1,202 research outputs found
There are no abnormal solutions of the BetheSalpeter equation in the static model
The four-point Green's function of static QED, where a fermion and an
antifermion are located at fixed space positions, is calculated in covariant
gauges. The bound state spectrum does not display any abnormal state
corresponding to excitations of the relative time. The equation that was
established by Mugibayashi in this model and which has abnormal solutions does
not coincide with the BetheSalpeter equation. Gauge transformation from the
Coulomb gauge also confirms the absence of abnormal solutions in the
BetheSalpeter equation.Comment: 11 pages, late
Does Pulsar B1757--24 Have a Fallback Disk?
Radio pulsars are thought to spin-down primarily due to torque from magnetic
dipole radiation (MDR) emitted by the time-varying stellar magnetic field as
the star rotates. This assumption yields a `characteristic age' for a pulsar
which has generally been assumed to be comparable to the actual age. Recent
observational limits on the proper motion of pulsar B1757-24, however, revealed
that the actual age (>39 kyr) of this pulsar is much greater than its MDR
characteristic age (16 kyr) - calling into question the assumption of pure MDR
spin-down for this and other pulsars. To explore the possible cause of this
discrepancy, we consider a scenario in which the pulsar acquired an accretion
disk from supernova ejecta, and the subsequent spin-down occurred under the
combined action of MDR and accretion torques. A simplified model of the
accretion torque involving a constant mass inflow rate at the pulsar
magnetosphere can explain the age and period derivative of the pulsar for
reasonable values of the pulsar magnetic field and inflow rate. We discuss
testable predictions of this model.Comment: Accepted by ApJ Letters. 15 pages with 1 figur
A Pilot Survey for the HO Southern Galactic Plane Survey (HOPS)
We describe observations with the Mopra radiotelescope designed to assess the
feasibility of the HO maser southern Galactic plane survey (HOPS). We
mapped two one-square-degree regions along the Galactic plane using the new 12
mm receiver and the UNSW Mopra spectrometer (MOPS). We covered the entire
spectrum between 19.5 and 27.5 GHz using this setup with the main aims of
finding out which spectral lines can be detected with a quick mapping survey.
We report on detected emission from HO masers, NH inversion transitions
(1,1), (2,2) and (3,3), HCN (3-2), as well as several radio recombination
lines.Comment: accepted by PAS
Decapitating the Duck: The (non)association of PSR B1757-24 and supernova remnant G5.4-1.2
We have made the first direct interferometric proper motion measurements of
the radio pulsar B1757-24, which sits at the tip of the "beak" of the putative
"Duck" supernova remnant. The peculiar morphology of this radio complex has
been used to argue alternately that the pulsar's space motion was either
surprisingly high or surprisingly low. In fact, we show that the pulsar's
motion is so small that it and its associated nonthermal nebula G5.27-0.9 (the
"head") are almost certainly unrelated to the much larger G5.4-1.2 (the
"wings").Comment: 10 pages, 1 figure, accepted for publication in ApJ
HI Emission and Absorption in the Southern Galactic Plane Survey
We present preliminary results from the Southern Galactic Plane Survey (SGPS)
Test Region and Parkes data. As part of the pilot project for the Southern
Galactic Plane Survey, observations of a Test Region (325.5 deg < l < 333.5
deg; -0.5 deg < b < 3.5 deg) were completed in December 1998. Single dish
observations of the full survey region (253 deg < l < 358 deg; |b| <1 deg) with
the Parkes Radio Telescope were completed in March 2000. We present a sample of
SGPS HI data with particular attention to the smallest and largest scale
structures seen in absorption and emission, respectively. On the large scale,
we detect many prominent HI shells. On the small scale, we note extremely
compact, cold clouds seen in HI self-absorption. We explore how these two
classes of objects probe opposite ends of the HI spatial power spectrum.Comment: 9 pages, 3 embedded postscript & 4 jpeg figures. Presented at the
Astronomical Society of Australia, Hobart, Tasmania, July 4-7 2000. To appear
in PASA Vol. 18(1
The 69 ms Radio Pulsar Near the Supernova Remnant RCW 103
We report the detection of the radio pulsar counterpart to the 69 ms X-ray
pulsar discovered near the supernova remnant RCW 103 (G332.4-0.4). Our
detection confirms that the pulsations arise from a rotation-powered neutron
star, which we name PSR J1617-5055. The observed barycentric period derivative
confirms that the pulsar has a characteristic age of only 8 kyr, the sixth
smallest of all known pulsars. The unusual apparent youth of the pulsar and its
proximity to a young remnant requires that an association be considered.
Although the respective ages and distances are consistent within substantial
uncertainties, the large inferred pulsar transverse velocity is difficult to
explain given the observed pulsar velocity distribution, the absence of
evidence for a pulsar wind nebula, and the symmetry of the remnant. Rather, we
argue that the objects are likely superposed on the sky; this is reasonable
given the complex area. Without an association, the question of where is the
supernova remnant left behind following the birth of PSR J1617-5055 remains
open. We also discuss a possible association between PSR J1617-5055 and the
gamma-ray source 2CG 333+01. Though an association is energetically plausible,
it is unlikely given that EGRET did not detect 2CG 333+01.Comment: 18 pages, 2 encapsulated Postscript figures, uses AAS LaTeX style
files. Accepted for publication in The Astrophysical Journal Letter
New OH Zeeman measurements of magnetic field strengths in molecular clouds
We present the results of a new survey of 23 molecular clouds for the Zeeman
effect in OH undertaken with the ATNF Parkes 64-m radio telescope and the NRAO
Green Bank 43-m radio telescope. The Zeeman effect was clearly detected in the
cloud associated with the HII region RCW 38, with a field strength of 38+/-3
micro-Gauss, and possibly detected in a cloud associated with the HII region
RCW 57, with a field strength of -203+/-24 micro-Gauss. The remaining 21
measurements give formal upper limits to the magnetic field strength, with
typical 1-sigma sensitivities <20 micro-Gauss. For 22 of the molecular clouds
we are also able to determine thecolumn density of the gas in which we have
made a sensitive search for the Zeeman effect. We combine these results with
previous Zeeman studies of 29 molecular clouds, most of which were compiled by
Crutcher (1999), for a comparsion of theoretical models with the data. This
comparison implies that if the clouds can be modeled as initially spherical
with uniform magnetic fields and densities that evolve to their final
equilibrium state assuming flux-freezing then the typical cloud is magnetically
supercritical, as was found by Crutcher (1999). If the clouds can be modeled as
highly flattened sheets threaded by uniform perpendicular fields, then the
typical cloud is approximately magnetically critical, in agreement with Shu et
al. (1999), but only if the true values of the field for the non-detections are
close to the 3-sigma upper limits. If instead these values are significantly
lower (for example, similar to the 1-sigma limits), then the typical cloud is
generally magnetically supercritical.Comment: 39 pages, 7 figures. Accepted for publication in Ap
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 Compact X-ray Source 1E 1547.0-5408 and the Radio Shell G327.24-0.13: A New Proposed Association between a Candidate Magnetar and a Candidate Supernova Remnant
We present X-ray, infrared and radio observations of the field centered on
X-ray source 1E 1547.0-5408 in the Galactic Plane. A new Chandra observation of
this source shows it is unresolved at arc-second resolution, and a new XMM
observation shows that its X-ray spectrum is best described by an absorbed
power-law and blackbody model. A comparison of the X-ray flux observed from
this source between 1980 and 2006 reveals that its absorbed 0.5-10 keV X-ray
flux decreased from ~2x10^-12 ergs cm-2 s-1 to ~3x10^-13 ergs cm-2 during this
period. The most recent XMM observation allows us to put a 5 sigma confidence
upper limit of 14% for the 0.5-10 keV peak-to-peak pulsed fraction. A
near-infrared observation of this field shows a source with magnitude Ks =
15.9+/-0.2 near the position of 1E 1547.0-5408, but the implied X-ray to
infrared flux ratio indicates the infrared emission is from an unrelated field
source, allowing us to limit the IR magnitude of 1E 1547.0-5408 to >17.5.
Archival radio observations reveal that 1E 1547.0-5408 sits at the center of a
faint, small (4' diameter) radio shell, G327.24-0.13, which is possibly a
previously unidentified supernova remnant. The X-ray properties of 1E
1547.0-5408 suggest that this source is a magnetar - a young neutron star whose
X-ray emission is powered by the decay of its extremely strong magnetic field.
The spatial coincidence between this source and G327.24-0.13 suggests that 1E
1547.0-5408 is associated with a young supernova remnant, supporting a neutron
star interpretation. Additional observations are needed to confirm the nature
of both 1E 1547.0-5408 and G327.24-0.13, and to determine if these sources are
associated. If so, this pair will be an important addition to the small number
of known associations between magnetars and supernova remnants.Comment: 11 pages, 5 figures, ApJ accepte
The Duck Redux: An Improved Proper Motion Upper Limit for the Pulsar B1757-24 Near the Supernova Remnant G5.4-1.2
"The Duck" is a complicated non-thermal radio system, consisting of the
energetic radio pulsar B1757-24, its surrounding pulsar wind nebula G5.27-0.90
and the adjacent supernova remnant (SNR) G5.4-1.2. PSR B1757-24 was originally
claimed to be a young (~15 000 yr) and extreme velocity (>~1500 km/s) pulsar
which had penetrated and emerged from the shell of the associated SNR G5.4-1.2,
but recent upper limits on the pulsar's motion have raised serious difficulties
with this interpretation. We here present 8.5 GHz interferometric observations
of the nebula G5.27-0.90 over a 12-year baseline, doubling the time-span of
previous measurements. These data correspondingly allow us to halve the
previous upper limit on the nebula's westward motion to 14 milliarcseconds/yr
(5-sigma), allowing a substantive reevaluation of this puzzling object. We rule
out the possibility that the pulsar and SNR were formed from a common supernova
explosion ~15 000 yrs ago as implied by the pulsar's characteristic age, but
conclude that an old (>~70 000 yr) pulsar / SNR association, or a situation in
which the pulsar and SNR are physically unrelated, are both still viable
explanations.Comment: 9 pages, including 1 color and 1 B/W figure. Minor changes following
referee's report. ApJ, in pres
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