411 research outputs found
Superlinear convergence for PCG using band plus algebra preconditioners for Toeplitz systems
AbstractThe paper studies fast and efficient solution algorithms for n×n symmetric ill conditioned Toeplitz systems Tn(f)x=b where the generating function f is known a priori, real valued, nonnegative, and has isolated roots of even order. The preconditioner that we propose is a product of a band Toeplitz matrix and matrices that belong to a certain trigonometric algebra. The basic idea behind the proposed scheme is to combine the advantages of all components of the product that are well known when every component is used as a stand-alone preconditioner. As a result we obtain a flexible preconditioner which can be applied to the system Tn(f)x=b infusing superlinear convergence to the PCG method. The important feature of the proposed technique is that it can be extended to cover the 2D case, i.e. ill-conditioned block Toeplitz matrices with Toeplitz blocks. We perform many numerical experiments, whose results confirm the theoretical analysis and effectiveness of the proposed strategy
Low-Frequency Spectral Turn-Overs in Millisecond Pulsars Studied from Imaging Observations
Measurements of pulsar flux densities are of great importance for
understanding the pulsar emission mechanism and for predictions of pulsar
survey yields and the pulsar population at large. Typically these flux
densities are determined from phase-averaged "pulse profiles", but this method
has limited applicability at low frequencies because the observed pulses can
easily be spread out by interstellar effects like scattering or dispersion,
leading to a non-pulsed continuum component that is necessarily ignored in this
type of analysis. In particular for the class of the millisecond pulsars (MSPs)
at frequencies below 200MHz, such interstellar effects can seriously compromise
de- tectability and measured flux densities. In this paper we investigate MSP
spectra based on a complementary approach, namely through investigation of
archival con- tinuum imaging data. Even though these images lose sensitivity to
pulsars since the on-pulse emission is averaged with off-pulse noise, they are
insensitive to effects from scattering and provide a reliable way to determine
the flux density and spectral indices of MSPs based on both pulsed and unpulsed
components. Using the 74MHz VLSSr as well as the 325MHz WENSS and 1.4GHz NVSS
catalogues, we investigate the imaging flux densities of MSPs and evaluate the
likelihood of spectral turn-overs in this population. We determine three new
MSP spectral indices and identify six new MSPs with likely spectral turn-overs.Comment: 10 pages, 4 figures, 3 tables, accepted for publication in MNRA
Selection of radio pulsar candidates using artificial neural networks
Radio pulsar surveys are producing many more pulsar candidates than can be
inspected by human experts in a practical length of time. Here we present a
technique to automatically identify credible pulsar candidates from pulsar
surveys using an artificial neural network. The technique has been applied to
candidates from a recent re-analysis of the Parkes multi-beam pulsar survey
resulting in the discovery of a previously unidentified pulsar.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 9 pages, 7 figures, and 1 tabl
Rotation measure variations in Galactic Centre pulsars
We report the results of an observational campaign using the Effelsberg 100-m
telescope of the pulsars J17462849, J17462850, J17462856 and
J17452912 located in the Central Molecular Zone (CMZ) close to the Galactic
centre in order to study rotation measure (RM) variations. We report for the
first time the RM value of PSR J17462850 to be rad
m. This pulsar shows significant variations of RM of rad
m over the course of months to years that suggest a strongly magnetized
environment. The structure function analysis of the RM of PSR J17462850
revealed a steep power-law index of comparable to the
value expected for isotropic turbulence. This pulsar also showed large
dispersion measure (DM) variation of pc cm in an event lasting
a few months where the RM increased by rad m. The large
difference in RM between PSR J17462849 and PSR J17462850 despite the
small angular separation reveals the presence of a magnetic field of at least
70 G in the CMZ and can explain the lack of polarization in the radio
images of the region. These results contribute to our understanding of the
magnetic field in the CMZ and show similarities between the RM behaviours of
these pulsars and some fast radio bursts (FRBs).Comment: Accepted for publication on Monthly Notices of the Royal Astronomical
Society, 13 pages, 7 figure
Insight into the OH polarimetric structure of OH 26.5+0.6
We present the first view of the magnetic field structure in the OH shell of
the extreme OH/IR star OH 26.5+0.6. MERLIN interferometric observations of this
object were obtained in December 1993 in full polarisation, at 1612, 1665 and
1667 MHz. The maser spots show a spheroidal distribution both at 1612 and 1667
MHz, while at 1665 MHz emission from the blue-shifted maser peak is
concentrated on the stellar position, and the red-shifted peak emission
exhibits a filamentary structure oriented on a SE-NW axis. The linear
polarisation in both main lines is rather faint, ranging from 9 to 20% at 1665
MHz and from 0 to 30% at 1667 MHz. At 1612 MHz most maser spots exhibit a
similar range of linear polarisation although those in the outermost parts of
the envelope reach values as high as 66%. This is particularly apparent in the
southern part of the shell. The detailed distribution of the polarisation
vectors could only be obtained at 1612 MHz. The polarisation vectors show a
highly structured distribution indicative of a poloidal magnetic field inclined
by 40-60 to the line of sight. The velocity distribution of the maser
spots with respect to the radial distance is well explained by an isotropic
outflow at constant velocity in the case of a prolate shaped spheroid envelope,
also tilted about 45-65 to the line of sight.Comment: 20 pages, 16 figures, accepted for publication in MNRA
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
BlackHoleCam -- Testing general relativity with pulsars orbiting Sagittarius A*
BlackHoleCam is a project funded by a European Research Council Synergy Grant
to build a complete astrophysical description of nearby supermassive black
holes by using a combination of radio imaging, pulsar observations, stellar
astrometry and general relativistic magneto-hydrodynamic models. BlackHoleCam
scientists are active partners of the Event Horizon Telescope Consortium. In
this talk I will discuss the use of pulsars orbiting Sagittarius A* for tests
of General Relativity, the current difficulties in detecting such sources,
recent results from the Galactic Centre magnetar PSR J1745-2900 and how
BlackHoleCam aims to search for undiscovered pulsars in the Galactic Centre.Comment: 4 pages, 3 figures, Proceedings of The Fifteenth Marcel Grossmann
Meeting on General Relativity, University of Rome La Sapienza, Italy, 1 - 7
July 201
Pulsed Gamma Rays from the Original Millisecond and Black Widow Pulsars: a case for Caustic Radio Emission?
We report the detection of pulsed gamma-ray emission from the fast
millisecond pulsars (MSPs) B1937+21 (also known as J1939+2134) and B1957+20
(J1959+2048) using 18 months of survey data recorded by the \emph{Fermi} Large
Area Telescope (LAT) and timing solutions based on radio observations conducted
at the Westerbork and Nan\c{c}ay radio telescopes. In addition, we analyzed
archival \emph{RXTE} and \emph{XMM-Newton} X-ray data for the two MSPs,
confirming the X-ray emission properties of PSR B1937+21 and finding evidence
() for pulsed emission from PSR B1957+20 for the first time. In
both cases the gamma-ray emission profile is characterized by two peaks
separated by half a rotation and are in close alignment with components
observed in radio and X-rays. These two pulsars join PSRs J0034-0534 and
J2214+3000 to form an emerging class of gamma-ray MSPs with phase-aligned peaks
in different energy bands. The modeling of the radio and gamma-ray emission
profiles suggests co-located emission regions in the outer magnetosphere.Comment: Accepted for publication in the Astrophysical Journa
- …