3,621 research outputs found
The giant radio galaxy 8C0821+695 and its environment
We present new VLA and Effelsberg observations of the radio galaxy
8C0821+695. We have obtained detailed images in total intensity and
polarization of this 2 Mpc sized giant. The magnetic field has a configuration
predominantly parallel to the source main axis. We observe Faraday rotation at
low frequencies, most probably produced by an ionized medium external to the
radio source. The spectral index distribution is that typical of FR II radio
galaxies, with spectral indices gradually steepening from the source extremes
towards the core. Modeling the spectrum in the lobes using standard synchrotron
loss models yields the spectral age of the source and the mean velocity of the
jet-head with respect to the lobe material. The existence of a possible
backflow in the lobe is considered to relate spectral with dynamical
determinations of the age and the velocity with respect to the external medium.
Through a very simple model, we obtain a physical characterization of the jets
and the external medium in which the radio galaxy expands. The results in
8C0821+695 are consistent with a relativistic jet nourishing the lobes which
expand in a hot, low density halo. We infer a deceleration of the source
expansion velocity which we explain through a progressive increase in the
hot-spot size.Comment: 11 pages; 8 figures; accepted in A&
Flaring Activity of Sgr A*: Expanding Hot Blobs
Sgr A* is considered to be a massive black hole at the Galactic center and is
known to be variable in radio, millimeter, near-IR and X-rays. Recent
multi-wavelength observing campaigns show a simultaneous X-ray and near-IR
flare, as well as sub-millimeter and near-IR flares from Sgr A*. The flare
activity is thought to be arising from the innermost region of Sgr A*. We have
recently argued that the duration of flares in near-IR and submillimeter
wavelengths implies that the burst of emission expands and cools on a dynamical
time scale before the flares leave Sgr A*. The detection of radio flares with a
time delay in the range of 20 and 40 minutes between 7 and 12mm peak emission
implies adiabatic expansion of a uniform, spherical hot blob due to flare
activity. We suspect that this simple outflow picture shows some of the
characteristics that are known to take place in microquasars, thus we may learn
much from comparative study of Sgr A* and its environment vs. microquasars.Comment: 10 pages, 5 figures, to be published in IV Microquasar Workshop:
Microquasars and Beyond, September 18-22 2006, Como, Ital
Subsynoptic-scale features associated with extreme surface gusts in UK extratropical cyclone events
Numerous studies have addressed the mesoscale features within extratropical cyclones (ETCs) that are responsible for the most destructive winds, though few have utilized surface observation data, and most are based on case studies. By using a 39-station UK surface observation network, coupled with in-depth analysis of the causes of extreme gusts during the period 2008–2014, we show that larger-scale features (warm and cold conveyer belts) are most commonly associated with the top 1% of UK gusts but smaller-scale features generate the most extreme winds. The cold conveyor belt is far more destructive when joining the momentum of the ETC, rather than earlier in its trajectory, ahead of the approaching warm front. Sting jets and convective lines account for two thirds of severe surface gusts in the UK
Simultaneous Chandra, CSO and VLA Observations of Sgr A*: The Nature of Flaring Activity
Sgr A*, the massive black hole at the center of the Galaxy, varies in radio
through X-ray emission on hourly time scales. The flare activity is thought to
arise from the innermost region of an accretion flow onto Sgr A*. We present
simultaneous light curves of Sgr A* in radio, sub-mm and X-rays that show a
possible time delay of 110 minutes between X-ray and 850 m
suggesting that the sub-mm flare emission is optically thick. At radio
wavelengths, we detect time lags of of and 20
minutes between the flare peaks observed at 13 and 7 mm (22 and 43 GHz) in
three different epochs using the VLA. Linear polarization of 1% and
0.7% is detected at 7 and 13 mm, respectively, when averaged over the
entire observation on 2006 July 17. A simple model of a bubble of synchrotron
emitting electrons cooling via adiabatic expansion can explain the time delay
between various wavelengths, the asymmetric shape of the light curves, and the
observed polarization of the flare emission at 43 and 22 GHz. The derived
physical quantities that characterize the emission give an expansion speed of
vc, magnetic field of B10-70 Gauss and particle
spectral index p1-2. These parameters suggest that the associated plasma
cannot escape from Sgr A* unless it has a large bulk motion.Comment: 28 pages, 9 figures, 2 tables, ApJ July 20, 200
The General Supersymmetric Solution of Topologically Massive Supergravity
We find the general fully non-linear solution of topologically massive
supergravity admitting a Killing spinor. It is of plane-wave type, with a null
Killing vector field. Conversely, we show that all solutions with a null
Killing vector are supersymmetric for one or the other choice of sign for the
Chern-Simons coupling constant \mu. If \mu does not take the critical value
\mu=\pm 1, these solutions are asymptotically regular on a Poincar\'e patch,
but do not admit a smooth global compactification with boundary S^1\times\R. In
the critical case, the solutions have a logarithmic singularity on the boundary
of the Poincar\'e patch. We derive a Nester-Witten identity, which allows us to
identify the associated charges, but we conclude that the presence of the
Chern-Simons term prevents us from making a statement about their positivity.
The Nester-Witten procedure is applied to the BTZ black hole.Comment: Minor correction
Magnetic Fields in Quasar Cores II
Multi-frequency polarimetry with the Very Long Baseline Array (VLBA)
telescope has revealed absolute Faraday Rotation Measures (RMs) in excess of
1000 rad/m/m in the central regions of 7 out of 8 strong quasars studied (e.g.,
3C 273, 3C 279, 3C 395). Beyond a projected distance of ~20 pc, however, the
jets are found to have |RM| < 100 rad/m/m. Such sharp RM gradients cannot be
produced by cluster or galactic-scale magnetic fields, but rather must be the
result of magnetic fields organized over the central 1-100 pc. The RMs of the
sources studied to date and the polarization properties of BL Lacs, quasars and
galaxies are shown to be consistent so far with the predictions of unified
schemes. The direct detection of high RMs in these quasar cores can explain the
low fractional core polarizations usually observed in quasars at centimeter
wavelengths as the result of irregularities in the Faraday screen on scales
smaller than the telescope beam. Variability in the RM of the core is reported
for 3C 279 between observations taken 1.5 years apart, indicating that the
Faraday screen changes on that timescale, or that the projected superluminal
motion of the inner jet components samples a new location in the screen with
time. Either way, these changes in the Faraday screen may explain the dramatic
variability in core polarization properties displayed by quasars.Comment: Accepted to the ApJ. 27 pages, 9 figures including figure 6 in colo
Implications of a High Angular Resolution Image of the Sunyaev-Zel'dovich Effect in RXJ1347-1145
The most X-ray luminous cluster known, RXJ1347-1145 (z=0.45), has been the
object of extensive study across the electromagnetic spectrum. We have imaged
the Sunyaev-Zel'dovich Effect (SZE) at 90 GHz (3.3 mm) in RXJ1347-1145 at 10"
resolution with the 64-pixel MUSTANG bolometer array on the Green Bank
Telescope (GBT), confirming a previously reported strong, localized enhancement
of the SZE 20" to the South-East of the center of X-ray emission. This
enhancement of the SZE has been interpreted as shock-heated (> 20 keV) gas
caused by an ongoing major (low mass-ratio) merger event. Our data support this
interpretation. We also detect a pronounced asymmetry in the projected cluster
pressure profile, with the pressure just east of the cluster core ~1.6 times
higher than just to the west. This is the highest resolution image of the SZE
made to date.Comment: 9 pages, 7 figures; accepted for publication in The Astrophysical
Journa
A View through Faraday's Fog 2: Parsec Scale Rotation Measures in 40 AGN
Results from a survey of the parsec scale Faraday rotation measure properties
for 40 quasars, radio galaxies and BL Lac objects are presented. Core rotation
measures for quasars vary from approximately 500 to several thousand radians
per meter squared. Quasar jets have rotation measures which are typically 500
radians per meter squared or less. The cores and jets of the BL Lac objects
have rotation measures similar to those found in quasar jets. The jets of radio
galaxies exhibit a range of rotation measures from a few hundred radians per
meter squared to almost 10,000 radians per meter squared for the jet of M87.
Radio galaxy cores are generally depolarized, and only one of four radio
galaxies (3C-120) has a detectable rotation measure in the core. Several
potential identities for the foreground Faraday screen are considered and we
believe the most promising candidate for all the AGN types considered is a
screen in close proximity to the jet. This constrains the path length to
approximately 10 parsecs, and magnetic field strengths of approximately 1
microGauss can account for the observed rotation measures. For 27 out of 34
quasars and BL Lacs their optically thick cores have good agreement to a lambda
squared law. This requires the different tau = 1 surfaces to have the same
intrinsic polarization angle independent of frequency and distance from the
black hole.Comment: Accepted to the Astrophysical Journal: 71 pages, 40 figure
90GHz and 150GHz observations of the Orion M42 region. A sub-millimeter to radio analysis
We have used the new 90GHz MUSTANG camera on the Robert C. Byrd Green Bank
Telescope (GBT) to map the bright Huygens region of the star-forming region M42
with a resolution of 9" and a sensitivity of 2.8mJy/beam. 90GHz is an
interesting transition frequency, as MUSTANG detects both the free-free
emission characteristic of the HII region created by the Trapezium stars,
normally seen at lower frequencies, and thermal dust emission from the
background OMC1 molecular cloud, normally mapped at higher frequencies. We also
present similar data from the 150GHz GISMO camera taken on the IRAM telescope.
This map has 15" resolution. By combining the MUSTANG data with 1.4, 8, and
21GHz radio data from the VLA and GBT, we derive a new estimate of the emission
measure (EM) averaged electron temperature of Te = 11376K by an original method
relating free-free emission intensities at optically thin and optically thick
frequencies. Combining ISO-LWS data with our data, we derive a new estimate of
the dust temperature and spectral emissivity index within the 80" ISO-LWS beam
toward OrionKL/BN, Td = 42K and Beta=1.3. We show that both Td and Beta
decrease when going from the HII region and excited OMC1 interface to the
denser UV shielded part of OMC1 (OrionKL/BN, Orion S). With a model consisting
of only free-free and thermal dust emission we are able to fit data taken at
frequencies from 1.5GHz to 854GHz.Comment: 18 pages, 8 figures, submitted to the Astrophysical Journa
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