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X. Haubois, K. Dodds-Eden,
A. Weiss,
T. Paumard,
G. Perrin,
Y. Clénet,
S. Gillessen,
P. Kervella,
F. Eisenhauer,
R. Genzel,
D. Rouan
[show abstract]
[hide abstract]
ABSTRACT: 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.
01/2013;
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[show abstract]
[hide abstract]
ABSTRACT: The X-ray and near-IR emission from Sgr A* is dominated by flaring, while a
quiescent component dominates the emission at radio and sub-mm wavelengths. The
spectral energy distribution of the quiescent emission from Sgr A* peaks at
sub-mm wavelengths and is modeled as synchrotron radiation from a thermal
population of electrons in the accretion flow, with electron temperatures
ranging up to $\sim 5-20$\,MeV. Here we investigate the mechanism by which
X-ray flare emission is produced through the interaction of the quiescent and
flaring components of Sgr A*. The X-ray flare emission has been interpreted as
inverse Compton, self-synchrotron-Compton, or synchrotron emission. We present
results of simultaneous X-ray and near-IR observations and show evidence that
X-ray peak flare emission lags behind near-IR flare emission with a time delay
ranging from a few to tens of minutes. Our Inverse Compton scattering modeling
places constraints on the electron density and temperature distributions of the
accretion flow and on the locations where flares are produced. In the context
of this model, the strong X-ray counterparts to near-IR flares arising from the
inner disk should show no significant time delay, whereas near-IR flares in the
outer disk should show a broadened and delayed X-ray flare.
03/2012;
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[show abstract]
[hide abstract]
ABSTRACT: While the existence of a starburst-AGN connection is undisputed, there is no
consensus on what the connection is. In this contribution, we begin by noting
that the mechanisms which drive gas inwards in disk galaxies are generally
inefficient at removing angular momentum, leading to stalled inflows. Thus, a
tiered series of such processes is required to bring gas to the smallest
scales, each of which on its own may not correlate with the presence of an AGN.
Similarly, each may be associated with a starburst event, making it important
to discriminate between 'circumnuclear' and 'nuclear' star formation. In this
contribution, we show that stellar feedback on scales of tens of parsecs plays
a critical role in first hindering and then helping accretion. We argue that it
is only after the initial turbulent phases of a starburst that gas from slow
stellar winds can accrete efficiently to smaller scales. This would imply that
the properties of the obscuring torus are directly coupled to star formation
and that the torus must be a complex dynamical entity. We finish by remarking
on other contexts where similar processes appear to be at work.
01/2012;
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S Gillessen,
R Genzel,
T K Fritz,
E Quataert,
C Alig,
A Burkert,
J Cuadra,
F Eisenhauer,
O Pfuhl, K Dodds-Eden,
C F Gammie,
T Ott
[show abstract]
[hide abstract]
ABSTRACT: Measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low. Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only ∼3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole's gravitational force. The cloud's dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.
Nature 12/2011; 481(7379):51-4. · 36.28 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present spatially resolved imaging and integral field spectroscopy data
for 450 cool giant stars within 1\,pc from Sgr\,A*. We use the prominent CO
bandheads to derive effective temperatures of individual giants. Additionally
we present the deepest spectroscopic observation of the Galactic Center so far,
probing the number of B9/A0 main sequence stars ($2.2-2.8\,M_\odot$) in two
deep fields. From spectro-photometry we construct a Hertzsprung-Russell diagram
of the red giant population and fit the observed diagram with model populations
to derive the star formation history of the nuclear cluster.
We find that (1) the average nuclear star-formation rate dropped from an
initial maximum $\sim10$\,Gyrs ago to a deep minimum 1-2\,Gyrs ago and
increased again during the last few hundred Myrs, and (2) that roughly 80% of
the stellar mass formed more than 5\,Gyrs ago; (3) mass estimates within $\rm
R\sim1\,pc$ from Sgr\,A* favor a dominant star formation mode with a 'normal'
Chabrier/Kroupa initial mass function for the majority of the past star
formation in the Galactic Center. The bulk stellar mass seems to have formed
under conditions significantly different from the young stellar disks, perhaps
because at the time of the formation of the nuclear cluster the massive black
hole and its sphere of influence was much smaller than today.
10/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We derive the extinction curve toward the Galactic center (GC) from 1 to 19 μm. We use hydrogen emission lines of the minispiral observed by ISO-SWS and SINFONI. The extinction-free flux reference is the 2 cm continuum emission observed by the Very Large Array. Toward the inner 14'' × 20'', we find an extinction of A 2.166 μm = 2.62 ± 0.11, with a power-law slope of α = –2.11 ± 0.06 shortward of 2.8 μm, consistent with the average near-infrared slope from the recent literature. At longer wavelengths, however, we find that the extinction is grayer than shortward of 2.8 μm. We find that it is not possible to fit the observed extinction curve with a dust model consisting of pure carbonaceous and silicate grains only, and the addition of composite particles, including ices, is needed to explain the observations. Combining a distance-dependent extinction with our distance-independent extinction, we derive the distance to the GC to be R 0 = 7.94 ± 0.65 kpc. Toward Sgr A* (r < 05), we obtain AH = 4.21 ± 0.10, AKs = 2.42 ± 0.10, and A L' = 1.09 ± 0.13.
The Astrophysical Journal 08/2011; 737(2):73. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We derive the extinction curve towards the Galactic Center from 1 to 19 µm. We use hydrogen emission lines of the minispiral observed by ISO-SWS and SINFONI. The extinction free flux reference is the 2 cm continuum emission observed by the VLA. Towards the inner 14 ′′ ×20 ′′ we find an extinction of A 2.166 µm = 2.62 ± 0.11, with a power-law slope of α = −2.11 ± 0.06 shortward of 2.8 µm, consistent with the average near infrared slope from the recent literature. At longer wavelengths, however, we find that the extinction is grayer than shortward of 2.8 µm. We find it is not possible to fit the observed extinction curve with a dust model consisting of pure carbonaceous and silicate grains only, and the addition of composite particles, including ices, is needed to explain the observations. Combining a distance dependent extinction with our distance independent extinction we derive the distance to the GC to be R 0 = 7.94±0.65 kpc. Towards Sgr A* (r < 0.5 ′′) we obtain A H = 4.21±0.10, A Ks = 2.42 ± 0.10 and A L ′ = 1.09 ± 0.13.
04/2011;
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F Eisenhauer,
G Perrin,
W Brandner,
C Straubmeier,
K Perraut,
A Amorim,
M Schöller,
S Gillessen,
P Kervella,
M Benisty, [......],
R Davies,
D Rouan,
M Wittkowski, K Dodds-Eden,
D Ziegler,
F Cassaing,
H Bonnet,
M Casali,
R Genzel,
P Lena
The Messenger. 03/2011; 143:16-24.
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G. Trap,
A. Goldwurm, K. Dodds-Eden,
A Weiss,
R. Terrier,
G. Ponti,
S. Gillessen,
R. Genzel,
P. Ferrando,
G. Belanger,
Y. Clenet,
D. Rouan,
P. Predehl,
R. Capelli,
F. Melia,
F. Yusef-Zadeh
[show abstract]
[hide abstract]
ABSTRACT: The radiative counterpart of the supermassive black hole at the Galactic
center (GC), Sgr A*, is subject to frequent flares visible simultaneously in
X-rays and near-infrared (NIR). Often, enhanced radio variability from
centimeter to sub-millimeter wavelengths is observed to follow these X-ray/NIR
eruptions. We present here a multi-wavelength campaign carried out in April
2009, with the aim of characterizing this broadband flaring activity.
Concurrent data from the XMM-Newton/EPIC (2-10 keV), VLT/NACO (2.1 microns, 3.8
microns), APEX/LABOCA (870 microns), and Fermi/LAT (0.1-200 GeV) instruments
are employed to derive light curves and spectral energy distributions of new
flares from Sgr A*. We detected two relatively bright NIR flares both
associated with weak X-ray activity, one of which was followed by a strong
sub-mm outburst 200 min later. Photometric spectral information on a NIR flare
was obtained for the first time with NACO giving a power-law photon index
alpha=-0.4\pm0.3. The first attempt to detect flaring activity from the Fermi
GC source 1FGL J1745.6-2900 is also reported. NIR, X-ray, and sub-mm flares are
finally modeled in the context of non-thermal emission processes. It is found
that the simplest scenario involving a single expanding plasmoid releasing
synchrotron NIR/sub-mm and synchrotron self-Compton X-ray radiation is
inadequate to reproduce the data, but suggestions to reconcile the basic
elements of the theory and the observations are proposed.
02/2011;
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K. Dodds-Eden,
S. Gillessen,
T. K. Fritz,
F. Eisenhauer,
S. Trippe,
R. Genzel,
T. Ott,
H. Bartko,
O. Pfuhl,
G. Bower,
A. Goldwurm,
D. Porquet,
G. Trap,
F. Yusef-Zadeh
[show abstract]
[hide abstract]
ABSTRACT: 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 variable (meaning the source is always `on' and varying) in the near-infrared, and there also appears to be some medium-term variability on timescales of weeks to months. The flux distribution can be described by a lognormal distribution at low intrinsic fluxes (less than about 5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a median flux of ~1.6 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 in the Ks-band, and 40% brighter than the next brightest flare ever observed from Sgr A*. Comment: 16 pages, 9 figures, submitted to ApJ
08/2010;
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[show abstract]
[hide abstract]
ABSTRACT: To understand the correlation and the radiation mechanism of flare emission in different wavelength bands, we have coordinated a number of telescopes to observe SgrA* simultaneously. We focus only on one aspect of the preliminary results of our multi-wavelength observing campaigns, namely, the short time scale variability of emission from SgrA* in near-IR, X-ray and radio wavelengths. The structure function analysis indicate most of the power spectral density is detected on hourly time scales in all wavelength bands. We also report minute time scale variability at 7 and 13mm placing a strong constraint on the nature of the variable emission. The hourly time scale variability can be explained in the context of a model in which the peak frequency of emission shifts toward lower frequencies as a self-absorbed synchrotron source expands adiabatically near the acceleration site. The short time scale variability, on the other hand, places a strong constraint on the size of the emitting region. Assuming that rapid minute time scale fluctuations of the emission is optically thick in radio wavelength, light travel arguments requires relativistic particle energy, thus suggesting the presence of outflow from SgrA*. Comment: 9 pages, 4 figures, The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies ASP Conference Series, 2010 eds: M. Morris, D. Q. Wang and F. Yuan
04/2010;
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K. Dodds-Eden,
D. Porquet,
G. Trap,
E. Quataert,
S. Gillessen,
N. Grosso,
R. Genzel,
A. Goldwurm,
F. Yusef-Zadeh,
S. Trippe,
H. Bartko,
F. Eisenhauer,
T. Ott,
T. K. Fritz,
O. Pfuhl
[show abstract]
[hide abstract]
ABSTRACT: We summarize recent observations and modeling of the brightest Sgr A* flare to be observed simultaneously in (near)-infrared and X-rays to date. Trying to explain the spectral characteristics of this flare through inverse Compton mechanisms implies physical parameters that are unrealistic for Sgr A*. Instead, a "cooling break" synchrotron model provides a more feasible explanation for the X-ray emission. In a magnetic field of about 5-30 Gauss the X-ray emitting electrons cool very quickly on the typical dynamical timescale while the NIR-emitting electrons cool more slowly. This produces a spectral break in the model between NIR and X-ray wavelengths that can explain the differences in the observed spectral indices. Comment: Proceedings of the Galactic Center Workshop 2009, Shanghai
02/2010;
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[show abstract]
[hide abstract]
ABSTRACT: 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, Shanghai
02/2010;
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[show abstract]
[hide abstract]
ABSTRACT: We systematically investigate the error sources for high-precision astrometry from adaptive optics (AO) based near-infrared imaging data. We focus on the application in the crowded stellar field in the Galactic Centre. We show that at the level of ≲100 μas a number of effects are limiting the accuracy. Most important are the imperfectly subtracted seeing haloes of neighbouring stars, residual image distortions and unrecognized confusion of the target source with fainter sources in the background. Further contributors to the error budget are the uncertainty in estimating the point-spread function, the signal-to-noise ratio induced statistical uncertainty, coordinate transformation errors, the chromaticity of refraction in Earth's atmosphere, the post-AO differential tilt jitter and anisoplanatism. For stars as bright as mK= 14, residual image distortions limit the astrometry, for fainter stars the limitation is set by the seeing haloes of the surrounding stars. In order to improve the astrometry substantially at the current generation of telescopes, an AO system with high performance and weak seeing haloes over a relatively small field (r≲ 3 arcsec) is suited best. Furthermore, techniques to estimate or reconstruct the seeing halo could be promising.
Monthly Notices of the Royal Astronomical Society 01/2010; 401(2):1177 - 1188. · 4.90 Impact Factor
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H. Bartko,
F. Martins,
S. Trippe,
T. K. Fritz,
R. Genzel,
T. Ott,
F. Eisenhauer,
S. Gillessen,
T. Paumard,
T. Alexander, [......],
Y. Levin,
L. Mascetti,
S. Nayakshin,
H. B. Perets,
G. Perrin,
O. Pfuhl,
M. J. Reid,
D. Rouan,
M. Zilka,
and A. Sternberg
[show abstract]
[hide abstract]
ABSTRACT: We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early- and late-type stars to mK ≥ 16, more than 2 mag deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O-, and B-stars reside in two strongly warped disks between 08 and 12'' from Sgr A*, as well as a central compact concentration (the S-star cluster) centered on Sgr A*. The later type B-stars (mK >15) in the radial interval between 08 and 12'' seem to be in a more isotropic distribution outside the disks. The observed dearth of late-type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best-fit power law of dN/dm m –0.45± 0.3. WR/O-stars were formed in situ in a single star formation event ~6 Myr ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 08 and of the early-type stars at distances beyond 12'' are compatible with a standard Salpeter/Kroupa IMF (best-fit power law of dN/dm m –2.15± 0.3).
The Astrophysical Journal 12/2009; 708(1):834. · 6.02 Impact Factor
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F. Yusef-Zadeh,
H. Bushouse,
M. Wardle,
C. Heinke,
D. A. Roberts,
C. D. Dowell,
A. Brunthaler,
M. J. Reid,
C. L. Martin,
D. P. Marrone, [......],
N. Grosso, K. Dodds-Eden,
G. C. Bower,
H. Wiesemeyer,
A. Miyazaki,
S. Pal,
S. Gillessen,
A. Goldwurm,
G. Trap,
and H. Maness
[show abstract]
[hide abstract]
ABSTRACT: We report the detection of variable emission from Sgr A* in almost all wavelength bands (i.e., centimeter, millimeter, submillimeter, near-IR, and X-rays) during a multi-wavelength observing campaign. Three new moderate flares are detected simultaneously in both near-IR and X-ray bands. The ratio of X-ray to near-IR flux in the flares is consistent with inverse Compton scattering of near-IR photons by submillimeter emitting relativistic particles which follow scaling relations obtained from size measurements of Sgr A*. We also find that the flare statistics in near-IR wavelengths is consistent with the probability of flare emission being inversely proportional to the flux. At millimeter wavelengths, the presence of flare emission at 43 GHz (7 mm) using the Very Long Baseline Array with milliarcsecond spatial resolution indicates the first direct evidence that hourly timescale flares are localized within the inner 30 × 70 Schwarzschild radii of Sgr A*. We also show several cross-correlation plots between near-IR, millimeter, and submillimeter light curves that collectively demonstrate the presence of time delays between the peaks of emission up to 5 hr. The evidence for time delays at millimeter and submillimeter wavelengths are consistent with the source of emission initially being optically thick followed by a transition to an optically thin regime. In particular, there is an intriguing correlation between the optically thin near-IR and X-ray flare and optically thick radio flare at 43 GHz that occurred on 2007 April 4. This would be the first evidence of a radio flare emission at 43 GHz delayed with respect to the near-IR and X-ray flare emission. The time delay measurements support the expansion of hot self-absorbed synchrotron plasma blob and weaken the hot spot model of flare emission. In addition, a simultaneous fit to 43 and 84 GHz light curves, using an adiabatic expansion model of hot plasma, appears to support a power law rather than a relativistic Maxwellian distribution of particles.
The Astrophysical Journal 10/2009; 706(1):348. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: 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 ApJL
10/2009;
-
G. Trap,
A. Goldwurm,
R. Terrier, K. Dodds-Eden,
S. Gillessen,
R. Genzel,
E. Pantin,
P.O. Lagage,
P. Ferrando,
G. Bélanger,
D. Porquet,
N. Grosso,
F. Yusef-Zadeh,
F. Melia
[show abstract]
[hide abstract]
ABSTRACT: Sagittarius is the supermassive black hole residing at the center of the Milky Way. It has been the main target of an extensive multiwavelength campaign we carried out in April 2007. Herein, we report the detection of a bright flare from the vicinity of the horizon, observed simultaneously in X-rays (XMM-Newton/EPIC) and near infrared (VLT/NACO) on April 4th for 1–2 h. For the first time, such an event also benefitted from a soft γ-rays (INTEGRAL/ISGRI) and mid infrared (VLT/VISIR) coverage, which enabled us to derive upper limits at both ends of the flare spectral energy distribution (SED). We discuss the physical implications of the contemporaneous light curves as well as the SED, in terms of synchrotron, synchrotron self-Compton and external Compton emission processes.
Advances in Space Research. 10/2009;
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H. Bartko,
F. Martins,
S. Trippe,
T. K. Fritz,
R. Genzel,
T. Ott,
F. Eisenhauer,
S. Gillessen,
T. Paumard,
T. Alexander, [......],
Y. Levin,
L. Mascetti,
S. Nayakshin,
H. B. Perets,
G. Perrin,
O. Pfuhl,
M. J. Reid,
D. Rouan,
M. Zilka,
A. Sternberg
[show abstract]
[hide abstract]
ABSTRACT: We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early and late type stars to m_K >= 16, more than 2 magnitudes deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O- and B- stars reside in two strongly warped disks between 0.8" and 12" from SgrA*, as well as a central compact concentration (the S-star cluster) centered on SgrA*. The later type B stars (m_K>15) in the radial interval between 0.8" and 12" seem to be in a more isotropic distribution outside the disks. The observed dearth of late type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best fit power law of dN/dm ~ m^(-0.45+/-0.3). Since at least the WR/O-stars were formed in situ in a single star formation event ~6 Myrs ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 0.8" and of the early-type stars at distances beyond 12" are compatible with a standard Salpeter/Kroupa IMF (best fit power law of dN/dm ~ m^(-2.15+/-0.3)). Comment: ApJ, in press
08/2009;
-
K. Dodds-Eden,
D. Porquet,
G. Trap,
E. Quataert,
X. Haubois,
S. Gillessen,
N. Grosso,
E. Pantin,
H. Falcke,
D. Rouan, [......],
S. Trippe,
P.-O. Lagage,
H. Bartko,
F. Eisenhauer,
T. Ott,
T. Paumard,
G. Perrin,
F. Yuan,
T.K. Fritz,
and L. Mascetti
[show abstract]
[hide abstract]
ABSTRACT: This paper reports measurements of Sgr A* made with NACO in L' band (3.80 μm), Ks band (2.12 μm), and H band (1.66 μm), and with VISIR in N band (11.88 μm) at the ESO VLT, as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 2007 April 4, a very bright flare was observed from Sgr A* simultaneously at L' band and X-ray wavelengths. No emission was detected using VISIR. The resulting spectral energy distribution has a blue slope (β>0 for νL ν νβ, consistent with νL ν ν0.4) between 12 μm and 3.8 μm. For the first time, our high-quality data allow a detailed comparison of infrared (IR) and X-ray light curves with a resolution of a few minutes. The IR and X-ray flares are simultaneous to within 3 minutes. However, the IR flare lasts significantly longer than the X-ray flare (both before and after the X-ray peak), and prominent substructures in the 3.8 μm light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L'-band light curve, we find that the flaring region must be no more than 1.2RS in size. The high X-ray to IR flux ratio, blue νL ν slope MIR to L' band, and the soft νL ν spectral index of the X-ray flare together place strong constraints on possible flare emission mechanisms. We find that it is quantitatively difficult to explain this bright X-ray flare with inverse Compton processes. A synchrotron emission scenario from an electron distribution with a cooling break is a more viable scenario.
The Astrophysical Journal 05/2009; 698(1):676. · 6.02 Impact Factor
