36 research outputs found
A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*
The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the
existence of a compact emitting component within several Schwarzschild radii,
, of the nucleus at the Galactic Center. This is interesting in view of
the predicted circularized flow within , based on detailed
multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this
unusual object. In this paper, we examine the physics of magnetic field
generation by a Keplerian dynamo subject to the conditions pertaining to Sgr
A*, and show that the sub-mm bump can be produced by thermal synchrotron
emission in this inner region. This spectral feature may therefore be taken as
indirect evidence for the existence of this circularization. In addition, the
self-Comptonization of the sub-mm bump appears to produce an X-ray flux
exceeding that due to bremsstrahlung from this region, which may account for
the X-ray counterpart to Sgr A* discovered recently by {\it Chandra}. However,
the required accretion rate in the Keplerian flow is orders of magnitude
smaller than that predicted by the Bondi-Hoyle simulations. We speculate that
rapid evaporation, in the form of a wind, may ensue from the heating associated
with turbulent mixing of gas elements with large eccentricity as they settle
down into a more or less circular (i.e., low eccentricity) trajectory. The
spectrum of Sgr A* longward of mm may be generated outside of the
Keplerian flow, where the gas is making a transition from a quasi-spherical
infall into a circularized pattern.Comment: 40 pages, 9 figure
Polarized mm And sub-mm Emission From Sgr A* At The Galactic Center
The recent detection of significant linear polarization at mm and sub-mm
wavelengths in the spectrum of Sgr A* (if confirmed) will be a useful probe of
the conditions within several Schwarzschild radii () of the event horizon
at the Galactic Center. Hydrodynamic simulations of gas flowing in the vicinity
of this object suggest that the infalling gas circularizes when it approaches
within of the black hole. We suggest that the sub-mm ``excess'' of
emission seen in the spectrum of Sgr A* may be associated with radiation
produced within the inner Keplerian region and that the observed polarization
characteristics provide direct evidence for this phenomenon. The overall
spectrum from this region, including the high-energy component due to
bremsstrahlung and inverse Compton scattering processes, is at or below the
recent {\it Chandra} measurement, and may account for the X-ray source if it
turns out to be the actual counterpart to Sgr A*.Comment: 12 pages, 2 figures. published in APJ Letter
The simultaneous spectrum of Sgr A* from 20cm to 1mm and the nature of the mm-excess
We report results from a multiwavelength campaign to measure the simultaneous
spectrum of the super-massive black hole candidate Sgr A* in the Galactic
Center from cm to mm-wavelengths using the VLA, BIMA, the Nobeyama 45m, and the
IRAM 30m telescopes. The observations confirm that the previously detected
mm-excess is an intrinsic feature of the spectrum of Sgr A*. The excess can be
interpreted as due to the presence of an ultra-compact component of
relativistic plasma with a size of a few Schwarzschild radii near the black
hole. If so, Sgr A* might offer the unique possibility to image the putative
black hole against the background of this component with future mm-VLBI
experiments.Comment: Apj, in press, (AAS, emulateapj) LaTex, 6 pages, preprint also
available at
http://www.mpifr-bonn.mpg.de/mpivlb/falcke/publications.html#campaig
Advection-Dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center
Sgr A* at the Galactic Center is a puzzling source. It has a mass
M=(2.5+/-0.4) x 10^6 solar masses which makes it an excellent black hole
candidate. Observations of stellar winds and other gas flows in its vicinity
suggest a mass accretion rate approximately few x 10^{-6} solar masses per
year. However, such an accretion rate would imply a luminosity > 10^{40} erg/s
if the radiative efficiency is the usual 10 percent, whereas observations
indicate a bolometric luminosity <10^{37} erg/s. The spectrum of Sgr A* is
unusual, with emission extending over many decades of wavelength. We present a
model of Sgr A* which is based on a two-temperature optically-thin
advection-dominated accretion flow. The model is consistent with the estimated
mass and accretion rate, and fits the observed fluxes in the cm/mm and X-ray
bands as well as upper limits in the sub-mm and infrared bands; the fit is less
good in the radio below 86 GHz and in gamma-rays above 100 MeV. The very low
luminosity of Sgr A* is explained naturally in the model by means of advection.
Most of the viscously dissipated energy is advected into the central mass by
the accreting gas, and therefore the radiative efficiency is extremely low,
approximately 5 x 10^{-6}. A critical element of the model is the presence of
an event horizon at the center which swallows the advected energy. The success
of the model could thus be viewed as confirmation that Sgr A* is a black hole.Comment: 41 pages (Latex) including 6 Figures and 2 Tables. Final Revised
Version changes to text, tables and figures. ApJ, 492, in pres
The Role of Magnetic Field Dissipation in the Black Hole Candidate Sgr A*
The compact, nonthermal radio source Sgr A* at the Galactic Center appears to
be coincident with a 2.6 million solar mass point-like object. Its energy
source may be the release of gravitational energy as gas from the interstellar
medium descends into its potential well. Simple attempts at calculating the
spectrum and flux based on this picture have come close to the observations,
yet have had difficulty in accounting for the low efficiency in this source.
There now appear to be two reasons for this low conversion rate: (1) the plasma
separates into two temperatures, with the protons attaining a significantly
higher temperature than that of the radiating electrons, and (2) the magnetic
field, B, is sub-equipartition, which reduces the magnetic bremsstrahlung
emissivity, and therefore the overall power of Sgr A*. We investigate the
latter with improvement over what has been attempted before: rather than
calculating B based on a presumed model, we instead infer its distribution with
radius empirically with the requirement that the resulting spectrum matches the
observations. Our ansatz for B(r) is motivated in part by earlier calculations
of the expected magnetic dissipation rate due to reconnection in a compressed
flow. We find reasonable agreement with the observed spectrum of Sgr A* as long
as its distribution consists of 3 primary components: an outer equipartition
field, a roughly constant field at intermediate radii (~1000 Schwarzschild
radii), and an inner dynamo (more or less within the last stable orbit for a
non-rotating black hole) which increases B to about 100 Gauss. The latter
component accounts for the observed sub-millimiter hump in this source.Comment: 33 pages including 2 figures; submitted to Ap
Molecules in G1.6-0.025 - 'Hot' Chemistry in the Absence of Star Formation at the Periphery of the Galactic Center Region
We present molecular line mapping of the Giant Molecular Cloud G1.6-0.025,
which is located at the high longitude end of the Central Molecular Zone of our
Galaxy. We assess the degree of star formation activity in that region using
several tracers and find very little. We made a large scale, medium (2')
resolution map in the J = 2-1 transition of SiO for which we find clumpy
emission over a ~0.8 x 0.3 degree-sized region stretching along the Galactic
plane. Toward selected positions we also took spectra in the easy to excite
J_k=2_k-1_k quartet of CH3OH and the CS 2-1 line. Throughout the cloud these
\meth lines are, remarkably, several times stronger than, both, the CS and the
SiO lines. The large widths of all the observed lines, similar to values
generally found in the Galactic center, indicate a high degree of turbulence.
Several high LSR velocity clumps that have 0-80 km/s higher velocities than the
bulk of the molecular cloud appear at the same projected position as "normal"
velocity material; this may indicate cloud-cloud collisions. Statistical
equilibrium modeling of the CH3OH lines observed by us and others yield
relatively high densities and moderate temperatures for a representative dual
velocity position. We find 8 10^4 cm-3/30 K for material in the G1.6-0.025
cloud and a higher temperature (190 K), but a 50% lower density in a high
velocity clump projected on the same location. Several scenarios are discussed
in which shock chemistry might enhance the CH3OH and SiO abundances in
G1.6-0.025 and elsewhere in the Central Molecular Zone.Comment: 51 pages incl. 9 figures and 6 Tables, ApJ (in press
MAMBO 1.2mm observations of luminous starbursts at z~2 in the SWIRE fields
We report on--off pointed MAMBO observations at 1.2 mm of 61 Spitzer-selected
star-forming galaxies from the SWIRE survey. The sources are selected on the
basis of bright 24um fluxes (f_24um>0.4mJy) and of stellar dominated
near-infrared spectral energy distributions in order to favor z~2 starburst
galaxies. The average 1.2mm flux for the whole sample is 1.5+/-0.2 mJy. Our
analysis focuses on 29 sources in the Lockman Hole field where the average
1.2mm flux (1.9+/-0.3 mJy) is higher than in other fields (1.1+/-0.2 mJy). The
analysis of the sources multi-wavelength spectral energy distributions
indicates that they are starburst galaxies with far-infrared luminosities
~10^12-10^13.3 Lsun, and stellar masses of ~0.2-6 x10^11 M_sun. Compared to
sub-millimeter selected galaxies (SMGs), the SWIRE-MAMBO sources are among
those with the largest 24um/millimeter flux ratios. The origin of such large
ratios is investigated by comparing the average mid-infrared spectra and the
stacked far-infrared spectral energy distributions of the SWIRE-MAMBO sources
and of SMGs. The mid-infrared spectra exhibit strong PAH features, and a warm
dust continuum. The warm dust continuum contributes to ~34% of the mid-infrared
emission, and is likely associated with an AGN component. This constribution is
consistent with what is found in SMGs. The large 24um/1.2mm flux ratios are
thus not due to AGN emission, but rather to enhanced PAH emission compared to
SMGs. The analysis of the stacked far-infrared fluxes yields warmer dust
temperatures than typically observed in SMGs. Our selection favors warm
ultra-luminous infrared sources at high-z, a class of objects that is rarely
found in SMG samples. Our sample is the largest Spitzer-selected sample
detected at millimeter wavelengths currently available.Comment: Accepted for publication in ApJ (51 pages; 16 figures). The quality
of some figures has been degraded for arXiv purposes. Full resolution version
available at this
http://www.iasf-milano.inaf.it/~polletta/mambo_swire/lonsdale08_ApJ_accepted.pd
Content analysis of press coverage during the H1N1 influenza pandemic in Germany 2009â2010
Husemann S, Fischer F. Content analysis of press coverage during the H1N1 influenza pandemic in Germany 2009â2010. BMC Public Health. 2015;15(1): 386.Background
The H1N1 influenza pandemic occurred in Germany between April 2009 and August 2010. Pandemics often lead to uncertainty amongst the public and so risk communication on health-related issues is one of the key areas of action for health authorities and other healthcare institutions. The mass media may contribute to risk communication, so this study analysed press coverage during the H1N1 pandemic in Germany.
Methods
A comprehensive analysis of the press coverage during the H1N1 pandemic was conducted in two steps. First, a temporal analysis was carried out of newspaper articles over the entire course of the pandemic, a total of 15,353 articles. The newspaper articles were obtained from the database Nexis. The total number of articles about the influenza pandemic during each individual week was plotted against the number of incident influenza cases during that week. Second, a quantitative content analysis of 140 newspaper articles from selected dates was conducted.
Results
This study indicates that media awareness seems to be strongly related to the actual situation in the pandemic, because changes in the number of infected people were associated with nearly identical changes in the number of newspaper articles. Few articles contained information on the agent of the influenza or support measures. Information on vaccination was included in 32.9% of all articles. Almost half of the articles (48.6%) used case reports. Fear appeals were used in only 10.7% of the newspaper articles; 32.9% of the articles contained the message characteristic âself-efficacyâ.
Conclusions
The newspaper articles that were analysed in the content analysis included different information and message characteristics. The extent of information provided differed during the pandemic. As current research indicates, the use of message characteristics such as fear appeals and self-efficacy, which were also included in the analysed newspaper articles, can help to make health messages effective
Photochemical activation of TRPA1 channels in neurons and animals
Optogenetics is a powerful research tool because it enables high-resolution optical control of neuronal activity. However, current optogenetic approaches are limited to transgenic systems expressing microbial opsins and other exogenous photoreceptors. Here, we identify optovin, a small molecule that enables repeated photoactivation of motor behaviors in wild type animals. Surprisingly, optovin's behavioral effects are not visually mediated. Rather, photodetection is performed by sensory neurons expressing the cation channel TRPA1. TRPA1 is both necessary and sufficient for the optovin response. Optovin activates human TRPA1 via structure-dependent photochemical reactions with redox-sensitive cysteine residues. In animals with severed spinal cords, optovin treatment enables control of motor activity in the paralyzed extremities by localized illumination. These studies identify a light-based strategy for controlling endogenous TRPA1 receptors in vivo, with potential clinical and research applications in non-transgenic animals, including humans