999 research outputs found
Testing the validity of the ray-tracing code GYOTO
In the next few years, the near-infrared interferometer GRAVITY will be able
to observe the Galactic center. Astrometric data will be obtained with an
anticipated accuracy of 10 as. To analyze these future data, we have
developed a code called GYOTO to compute orbits and images. We want to assess
the validity and accuracy of GYOTO in a variety of contexts, in particular for
stellar astrometry in the Galactic center. Furthermore, we want to tackle and
complete a study made on the astrometric displacements that are due to lensing
effects of a star of the central parsec with GYOTO. We first validate GYOTO in
the weak-deflection limit (WDL) by studying primary caustics and primary
critical curves obtained for a Kerr black hole. We compare GYOTO results to
available analytical approximations and estimate GYOTO errors using an
intrinsic estimator. In the strong-deflection limit (SDL), we choose to compare
null geodesics computed by GYOTO and the ray-tracing code named Geokerr.
Finally, we use GYOTO to estimate the apparent displacements of a star for
different angles from Sagittarius A* (Sgr A*). We have demonstrated that GYOTO
is accurate to a very high level, orders of magnitude better than the GRAVITY
requirements. GYOTO is also valid in weak- and strong-deflection regimes and
for very long integrations. At the astrometric precision that GRAVITY is aiming
for, lensing effects must always be taken into account when fitting stellar
orbits in the central parsec of the Galaxy.Comment: 11 pages, 12 figure
Hot molecular hydrogen in the central parsec of the Galaxy through near-infrared 3D fitting
Aims. We have investigated neutral gas in the central cavity of the
circumnuclear disk (CND) at the Galactic Center, where the ionized minispiral
lies, to describe the H2 distribution and properties in this ionized
environment. Methods. This study was carried out through a spectro-imaging data
cube of the central cavity obtained with SPIFFI on the VLT. The observed field
of view is 36"x 29" , with a spectral resolution R = 1 300 in the
near-infrared. These observations cover several H2 lines. To preserve the
spatial resolution and avoid edge effects, we applied a new line-fitting method
that consists of a regularized 3D fitting. We also applied a more classical 1D
fitting to compare the relative strength of the H2 lines. Results. We present
high spatial and spectral resolution maps of the intensity, velocity, and width
of five H2 lines and an extinction map derived from H2. Molecular gas is
detected everywhere in the field. In particular, in addition to the known CND
features, we detected an emission from the northern arm cloud and from the
minicavity. The excitation diagrams allow us to estimate the temperature, mass,
and density of these features. Conclusions. We interpret the CND emission as
coming from a hot, thermalized, thin layer at the surface of the clouds. The
observed H2 corresponds only to a small fraction of the total H2 mass. The
emission remains fairly strong in the whole central cavity, but it is not
thermalized. A strong deviation from thermal equilibrium is detected near the
minicavity. We suggest that this emission is caused by constantly forming H2
that is destroyed again before it reaches ortho/para equilibrium
Flux Modulation from the Rossby Wave Instability in microquasars accretion disks: toward a HFQPO model
Context. There have been a long string of efforts to understand the source of
the variability observed in microquasars, especially concerning the elusive
High-Frequency Quasi-Periodic Oscillation. These oscillations are among the
fastest phenomena that affect matter in the vicinity of stellar black holes and
therefore could be used as probes of strong-field general relativity.
Nevertheless, no model has yet gained wide acceptance. Aims. The aim of this
article is to investigate the model derived from the occurrence of the Rossby
wave instability at the inner edge of the accretion disk. In particular, our
goal here is to demonstrate the capacity of this instability to modulate the
observed flux in agreement with the observed results. Methods. We use the
AMRVAC hydrodynamical code to model the instability in a 3D optically thin
disk. The GYOTO ray-tracing code is then used to compute the associated light
curve. Results. We show that the 3D Rossby wave instability is able to modulate
the flux well within the observed limits.We highlight that 2D simulations allow
us to obtain the same general characteristics of the light curve as 3D
calculations. With the time resolution we adopted in this work, three
dimensional simulations do not give rise to any new observable features that
could be detected by current instrumentation or archive data.Comment: 10 pages, 10 figures, accepted by A&
The efficiency of resonant relaxation around a massive black hole
Resonant relaxation (RR) is a rapid relaxation process that operates in the
nearly-Keplerian potential near a massive black hole (MBH). RR dominates the
dynamics of compact remnants that inspiral into a MBH and emit gravitational
waves (extreme mass ratio inspiral events, EMRIs). RR can either increase the
EMRI rate, or strongly suppress it, depending on its still poorly-determined
efficiency. We use small-scale Newtonian N-body simulations to measure the RR
efficiency and to explore its possible dependence on the stellar number density
profile around the MBH, and the mass-ratio between the MBH and a star (a
single-mass stellar population is assumed). We develop an efficient and robust
procedure for detecting and measuring RR in N-body simulations. We present a
suite of simulations with a range of stellar density profiles and mass-ratios,
and measure the mean RR efficiency in the near-Keplerian limit. We do not find
a statistically significant dependence on the density profile or the
mass-ratio. Our numerical determination of the RR efficiency in the Newtonian,
single-mass population approximations, suggests that RR will likely enhance the
EMRI rate by a factor of a few over the rates predicted assuming only slow
stochastic two-body relaxation.Comment: 5 pp, 6 figs, ApJ submitte
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
Variable accretion and emission from the stellar winds in the Galactic centre
We present numerical simulations of stellar wind dynamics in the central
parsec of the Galactic centre, studying in particular the accretion of gas on
to Sgr A*, the super-massive black hole. Unlike our previous work, here we use
state-of-the-art observational data on orbits and wind properties of individual
wind-producing stars. Since wind velocities were revised upwards and non-zero
eccentricities were considered, our new simulations show fewer clumps of cold
gas and no conspicuous disc-like structure. The accretion rate is dominated by
a few close `slow wind stars' v_w < 750 km/s, and is consistent with the Bondi
estimate, but variable on time-scales of tens to hundreds of years. This
variability is due to the stochastic in-fall of cold clumps of gas, as in
earlier simulations, and to the eccentric orbits of stars. The present models
fail to explain the high luminosity of Sgr A* a few hundred years ago implied
by Integral observations, but we argue that the accretion of a cold clump with
a small impact parameter could have caused it. Finally, we show the possibility
of constraining the total mass-loss rate of the `slow wind stars' using near
infra-red observations of gas in the central few arcseconds.Comment: MNRAS, accepted. 10 pages, 11 figures. PS version is better (Fig 11
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