Andrew Shearer - Academia.edu (original) (raw)

Papers by Andrew Shearer

Astrophys J, Dec 16, 1999

Using the high speed 2-d TRIFFID photometer, we have obtained phase resolved photometry of the Cr... more Using the high speed 2-d TRIFFID photometer, we have obtained phase resolved photometry of the Crab pulsar in UBV that allows us to flux the unpulsed light curve component. Following de-extinction, weighted least-square fitting indicates a power-law exponent of alpha = -0.62 +/- 0.49. This is steeper than that reported for the peak components and its origin remains unclear with respect to contemporary magnetospheric theory.

We have detected an enhancement in the optical emission from the Crab pulsar which is correlated ... more We have detected an enhancement in the optical emission from the Crab pulsar which is correlated with the presence of the giant radio pulses. Coincident optical pulses were found to be on average 3% brighter than those coincident with normal radio pulses. This enhancement, combined with the lack of any other pulse profile changes, indicates that both the giant radio pulses and the increased optical emission are most likely linked to an increase in the electron-positron plasma density.

ABSTRACT Although optical pulsar studies are currently limited a few favoured objects, the observ... more ABSTRACT Although optical pulsar studies are currently limited a few favoured objects, the observation of pulsars at optical wavelengths provides an opportunity to derive a number of important pulsar characteristics. These parameters will be vital for a comprehensive model of pulsar emission mechanisms. The exploitation of the data available through optical pulsar studies requires that high-time-resolution instruments suitable for the optical domain, and for faint pulsar targets, are developed and thoroughly tested. Observations of the Crab pulsar with the high-time-resolution photon-tagging photometer IquEYE show an optical-radio delay of ˜178 μs. In conjunction with simultaneous Jodrell Bank radio observations, a correlation between giant radio pulses and enhanced optical pulses was detected for this pulsar, which is possible evidence for the reprocessing of radio photons. Comparison with optical linear polarisation measurements indicate a shift between linear and circular polarisation at the radio precursor phase.

Observations of the 5 confirmed optical pulsars indicate that the peak emission scales according ... more Observations of the 5 confirmed optical pulsars indicate that the peak emission scales according to the outer field strength. We show that this gives gives further confirmation that a simple phenomenological models such as Pacini and Salvati (1987) still have validity. Furthermore we show that the Crab pulsar exhibits unpulsed emission which further complicates any studies of the thermal emission from Isolated Neutron Stars.

![Research paper thumbnail of Giant` Optical Pulses from the Crab Pulsar](https://a.academia-assets.com/images/blank-paper.jpg)

At present there are eight known optical pulsars. Over the last number of years attention has bee... more At present there are eight known optical pulsars. Over the last number of years attention has been focused more on the high energy (X-ray and gamma-ray) emission. Analysis of pulsed emission at optical wavelengths has been confined mainly to analytical/phenomenological models. This poster presents initial simulation results of a quasi phenomenological-physical model of pulsed optical emission from neutron stars. The underlying magnetic field structure is taken to be that of a three dimensional retarded dipole. Emission areas are located by combining geometrical mapping of beams from within the magnetosphere with viewing angle data (taken from standard radio data). Models are constructed and simulations carried out using these specific viewing geometries - applied particularly to the Crab.

Integrated ground-based and HST optical studies of isolated neutron stars have provided important... more Integrated ground-based and HST optical studies of isolated neutron stars have provided important independent datasets in the determination of emission activity, particularly in the fitting of anticipated Rayleigh-Jeans extrapolations from EUV/soft X-ray datasets, despite their intrinsic faintness. Differentiation of the pulsed and unpulsed fluxes and consequently of the nonthermal and thermal modes of emission could provide definitive data with which to constrain this blackbody continuum. Based upon high speed photometric observations of Geminga and PSR B0656+14 in the B band, we have combined upper limits of unpulsed emission with recently published model-fits with a view to assessing possible implications for the R/d parameter. For Geminga, with a known distance of ~ 160 pc, we find that R_{infty} < 9.5 km for a ~ blackbody source, and R_{\infty} < 10.0 km with the presence of a magnetized H atmosphere. In addition, we suggest that PSR B0656+14 is some ~ 4 - 5 times closer than the 760 pc estimated from DM measurements alone.

PSR B0656+14's high energy emission is consistent with that of combined magnetospheric and therma... more PSR B0656+14's high energy emission is consistent with that of combined magnetospheric and thermal (surface & polar cap) emission. Uncertainties with the radio-derived distance and X-ray instrumentation sensitivities complicate a definitive thermal characterisation however. A re-analysis of combined ROSAT/EUVE archival data in conjunction with integrated & phase-resolved optical photometry is shown to constrain this characterisation.

High speed optical photometry allows differentiation of the pulsed and unpulsed components of emi... more High speed optical photometry allows differentiation of the pulsed and unpulsed components of emission from isolated rotation-powered neutron stars. The predominately thermal unpulsed flux can critically constrain (Tsurface, R/d) space of the blackbody continuum. For Geminga, using upper limits to this flux in B combined with published model-fits we conclude that Rinfty <= 9.5 km (d ~160 pc). Similarly, we suggest a distance some 4-5 times closer than the DM distance of 760 pc for PSR B0656+14. For the Crab pulsar, there is evidence of significant unpulsed emission consistent spectrally with a magnetospheric origin, in contrast to expected unpulsed gamma-ray emission beyond the light cylinder. Such results demand further observations, with important future implications for equation of state and emission models.

Journal of Biological Chemistry, 2015

Neuromyelitis optica immunoglobulin G (NMO-IgG) binds to aquaporin-4 (AQP4) water channels in the... more Neuromyelitis optica immunoglobulin G (NMO-IgG) binds to aquaporin-4 (AQP4) water channels in the central nervous system leading to immune-mediated injury. We have previously demonstrated that a high proportion of CSF plasma cells of NMO patients produce antibody to extracellular domains of the AQP4 protein and that recombinant IgG (rAb) derived from these cells recapitulate pathogenic features of disease. We performed a comprehensive mutational analysis of the three extracellular loops of the M23 isoform of human AQP4 using both serial and single point mutations and evaluated the effects on binding of NMO AQP4-reactive rAbs by quantitative immunofluorescence. Whereas all NMO rAbs required conserved loop C ((137)TP(138) and (150)V) and loop E ((230)HW(231)) amino acids for binding, two broad patterns of NMO-IgG recognition could be distinguished based on differential sensitivity to loop A amino acid changes. Pattern 1 NMO rAbs were insensitive to loop A mutations and could be further discriminated by differential sensitivity to amino acid changes in loop C ((148)TM(149) and (151)H) and loop E ((226)N and (228)E). Alternatively, Pattern 2 NMO rAbs showed significantly reduced binding following amino acid changes in loop A ((63)EKP(65) and (69)D) and loop C ((141)V, (151)H, and (154)L). Amino acid substitutions at (137)TP(138) altered loop C conformation and abolished the binding of all NMO rAbs and NMO-IgG, indicating the global importance of loop C conformation to the recognition of AQP4 by pathogenic NMO Abs. The generation of human NMO rAbs has allowed the first high resolution mapping of extracellular loop amino acids critical for NMO-IgG binding and identified regions of AQP4 extracellular structure that may represent prime targets for drug therapy.

Developmental Biology, 2015

Extraocular muscles (EOMs) are highly specialized skeletal muscles that originate from the head m... more Extraocular muscles (EOMs) are highly specialized skeletal muscles that originate from the head mesoderm and control eye movements. EOMs are uniquely spared in Duchenne muscular dystrophy and animal models of dystrophin deficiency. Specific traits of myogenic progenitors may be determinants of this preferential sparing, but very little is known about the myogenic cells in this muscle group. While satellite cells (SCs) have long been recognized as the main source of myogenic cells in adult muscle, most of the knowledge about these cells comes from the prototypic limb muscles. In this study, we show that EOMs, regardless of their distinctive Pax3-negative lineage origin, harbor SCs that share a common signature (Pax7(+), Ki67(-), Nestin-GFP(+), Myf5(nLacZ+), MyoD-positive lineage origin) with their limb and diaphragm somite-derived counterparts, but are remarkably endowed with a high proliferative potential as revealed in cell culture assays. Specifically, we demonstrate that in adult as well as in aging mice, EOM SCs possess a superior expansion capacity, contributing significantly more proliferating, differentiating and renewal progeny than their limb and diaphragm counterparts. These robust growth and renewal properties are maintained by EOM SCs isolated from dystrophin-null (mdx) mice, while SCs from muscles affected by dystrophin deficiency (i.e., limb and diaphragm) expand poorly in vitro. EOM SCs also retain higher performance in cell transplantation assays in which donor cells were engrafted into host mdx limb muscle. Collectively, our study provides a comprehensive picture of EOM myogenic progenitors, showing that while these cells share common hallmarks with the prototypic SCs in somite-derived muscles, they distinctively feature robust growth and renewal capacities that warrant the title of high performance myo-engines and promote consideration of their properties for developing new approaches in cell-based therapy to combat skeletal muscle wasting.

Basic Cell Culture Protocols, 2004

Journal of Neuroimmunology, 2008

Using FACS and single cell reverse transcriptase polymerase chain reaction, we examined the cereb... more Using FACS and single cell reverse transcriptase polymerase chain reaction, we examined the cerebrospinal fluid (CSF) IgG VH repertoires from 10 subjects with a clinically isolated demyelinating syndrome (CIS). B and plasma cell repertoires from individual subjects showed similar VH family germline usage, nearly identical levels of post-germinal center somatic hypermutation, and significant overlap in their clonal populations. Repertoires from 7 of 10 CIS subjects demonstrated a biased usage of VH4 and/or VH2 family gene segments in their plasma or B cell repertoires. V region bias, however, was not observed in the corresponding peripheral blood CD19+ B cell repertoires from 2 CIS subjects or in normal healthy adults. Clinically, subjects with VH4 or VH2 CSF IgG repertoire bias rapidly progressed to definite MS, whereas individuals without repertoire bias did not develop MS after a minimum of 2 years of follow-up (p = 0.01).

Journal of Immunological Methods, 2007

Real-time immuno-PCR (RT-IPCR) is a powerful technique that combines ELISA with the specificity a... more Real-time immuno-PCR (RT-IPCR) is a powerful technique that combines ELISA with the specificity and sensitivity of PCR. RT-IPCR of phage-displayed peptides exploits the unique physical associations between phenotype (the displayed peptide) and genotype (the encoding DNA) within the same phage particle. Previously, we identified phage peptides specific for recombinant antibodies (rAbs) prepared from clonally expanded plasma cells in multiple sclerosis (MS) cerebrospinal fluid (CSF) and subacute sclerosing panencephalitis (SSPE) brain. Herein, we applied phage-mediated RT-IPCR to study reactivity of these specific phage peptides for the rAbs. Compared to standard ELISA, which required greater than 10 4 or 10 5 phage particles to detect binding to rAbs, RT-IPCR detected binding with as few as 100 phage particles. RT-IPCR was also superior to ELISA in determining relative affinities of rAbs for phage peptides and was effective in screening MS CSF for IgG reactivity to phage peptides. Phage-mediated RT-IPCR is a rapid, high-throughput technology that avoids the requirement for synthetic peptides and will facilitate the identification of candidate peptides that react with the IgG in MS CSF.

Developmental Biology, 2010

Satellite cells are myogenic progenitors residing on the myofiber surface that support skeletal m... more Satellite cells are myogenic progenitors residing on the myofiber surface that support skeletal muscle repair. We used mice in which satellite cells were detected by GFP expression driven by nestin gene regulatory elements to define age-related changes in both numbers of satellite cells that occupy hindlimb myofibers and their individual performance. We demonstrate a reduction in satellite cells per myofiber with age that is more prominent in females compared to males. Satellite cell loss also persists with age in myostatin-null mice regardless of increased muscle mass. Immunofluorescent analysis of isolated myofibers from nestin-GFP/Myf5 nLacZ/+ mice reveals a decline with age in the number of satellite cells that express detectable levels of βgal. Nestin-GFP expression typically diminishes in primary cultures of satellite cells as myogenic progeny proliferate and differentiate, but GFP subsequently reappears in the Pax7 + reserve population. Clonal analysis of sorted GFP + satellite cells from hindlimb muscles shows heterogeneity in the extent of cell density and myotube formation among colonies. Reserve cells emerge primarily within high-density colonies, and the number of clones that produce reserve cells is reduced with age. Thus, satellite cell depletion with age could be attributed to a reduced capacity to generate a reserve population.

Developmental Biology, 2014

Extraocular muscles (EOM) represent a unique muscle group that controls eye movements and origina... more Extraocular muscles (EOM) represent a unique muscle group that controls eye movements and originates from head mesoderm, while the more typically studied body and limb muscles are somite-derived. Aiming to investigate myogenic progenitors (satellite cells) in EOM versus limb and diaphragm of adult mice, we have been using flow cytometry in combination with myogenic-specific Cre-loxP lineage marking for cell isolation. While analyzing cells from the EOM of mice that harbor Myf5(Cre)-driven GFP expression, we identified in addition to the expected GFP(+) myogenic cells (presumably satellite cells), a second dominant GFP(+) population distinguished as being Sca1(+), non-myogenic, and exhibiting a fibro/adipogenic potential. This unexpected population was not only unique to EOM compared to the other muscles but also specific to the Myf5(Cre)-driven reporter when compared to the MyoD(Cre) driver. Histological studies of periocular tissue preparations demonstrated the presence of Myf5(Cre)-driven GFP(+) cells in connective tissue locations adjacent to the muscle masses, including cells in the vasculature wall. These vasculature-associated GFP(+) cells were further identified as mural cells based on the presence of the specific XLacZ4 transgene. Unlike the EOM satellite cells that originate from a Pax3-negative lineage, these non-myogenic Myf5(Cre)-driven GFP(+) cells appear to be related to cells of a Pax3-expressing origin, presumably derived from the neural crest. In all, our lineage tracing based on multiple reporter lines has demonstrated that regardless of common ancestral expression of Myf5, there is a clear distinction between periocular myogenic and non-myogenic cell lineages according to their mutually exclusive antecedence of MyoD and Pax3 gene activity.

British Journal of Haematology, 1994

We report a case of left atrial myxoma associated coagulation system around the myxoma fluctuated... more We report a case of left atrial myxoma associated coagulation system around the myxoma fluctuated, causing with a fluctuating level of cryofibrinogen. Her initial variable conversion of fibrinogen to fibrin, so that at times symptoms and signs were consistent with primary cryo-partially cross-linked fibrin(ogen) resulted in cryofibrinogen, fibrinogenaemia, but a repeat episode occurred without the but at other t i e s frank emboli from more complete clotting cryofibrinogen being detectable. A more typically embolic occurred. cerebro-vascular accident (CVA) occurred, with the subsequent discovery of the patient's myxoma. The cryofibrinogen Keywords: symptomatic cryofibrinogenaemia, left atrial was again present but it disappeared with resection of the myxoma. We propose that the degree of activation of the myxoma.

Annals of Neurology, 2009

Objective-Intrathecal IgG synthesis, persistence of bands of oligoclonal IgG, and memory Bcell cl... more Objective-Intrathecal IgG synthesis, persistence of bands of oligoclonal IgG, and memory Bcell clonal expansion are well-characterized features of the humoral response in multiple sclerosis (MS). Nevertheless, the target antigen of this response remains enigmatic.

Proceedings of the International Astronomical Union, 2006

Supernova remnants are the remains of the outer envelope of the supernova progenitor star. Neutro... more Supernova remnants are the remains of the outer envelope of the supernova progenitor star. Neutron stars are one of the possible remnants of the region where the explosion detonated. Our knowledge, from galactic studies, of SNRs is well established as their morphology can be understood in terms of the initial blast and out-gassing during the last stages of the progenitor. However, observationally the link between neutron stars and SNRs is only poorly established. During a supernova there are a number of possibilities to produce a condensed remnant - no remnant; a neutron star (pulsar?); a magnetar; a black hole or something more exotic. We do not know what fraction of supernovae go down these possible paths. In the ELT era we will have the first real opportunity to sample the pulsar population in external galaxies and get a more comprehensive survey of optical emission from local pulsars. Such a survey would have significantly reduced biases compared to the current state of radio surveys particularly in the area of pulsar-SNR statistics. Furthermore, a 50m telescope will be able to survey galaxies out to at least 20 Mpc for young SNRs using Hα:[OIII] and Hα[SII] ratios. Currently there are over 1500 radio pulsars detected, 14 of which have been observed at optical wavelengths. Although small in number the family of optical pulsars yield much useful information in bridging the gap between the long wavelength radio emission and the high energy gamma-ray emission from pulsars. Specifically optical techniques are currently the only way of detecting polarisation in the high-energy regime. The advent of ELTs will increase the detection rate of local galactic pulsars and provides the possibility of detecting a significant number of extragalactic optical pulsars. Phenomenologically, the Pacini scaling law predicts ˜150 galactic pulsars to have pulsed optical emission with an m_{V} 32. Using SKA, Crab-like giant radio pulses should be detectable out to 7 Mpc. In contrast Crab-l ike pulsars would have a normal peak pulsed m_{V} ˜31 at 10 Mpc making ELT optical observations more sensitive than radio observations and the best method for extragalactic pulsar discovery. To date only five AXPs have observed IR emission and two optical emission. ELTs will be able to sample the AXP population within the Galaxy as well as the local group again providing better statistics for the birth rate of AXPs compared to ‘normal’ pulsars. A combination of an AXP and ‘normal’ pulsar survey will make a significant contribution to the birthrate question - what fraction of supernovae produce pulsars compared to AXPs and other condensed objects?