2,350 research outputs found
Searching for Hyperbolicity
This is an expository paper, based on by a talk given at the AWM Research
Symposium 2017. It is intended as a gentle introduction to geometric group
theory with a focus on the notion of hyperbolicity, a theme that has inspired
the field from its inception to current-day research
Detection of Bursts from FRB 121102 with the Effelsberg 100-m Radio Telescope at 5 GHz and the Role of Scintillation
FRB 121102, the only repeating fast radio burst (FRB) known to date, was
discovered at 1.4 GHz and shortly after the discovery of its repeating nature,
detected up to 2.4 GHz. Here we present three bursts detected with the 100-m
Effelsberg radio telescope at 4.85 GHz. All three bursts exhibited frequency
structure on broad and narrow frequency scales. Using an autocorrelation
function analysis, we measured a characteristic bandwidth of the small-scale
structure of 6.41.6 MHz, which is consistent with the diffractive
scintillation bandwidth for this line of sight through the Galactic
interstellar medium (ISM) predicted by the NE2001 model. These were the only
detections in a campaign totaling 22 hours in 10 observing epochs spanning five
months. The observed burst detection rate within this observation was
inconsistent with a Poisson process with a constant average occurrence rate;
three bursts arrived in the final 0.3 hr of a 2 hr observation on 2016 August
20. We therefore observed a change in the rate of detectable bursts during this
observation, and we argue that boosting by diffractive interstellar
scintillations may have played a role in the detectability. Understanding
whether changes in the detection rate of bursts from FRB 121102 observed at
other radio frequencies and epochs are also a product of propagation effects,
such as scintillation boosting by the Galactic ISM or plasma lensing in the
host galaxy, or an intrinsic property of the burst emission will require
further observations.Comment: Accepted to ApJ. Minor typos correcte
Time-Correlated Structure in Spin Fluctuations in Pulsars
We study statistical properties of stochastic variations in pulse arrival
times, timing noise, in radio pulsars using a new analysis method applied in
the time domain. The method proceeds in two steps. First, we subtract
low-frequency wander using a high-pass filter. Second, we calculate the
discrete correlation function of the filtered data. As a complementary method
for measuring correlations, we introduce a statistic that measures the
dispersion of the data with respect to the data translated in time. The
analysis methods presented here are robust and of general usefulness for
studying arrival time variations over timescales approaching the average
sampling interval. We apply these methods to timing data for 32 pulsars. In two
radio pulsars, PSRs B1133+16 and B1933+16, we find that fluctuations in arrival
times are correlated over timescales of 10 - 20 d with the distinct signature
of a relaxation process. Though this relaxation response could be
magnetospheric in origin, we argue that damping between the neutron star crust
and interior liquid is a more likely explanation. Under this interpretation,
our results provide the first evidence independent from pulsar spin glitches of
differential rotation in neutron stars. PSR B0950+08, shows evidence for
quasi-periodic oscillations that could be related to mode switching.Comment: 25 pages, Final journal version (MNRAS
Correction : Assessing dimerisation degree and cooperativity in a biomimetic small-molecule model by pulsed EPR
Correction for ‘Assessing dimerisation degree and cooperativity in a biomimetic small-molecule model by pulsed EPR’ by K. Ackermann et al., Chem. Commun., 2015, 51, 5257–5260.Publisher PDFPeer reviewe
Exploring the scope of tandem palladium and isothiourea relay catalysis for the synthesis of α-amino acid derivatives
Funding: This research was funded by the School of Chemistry, University of St Andrews. A.D.S. thanks the Royal Society for a Wolfson Research Merit Award.The scope and limitations of a tandem N-allylation/[2,3]-rearrangementprotocol are investigated through the incorporation of a variety of functionalgroups within an allylic phosphate precursor. This method uses readilyaccessible N,N-dimethylglycine aryl esters and functionalizedallylic phosphates, forming quaternary ammonium salts in situ in the presence of a palladium catalyst. Subsequentenantioselective [2,3]-sigmatropic rearrangement, promoted by the chiralisothiourea tetramisole, generates α-amino acid derivatives with two contiguousstereocenters. The incorporation of electron withdrawing ester and amide groupsgave the best results, furnishing the desired products in moderate to goodyields (29-70%), with low diastereocontrol (typically 60:40 dr) but highenantioselectivity (up to 90:10 er). These results indicate that substrate-catalystinteractions in the proposed transition state are sensitive to the substitutionpattern of the substrates.Publisher PDFPeer reviewe
Discovery of an optical bow-shock around pulsar B0740-28
We report the discovery of a faint H-alpha pulsar wind nebula (PWN) powered
by the radio pulsar B0740-28. The characteristic bow-shock morphology of the
PWN implies a direction of motion consistent with the previously measured
velocity vector for the pulsar. The PWN has a flux density more than an order
of magnitude lower than for the PWNe seen around other pulsars, but, for a
distance 2 kpc, it is consistent with propagation through a medium of atomic
density n_H ~ 0.25 cm^{-3}, and neutral fraction of 1%. The morphology of the
PWN in the area close to the pulsar is distinct from that in downstream
regions, as is also seen for the PWN powered by PSR B2224+65. In particular,
the PWN associated with PSR B0740-28 appears to close at its rear, suggesting
that the pulsar has recently passed through a transition from low density to
high density ambient gas. The faintness of this source underscores that deep
searches are needed to find further examples of optical pulsar nebulae.Comment: 5 pages, 1 figure, to appear in Astronomy & Astrophysics Letter
Pulsar Searches And Timing With The Square Kilometre Array
The Square Kilometre Array (SKA) is a planned multi purpose radio telescope
with a collecting area approaching 1 million square metres. One of the key
science objectives of the SKA is to provide exquisite strong-field tests of
gravitational physics by finding and timing pulsars in extreme binary systems
such as a pulsar-black hole binary. To find out how three preliminary SKA
configurations will affect a pulsar survey, we have simulated SKA pulsar
surveys for each configuration. We estimate that the total number of normal
pulsars the SKA will detect, using only the 1-km core and 30 minutes
integration time, is around 14000 normal pulsar and 6000 millisecond pulsars.
We describe a simple strategy for follow-up timing observations and find that,
depending on the configuration, it would take 1-6 days to obtain a single
timing point for 14000 pulsars. Obtaining a single timing point for the
high-precision timing projects of the SKA, will take less than 14 hours, 2
days, or 3 days, depending on the configuration. The presence of aperture
arrays will be of great benefit here. We also study the computational
requirements for beam forming and data analysis for a pulsar survey. Beam
forming of the full field of view of the single-pixel feed 15-m dishes using
the 1-km core of the SKA requires about 2.2*10^15 operations per second. The
corresponding data rate from such a pulsar survey is about 4.7*10^11 bytes per
second. The required computational power for a deep real time analysis is
estimated to be 1.2*10^16 operations per second. For an aperture array or
dishes equipped with phased array feeds, the survey can be performed faster,
but the computational requirements and data rates will go up.Comment: 12 pages, 9 figures, accepted for publication in A&
Accelerating incoherent dedispersion
Incoherent dedispersion is a computationally intensive problem that appears
frequently in pulsar and transient astronomy. For current and future transient
pipelines, dedispersion can dominate the total execution time, meaning its
computational speed acts as a constraint on the quality and quantity of science
results. It is thus critical that the algorithm be able to take advantage of
trends in commodity computing hardware. With this goal in mind, we present
analysis of the 'direct', 'tree' and 'sub-band' dedispersion algorithms with
respect to their potential for efficient execution on modern graphics
processing units (GPUs). We find all three to be excellent candidates, and
proceed to describe implementations in C for CUDA using insight gained from the
analysis. Using recent CPU and GPU hardware, the transition to the GPU provides
a speed-up of 9x for the direct algorithm when compared to an optimised
quad-core CPU code. For realistic recent survey parameters, these speeds are
high enough that further optimisation is unnecessary to achieve real-time
processing. Where further speed-ups are desirable, we find that the tree and
sub-band algorithms are able to provide 3-7x better performance at the cost of
certain smearing, memory consumption and development time trade-offs. We finish
with a discussion of the implications of these results for future transient
surveys. Our GPU dedispersion code is publicly available as a C library at:
http://dedisp.googlecode.com/Comment: 15 pages, 4 figures, 2 tables, accepted for publication in MNRA
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