656 research outputs found
A molecular shell with star formation toward the supernova remnant G349.7+0.2
A field of ~38'x38' around the supernova remnant (SNR) G349.7+0.2 has been
surveyed in the CO J=1-0 transition with the 12 Meter Telescope of the NRAO,
using the On-The-Fly technique. The resolution of the observations is 54". We
have found that this remnant is interacting with a small CO cloud which, in
turn, is part of a much larger molecular complex, which we call the ``Large CO
Shell''. The Large CO Shell has a diameter of about 100 pc, an H_2 mass of
930,000 solar masses, and a density of 35 cm-3. We investigate the origin of
this structure and suggest that an old supernova explosion ocurred about 4
million years ago, as a suitable hypothesis. Analyzing the interaction between
G349.7+0.2 and the Large CO Shell, it is possible to determine that the shock
front currently driven into the molecular gas is a non-dissociative shock
(C-type), in agreement with the presence of OH 1720 MHz masers. The positional
and kinematical coincidence among one of the CO clouds that constitute the
Large CO Shell, an IRAS point-like source and an ultracompact H II region,
indicate the presence of a recently formed star. We suggest that the formation
of this star was triggered during the expansion of the Large CO Shell, and
suggest the possibility that the same expansion also created the progenitor
star of G349.7+0.2. The Large CO Shell would then be one of the few
observational examples of supernova-induced star formation.Comment: accepted in Astronomical Journal, corrected typo in the abstract (in
first line, 38' instead of 38"
The neutral gas in the environs of the Geminga gamma-ray pulsar
We present a high-resolution (24 arcsec) study of the HI interstellar gas
distribution around the radio-quiet neutron star Geminga. Based on Very Large
Array (VLA) and MPIfR Effelsberg telescope data, we analyzed a 40' x 40' field
around Geminga. These observations have revealed the presence of a neutral gas
shell, 0.4 pc in radius, with an associated HI mass of 0.8 Msun, which
surrounds Geminga at a radial velocity compatible with the kinematical distance
of the neutron star. In addition, morphological agreement is observed between
the internal face of the HI shell and the brightest structure of Geminga's tail
observed in X-rays.We explore the possibility that this morphological agreement
is the result of a physical association.Comment: One tarfile including a Latex file (7 pages) and two figures. Paper
accepted for publication in Advances in Space Research; typos corrected;
changes in section Results and Discussion after referee's suggestions. S.
Johnston's affilation correcte
Magnetic field effects on neutrino production in microquasars
We investigate the effects of magnetic fields on neutrino production in
microquasars. We calculate the steady particle distributions for the pions and
muons generated in p-gamma and p-p interactions in the jet taking the effects
of all energy losses into account. The obtained neutrino emission is
significantly modified due to the synchrotron losses suffered by secondary
pions and muons. The estimates made for neutrino fluxes arriving on the Earth
imply that detection of high-energy neutrinos from the vicinity of the compact
object can be difficult. However, in the case of windy microquasars, the
interaction of energetic protons in the jet with matter of dense clumps of the
wind could produce detectable neutrinos. This is because the pions and muons at
larger distances from the compact object will not be affected by synchrotron
losses.Comment: 12 pages, 11 figures, accepted for publication in A&
Imaging transverse electron focusing in semiconducting heterostructures with spin-orbit coupling
Transverse electron focusing in two-dimensional electron gases (2DEGs) with
strong spin-orbit coupling is revisited. The transverse focusing is related to
the transmission between two contacts at the edge of a 2DEG when a
perpendicular magnetic field is applied. Scanning probe microscopy imaging
techniques can be used to study the electron flow in these systems. Using
numerical techniques we simulate the images that could be obtained in such
experiments. We show that hybrid edge states can be imaged and that the
outgoing flux can be polarized if the microscope tip probe is placed in
specific positions.Comment: Contribution to the Book/Proceedings of the PITP Les Houches School
on "Quantum Magnetism" held on June, 2006. Final forma
The antenna DSA 3 and its potential use for Radio Astronomy
The European Space Agency (ESA) will inaugurate its third Deep Space Antenna
(DSA 3) by the end of 2012. DSA 3 will be located in Argentina near the city of
Malarg"ue in the Mendoza province. While the instrument will be primarily
dedicated to communications with interplanetary missions, the characteristics
of its antenna and receivers will also enable standalone leading scientific
contributions, with a high scientific-technological return. We outline here
scientific proposals for a radio astronomical use of DSA 3.Comment: 4 pages, submitted as Proceedings for the BAA
High resolution CO observations towards the Bright Eastern Knot of the SNR Puppis A
This paper reports molecular observations towards the Bright Eastern Knot
(BEK) in the SNR Puppis A, a feature where radio and X-ray studies suggest that
the shock front is interacting with a dense molecular clump. We performed
high-resolution millimetric observations towards the BEK of Puppis A using the
SEST telescope in the 12CO J=1-0 and 2-1 lines (beams of 45" and 23"
respectively). More extended, lower angular resolution 12CO J=1-0 observations
taken from NANTEN archival data were also analyzed to obtain a complete
picture. In the velocity range near 16 km/s, the Puppis A systemic velocity,
our study revealed two important properties: (i) no dense molecular gas is
detected immediately adjacent to the eastern border of the BEK and (ii) the
molecular clump detected very close to the radiocontinuum maximum is probably
located in the foreground along the line of sight and has not yet been reached
by the SNR shock front. We propose two possible scenarios to explain the
absence of molecular emission eastwards of the BEK border of Puppis A. Either
the shock front has completely engulfed and destroyed a molecular clump or the
shock front is interacting with part of a larger cloud and we do not detect CO
emission immediately beyond it because the molecules have been dissociated by
photodissociation and by reactions with photoionized material due to the
radiative precursor.Comment: 10 pages, 7 figures. Accepted for publication in A&
Gravitational radiation from precessing accretion disks in gamma-ray bursts
We study the precession of accretion disks in the context of gamma-ray burst
inner engines. Our aim is to quantitatively estimate the characteristics of
gravitational waves produced by the precession of the transient accretion disk
in gamma-ray bursts. We evaluate the possible periods of disk precession caused
by the Lense-Thirring effect using an accretion disk model that allows for
neutrino cooling. Assuming jet ejection perpendicular to the disk plane and a
typical intrinsic time-dependence for the burst, we find gamma-ray light curves
that have a temporal microstructure similar to that observed in some reported
events. The parameters obtained for the precession are then used to evaluate
the production of gravitational waves. We find that the precession of accretion
disks of outer radius smaller than cm and accretion rates above 1 solar
mass per second could be detected by Advanced LIGO if they occur at distances
of less than 100 Mpc. We conclude that the precession of a neutrino-cooled
accretion disk in long gamma-ray bursts can be probed by gravitational wave
astronomy. Precession of the disks in short gamma-ray events is undetectable
with the current technology.Comment: 5 pages, 5 figures, accepted for publication in A&
A two-component model for the high-energy variability of blazars. Application to PKS 2155-304
We study the production of VHE emission in blazars as a superposition of a
steady component from a baryonic jet and a time- dependent contribution from an
inner e-e+ beam launched by the black hole. Both primary relativistic electrons
and protons are injected in the jet, and the particle distributions along it
are found by solving a one-dimensional transport equation that accounts for
convection and cooling. The short-timescale variability of the emission is
explained by local pair injections in turbulent regions of the inner beam. For
illustration, we apply the model to the case of PKS 2155-304, reproducing a
quiescent state of emission with inverse Compton and synchrotron radiation from
primary electrons, as well as proton-proton interactions in the jet. The latter
also yield an accompanying neutrino flux that could be observed with a new
generation km-scale detector in the northern hemisphere such as KM3NeT.Comment: 9 pages, 12 figures, published in A&A (final version
Magnetic Field Amplification in Tycho and other Shell-type Supernova Remnants
It is shown that amplification of the magnetic field in supernova remnants
(SNRs) occurs in all six objects where morphological measurements are presently
available in the hard X-ray continuum at several keV. For the three
archetypical objects (SN 1006, Cas A and Tycho's SNR) to which nonlinear
time-dependent acceleration theory has been successfully applied up to now, the
global theoretical and the local observational field strengths agree very well,
suggesting in addition that all young SNRs exhibit the amplification effect as
a result of very efficient acceleration of nuclear cosmic rays (CRs) at the
outer shock. Since this appears to be empirically the case, we may reverse the
argument and consider field amplification as a measure of nuclear CR
acceleration and it has indeed been argued that acceleration in the amplified
fields allows the CR spectrum from SNRs to reach the knee in the spectrum or,
in special objects, even beyond. The above results are furthermore used to
investigate the time evolution of field amplification in young SNRs. Although
the uncertainties in the data do not allow precise conclusions regarding this
point, they rather clearly show that the ratio of the magnetic field energy
density and the kinetic energy density of gas flow into the shock is of the
order of a few percent if the shock speed is high enough V_s > 10^3 km/s, and
this ratio remains nearly constant during the SNR evolution. The escape of the
highest energy nuclear particles from their sources becomes progressively
important with age, reducing also the cutoff in the \pi^0 -decay gamma-ray
emission spectrum with time after the end of the sweep-up phase. Simultaneously
the leptonic gamma-ray channels will gain in relative importance with
increasing age of the sources.Comment: 13 pages, 8 figures, accepted for publication in A&
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