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 6.4$\pm$1.6 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