68 research outputs found
Fast radio bursts and the radio counterparts of neutron star mergers
No full textMany of the most energetic phenomena in the Universe can be seen by radio telescopes and are transient in nature. Recent scientific and technological advancements have enabled the breakthrough detections of two new types of exciting transient: fast radio bursts (FRBs) and neutron star mergers. This thesis focuses on devising new observational strategies for these two young topics, and exploring possible connections between them. What causes FRBs is not known. Some FRB sources repeat by emitting multiple times, whereas most are only observed to emit once. A highly debated question is whether there are multiple types of FRB, or whether they all have the same physical explanation. Part of this thesis explores this question, focusing on observations of the oldest known repeating FRB (Ch. 2) and comparing observations of other FRBs to models related to neutron star mergers (Ch. 3). We devise a new strategy using the Low-Frequency Array (LOFAR) to catch coherent radio emission, like FRBs, immediately following a gamma-ray burst/neutron star merger and provide the deepest limits yet (Ch. 4 & 5). We also look at the elusive longer lasting synchrotron radio afterglow known to follow binary neutron star mergers. We use LOFAR to study GW170817, the only merger radio afterglow ever to be associated with detected gravitational waves (Ch. 6). Such radio afterglows are encoded with rich high-energy physics but are difficult to find owing to their poorly constrained locations. We develop a LOFAR strategy in Ch. 7 that will enable us to probe the full location uncertainty region in the next gravitational wave observing run
Proline-rich tyrosine kinase 2 mediates gonadotropin-releasing hormone signaling to a specific extracellularly regulated kinase-sensitive transcriptional locus in the luteinizing hormone beta-subunit gene
No full textG protein-coupled receptor regulation of gene transcription primarily occurs through the phosphorylation of transcription factors by MAPKs. This requires transduction of an activating signal via scaffold proteins that can ultimately determine the outcome by binding signaling kinases and adapter proteins with effects on the target transcription factor and locus of activation. By investigating these mechanisms, we have elucidated how pituitary gonadotrope cells decode an input GnRH signal into coherent transcriptional output from the LH beta-subunit gene promoter. We show that GnRH activates c-Src and multiple members of the MAPK family, c-Jun NH2-terminal kinase 1/2, p38MAPK, and ERK1/2. Using dominant-negative point mutations and chemical inhibitors, we identified that calcium-dependent proline-rich tyrosine kinase 2 specifically acts as a scaffold for a focal adhesion/cytoskeleton-dependent complex comprised of c-Src, Grb2, and mSos that translocates an ERK-activating signal to the nucleus. The locus of action of ERK was specifically mapped to early growth response-1 (Egr-1) DNA binding sites within the LH beta-subunit gene proximal promoter, which was also activated by p38MAPK, but not c-Jun NH2-terminal kinase 1/2. Egr-1 was confirmed as the transcription factor target of ERK and p38MAPK by blockade of protein expression, transcriptional activity, and DNA binding. We have identified a novel GnRH-activated proline-rich tyrosine kinase 2-dependent ERK-mediated signal transduction pathway that specifically regulates Egr-1 activation of the LH beta-subunit proximal gene promoter, and thus provide insight into the molecular mechanisms required for differential regulation of gonadotropin gene expression
Cl NQR and calorimetric reinvestigation of the incommensurate phase of bis(4-chlorophenyl) sulfone : evidence for no lock-in transition
No full textCl NQR and calorimetric measurements in BCPS have been reinvestigated between 77 K and room temperature (NQR) and between 100 K and 180 K (). It is shown that, in contradiction with conclusions found in literature, there is no lock-in transition at 115 K. These new results are in agreement with those obtained with other experimental techniques: Raman scattering, X-ray diffraction, neutron diffraction and proton NMR; the latter indicate that the incommensurate phase, appearing on cooling at 150 K, persists down to 4.2 K, at least. The shape of the Cl NQR spectrum in the BCPS incommensurate phase shows that the frequency of a chlorine nucleus is a function of the order parameter, through linear and quadratic terms
Optical outburst of the B[e]/X-Ray system CI Cam/XTE J0421+560
No full textWe report a very strong optical outburst of the B[e]/transient X-ray system CI Cam/XTE J0421+560. On 2016 Oct 09.064 we measured H alpha at 80 times the continuum flux level, similar to the previous reported outbursts in 1998 (Hynes et al., 2002) and 2005 (Yan et al., 2007)
Constraining a neutron star merger origin for localized fast radio bursts
No full textWhat the progenitors of fast radio bursts (FRBs) are, and whether there are multiple types of progenitors are open questions. The advent of localized FRBs with host galaxy redshifts allows the various emission models to be directly tested for the first time. Given the recent localizations of two non-repeating FRBs (FRB 180924 and FRB 190523), we discuss a selection of FRB emission models and demonstrate how we can place constraints on key model parameters such as e magnetic field strength and age of the putative FRB-emitting neutron star. In particular, we focus on models related to compact binary merger events involving at least one neutron star, motivated by commonalities between the host galaxies of the FRBs and the hosts of such merger events/short gamma-ray bursts (SGRBs). We rule out the possibility that either FRB was produced during the final inspiral stage of a merging binary system. Where possible, we predict the light curve of electromagnetic emission associated with a given model and use it to recommend multiwavelength follow-up strategies that may help confirm or rule out models for future FRBs. In addition, we conduct a targeted sub-threshold search in Fermi Gamma-ray Burst Monitor data for potential SGRB candidates associated with either FRB, and show what a non-detection means for relevant models. The methodology presented in this study may be easily applied to future localized FRBs, and adapted to sources with possibly core-collapse supernova progenitors, to help constrain potential models for the FRB population at large
Optical outburst of the B[e]/X-Ray system CI Cam/XTE J0421+560
No full textWe report a very strong optical outburst of the B[e]/transient X-ray system CI Cam/XTE J0421+560. On 2016 Oct 09.064 we measured H alpha at 80 times the continuum flux level, similar to the previous reported outbursts in 1998 (Hynes et al., 2002) and 2005 (Yan et al., 2007)
Preferential role of calcium in the regulation of prolactin gene transcription by thyrotropin-releasing hormone in GH3 pituitary cells
No full textpeer reviewedTRH induces two separate events in pituitary PRL cells. It increases the release of stored PRL and enhances the rate of PRL gene transcription, which results in an increased steady state concentration of PRL messenger RNA (mRNA) and a concomitant augmentation of PRL production. The mechanisms underlying the release process involve the activation of phosphatidylinositol turnover which generates inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. In order to determine whether these intracellular messengers also mediate the stimulation of PRL gene expression by TRH, we have correlated the level of receptor occupancy with the rate of gene transcription and investigated the action of drugs which increase cytosolic calcium or activate protein kinase C. We have determined that sustained stimulation of transcription requires the persistent occupancy of a limited number of TRH receptor sites and that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), calcium ionophores (A23187, ionomycin), and the calcium channel agonist BAY K 8644 enhance PRL gene transcription. However, TPA is less potent and ionomycin requires a low concentration of TPA to fully mimic TRH action, whereas BAY K 8644 alone displays the same potency as TRH. The effects of BAY K 8644 and TRH are not additive and thus suggest that the influx of calcium plays a predominant role in the regulation of PRL gene transcription by TRH
LOFAR observations of gravitational wave merger events: O3 results and O4 strategy
No full textThe electromagnetic counterparts to gravitational wave (GW) merger events
hold immense scientific value, but are difficult to detect due to the typically
large localisation errors associated with GW events. The Low-Frequency Array
(LOFAR) is an attractive GW follow-up instrument owing to its high sensitivity,
large instantaneous field of view, and ability to automatically trigger on
events to probe potential prompt emission within minutes. Here, we report on
144-MHz LOFAR radio observations of three GW merger events containing at least
one neutron star that were detected during the third GW observing run.
Specifically, we probe 9 and 16 per cent of the location probability density
maps of S190426c and S200213t, respectively, and place limits at the location
of an interesting optical transient (PS19hgw/AT2019wxt) found within the
localisation map of S191213g. While these GW events are not particularly
significant, we use multi-epoch LOFAR data to devise a sensitive wide-field GW
follow-up strategy to be used in future GW observing runs. In particular, we
improve on our previously published strategy by implementing direction
dependent calibration and mosaicing, resulting in nearly an order of magnitude
increase in sensitivity and more uniform coverage. We achieve a uniform
sensitivity of Jy across a single instantaneous LOFAR
pointing's 21 deg core, and a median sensitivity of 1.1 mJy when
including the full 89 deg hexagonal beam pattern. We also place the
deepest transient surface density limits yet on of order month timescales for
surveys between 60--340 MHz (0.017 deg above mJy and 0.073
deg above mJy).Comment: Replaced with accepted version for publication in MNRA
A quantitative approach for comparing modeled biospheric carbon flux estimates across regional scales
No full textAbstract. Given the large differences between biospheric model estimates of regional carbon exchange, there is a need to understand and reconcile the predicted spatial variability of fluxes across models. This paper presents a set of quantitative tools that can be applied for comparing flux estimates in light of the inherent differences in model formulation. The presented methods include variogram analysis, variable selection, and geostatistical regression. These methods are evaluated in terms of their ability to assess and identify differences in spatial variability in flux estimates across North America among a small subset of models, as well as differences in the environmental drivers that appear to have the greatest control over the spatial variability of predicted fluxes. The examined models are the Simple Biosphere (SiB 3.0), Carnegie Ames Stanford Approach (CASA), and CASA coupled with the Global Fire Emissions Database (CASA GFEDv2), and the analyses are performed on model-predicted net ecosystem exchange, gross primary production, and ecosystem respiration. Variogram analysis reveals consistent seasonal differences in spatial variability among modeled fluxes at a 1°×1° spatial resolution. However, significant differences are observed in the overall magnitude of the carbon flux spatial variability across models, in both net ecosystem exchange and component fluxes. Results of the variable selection and geostatistical regression analyses suggest fundamental differences between the models in terms of the factors that control the spatial variability of predicted flux. For example, carbon flux is more strongly correlated with percent land cover in CASA GFEDv2 than in SiB or CASA. Some of these factors can be linked back to model formulation, and would have been difficult to identify simply by comparing net fluxes between models. Overall, the quantitative approach presented here provides a set of tools for comparing predicted grid-scale fluxes across models, a task that has historically been difficult unless standardized forcing data were prescribed or a detailed sensitivity analysis was performed.
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Pulsar revival in neutron star mergers: multimessenger prospects for the discovery of pre-merger coherent radio emission
No full textWe investigate pre-merger coherent radio emission from neutron star mergers arising due to the magnetospheric interaction between compact objects. We consider two plausible radiation mechanisms, and show that if one neutron star has a surface magnetic field Bs ≥ 1012G, coherent millisecond radio bursts with characteristic temporal morphology and inclination angle dependence are observable to Gpc distances with next-generation radio facilities. We explore multi-messenger and multi-wavelength methods of identification of a neutron star merger origin of radio bursts, such as in fast radio burst surveys, triggered observations of gamma-ray bursts and gravitational wave events, and optical/radio follow-up of fast radio bursts in search of kilonova and radio afterglow emission. We present our findings for current and future observing facilities, and make recommendations for verifying or constraining the model
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