1,371 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
Cетевой мониторинг программного обеспечения в информационной инфраструктуре предприятия
The article is devoted to network monitoring software enterprises. The process of providing functional and technical state of the system are described the actual business needs of the company. The paper proposes a solution to the optimization of this process, based on the scope of activities of the enterprise
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
Performance of astrometric detection of a hotspot orbiting on the innermost stable circular orbit of the galactic centre black hole
The galactic central black hole Sgr A* exhibits outbursts of radiation in the
near infrared (so-called IR flares). One model of these events consists in a
hotspot orbiting on the innermost stable circular orbit (ISCO) of the hole.
These outbursts can be used as a probe of the central gravitational potential.
One main scientific goal of the second generation VLTI instrument GRAVITY is to
observe these flares astrometrically. Here, the astrometric precision of
GRAVITY is investigated in imaging mode, which consists in analysing the image
computed from the interferometric data. The capability of the instrument to put
in light the motion of a hotspot orbiting on the ISCO of our central black hole
is then discussed.
We find that GRAVITY's astrometric precision for a single star in imaging
mode is smaller than the Schwarzschild radius of Sgr A*. The instrument can
also demonstrate that a body orbiting on the last stable orbit of the black
hole is indeed moving. It yields a typical size of the orbit, if the source is
as bright as m_K=14.
These results show that GRAVITY allows one to study the close environment of
Sgr A*. Having access to the ISCO of the central massive black hole probably
allows constraining general relativity in its strong regime. Moreover, if the
hotspot model is appropriate, the black hole spin can be constrained.Comment: 13 pages, 11 figures ; accepted by MNRA
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
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
Locating the VHE source in the Galactic Centre with milli-arcsecond accuracy
Very high-energy gamma-rays (VHE; E>100 GeV) have been detected from the
direction of the Galactic Centre up to energies E>10 TeV. Up to now, the origin
of this emission is unknown due to the limited positional accuracy of the
observing instruments. One of the counterpart candidates is the super-massive
black hole (SMBH) Sgr A*. If the VHE emission is produced within ~10^{15} cm
~1000 r_G (r_G=G M/c^2 is the Schwarzschild radius) of the SMBH, a decrease of
the VHE photon flux in the energy range 100--300 GeV is expected whenever an
early type or giant star approaches the line of sight within ~ milli-arcseconds
(mas). The dimming of the flux is due to absorption by pair-production of the
VHE photons in the soft photon field of the star, an effect we refer to as
pair-production eclipse (PPE). Based upon the currently known orbits of stars
in the inner arcsecond of the Galaxy we find that PPEs lead to a systematic
dimming in the 100--300 GeV band at the level of a few per cent and lasts for
several weeks. Since the PPE affects only a narrow energy band and is well
correlated with the passage of the star, it can be clearly discriminated
against other systematic or even source-intrinsic effects. While the effect is
too small to be observable with the current generation of VHE detectors,
upcoming high count-rate experiments like the Cherenkov telescope array (CTA)
will be sufficiently sensitive. Measuring the temporal signature of the PPE
bears the potential to locate the position and size of the VHE emitting region
within the inner 1000 r_G or in the case of a non-detection exclude the
immediate environment of the SMBH as the site of gamma-ray production
altogether.Comment: 7 pages, published in MNRAS 402, pg. 1342-134
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
Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY
The black hole at the Galactic centre exhibits regularly flares of radiation,
the origin of which is still not understood. In this article, we study the
ability of the near-future GRAVITY infrared instrument to constrain the nature
of these events. We develop realistic simulations of GRAVITY astrometric data
sets for various flare models. We show that the instrument will be able to
distinguish an ejected blob from alternative flare models, provided the blob
inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and
the flare duration is >= 1h30.Comment: 11 pages, 9 figures, accepted by MNRA
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