338 research outputs found
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&
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
GRAVITY: The AO-Assisted, Two-Object Beam-Combiner Instrument
We present the proposal for the infrared adaptive optics (AO) assisted,
two-object, high-throughput, multiple-beam-combiner GRAVITY for the VLTI. This
instrument will be optimized for phase-referenced interferometric imaging and
narrow-angle astrometry of faint, red objects. Following the scientific
drivers, we analyze the VLTI infrastructure, and subsequently derive the
requirements and concept for the optimum instrument. The analysis can be
summarized with the need for highest sensitivity, phase referenced imaging and
astrometry of two objects in the VLTI beam, and infrared wavefront-sensing.
Consequently our proposed instrument allows the observations of faint, red
objects with its internal infrared wavefront sensor, pushes the optical
throughput by restricting observations to K-band at low and medium spectral
resolution, and is fully enclosed in a cryostat for optimum background
suppression and stability. Our instrument will thus increase the sensitivity of
the VLTI significantly beyond the present capabilities. With its two fibers per
telescope beam, GRAVITY will not only allow the simultaneous observations of
two objects, but will also push the astrometric accuracy for UTs to 10
micro-arcsec, and provide simultaneous astrometry for up to six baselines.Comment: 12 pages, to be published in the Proceedings of the ESO Workshop on
"The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd
Generation VLTI Instrumentation", eds. F. Paresce, A. Richichi, A. Chelli and
F. Delplancke, held in Garching, Germany, 4-8 April 200
New Results on the Helium Stars in the Galactic Center Using BEAR Spectro-Imagery
Integral field spectroscopy of the central parsec of the Galactic Center was obtained at 2.06 microns using BEAR, an imaging Fourier Transform Spectrometer, at a spectral resolution of 74 km/s. Sixteen stars were confirmed as helium stars by detecting the He I 2.058 microns line in emission, providing a homogeneous set of fully resolved line profiles. These observations allow us to discard some of the earlier detections of such stars in the central cluster and to add three new stars. The sources detected in the BEAR data were compared with adaptive optics images in the K band to determine whether the emission was due to single stars. Two sub-classes of almost equal number are clearly identified from the width of their line profiles, and from the brightness of their continuum. Most of the emission lines show a P Cygni profile. From these results, we propose that the latter group is formed of stars in or near the LBV phase, and the other one of stars at the WR stage. The division into two groups is also shown by their spatial distribution, with the narrow-line stars in a compact central cluster (IRS 16) and the other group distributed at the periphery of the central cluster of hot stars. In the same data cube, streamers of interstellar helium gas are also detected. The helium emission traces the densest parts of the SgrA West Mini-Spiral. Several helium stars have a radial velocity comparable to the velocity of the interstellar gas in which they are embedded. In the final discussion, all these findings are examined to present a possible scenario for the formation of very massive stars in the exceptional conditions of the vicinity of the central Black Hole
Flares and variability from Sagittarius A*: five nights of simultaneous multi-wavelength observations
Aims. We report on simultaneous observations and modeling of mid-infrared
(MIR), near-infrared (NIR), and submillimeter (submm) emission of the source
Sgr A* associated with the supermassive black hole at the center of our Galaxy.
Our goal was to monitor the activity of Sgr A* at different wavelengths in
order to constrain the emitting processes and gain insight into the nature of
the close environment of Sgr A*. Methods. We used the MIR instrument VISIR in
the BURST imaging mode, the adaptive optics assisted NIR camera NACO, and the
sub-mm antenna APEX to monitor Sgr A* over several nights in July 2007.
Results. The observations reveal remarkable variability in the NIR and sub-mm
during the five nights of observation. No source was detected in the MIR, but
we derived the lowest upper limit for a flare at 8.59 microns (22.4 mJy with
A_8.59mu = 1.6+/- 0.5). This observational constraint makes us discard the
observed NIR emission as coming from a thermal component emitting at sub-mm
frequencies. Moreover, comparison of the sub-mm and NIR variability shows that
the highest NIR fluxes (flares) are coincident with the lowest sub-mm levels of
our five-night campaign involving three flares. We explain this behavior by a
loss of electrons to the system and/or by a decrease in the magnetic field, as
might conceivably occur in scenarios involving fast outflows and/or magnetic
reconnection.Comment: 10 pages, 7 figures, published in A&
The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared
High spatial resolution observations in the 1 to 3.5 micron region of the
Galactic Center source known historically as IRS 13 are presented. They include
ground-based adaptive optics images in the H, Kp (2.12/0.4 micron) and L bands,
NICMOS data in filters between 1.1 and 2.2 micron, and integral field
spectroscopic data from BEAR, an Imaging FTS, in the HeI 2.06 micron and the
Br line regions. Analysis of all these data provides a completely new
picture of the main component, IRS 13E, which appears as a cluster of seven
individual stars within a projected diameter of ~0.5'' (0.02 pc). The brightest
sources, 13E1, 13E2, 13E3 (a binary), and 13E4, are all massive stars, 13E1 a
blue object, with no detected emission line while 13E2 and 13E4 are high-mass
emission line stars. 13E2 is at the WR stage and 13E4 a massive O-type star.
13E3A and B are extremely red objects, proposed as other examples of dusty WR
stars. All these sources have a common westward proper motion. 13E5, is a red
source similar to 13E3A/B. This concentration of comoving massive hot stars,
IRS 13E, is proposed as the remaining core of a massive star cluster, which
could harbor an intermediate-mass black hole (IMBH) of ~1300 M_sol. This
detection plays in favor of a scenario in which the helium stars and the other
hot stars in the central pc originate from the stripping of a massive cluster
formed several tens of pc from the center. The detection of a discrete X-ray
emission (Baganoff et al. 2003) at the IRS~13 position is examined in this
context.Comment: 14 pages, 6 figures (3 in color), LaTeX2e, accepted in A&
GCIRS 7, a pulsating M1 supergiant at the Galactic centre. Physical properties and age
The stellar population in the central parsec of the Galaxy is dominated by an
old (several Gyr) population, but young, massive stars dominate the luminosity
function. We have studied the most luminous of these stars, GCIRS 7, in order
to constrain the age of the recent star formation event in the Galactic Centre
and to characterise it as an interferometric reference for observations of the
Galactic Centre with the instrument GRAVITY, which will equip the Very Large
Telescope Interferometer in the near future. We present the first H-band
interferometric observations of GCIRS 7, obtained using the PIONIER visitor
instrument on the VLTI using the four 8.2-m unit telescopes. In addition, we
present unpublished K-band VLTI/AMBER data, build JHKL light-curves based on
data spanning 4 decades, and measured the star's effective temperature using
SINFONI spectroscopy. GCIRS 7 is marginally resolved at H-band (in 2013:
uniform-disk diameter=1.076+/-0.093mas, R=960+/-92Rsun at 8.33+/-0.35kpc). We
detect a significant circumstellar contribution at K-band. The star and its
environment are variable in brightness and in size. The photospheric H-band
variations are well modelled with two periods: P0~470+/-10 days (amplitude
~0.64mag) and long secondary period LSP~2700-2850 days (~1.1mag). As measured
from CO equivalent width, =3600+/-195K. The size, periods, luminosity
(=-8.44+/-0.22) and effective temperature are consistent with an M1
supergiant with an initial mass of 22.5+/-2.5Msun and an age of 6.5-10Myr
(depending on rotation). This age is in remarkable agreement with most
estimates for the recent star formation event in the central parsec. Caution
should be taken when using this star as an interferometric reference as it is
variable in size, is surrounded by a variable circumstellar environment and
large convection cells may form on its photosphere.Comment: Accepted for publication in A&A. 10 pages, 12 figure
- …