4,532 research outputs found
No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2
The gas cloud G2 falling toward Sagittarius A* (Sgr A*), the supermassive
black hole at the center of the Milky Way, is supposed to provide valuable
information on the physics of accretion flows and the environment of the black
hole. We observed Sgr A* with four European stations of the Global Millimeter
Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when
parts of G2 had already passed the pericenter. We searched for possible
transient asymmetric structure -- such as jets or winds from hot accretion
flows -- around Sgr A* caused by accretion of material from G2. The
interferometric closure phases remained zero within errors during the
observation time. We thus conclude that Sgr A* did not show significant
asymmetric (in the observer frame) outflows in late 2013. Using simulations, we
constrain the size of the outflows that we could have missed to ~2.5 mas along
the major axis, ~0.4 mas along the minor axis of the beam, corresponding to
approximately 232 and 35 Schwarzschild radii, respectively; we thus probe
spatial scales on which the jets of radio galaxies are suspected to convert
magnetic into kinetic energy. As probably less than 0.2 Jy of the flux from Sgr
A* can be attributed to accretion from G2, one finds an effective accretion
rate eta*Mdot < 1.5*10^9 kg/s ~ 7.7*10^-9 Mearth/yr for material from G2.
Exploiting the kinetic jet power--accretion power relation of radio galaxies,
one finds that the rate of accretion of matter that ends up in jets is limited
to Mdot < 10^17 kg/s ~ 0.5 Mearth/yr, less than about 20% of the mass of G2.
Accordingly, G2 appears to be largely stable against loss of angular momentum
and subsequent (partial) accretion at least on time scales < 1 year.Comment: 5 pages, 2 figures, 1 table; A&A Letter, in press (submitted 2015
February 26; accepted 2015 March 31
Dust in 3C324
The results of a deep submillimetre observation using SCUBA of the powerful
radio galaxy 3C324, at redshift z=1.206, are presented. At 850 microns,
emission from the location of the host radio galaxy is marginally detected at
the 4.2 sigma level, 3.01 +/- 0.72 mJy, but there is no detection of emission
at 450 microns to a 3 sigma limit of 21 mJy. A new 32 GHz radio observation
using the Effelsberg 100m telescope confirms that the sub-millimetre signal is
not associated with synchrotron emission. These observations indicate that both
the mass of warm dust within 3C324, and the star formation rate, lie up to an
order of magnitude below the values recently determined for radio galaxies at z
= 3 to 4. The results are compared with dust masses and star formation rates
derived in other ways for 3C324.Comment: 5 pages LaTeX, including 1 figure. Accepted for publication in MNRA
5 year Global 3-mm VLBI survey of Gamma-ray active blazars
The Global mm-VLBI Array (GMVA) is a network of 14 3\,mm and 7\,mm capable
telescopes spanning Europe and the United States, with planned extensions to
Asia. The array is capable of sensitive maps with angular resolution often
exceeding 50\,as. Using the GMVA, a large sample of prominent -ray
blazars have been observed approximately 6 monthly from later 2008 until now.
Combining 3\,mm maps from the GMVA with near-in-time 7\,mm maps from the
VLBA-BU-BLAZAR program and 2\,cm maps from the MOJAVE program, we determine the
sub-pc morphology and high frequency spectral structure of -ray
blazars. The magnetic field strength can be estimated at different locations
along the jet under the assumption of equipartition between magnetic field and
relativistic particle energies. Making assumptions on the jet magnetic field
configuration (e.g. poloidal or toroidal), we can estimate the separation of
the mm-wave "core" and the jet base, and estimate the strength of the magnetic
field there. The results of this analysis show that on average, the magnetic
field strength decreases with a power-law , .
This suggests that on average, the mm-wave "core" is \,pc downstream
of the de-projected jet apex and that the magnetic field strength is of the
order \,kG, broadly consistent with the predictions of
magnetic jet launching (e.g. via magnetically arrested disks (MAD)).Comment: 6 pages, 1 figur
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Long-term variations of the mesospheric wind field at mid-latitudes
Continuous MF radar observations at the station Juliusruh (54.6° N; 13.4° E) have been analysed for the time interval between 1990 and 2005, to obtain information about solar activity-induced variations, as well as long-term trends in the mesospheric wind field. Using monthly median values of the zonal and the meridional prevailing wind components, as well as of the amplitude of the semidiurnal tide, regression analyses have been carried out with a dependence on solar activity and time. The solar activity causes a significant amplification of the zonal winds during summer (increasing easterly winds) and winter (increasing westerly winds). The meridional wind component is positively correlated with the solar activity during summer but during winter the correlation is very small and non significant. Also, the solar influence upon the amplitude of the semidiurnal tidal component is relatively small (in dependence on height partly positive and partly negative) and mostly non-significant.
The derived trends in the zonal wind component during summer are below an altitude of about 83 km negative and above this height positive. During the winter months the trends are nearly opposite compared with the trends in summer (transition height near 86 km). The trends in the meridional wind components are below about 85 km positive in summer (significant) and near zero (nonsignificant) in winter; above this height during both seasons negative trends have been detected. The trends in the semidiurnal tidal amplitude are at all heights positive, but only partly significant.
The detected trends and solar cycle dependencies are compared with other experimental results and model calculations. There is no full agreement between the different results, probably caused by different measuring techniques and evaluation methods used. Also, different heights and observation periods investigated may contribute to the detected differences
Measuring the Virial Temperature of Galactic Halos Through Electron Scattering of Quasar Emission Lines
Semi-analytic models of galaxy formation postulate the existence of
virialized gaseous halos around galaxies at high redshifts. A small fraction of
the light emitted by any high-redshift quasar is therefore expected to scatter
off the free electrons in the halo of its host galaxy. The broadening of the
scattered emission lines of the quasar can be used to measure the temperature
of these electrons. For gas in virial equilibrium, the velocity width of the
scattered line profile is larger by a factor of (m_p/m_e)^{1/2}=43 than the
velocity dispersion of the host galaxy and reaches >10,000 km/s for the massive
galaxies and groups in which bright quasars reside. In these systems, the
scattered width exceeds the intrinsic width of the quasar lines and hence can
be used to measure the virial temperature of the quasar host. The high degree
of polarization of the scattered radiation can help filter out the extended
scattered light from the central emission by the quasar and its host galaxy.
The signal-to-noise ratio of the spectral broadening can be enhanced by
matching the full spectrum of the scattered radiation to a template of the
unscattered quasar spectrum. Although the central fuzz around low-redshift
quasars is dominated by starlight, the fuzz around high-redshift quasars might
be dominated by scattering before galaxies have converted most of their gas
reservoirs into stars.Comment: 10 pages, 1 figures, submitted to ApJ Letter
Reflectivity Anisotropy Spectra of Cu- and Ag- (110) surfaces from {\it ab initio} theory
We are able to disentagle the effects of the intraband and interband parts of
the bulk dielectric function on the bare dielectric anisotropy of the surface.
We show how the position, sign and amplitude of the structures observed in such
spectra depend on the above quantities. The lineshape of all the calculated
structures agree very well with the ones observed experimentally for samples
treated by suitable surface cleaning. In particular, we reproduce the observed
single peak structure of Ag at high energy, found to represent a state of the
clean surface different from the one giving the originally observed double peak
structure. This results is not reproduced by the 'local field' model.Comment: 4 pages, 3 figures. submitted to Phys. Rev. Let
The 1.4 GHz light curve of GRB 970508
We report on Westerbork 1.4 GHz radio observations of the radio counterpart
to -ray burst GRB~970508, between 0.80 and 138 days after this event.
The 1.4 GHz light curve shows a transition from optically thick to thin
emission between 39 and 54 days after the event. We derive the slope of the
spectrum of injected electrons () in two
independent ways which yield values very close to . This is in agreement
with a relativistic dynamically near-adiabatic blast wave model whose emission
is dominated by synchrotron radiation and in which a significant fraction of
the electrons cool fast.Comment: Paper I. Accepted for publication in the Astrophysical Journal
Letter
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