8,638 research outputs found
Radio continuum and polarization study of SNR G57.2+0.8 associated with magnetar SGR1935+2154
We present a radio continuum and linear polarization study of the Galactic
supernova remnant G57.2+0.8, which may host the recently discovered magnetar
SGR1935+2154. The radio SNR shows the typical radio continuum spectrum of a
mature supernova remnant with a spectral index of and
moderate polarized intensity. Magnetic field vectors indicate a tangential
magnetic field, expected for an evolved SNR, in one part of the SNR and a
radial magnetic field in the other. The latter can be explained by an
overlapping arc-like feature, perhaps a pulsar wind nebula, emanating from the
magnetar. The presence of a pulsar wind nebula is supported by the low average
braking index of 1.2, we extrapolated for the magnetar, and the detection of
diffuse X-ray emission around it. We found a distance of 12.5 kpc for the SNR,
which identifies G57.2+0.8 as a resident of the Outer spiral arm of the Milky
Way. The SNR has a radius of about 20 pc and could be as old as 41,000 years.
The SNR has already entered the radiative or pressure-driven snowplow phase of
its evolution. We compared independently determined characteristics like age
and distance for both, the SNR and SGR1935+2154, and conclude that they are
physically related.Comment: accepted by The Astrophysical Journal, 16 pages, 10 figure
G55.0+0.3: A Highly Evolved Supernova Remnant
Multi-frequency analysis has revealed the presence of a new supernova
remnant, G55.0+0.3, in the Galactic plane. A kinematic distance of 14 kpc has
been measured from HI spectral line data. The faint, clumpy half-shell is
non-thermal and has a physical radius of 70 pc. Using an evolutionary model,
the age of the remnant is estimated to be on the order of one million years,
which exceeds conventional limits by a factor of five. The remnant may be
associated with the nearby pulsar J1932+2020, which has a spin-down age of 1.1
million years. This work implies that the radiative lifetimes of remnants could
be much longer than previously suggested.Comment: 27 pages, 7 figures in 9 files (figures 1 and 2 require 2 files
each), Accepted for publication in The Astrophysical Journal (Jan. 20, 1998
volume
The sino-german 6cm polarization survey of the galactic plane: A summary
We have finished the 6cm polarization survey of the Galactic plane using the
Urumqi 25m radio telescope. It covers 10deg<l<230deg in Galactic longitude and
|b| <5deg in Galactic latitude. The new polarization maps not only reveal new
properties of the diffuse magnetized interstellar medium, but also are very
useful for studying individual objects such as Hii regions, which may act as
Faraday screens with strong regular magnetic fields inside, and supernova
remnants for their polarization properties and spectra. The high sensitivity of
the survey enables us to discover two new SNRs G178.2-4.2 and G25.3-2.1 and a
number of Hii regions.Comment: 10 pages, 1 figure. International Journal of Modern Physics:
Conference Series (IJMPCS) for Proceedings of 3rd Galileo-Xu Guangqi meetin
Radio and gamma-ray constraints on dark matter annihilation in the Galactic center
We determine upper limits on the dark matter (DM) self-annihilation cross
section for scenarios in which annihilation leads to the production of
electron--positron pairs. In the Galactic centre (GC), relativistic electrons
and positrons produce a radio flux via synchroton emission, and a gamma ray
flux via bremsstrahlung and inverse Compton scattering. On the basis of
archival, interferometric and single-dish radio data, we have determined the
radio spectrum of an elliptical region around the Galactic centre of extent 3
degrees semi-major axis (along the Galactic plane) and 1 degree semi-minor axis
and a second, rectangular region, also centered on the GC, of extent 1.6
degrees x 0.6 degrees. The radio spectra of both regions are non-thermal over
the range of frequencies for which we have data: 74 MHz -- 10 GHz. We also
consider gamma-ray data covering the same region from the EGRET instrument
(about GeV) and from HESS (around TeV). We show how the combination of these
data can be used to place robust constraints on DM annihilation scenarios, in a
way which is relatively insensitive to assumptions about the magnetic field
amplitude in this region. Our results are approximately an order of magnitude
more constraining than existing Galactic centre radio and gamma ray limits. For
a DM mass of m_\chi =10 GeV, and an NFW profile, we find that the
velocity-averaged cross-section must be less than a few times 10^-25 cm^3 s^-1.Comment: 14 pages, 9 figures. Version accepted for publication in PRD.
Reference section updated/extended
A 1.4 GHz radio continuum and polarization survey at medium Galactic latitudes: I. Observation and reduction technique
A radio continuum survey at medium Galactic latitudes with the Effelsberg
100-m telescope is being carried out at a centre frequency of 1.4 GHz in total
power and linear polarization. Areas up to +/- 20 degree of Galactic latitude
are now being observed at a sensitivity of 15 mK TB total intensity and 8 mK TB
in linear polarization with an angular resolution of 9'35. This paper describes
the observing and reduction technique applied which results in absolutely
calibrated maps. The methods are illustrated by examples of images from the
survey.Comment: 12 pages, 9 figures, Accepted for publication in Astronomy and
Astrophysics Supp. Se
A Radio Continuum and Polarisation Study of the pulsar wind nebula CTB87 (G74.9+1.2)
We present radio continuum and linear polarisation observations of the pulsar
wind nebula CTB87 (G74.9+1.2) with the Effelsberg 100-m radio telescope between
4.75 and 32 GHz. An analysis of these new data including archived low-frequency
observations at 1420 MHz and 408 MHz from the Canadian Galactic Plane Survey
shows that CTB87 consists of two distinct emission components: a compact
kidney-shaped component, 14 pc x 8.5 pc (7.8' x 4.8') in size and a larger
diffuse, spherical and centrally peaked component of about 30 pc (17') in
diameter. The kidney-shaped component with a much steeper radio continuum
spectrum is highly linearly polarised and likely represents a relic pulsar wind
nebula. The diffuse component represents the undisturbed part of the PWN
expanding inside a cavity or stellar wind bubble. The previously reported
spectral break above 10 GHz is likely the result of missing large-scale
emission and insufficient sensitivity of the high-frequency radio continuum
observations. The simulation of the system's evolution yields an age of about
18,000 years as the result of a type II supernova explosion with an ejecta mass
of about 12 solar masses and an explosion energy of about 7 x 10^50 erg. We
also found evidence for a radio shell in our polarisation data which represents
the blast wave that entered the molecular cloud complex at a radius of about 13
pc.Comment: 17 pages, 15 figures, 5 tables, MNRAS, accepted for publicatio
Theory of rigid-plane phonon modes in layered crystals
The lattice dynamics of low-frequency rigid-plane modes in metallic (graphene
multilayers, GML) and in insulating (hexagonal boron-nitride multilayers, BNML)
layered crystals is investigated. The frequencies of shearing and compression
(stretching) modes depend on the layer number {\EuScript N} and are presented
in the form of fan diagrams. The results for GML and BNML are very similar. In
both cases only the interactions (van der Waals and Coulomb) between
nearest-neighbor planes are effective, while the interactions between more
distant planes are screened. A comparison with recent Raman scattering results
on low-frequency shear modes in GML [Tan {\it et al.}, arXiv:1106.1146v1
(2011)] is made. Relations with the low-lying rigid-plane phonon dispersions in
the bulk materials are established. Master curves which connect the fan diagram
frequencies for any given {\EuScript N} are derived. Static and dynamic
thermal correlation functions for rigid-layer shear and compression modes are
calculated. The results might be of use for the interpretation of friction
force experiments on multilayer crystals
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