We present a detailed analysis of high-resolution near-infrared imaging and spectroscopy of the potential star cluster IRS13E very close to the massive black hole in the Galactic center. We detect 19 ...objects in IRS13E from Ks-band images, 15 of which are also detected reliably in the H band. We derive consistent proper motions for these objects from the two bands. Most objects share a similar westward proper motion. We characterize the objects using spectroscopy (1.45-2.45 mum) and (narrowband) imaging from the H (1.66 mum) to the L' band (3.80 mum). Nine of the objects detected in both the Ks and H bands are very red, and we find that they are all consistent with being warm dust clumps. The dust emission may be caused by the colliding winds of the two Wolf-Rayet stars in the cluster. Three of the six detected stars do not share the motion or spectral properties of the three bright stars. This leaves only the three bright, early-type stars as potential cluster members. It is unlikely that these stars are a chance configuration. Assuming the presence of an intermediate mass black hole (IMBH), a mass of about 14,000 M sun follows from the velocities and positions of these three stars. However, our acceleration limits make such an IMBH nearly as unlikely as a chance occurrence of such a star association. Furthermore, there is no variable X-ray source in IRS13E despite the high density of dust and gas. Therefore, we conclude that is unlikely that IRS13E hosts a black hole massive enough to bind the three stars.
Using 25 years of data from uninterrupted monitoring of stellar orbits in the Galactic Center, we present an update of the main results from this unique data set: a measurement of mass and distance ...to Sgr A*. Our progress is not only due to the eight-year increase in time base, but also to the improved definition of the coordinate system. The star S2 continues to yield the best constraints on the mass of and distance to Sgr A*; the statistical errors of 0.13 × 10 6 M and 0.12 kpc have halved compared to the previous study. The S2 orbit fit is robust and does not need any prior information. Using coordinate system priors, the star S1 also yields tight constraints on mass and distance. For a combined orbit fit, we use 17 stars, which yields our current best estimates for mass and distance: M = 4.28 0.10 stat . 0.21 sys × 10 6 M and R 0 = 8.32 0.07 stat . 0.14 sys kpc . These numbers are in agreement with the recent determination of R0 from the statistical cluster parallax. The positions of the mass, of the near-infrared flares from Sgr A*, and of the radio source Sgr A* agree to within 1 mas. In total, we have determined orbits for 40 stars so far, a sample which consists of 32 stars with randomly oriented orbits and a thermal eccentricity distribution, plus eight stars that we can explicitly show are members of the clockwise disk of young stars, and which have lower-eccentricity orbits.
We derive new constraints on the mass, rotation, orbit structure, and statistical parallax of the Galactic old nuclear star cluster and the mass of the supermassive black hole. We combine star counts ...and kinematic data from Fritz et al., including 2500 line-of-sight velocities and 10 000 proper motions obtained with VLT instruments. We show that the difference between the proper motion dispersions σ
l
and σ
b
cannot be explained by rotation, but is a consequence of the flattening of the nuclear cluster. We fit the surface density distribution of stars in the central 1000 arcsec by a superposition of a spheroidal cluster with scale ∼100 arcsec and a much larger nuclear disc component. We compute the self-consistent two-integral distribution function f(E, L
z
) for this density model, and add rotation self-consistently. We find that (i) the orbit structure of the f(E, L
z
) gives an excellent match to the observed velocity dispersion profiles as well as the proper motion and line-of-sight velocity histograms, including the double-peak in the v
l
-histograms. (ii) This requires an axial ratio near q
1 = 0.7 consistent with our determination from star counts, q
1 = 0.73 ± 0.04 for r < 70 arcsec. (iii) The nuclear star cluster is approximately described by an isotropic rotator model. (iv) Using the corresponding Jeans equations to fit the proper motion and line-of-sight velocity dispersions, we obtain best estimates for the nuclear star cluster mass, black hole mass, and distance M
*(r < 100 arcsec) = (8.94 ± 0.31|stat ± 0.9|syst) × 106 M⊙, M
• = (3.86 ± 0.14|stat ± 0.4|syst) × 106 M⊙, and R
0 = 8.27 ± 0.09|stat ± 0.1|syst kpc, where the estimated systematic errors account for additional uncertainties in the dynamical modelling. (v) The combination of the cluster dynamics with the S-star orbits around Sgr A* strongly reduces the degeneracy between black hole mass and Galactic Centre distance present in previous S-star studies. A joint statistical analysis with the results of Gillessen et al., gives M
• = (4.23 ± 0.14) × 106 M⊙ and R
0 = 8.33 ± 0.11 kpc.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted)We obtain the basic properties of the nuclear cluster of the Milky Way. First, we investigate the structural properties by constructing ...a stellar density map of the central 1000'' using extinction-corrected old star counts from VISTA, WFC3/IR, and VLT/NACO data. We describe the data using two components. The inner, slightly flattened (axis ratio of q= 0.80 + or - 0.04) 7 component is the nuclear cluster, while the outer component corresponds to the stellar component of the circumnuclear zone. For the nuclear cluster, we measure a half-light radius of 178 + or - 0.04 51" thickapproximate + or - 2 pc and a luminosity of M sub(ks)= - 16.0 + or - 0.5. Second, we measure detailed dynamics out to 4 pc. We obtain 10,351 proper motions from AO data, and 2513 radial velocities from VLT/SINFONI data. We determine the cluster mass by means of isotropic spherical Jeans modeling. We fix the distance to the Galactic Center and the mass of the supermassive black hole. We model the cluster either with a constant M/L or with a power law. For the latter case, we obtain a slope of 1.18 + or - 0.06. We get a cluster mass within 100'' of ... for both modeling approaches. A model which includes the observed flattening gives a 47% larger mass (see Chatzopoulos et al.). Our results slightly favor a core over a cusp in the mass profile. By minimizing the number of unbound stars within 8'', we obtain a distance of ... kpc when using an R sub(0) supermassive black hole mass relation from stellar orbits. Combining our results, we obtain ... , which is roughly consistent with a Chabrier IMF.
Two recent papers (Ghez et al. 2008; Gillessen et al. 2009) have estimated the mass of and the distance to the massive black hole (MBH) 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 we allow 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. The so-combined data set allows for a moderate improvement of the statistical errors of the mass of and the 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 R{sub 0} = 8.28 +- 0.15|{sub stat} +- 0.29|{sub sys} kpc and M{sub MBH} = 4.30 +- 0.20|{sub stat} +- 0.30|{sub sys} x 10{sup 6} M{sub sun} as best estimates from a multi-star fit.
Abstract
We present the first fully simultaneous fits to the near-infrared (NIR) and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A*, the supermassive black hole at ...the Milky Way's centre. Our study arises from ambitious multiwavelength monitoring campaigns with XMM–Newton, NuSTAR and SINFONI. The average multiwavelength spectrum is well reproduced by a broken power law with ΓNIR = 1.7 ± 0.1 and ΓX = 2.27 ± 0.12. The difference in spectral slopes (ΔΓ = 0.57 ± 0.09) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about 103 s, when a very steep X-ray spectrum (ΔΓ = 1.8 ± 0.4) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high-energy cut-off in the electron distribution (γmax) induces an initial cut-off in the optical–UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of γmax. We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from B ∼ 30 ± 8 G to B ∼ 4.8 ± 1.7 G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A*'s flaring emission.
The Galactic Center black hole Sgr A* is the archetypical example of an underfed massive black hole. The extremely low accretion rate can be understood in radiatively inefficient accretion flow ...models. Testing those models has proven to be difficult due to the lack of suitable probes. Radio and submillimeter polarization measurements constrain the flow very close to the event horizon. X-ray observations resolving the Bondi radius yield an estimate roughly four orders of magnitude further out. Here, we present a new, indirect measurement of the accretion flow density at intermediate radii. We use the dynamics of the gas cloud G2 to probe the ambient density. We detect the presence of a drag force slowing down G2 with a statistical significance of 9 . This probes the accretion flow density at around 1000 Schwarzschild radii and yields a number density of 4 × 103 cm−3. Self-similar accretion models where the density follows a power-law radial profile between the inner zone and the Bondi radius have predicted similar values.
The broadening of atomic emission lines by high-velocity motion of gas near accreting supermassive black holes is an observational hallmark of quasars
. Observations of broad emission lines could ...potentially constrain the mechanism for transporting gas inwards through accretion disks or outwards through winds
. The size of regions for which broad emission lines are observed (broad-line regions) has been estimated by measuring the delay in light travel time between the variable brightness of the accretion disk continuum and the emission lines
-a method known as reverberation mapping. In some models the emission lines arise from a continuous outflow
, whereas in others they arise from orbiting gas clouds
. Directly imaging such regions has not hitherto been possible because of their small angular size (less than 10
arcseconds
). Here we report a spatial offset (with a spatial resolution of 10
arcseconds, or about 0.03 parsecs for a distance of 550 million parsecs) between the red and blue photo-centres of the broad Paschen-α line of the quasar 3C 273 perpendicular to the direction of its radio jet. This spatial offset corresponds to a gradient in the velocity of the gas and thus implies that the gas is orbiting the central supermassive black hole. The data are well fitted by a broad-line-region model of a thick disk of gravitationally bound material orbiting a black hole of 3 × 10
solar masses. We infer a disk radius of 150 light days; a radius of 100-400 light days was found previously using reverberation mapping
. The rotation axis of the disk aligns in inclination and position angle with the radio jet. Our results support the methods that are often used to estimate the masses of accreting supermassive black holes and to study their evolution over cosmic time.
We study the young S-stars within a distance of 0.04 pc from the supermassive black hole in the center of our Galaxy. Given how inhospitable the region is for star formation, their presence is more ...puzzling the younger we estimate their ages. In this study, we analyze the result of 12 years of high-resolution spectroscopy within the central arcsecond of the Galactic Center (GC). By co-adding between 55 and 105 hr of spectra we have obtained high signal-to-noise H- and K-band spectra of eight stars orbiting the central supermassive black hole. Using deep H-band spectra, we show that these stars must be high surface gravity (dwarf) stars. We compare these deep spectra to detailed model atmospheres and stellar evolution models to infer the stellar parameters. Our analysis reveals an effective temperature of 21,000-28,500 K, a rotational velocity of 60-170 km s−1, and a surface gravity of 4.1-4.2. These parameters imply a spectral type of B0-B3V for these stars. The inferred masses lie within 8-14 . We derive an age of Myr for the star S2, which is compatible with the age of the clockwise-rotating young stellar disk in the GC. We estimate the ages of all other studied S-stars to be less than 15 Myr, which is compatible with the age of S2 within the uncertainties. The relatively low ages for these S-stars favor a scenario in which the stars formed in a local disk rather than a field binary-disruption scenario that occurred over a longer period of time.
We present new observations of the recently discovered gas cloud G2 currently falling toward the massive black hole in the Galactic Center. The new data confirm that G2 is on a highly elliptical ...orbit with a predicted pericenter passage mid-2013. The updated orbit has an even larger eccentricity of 0.966, an epoch of pericenter two months later than estimated before, and a nominal minimum distance of 2200 Schwarzschild radii only. The velocity gradient of G2 has developed further to 600 km s super(-1) FWHM in summer 2012. We also detect the tail of similar total flux and on the same orbit as G2 along the trajectory at high significance. No hydrodynamic effects are detected yet, since the simple model of a tidally shearing gas cloud still describes the data very well. The flux of G2 has not changed by more than 10% between 2008 and 2012, disfavoring models where additional gas from a reservoir is released to the disrupting diffuse gas component.
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