2,528 research outputs found
Inertial Range Scaling, Karman-Howarth Theorem and Intermittency for Forced and Decaying Lagrangian Averaged MHD in 2D
We present an extension of the Karman-Howarth theorem to the Lagrangian
averaged magnetohydrodynamic (LAMHD-alpha) equations. The scaling laws
resulting as a corollary of this theorem are studied in numerical simulations,
as well as the scaling of the longitudinal structure function exponents
indicative of intermittency. Numerical simulations for a magnetic Prandtl
number equal to unity are presented both for freely decaying and for forced two
dimensional MHD turbulence, solving directly the MHD equations, and employing
the LAMHD-alpha equations at 1/2 and 1/4 resolution. Linear scaling of the
third-order structure function with length is observed. The LAMHD-alpha
equations also capture the anomalous scaling of the longitudinal structure
function exponents up to order 8.Comment: 34 pages, 7 figures author institution addresses added magnetic
Prandtl number stated clearl
The Radio Afterglow and Host Galaxy of the Dark GRB 020819
Of the fourteen gamma-ray bursts (GRBs) localized to better than 2' radius
with the SXC on HETE-2, only two lack optical afterglow detections, and the
high recovery rate among this sample has been used to argue that the fraction
of truly dark bursts is ~10%. While a large fraction of earlier dark bursts can
be explained by the failure of ground-based searches to reach appropriate
limiting magnitudes, suppression of the optical light of these SXC dark bursts
seems likely. Here we report the discovery and observation of the radio
afterglow of GRB 020819, an SXC dark burst, which enables us to identify the
likely host galaxy (probability of 99.2%) and hence the redshift (z=0.41) of
the GRB. The radio light curve is qualitatively similar to that of several
other radio afterglows, and may include an early-time contribution from the
emission of the reverse shock. The proposed host is a bright R = 19.5 mag
barred spiral galaxy, with a faint R ~ 24.0 mag "blob'' of emission, 3" from
the galaxy core (16 kpc in projection), that is coincident with the radio
afterglow. Optical photometry of the galaxy and blob, beginning 3 hours after
the burst and extending over more than 100 days, establishes strong upper
limits to the optical brightness of any afterglow or associated supernova.
Combining the afterglow radio fluxes and our earliest R-band limit, we find
that the most likely afterglow model invokes a spherical expansion into a
constant-density (rather than stellar wind-like) external environment; within
the context of this model, a modest local extinction of A_V ~ 1 mag is
sufficient to suppress the optical flux below our limits.Comment: 7 pages, 2 figures. ApJ, in press. For more info on dark bursts, see
http://www.astro.ku.dk/~pallja/dark.htm
Spitzer Observations of the New Luminous Red Nova M85OT2006-1
M85OT2006-1 is the latest and most brilliant addition to the small group of
known Luminous Red Novae (LRNe). An identifying characteristic of the
previously detected events (M31 RV, V4322 Sgr & V838 Mon) was a spectral
red-ward evolution connected with an emerging infrared component following the
optical decay. Here we report on the discovery of a similar feature in
Keck/NIRC and Spitzer photometry of M85OT2006-1 six months post-eruption. We
find that its 2.1-22 micrometer spectral energy distribution is best described
by a black body with effective temperature T_eff=950+-150 K and bolometric
luminosity L=2.9+-0.5x10^5 L_sun. Assuming spherical geometry, the black body
effective radius, R=2.0+-0.5x10^4 R_sun, and corresponding expansion velocity,
v=870+-260 km/s, are remarkably similar to the properties of M31 RV 70 days
after its eruption. Furthermore, we propose a search strategy for LRNe in the
local Universe making use of the longevity of their infrared excess emission
and discuss the expected number of events in the Spitzer Infrared Nearby
Galaxies Survey.Comment: manuscript after minor revisions, accepted for Ap
New search strategy for high z intervening absorbers: GRB021004, a pilot study
We present near-infrared narrow- and broad-band imaging of the field of
GRB021004, performed with ISAAC on the UT1 of the ESO Very Large Telescope. The
narrow-band filters were chosen to match prominent emission lines at the
redshift of the absorption-line systems found against the early-time afterglow
of GRB021004: [OIII] at z=1.38 and Halpha at z=1.60, respectively. For the
z=1.38 system we find an emission-line source at an impact parameter of 16",
which is somewhat larger than the typical impact parameters of a sample of MgII
absorbers at redshifts around unity. Assuming that this tentative
redshift-identification is correct, the star formation rate of the galaxy is 13
+- 2 Msun/year. Our study reaches star-formation rate limits (5 sigma) of 5.7
Msun/year at z=1.38, and 7.7 Msun/year at z=1.60. These limits correspond to a
depth of roughly 0.13 L*. Any galaxy counterpart of the absorbers nearer to the
line of sight either has to be fainter than this limit or not be an
emission-line source.Comment: 4 pages, 3 figures, accepted for publication in A&A letter
Recommended from our members
The Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS)
The CLASS experiment on Chandrayaan-2, the second Indian lunar mission, aims tomap the abundance of the major rock forming elements on the lunar surface using the technique of X-ray fluorescence during solar flare events. CLASS is a continuation of the successful C1XS [1] XRF experiment on Chandrayaan-1. CLASS is designed to provide lunar mapping of elemental abundances with a nominal spatial resolution of 25 km (FWHM) from a 200 km polar, circular orbit of Chandrayaan-2
Correlations between morphology, crystal structure and magnetization of epitaxial cobalt-platinum films grown with pulsed laser ablation
The effects of growth rate (G_r), deposition temperature (T_d), film
thickness (t_F), and substrate induced strain (epsilon) on morphological,
crystallographic and magnetic characteristics of equiatomic CoPt epitaxial
films synthesized with PLD are investigated. The (001) substrates of MgO, STO
and LAO provide different degree of epitaxial strain for growth of the
disordered face centered cubic (fcc) and ordered face centered tetragonal
(L1_0) phases of CoPt. The films deposited at T_d~600 ^0C on all three
substrates are fcc with in-plane magnetization and a narrow hysteresis loop of
width~200 Oe. The L1_0 phase, stabilized only at T_d~700 ^0C becomes
predominantly c-axis oriented as T_d is increased to 800 ^0C. While the
crystallographic structure of the films depends solely on the T_d, their
microstructure and magnetization characteristics are decided by the growth
rate. At the higher G_r (~1A/sec) the L1_0 films have a maze-like structure
which converts to a continuous film as the t_F is increased from 20 to 50 nm.
The H_c of these films increases as the L1_0 phase fraction grows with T_d and
its orientation becomes out of the film plane. The evolution of microstructure
with T_d is remarkably different at lower growth rate (~0.4A /sec). Here the
structure changes from a self-similar fractal pattern to an assembly of
nano-dots as the T_d is raised from 700 to 800 ^0C, and is understood in terms
of the imbalance between strain and interfacial energies. MFM of such films
reveals no distinct domain walls within the nano-islands while a clear contrast
is seen between the islands of reversed magnetization. The simple picture of
coherent rotation of moment appears incompatible with the time dependence of
the remanent magnetization in these films.Comment: 10 figure
Anomalies and Hawking radiation from the Reissner-Nordstr\"om black hole with a global monopole
We extend the work by S. Iso, H. Umetsu and F. Wilczek [Phys. Rev. Lett. 96
(2006) 151302] to derive the Hawking flux via gauge and gravitational anomalies
of a most general two-dimensional non-extremal black hole space-time with the
determinant of its diagonal metric differing from the unity () and use it to investigate Hawking radiation from the Reissner-Nordstrom
black hole with a global monopole by requiring the cancellation of anomalies at
the horizon. It is shown that the compensating energy momentum and gauge fluxes
required to cancel gravitational and gauge anomalies at the horizon are
precisely equivalent to the -dimensional thermal fluxes associated with
Hawking radiation emanating from the horizon at the Hawking temperature. These
fluxes are universally determined by the value of anomalies at the horizon.Comment: 18 pages, 0 figure. 1 footnote and 4 new reference adde
Canonical Particle Acceleration in FRI Radio Galaxies
Matched resolution multi-frequency VLA observations of four radio galaxies
are used to derive the asymptotic low energy slope of the relativistic electron
distribution. Where available, low energy slopes are also determined for other
sources in the literature. They provide information on the acceleration physics
independent of radiative and other losses, which confuse measurements of the
synchrotron spectra in most radio, optical and X-ray studies. We find a narrow
range of inferred low energy electron energy slopes, n(E)=const*E^-2.1 for the
currently small sample of lower luminosity sources classified as FRI (not
classical doubles). This distribution is close to, but apparently inconsistent
with, the test particle limit of n(E)=const*E^-2.0 expected from strong
diffusive shock acceleration in the non-relativistic limit. Relativistic shocks
or those modified by the back-pressure of efficiently accelerated cosmic rays
are two alternatives to produce somewhat steeper spectra. We note for further
study the possiblity of acceleration through shocks, turbulence or shear in the
flaring/brightening regions in FRI jets as they move away from the nucleus.
Jets on pc scales and the collimated jets and hot spots of FRII (classical
double) sources would be governed by different acceleration sites and
mechanisms; they appear to show a much wider range of spectra than for FRI
sources.Comment: 16 figures, including 5 color. Accepted to Astrophysical Journa
Velocity-Metallicity Correlation for high-z DLA Galaxies: Evidence for a Mass-Metallicity Relation?
We used our database of VLT-UVES quasar spectra to build up a sample of 70
Damped Lyman-alpha (DLA) or strong sub-DLA systems with total neutral hydrogen
column densities of log N(HI)>20 and redshifts in the range 1.7<z_abs<4.3. For
each of the systems, we measured in an homogeneous manner the metallicities
relative to Solar, [X/H] (with X=Zn, or S or Si), and the velocity widths of
low-ionization line profiles, Delta V. We provide for the first time evidence
for a correlation between DLA metallicity and line profile velocity width,
which is detected at the 6.1sigma significance level. This confirms the trend
previously observed in a much smaller sample by Wolfe & Prochaska (1998). The
best-fit linear relation is [X/H]=1.55(\pm 0.12) log Delta V -4.33(\pm 0.23)
with Delta V expressed in km/s. The slope of the DLA velocity-metallicity
relation is the same within uncertainties between the higher (z_abs>2.43) and
the lower (z_abs<2.43) redshift halves of our sample. However, the two
populations of systems are statistically different. There is a strong redshift
evolution in the sense that the mean metallicity and mean velocity width
increase with decreasing redshift. We argue that the existence of a DLA
velocity-metallicity correlation, over more than a factor of 100 spread in
metallicity, is probably the consequence of an underlying mass-metallicity
relation for the galaxies responsible for DLA absorption lines. Assuming a
simple linear scaling of the galaxy luminosity with the mass of the dark-matter
halo, we find that the slope of the DLA velocity-metallicity relation is
consistent with that of the luminosity-metallicity relation derived for local
galaxies. [...] abridged.Comment: 10 pages, 4 figures, A&A in pres
On the Origin of the Wide HI Absorption Line Toward Sgr A*
We have imaged a region of about 5' extent surrounding Sgr A* in the HI 21
cm-line absorption using the Very Large Array. A Gaussian decomposition of the
optical depth spectra at positions within about 2' (approx. 5 pc at 8.5 kpc) of
Sgr A* detects a wide line underlying the many narrow absorption lines. The
wide line has a mean peak optical depth of 0.32 +/- 0.12 centered at a mean
velocity of V(lsr) = -4 +/- 15 km/s. The mean full width at half maximum is 119
+/- 42 km/s. Such a wide line is absent in the spectra at positions beyond
about 2' from Sgr A*. The position-velocity diagrams in optical depth reveal
that the wide line originates in various components of the circumnuclear disk
(radius approx. 1.3') surrounding Sgr A*. These components contribute to the
optical depth of the wide line in different velocity ranges. The
position-velocity diagrams do not reveal any diffuse feature which could be
attributed to a large number of HI clouds along the line of sight to Sgr A*.
Consequently, the wide line has no implications either to a global population
of shocked HI clouds in the Galaxy or to the energetics of the interstellar
medium as was earlier thought.Comment: LaTeX, 12 pages and 9 figures, accepted for publication in J.
Astrophys. Ast
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