5,998 research outputs found
Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
We have carried out an analysis of the radio and optical properties of a
statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey
of nearby galaxies. We find that the space density of bright galaxies (-22 mag
<= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3}
Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy
types, radio spectra, and radio source sizes are uncorrelated with Seyfert
type, as predicted by the unified schemes for active galaxies. Approximately
half of the detected galaxies have flat or inverted radio spectra, more than
expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found
to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm,
particularly among the galaxies with the weakest nuclear activity. We suggest
that this difference can be accommodated in the unified schemes if a minimum
level of Seyfert activity is required for a radio source to emerge from the
vicinity of the active nucleus. Below this level, Seyfert radio sources might
be suppressed by free-free absorption associated with the nuclear torus or a
compact narrow-line region, thus accounting for both the weakness of the radio
emission and the preponderance of flat spectra. Alternatively, the flat spectra
and weak radio sources might indicate that the weak active nuclei are fed by
advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap
Wide-field LOFAR-LBA power-spectra analyses: Impact of calibration, polarization leakage and ionosphere
Contamination due to foregrounds (Galactic and Extra-galactic), calibration
errors and ionospheric effects pose major challenges in detection of the cosmic
21 cm signal in various Epoch of Reionization (EoR) experiments. We present the
results of a pilot study of a field centered on 3C196 using LOFAR Low Band
(56-70 MHz) observations, where we quantify various wide field and calibration
effects such as gain errors, polarized foregrounds, and ionospheric effects. We
observe a `pitchfork' structure in the 2D power spectrum of the polarized
intensity in delay-baseline space, which leaks into the modes beyond the
instrumental horizon (EoR/CD window). We show that this structure largely
arises due to strong instrumental polarization leakage () towards
{Cas\,A} ( kJy at 81 MHz, brightest source in northern sky), which is
far away from primary field of view. We measure an extremely small ionospheric
diffractive scale ( m at 60 MHz) towards {Cas\,A}
resembling pure Kolmogorov turbulence compared to
km towards zenith at 150 MHz for typical ionospheric conditions. This is one of
the smallest diffractive scales ever measured at these frequencies. Our work
provides insights in understanding the nature of aforementioned effects and
mitigating them in future Cosmic Dawn observations (e.g. with SKA-low and HERA)
in the same frequency window.Comment: 20 pages, 11 figures, accepted for publication in MNRA
A new sample of faint Gigahertz Peaked Spectrum radio sources
The Westerbork Northern Sky Survey (WENSS) has been used to select a sample
of Gigahertz Peaked Spectrum (GPS) radio sources at flux densities one to two
orders of magnitude lower than bright GPS sources investigated in earlier
studies. Sources with inverted spectra at frequencies above 325 MHz have been
observed with the WSRT at 1.4 and 5 GHz and with the VLA at 8.6 and 15 GHz to
select genuine GPS sources. This has resulted in a sample of 47 GPS sources
with peak frequencies ranging from ~500 MHz to >15 GHz, and peak flux densities
ranging from ~40 to ~900 mJy. Counts of GPS sources in our sample as a function
of flux density have been compared with counts of large scale sources from
WENSS scaled to 2 GHz, the typical peak frequency of our GPS sources. The
counts can be made similar if the number of large scale sources at 2 GHz is
divided by 250, and their flux densities increase by a factor of 10. On the
scenario that all GPS sources evolve into large scale radio sources, these
results show that the lifetime of a typical GPS source is ~250 times shorter
than a typical large scale radio source, and that the source luminosity must
decrease by a factor of ~10 in evolving from GPS to large scale radio source.
However, we note that the redshift distributions of GPS and large scale radio
sources are different and that this hampers a direct and straightforward
interpretation of the source counts. Further modeling of radio source evolution
combined with cosmological evolution of the radio luminosity function for large
sources is required.Comment: LaTeX, 15 pages, 8 figs. To be published in A&AS. For more info see
http://www.ast.cam.ac.uk/~snelle
VLA Observations of J1228+441, a Luminous Supernova Remnant in NGC 4449
The luminous, oxygen-rich supernova remnant, J1228+441, is located in the
irregular galaxy NGC 4449 and has been observed at radio wavelengths for thirty
years. An analysis of recent VLA observations of NGC 4449, combined with VLA
archive data and previously published VLA and WSRT observations, yields light
curves at 6 and 20 cm from 1972 to 2002. The light curves at all radio
frequencies exhibit a marked decline in radio emission, confirming past
findings. This paper presents and discusses the radio light curves and spectral
index, alpha, variations from 1972 to 2002 where nu propto nu^(-alpha) and
compares J1228+441 with other radio supernovae. The spectral index of J1228+441
appears to have steepened in the last five years at higher frequencies from
alpha=0.64 +- 0.02 in 1996 to alpha=1.01 +- 0.02 in 2001-2002.Comment: aastex, 19 pages, 6 figures, to be published in A
On the Correlations between Flavour Observables in Minimal U(2)^3 Models
The stringent correlations between flavour observables in models with CMFV
are consistent with the present data except for the correlation Delta
M_{s,d}-epsilon_K. Motivated by the recent work of Barbieri et al, we compare
the CMFV correlations with the ones present in a special class of models with
an approximate global U(2)^3 flavour symmetry, constrained by a minimal set of
spurions governing the breakdown of this symmetry and the assumption that only
SM operators are relevant in flavour physics. This analog of CMFV to be called
MU(2)^3 allows to avoid the Delta M_{s,d}-epsilon_K tension in question because
of reduced flavour symmetry and implied non-MFV contributions to Delta M_{s,d}.
While the patterns of flavour violation in K meson system is the same as in
CMFV models, the CP-violation in B_{s,d} meson systems can deviate from the one
in the SM and CMFV models. We point out a stringent triple S_{psi K_S}-S_{psi
phi}-|V_ub| correlation in this class of models that could in the future
provide a transparent distinction between different MU(2)^3 models and in the
context of these models determine |V_ub| by means of precise measurements of
S_{psi K_S} and S_{psi phi} with only small hadronic uncertainties. For fixed
S_{psi K_S} the correlation between B(B^+ -> tau^+nu_tau) and S_{psi phi}
follows. We also find that MU(2)^3 models could in principle accommodate a
negative value of S_{psi phi}, provided |V_ub| is found to be in the ballpark
of exclusive determinations and the particular MU(2)^3 model provides a 25%
enhancement of epsilon_K. A supersymmetric U(2)^3 model worked out in the
Barbieri-School appears to satisfy these requirements. However if B(B^+ ->
tau^+nu_tau)>1.0 10^{-4} will be confirmed by future experiments only positive
S_{psi phi} is allowed in this framework. We summarize briefly the pattern of
flavour violation in rare K and B_{s,d} decays in MU(2)^3 models.Comment: 28 pages, 6 figures; v2: Few references and discussion on CP
violation in B_s-> mu^+ mu^- added; v3: Several clarifying comments added,
conclusions unchanged, version accepted for publication in JHE
Dynamics and Selection of Giant Spirals in Rayleigh-Benard Convection
For Rayleigh-Benard convection of a fluid with Prandtl number \sigma \approx
1, we report experimental and theoretical results on a pattern selection
mechanism for cell-filling, giant, rotating spirals. We show that the pattern
selection in a certain limit can be explained quantitatively by a
phase-diffusion mechanism. This mechanism for pattern selection is very
different from that for spirals in excitable media
WN1626+5153: A Giant Radio Galaxy from the WENSS survey
An FRII radio source with an angular extension of 18.4 arcminutes has been
discovered in the Westerbork Northern Sky Survey that is being carried out at
325 MHz. This source is identified with Mrk 1498 at indicating that
its projected extension is 1.6 \h50\ Mpc. The optical spectrum of Mrk 1498 has
the typical emission lines of a narrow line radio galaxy; H is the only
permitted line that has a faint broad component. The discovery shows that the
good sensitivity of WENSS at low frequencies to low surface brightness features
at scales of a few minutes, is uniquely suited to provide well defined and
relatively large samples of giant radio galaxies. The future definition of such
samples will be important for our understanding of giant radio galaxies.Comment: 5 pages, 4 figure
Cornering New Physics in b --> s Transitions
We derive constraints on Wilson coefficients of dimension-six effective
operators probing the b --> s transition, using recent improved measurements of
the rare decays Bs --> mu+mu-, B --> K mu+mu- and B --> K* mu+mu- and including
all relevant observables in inclusive and exclusive decays. We consider
operators present in the SM as well as their chirality-flipped counterparts and
scalar operators. We find good agreement with the SM expectations. Compared to
the situation before winter 2012, we find significantly more stringent
constraints on the chirality-flipped coefficients due to complementary
constraints from B --> K mu+mu- and B --> K* mu+mu- and due to the LHCb
measurement of the angular observable S_3 in the latter decay. We also list the
full set of observables sensitive to new physics in the low recoil region of B
--> K* mu+mu-.Comment: 18 pages, 6 figures, 4 tables. v3: typos correcte
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