1,874 research outputs found
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 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
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
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&
A portable, modular, self-contained recirculating chamber to measure benthic processes under controlled water velocity
Citation: Ruegg, J., Brant, J. D., Larson, D. M., Trentman, M. T., & Dodds, W. K. (2015). A portable, modular, self-contained recirculating chamber to measure benthic processes under controlled water velocity. Freshwater Science, 34(3), 831-844. doi:10.1086/682328We report the design, construction, and functional characteristics of a sealable, portable chamber for measuring benthic metabolic process rates, particularly those under unidirectional flow as found in streams. The design optimizes inherent tradeoffs, such as size, stability, and cost, associated with chambers built for field-based measurements. The chamber is small enough to be portable and minimizes the water-volume: benthic surface-area ratio. In addition, the chamber is clear to allow measurement of photosynthetic rates. The design minimizes power draw to sustain water velocities found at stream field sites and is modular to allow easy disassembly and cleaning. The design is relatively simple, thereby increasing sturdiness, minimizing construction costs, and decreasing the expertise required to build the unit. We demonstrated the performance characteristics, specifically amperage needed to achieve desired water velocity, flow heterogeneity and turbulence in the working area, the degree of isolation from atmosphere, mixing rate of solute injectate, and heating rate of the chamber. We provide proof of concept with data for in situ benthic rates (gross community production, community respiration, and NH4+ uptake). Publications on metabolic chambers built for in situ use do not typically report performance characteristics, so it is difficult to compare our design to existing literature. We include chamber characteristics to clarify the advantages and limitations of benthic rates measured in such chambers
Clinical Spectrum of HLA-B27-associated Ocular Inflammation
Human leukocyte antigen (HLA)-B27-associated anterior uveitis (AU) is the most commonly diagnosed form of AU and represents the largest entity of non-infectious uveitis around the world. The most typical ocular manifestation associated with HLA-B27 consists of unilateral AU of acute onset. The HLA-B27-associated acute AU represents a distinct clinical entity occurring typically in young adults between the ages of 20 and 40 years. HLA-B27-associated acute AU is typically unilateral and lasts usually several weeks and diminishes within 3 months in the majority of patients. The anterior chamber shows typically severe cellular reaction and flare, as well as a fibrinous exudate. Frequently, posterior synechiae are formed and occasionally hypopyon is present. The pattern of the disease is recurrent with a full remission between the attacks. Intraocular pressure during active periods is typically low due to inflammation of ciliary body and decreased aqueous production. Less typical presentations are also recognized and include the development of chronic inflammation, posterior segmen
Geometry of River Networks I: Scaling, Fluctuations, and Deviations
This article is the first in a series of three papers investigating the
detailed geometry of river networks. Large-scale river networks mark an
important class of two-dimensional branching networks, being not only of
intrinsic interest but also a pervasive natural phenomenon. In the description
of river network structure, scaling laws are uniformly observed. Reported
values of scaling exponents vary suggesting that no unique set of scaling
exponents exists. To improve this current understanding of scaling in river
networks and to provide a fuller description of branching network structure, we
report here a theoretical and empirical study of fluctuations about and
deviations from scaling. We examine data for continent-scale river networks
such as the Mississippi and the Amazon and draw inspiration from a simple model
of directed, random networks. We center our investigations on the scaling of
the length of sub-basin's dominant stream with its area, a characterization of
basin shape known as Hack's law. We generalize this relationship to a joint
probability density and show that fluctuations about scaling are substantial.
We find strong deviations from scaling at small scales which can be explained
by the existence of linear network structure. At intermediate scales, we find
slow drifts in exponent values indicating that scaling is only approximately
obeyed and that universality remains indeterminate. At large scales, we observe
a breakdown in scaling due to decreasing sample space and correlations with
overall basin shape. The extent of approximate scaling is significantly
restricted by these deviations and will not be improved by increases in network
resolution.Comment: 16 pages, 13 figures, Revtex4, submitted to PR
Phase I study of TP300 in patients with advanced solid tumors with pharmacokinetic, pharmacogenetic and pharmacodynamic analyses
Background: A Phase I dose escalation first in man study assessed maximum tolerated dose (MTD), dose-limiting toxicity (DLT) and recommended Phase II dose of TP300, a water soluble prodrug of the Topo-1 inhibitor TP3076, and active metabolite, TP3011.
<p/>Methods: Eligible patients with refractory advanced solid tumors, adequate performance status, haematologic, renal, and hepatic function. TP300 was given as a 1-hour i.v. infusion 3-weekly and pharmacokinetic (PK) profiles of TP300, TP3076 and TP3011 were analysed. Polymorphisms in CYP2D6, AOX1 and UGT1A1 were studied and DNA strand-breaks measured in peripheral blood mononuclear cells (PBMCs).
<p/>Results: 32 patients received TP300 at 1, 2, 4, 6, 8, 10, 12 mg/m2. MTD was 10 mg/m2; DLTs at 12 (2/4 patients) and 10 mg/m2 (3/12) included thrombocytopenia and febrile neutropenia; diarrhea was uncommon. Six patients (five had received irinotecan), had stable disease for 1.5-5 months. TP3076 showed dose proportionality in AUC and Cmax from 1--10 mg/m2. Genetic polymorphisms had no apparent influence on exposure. DNA strand-breaks were detected after TP300 infusion.
<p/>Conclusions: TP300 had predictable hematologic toxicity, and diarrhea was uncommon. AUC at MTD is substantially greater than for SN38. TP3076 and TP3011 are equi-potent with SN38, suggesting a PK advantage
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