of Sgr A*, no evidence is found for an
increase in the TeV gamma -ray flux from this region. We find that an
increase in the gamma -ray flux of a factor of 2 or greater can be
excluded at a confidence level of 99%. This finding disfavours
scenarios in which the keV and TeV emission are associated with the
same population of accelerated particles and in which the bulk of the
gamma -ray emission is produced within 10^14 cm ( 100 R_S) of the
supermassive black hole.
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Email : uloeck@astro.uni-bonn.de
Title : Influence of a stellar cusp on the dynamics of young stellar
discs and the origin of the S-stars in the Galactic Centre
Author(s): Ulf Loeckmann, Holger Baumgardt, and Pavel Kroupa
Paper : MNRAS, 2009, in press
EPrint : 0906.0574
Abstract:
Observations of the Galactic Centre show evidence of one or two
disc-like structures of very young stars orbiting the central
super-massive black hole within a distance of a few 0.1 pc. A number of
analyses have been carried out to investigate the dynamical behaviour
and consequences of these discs, including disc thickness and
eccentricity growth as well as mutual interaction and warping. However,
most of these studies have neglected the influence of the stellar cusp
surrounding the black hole, which is believed to be 1-2 orders of
magnitude more massive than the disc(s). By means of N-body
integrations using our BHINT code, we study the impact of stellar cusps
of different compositions. We find that although the presence of a cusp
does have an important effect on the evolution of an otherwise isolated
flat disc, its influence on the evolution of disc thickness and warping
is rather mild in a two-disc configuration. However, we show that the
creation of highly eccentric orbits strongly depends on the graininess
of the cusp (i.e. the mean and maximum stellar masses): While Chang
(2008) recently found that full cycles of Kozai resonance are prevented
by the presence of an analytic cusp, we show that relaxation processes
play an important role in such highly dense regions and support
short-term resonances. We thus find that young disc stars on initially
circular orbits can achieve high eccentricities by resonant effects
also in the presence of a cusp of stellar remnants, yielding a
mechanism to create S-stars and hyper-velocity stars. Furthermore, we
discuss the underlying initial mass function (IMF) of the young stellar
discs and find no definite evidence for a non-canonical IMF.
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Email : uloeck@astro.uni-bonn.de
Title : On the number of young stellar discs in the Galactic Centre
Author(s): Ulf Loeckmann and Holger Baumgardt
Paper : MNRAS, 394, 1841-1846, 2009
EPrint : 0901.1660
Web : http://www3.interscience.wiley.com/cgi-bin/fulltext/122245841
/HTMLSTART
Abstract:
Observations of the Galactic Centre show evidence of disc-like
structures of very young stars orbiting the central super-massive black
hole within a distance of a few 0.1 pc. While it is widely accepted
that about half of the stars form a relatively flat disc rotating
clockwise on the sky, there is a substantial ongoing debate on whether
there is a second, counter-clockwise disc of stars. By means of N-body
simulations using our BHINT code, we show that two highly inclined
stellar discs with the observed properties cannot be recognised as two
flat circular discs after 5 Myr of mutual interaction. Instead, our
calculations predict a significant warping of the two discs, which we
show to be apparent among the structures observed in the Galactic
Centre. While the high eccentricities of the observed counter-clockwise
orbits suggest an eccentric origin of this system, we show the
eccentricity distribution in the inner part of the more massive
clockwise disc to be perfectly consistent with an initially circular
disc in which stellar eccentricities increase due to both non-resonant
and resonant relaxation. We conclude that the relevant question to ask
is therefore not whether there are two discs of young stars, but
whether there were two such discs to begin with.
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