3,084 research outputs found
Peace: Pulsar Evaluation Algorithm For Candidate Extraction – A Software Package For Post-Analysis Processing Of Pulsar Survey Candidates
Modern radio pulsar surveys produce a large volume of prospective candidates, the majority of which are polluted by human-created radio frequency interference or other forms of noise. Typically, large numbers of candidates need to be visually inspected in order to determine if they are real pulsars. This process can be labour intensive. In this paper, we introduce an algorithm called Pulsar Evaluation Algorithm for Candidate Extraction
A large-scale R-matrix calculation for electron-impact excitation of the Ne O-like ion
The five J levels within a or ground state complex provide
an excellent testing ground for the comparison of theoretical line ratios with
astrophysically observed values, in addition to providing valuable electron
temperature and density diagnostics. The low temperature nature of the line
ratios ensure that the theoretically derived values are sensitive to the
underlying atomic structure and electron-impact excitation rates. Previous
R-matrix calculations for the Ne O-like ion exhibit large spurious
structure in the cross sections at higher electron energies, which may affect
Maxwellian averaged rates even at low temperatures. Furthermore, there is an
absence of comprehensive excitation data between the excited states that may
provide newer diagnostics to compliment the more established lines discussed in
this paper. To resolve these issues, we present both a small scale 56-level
Breit-Pauli (BP) calculation and a large-scale 554 levels R-matrix Intermediate
Coupling Frame Transformation (ICFT) calculation that extends the scope and
validity of earlier JAJOM calculations both in terms of the atomic structure
and scattering cross sections. Our results provide a comprehensive
electron-impact excitation data set for all transitions to higher shells.
The fundamental atomic data for this O-like ion is subsequently used within a
collisional radiative framework to provide the line ratios across a range of
electron temperatures and densities of interest in astrophysical observations.Comment: 17 pages, 8 figure
Characterization and sequence analysis of the lsg (LOS synthesis genes) locus from Haemophilus influenzae type b
Analysis of the lsg (LOS synthesis genes) cluster in Escherichia coli strain K12 and mutations in the lsg locus in Haemophilus influenzae type b indicated the presence of 3 regions responsible for sequential modifications of E. coli lipopolysaccharide (LPS). Sequencing of the lsg region yielded 7,435 bp that encompassed 7 complete and 1 partial open reading frames (ORFs 1-8). The predicted product of ORF1 had homology to the consensus sequence of cytochrome b proteins (21% identity, 51% similarity) and to other transmembrane proteins. The products of ORF5 and ORF6 share overall 23% identity and 49% similarity with each other. The ORF6 protein had high homology with the product of ORF275 of the E. coli rfb gene cluster (40% identity, 58% similarity), whose function is not known. Multiple sequence alignment of the ORF5 and ORF6 proteins with the RfbB, RfbJ and RfbX proteins revealed conserved motifs over the N-terminal half region of all these proteins. The products of ORF7 and ORF8 are homologous with Azotobacter vinelandii MolA protein (30% identity, 51% similarity) and MolB protein (26% identity, 48% similarity), respectively. The promoter regions of ORF1, 7 and 8 were determined by primer extension analysis and found to be similar to bacterial σ70-dependent promoters. ORF7 and ORF8 are transcribed into diverse orientation. At least 5 of the encoded proteins have been identified using coupled E. coli transcription/translation system and labeling with [35S]-methionine. We conclude that the genetic organization of the lsg biosynthesis pathway involves multiple operons that lead to the assembly of an H. influenzae LOS structure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67077/2/10.1177_096805199400100305.pd
Gravitational wave astronomy with the SKA
On a time scale of years to decades, gravitational wave (GW) astronomy will
become a reality. Low frequency (nanoHz) GWs are detectable through long-term
timing observations of the most stable pulsars. Radio observatories worldwide
are currently carrying out observing programmes to detect GWs, with data sets
being shared through the International Pulsar Timing Array project. One of the
most likely sources of low frequency GWs are supermassive black hole binaries
(SMBHBs), detectable as a background due to a large number of binaries, or as
continuous or burst emission from individual sources. No GW signal has yet been
detected, but stringent constraints are already being placed on galaxy
evolution models. The SKA will bring this research to fruition.
In this chapter, we describe how timing observations using SKA1 will
contribute to detecting GWs, or can confirm a detection if a first signal
already has been identified when SKA1 commences observations. We describe how
SKA observations will identify the source(s) of a GW signal, search for
anisotropies in the background, improve models of galaxy evolution, test
theories of gravity, and characterise the early inspiral phase of a SMBHB
system.
We describe the impact of the large number of millisecond pulsars to be
discovered by the SKA; and the observing cadence, observation durations, and
instrumentation required to reach the necessary sensitivity. We describe the
noise processes that will influence the achievable precision with the SKA. We
assume a long-term timing programme using the SKA1-MID array and consider the
implications of modifications to the current design. We describe the possible
benefits from observations using SKA1-LOW. Finally, we describe GW detection
prospects with SKA1 and SKA2, and end with a description of the expectations of
GW astronomy.Comment: 19 pages, 3 figures, to be published in: "Advancing Astrophysics with
the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)03
The Baryonic Halos of Elliptical Galaxies: Radial Distribution of Globular Clusters and Diffuse Hot Gas
For a sample of 9 well-studied giant ellipticals we compare the projected
radial distribution of their red and blue globular cluster (GC) subpopulations
with their host galaxy stellar and X-ray surface brightness profiles. We
support previous findings that the surface density distribution of red
(metal-rich) GCs follows that of the host galaxy starlight. We find good
agreement between the outer slope of the blue GC surface density and that of
the galaxy X-ray emission. This coincidence of projected radial profiles is
likely due to the fact that both blue GCs and X-ray emitting hot gas share the
same gravitational potential in equilibrium. When deprojected the X-ray
emitting hot gas has a radial density dependence that is the square root of
that for the GC density. We further show that the energy per unit mass for blue
GCs is roughly half that of the hot gas.Comment: 11 pages, 5 figures, accepted for publication in MNRA
The shape of a moving fluxon in stacked Josephson junctions
We study numerically and analytically the shape of a single fluxon moving in
a double stacked Josephson junctions (SJJ's) for various junction parameters.
We show that the fluxon in a double SJJ's consists of two components, which are
characterized by different Swihart velocities and Josephson penetration depths.
The weight coefficients of the two components depend on the parameters of the
junctions and the velocity of the fluxon. It is shown that the fluxon in SJJ's
may have an unusual shape with an inverted magnetic field in the second
junction when the velocity of the fluxon is approaching the lower Swihart
velocity. Finally, we study the influence of fluxon shape on flux-flow
current-voltage characteristics and analyze the spectrum of Cherenkov radiation
for fluxon velocity above the lower Swihart velocity. Analytic expression for
the wavelength of Cherenkov radiation is derived.Comment: 12 pages, 12 figure
Rotation of the pre-stellar core L1689B
The search for the onset of star formation in pre-stellar cores has focussed
on the identification of an infall signature in the molecular line profiles of
tracer species. The classic infall signature is a double peaked line profile
with an asymmetry in the strength of the peaks such that the blue peak is
stronger. L1689B is a pre-stellar core and infall candidate but new JCMT HCO+
line profile data, presented here, confirms that both blue and red asymmetric
line profiles are present in this source. Moreover, a dividing line can be
drawn between the locations where each type of profile is found. It is argued
that it is unlikely that the line profiles can be interpreted with simple
models of infall or outflow and that rotation of the inner regions is the most
likely explanation. A rotational model is developed in detail with a new 3D
molecular line transport code and it is found that the best type of model is
one in which the rotational velocity profile is in between solid body and
Keplerian. It is firstly shown that red and blue asymmetric line profiles can
be generated with a rotation model entirely in the absence of any infall
motion. The model is then quantitively compared with the JCMT data and an
iteration over a range of parameters is performed to minmize the difference
between the data and model. The results indicate that rotation can dominate the
line profile shape even before the onset of infall.Comment: Accepted by MNRAS, 7 pages, 4 figure
An annular gap acceleration model for -ray emission of pulsars
If the binding energy of the pulsar's surface is not so high (the case of a
neutron star), both the negative and positive charges will flow out freely from
the surface of the star. The annular free flow model for -ray emission
of pulsars is suggested in this paper. It is emphasized that: (1). Two kinds of
acceleration regions (annular and core) need to be taken into account. The
annular acceleration region is defined by the magnetic field lines that cross
the null charge surface within the light cylinder. (2). If the potential drop
in the annular region of a pulsar is high enough (normally the cases of young
pulsars), charges in both the annular and the core regions could be accelerated
and produce primary gamma-rays. Secondary pairs are generated in both regions
and stream outwards to power the broadband radiations. (3). The potential drop
in the annular region grows more rapidly than that in the core region. The
annular acceleration process is a key point to produce wide emission beams as
observed. (4). The advantages of both the polar cap and outer gap models are
retained in this model. The geometric properties of the -ray emission
from the annular flow is analogous to that presented in a previous work by Qiao
et al., which match the observations well. (5). Since charges with different
signs leave the pulsar through the annular and the core regions, respectively,
the current closure problem can be partially solved.Comment: 11 pages 2 figures, accepted by Chinese Journal of Astronomy and
Astrophysic
A transient component in the pulse profile of PSR J0738-4042
One of the tenets of the radio pulsar observational picture is that the
integrated pulse profiles are constant with time. This assumption underpins
much of the fantastic science made possible via pulsar timing. Over the past
few years, however, this assumption has come under question with a number of
pulsars showing pulse shape changes on a range of timescales. Here, we show the
dramatic appearance of a bright component in the pulse profile of PSR
J0738-4042 (B0736-40). The component arises on the leading edge of the profile.
It was not present in 2004 but strongly present in 2006 and all observations
thereafter. A subsequent search through the literature shows the additional
component varies in flux density over timescales of decades. We show that the
polarization properties of the transient component are consistent with the
picture of competing orthogonal polarization modes. Faced with the general
problem of identifying and characterising average profile changes, we outline
and apply a statistical technique based on a Hidden Markov Model. The value of
this technique is established through simulations, and is shown to work
successfully in the case of low signal-to-noise profiles.Comment: Accepted for publication in MNRA
The soft X-ray absorption lines of the Seyfert 1 galaxy MCG--6-30-15
The absorption lines in the soft X-ray spectrum of MCG--6-30-15 are studied
using the Reflection Grating Spectrometer data from the 2001 XMM-Newton 320 ks
observation. A line search of the full time-averaged spectrum reveals 51
absorption lines and one emission line. The equivalent widths of the lines are
measured and the majority of the lines identified. We find lines produced by a
broad range of charge states for several elements, including almost all the
charge states oxygen and iron, suggesting a broad range of ionization
parameters is present in the warm absorber. The equivalent widths of the lines
are broadly consistent with the best fitting warm absorber models from Turner
et al (2003). The equivalent widths of the absorption lines allow confidence
limits on the column density of the species to be determined. For OVII a column
density of 10^18.36-10^18.86 cm^-2 is found. This column density of OVII, when
combined with the inferred FeI absorption, is sufficient to explain the drop in
flux at 0.7 keV as being due to absorption from the warm absorber. Fitting OI
K-edge absorption to the spectrum reveals a column of 10^17.51-10^17.67 cm^-2
of OI, suggesting an Fe:O ratio of ~1:2, consistent with the neutral iron being
in the form of iron oxide dust. Variability is seen in a few absorption lines,
but the majority of the absorption features, including the prominent absorption
edges, stay constant throughout the observation despite variability in the
continuum flux.Comment: Accepted for publication in MNRAS, 11 pages, 7 figure
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