1,164 research outputs found
Fe Ka line emission from the Arches cluster region - evidence for ongoing particle bombardment?
We present the results of eight years of XMM-Newton observations of the
region surrounding the Arches cluster in the Galactic Center. We study the
spatial distribution and temporal behaviour of the Fe-Ka line emission with the
objective of identifying the likely source of the excitation. We investigate
the variability of the 6.4-keV line emission of four clouds through spectral
fitting of the EPIC MOS data with the use of a modelled background, which
avoids many of the systematics inherent in local background subtraction. We
also employ spectral stacking of both EPIC PN and MOS data to search for
evidence of an Fe-K edge feature imprinted on the underlying X-ray continuum.
The lightcurves of the Fe-Ka line from three bright molecular knots close to
the Arches cluster are found to be constant over the 8-year observation window.
West of the cluster, however, we found a bright cloud exhibiting the fastest
Fe-Ka variability yet seen in a molecular cloud in the Galactic Center region.
The time-averaged spectra of the molecular clouds reveal no convincing evidence
of the 7.1-keV edge feature. The EW of the 6.4-keV line emitted by the clouds
near the cluster is found to be ~1.0 keV. The observed Fe-Ka line flux and the
high EW suggest the fluorescence has a photoionization origin, although
excitation by cosmic-ray particles is not specifically excluded. For the three
clouds nearest to the cluster, an identification of the source of
photo-ionizing photons with an earlier outburst of Sgr A* is however at best
tentative. The hardness of the nonthermal component associated with the 6.4-keV
line emission might be best explained in terms of bombardment by cosmic-ray
particles from the Arches cluster itself. The relatively short-timescale
variability seen in the 6.4-keV line emission from the cloud to the West of the
cluster is most likely the result of illumination by a nearby transient X-ray
source.Comment: 13 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
The orbit of the star S2 around SgrA* from VLT and Keck data
Two recent papers (Ghez et al. 2008, Gillessen et al. 2009) have estimated
the mass of and the distance to the massive black hole in the center of the
Milky Way using stellar orbits. The two astrometric data sets are independent
and yielded consistent results, even though the measured positions do not match
when simply overplotting the two sets. In this letter we show that the two sets
can be brought to excellent agreement with each other when allowing for a small
offset in the definition of the reference frame of the two data sets. The
required offsets in the coordinates and velocities of the origin of the
reference frames are consistent with the uncertainties given in Ghez et al.
(2008). The so combined data set allows for a moderate improvement of the
statistical errors of mass of and distance to Sgr A*, but the overall
accuracies of these numbers are dominated by systematic errors and the
long-term calibration of the reference frame. We obtain R0 = 8.28 +- 0.15(stat)
+- 0.29(sys) kpc and M(MBH) = 4.30 +- 0.20(stat) +- 0.30(sys) x 10^6 Msun as
best estimates from a multi-star fit.Comment: submitted to ApJ
A polarised infrared flare from Sagittarius A* and the signatures of orbiting plasma hotspots
In this article we summarise and discuss the infrared, radio, and X-ray
emission from the supermassive black hole in the Galactic Centre, SgrA*. We
include new results from near-infrared polarimetric imaging observations
obtained on May 31st, 2006. In that night, a strong flare in Ks band (2.08
microns) reaching top fluxes of ~16 mJy could be observed. This flare was
highly polarised (up to ~40%) and showed clear sub-structure on a time scale of
15 minutes, including a swing in the polarisation angle of about 70 degrees.
For the first time we were able to observe both polarised flux and short-time
variability, with high significance in the same flare event. This result adds
decisive information to the puzzle of the SgrA* activity. The observed
polarisation angle during the flare peak is the same as observed in two events
in 2004 and 2005. Our observations strongly support the dynamical emission
model of a decaying plasma hotspot orbiting SgrA* on a relativistic orbit. The
observed polarisation parameters and their variability with time might allow to
constrain the orientation of accretion disc and spin axis with respect to the
Galaxy.Comment: 9 pages, 8 figures, accepted for publication in MNRA
The power of monitoring stellar orbits
The center of the Milky Way hosts a massive black hole. The observational
evidence for its existence is overwhelming. The compact radio source Sgr A* has
been associated with a black hole since its discovery. In the last decade,
high-resolution, near-infrared measurements of individual stellar orbits in the
innermost region of the Galactic Center have shown that at the position of Sgr
A* a highly concentrated mass of 4 x 10^6 M_sun is located. Assuming that
general relativity is correct, the conclusion that Sgr A* is a massive black
hole is inevitable. Without doubt this is the most important application of
stellar orbits in the Galactic Center. Here, we discuss the possibilities going
beyond the mass measurement offered by monitoring these orbits. They are an
extremely useful tool for many scientific questions, such as a geometric
distance estimate to the Galactic Center or the puzzle, how these stars reached
their current orbits. Future improvements in the instrumentation will open up
the route to testing relativistic effects in the gravitational potential of the
black hole, allowing to take full advantage of this unique laboratory for
celestial mechanics.Comment: Proceedings of the Galactic Center Workshop 2009, Shangha
Massive binaries in the vicinity of Sgr A*
A long-term spectroscopic and photometric survey of the most luminous and
massive stars in the vicinity of the super-massive black hole Sgr A* revealed
two new binaries; a long-period Ofpe/WN9 binary, GCIRS 16NE, with a modest
eccentricity of 0.3 and a period of 224 days and an eclipsing Wolf-Rayet binary
with a period of 2.3 days. Together with the already identified binary GCIRS
16SW, there are now three confirmed OB/WR binaries in the inner 0.2\,pc of the
Galactic Center. Using radial velocity change upper limits, we were able to
constrain the spectroscopic binary fraction in the Galactic Center to at a confidence level of 95%, a massive binary
fraction similar to that observed in dense clusters. The fraction of eclipsing
binaries with photometric amplitudes is , which is consistent with local OB star clusters ().
Overall the Galactic Center binary fraction seems to be close to the binary
fraction in comparable young clusters.Comment: 5 figures, submitted to Ap
On the nature of the fast moving star S2 in the Galactic Center
We analyze the properties of the star S2 orbiting the supermassive black hole
at the center of the Galaxy. A high quality SINFONI H and K band spectrum
obtained from coadding 23.5 hours of observation between 2004 and 2007 reveals
that S2 is an early B dwarf (B0-2.5V). Using model atmospheres, we constrain
its stellar and wind properties. We show that S2 is a genuine massive star, and
not the core of a stripped giant star as sometimes speculated to resolve the
problem of star formation so close to the supermassive black hole. We give an
upper limit on its mass loss rate, and show that it is He enriched, possibly
because of the presence of a magnetic field.Comment: 4 pages, 5 figures, ApJ letters accepte
Disruption of a Proto-Planetary Disk by the Black Hole at the Milky Way Centre
Recently, an ionized cloud of gas was discovered plunging toward the
supermassive black hole, SgrA*, at the centre of the Milky Way. The cloud is
being tidally disrupted along its path to closest approach at ~3100
Schwarzschild radii from the black hole. Here, we show that the observed
properties of this cloud of gas can naturally be produced by a proto-planetary
disk surrounding a low-mass star, which was scattered from the observed ring of
young stars orbiting SgrA*. As the young star approaches the black hole, its
disk experiences both photo-evaporation and tidal disruption, producing a
cloud. Our model implies that planets form in the Galactic centre, and that
tidal debris from proto-planetary disks can flag low mass stars which are
otherwise too faint to be detected.Comment: Accepted to Nature Communications; new Figure 4b provides predicted
Br-gamma emission as a function of tim
Clockwise Stellar Disk and the Dark Mass in the Galactic Center
Two disks of young stars have recently been discovered in the Galactic
Center. The disks are rotating in the gravitational field of the central black
hole at radii r=0.1-0.3 pc and thus open a new opportunity to measure the
central mass. We find that the observed motion of stars in the clockwise disk
implies M=4.3+/-0.5 million solar masses for the fiducial distance to the
Galactic Center R_0=8 kpc and derive the scaling of M with R_0. As a tool for
our estimate we use orbital roulette, a recently developed method. The method
reconstructs the three-dimensional orbits of the disk stars and checks the
randomness of their orbital phases. We also estimate the three-dimensional
positions and orbital eccentricities of the clockwise-disk stars.Comment: Comments: 16 pages, 5 figures, ApJ, in pres
Hydrodynamical simulations of a compact source scenario for G2
The origin of the dense gas cloud G2 discovered in the Galactic Center
(Gillessen et al. 2012) is still a debated puzzle. G2 might be a diffuse cloud
or the result of an outflow from an invisible star embedded in it. We present
here detailed simulations of the evolution of winds on G2's orbit. We find that
the hydrodynamic interaction with the hot atmosphere present in the Galactic
Center and the extreme gravitational field of the supermassive black hole must
be taken in account when modeling such a source scenario. We find that the
hydrodynamic interaction with the hot atmosphere present in the Galactic Center
and the extreme gravitational field of the supermassive black hole must be
taken in account when modeling such a source scenario. We also find that in
this scenario most of the Br\gamma\ luminosity is expected to come from the
highly filamentary densest shocked wind material. G2's observational properties
can be used to constrain the properties of the outflow and our best model has a
mass outflow rate of Mdot,w=8.8 x 10^{-8} Msun/yr and a wind velocity of vw =
50 km/s. These values are compatible with those of a young TTauri star wind, as
already suggested by Scoville & Burkert (2013).Comment: 4 pages, 3 figures; Proceeding of the IAU 303: "The GC: Feeding and
Feedback in a Normal Galactic Nucleus" / September 30 - October 4, 2013,
Santa Fe, New Mexico (USA
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