973 research outputs found
A nova origin of the gas cloud at the Galactic Center ?
The recent discovery by Gillessen and collaborators of a cloud of gas falling
towards the Galactic Center on a highly eccentric orbit, diving nearly straight
into the immediate neighborhood of the central supermassive black hole, raises
the important question of its origin. Several models have already been
proposed. Here we suggest that a recent nova outburst has ejected a ring-like
shell of gas. Viewed at high inclination, that could account for the mass, head
and tail structure, and the unusually high eccentricity of the observed cloud
in a natural way, even as the nova moves on an orbit quite normal for the young
stars in the close neighborhood of the Galactic Center. We illustrate this by
calculating orbits for the head and tail parts of the ejecta and the nova that
has produced it. We briefly discuss some of the questions that this model, if
true, raises about the stellar environment close to the Galactic Center.Comment: 4 pages, 1 figure, Journal reference, correction of a minor erro
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
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
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
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
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
Reply to Yu Wei, Yao Zhu, and Dingwei Ye's Letter to the Editor re: Fabio Turco, Andrew Armstrong, Gerhardt Attard, et al. What Experts Think About Prostate Cancer Management During the COVID-19 Pandemic: Report from the Advanced Prostate Cancer Consensus Conference 2021. Eur Urol. 2022;82:6–11
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
The two states of Sgr A* in the near-infrared: bright episodic flares on top of low-level continuous variability
In this paper we examine properties of the variable source Sgr A* in the
near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT
observations taken 2004 to 2009. We investigate the variability of Sgr A* with
two different photometric methods and analyze its flux distribution. We find
Sgr A* is continuously emitting and continuously variable in the near-infrared,
with some variability occurring on timescales as long as weeks. The flux
distribution can be described by a lognormal distribution at low intrinsic
fluxes (<~5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a
median flux of approximately 1.1 mJy, but above 5 mJy the flux distribution is
significantly flatter (high flux events are more common) than expected for the
extrapolation of the lognormal distribution to high fluxes. We make a general
identification of the low level emission above 5 mJy as flaring emission and of
the low level emission as the quiescent state. We also report here the
brightest Ks-band flare ever observed (from August 5th, 2008) which reached an
intrinsic Ks-band flux of 27.5 mJy (m_{Ks}=13.5). This flare was a factor 27
increase over the median flux of Sgr A*, close to double the brightness of the
star S2, and 40% brighter than the next brightest flare ever observed from
Sgr~A*.Comment: 14 pages, 6 figures, accepted for publication in Ap
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