SEARCH FOR GRAVITATIONALLY REDSHIFTED 2.2 MEV LINE FROM 4U 1820-30

We have analyzed 1.7 Ms of the INTEGRAL data of the Low Mass X-Ray Binary (LMXB) 4U 1820-30 and searched for the redshifted 2.2 MeV neutron capture gamma-ray line. This source is unique in that it is thought to be accreting pure Helium and might be a powerful 2.2 MeV line source. If detected, this line would strongly constrain the neutron star equation of state, motivating this search. The line is expected to be redshifted to 1.30-1.72 MeV so we scanned the 1-2 MeV region. Although we failed to detect the redshifted 2.2 MeV line, mainly due to the intense background noise to which INTEGRAL is exposed, we placed upper limits on the source’s flux for different line widths. We plan to do analysis on the rest of the data (over 8 Ms) in the future

A Combined Compton and Coded-aperture Telescope for Medium-energy Gamma-ray Astrophysics

A future mission in medium-energy gamma-ray astrophysics would allow for many scientific advancements, e.g. a possible explanation for the excess positron emission from the Galactic Center, a better understanding of nucleosynthesis and explosion mechanisms in Type Ia supernovae, and a look at the physical forces at play in compact objects such as black holes and neutron stars. Additionally, further observation in this energy regime would significantly extend the search parameter space for low-mass dark matter. In order to achieve these objectives, an instrument with good energy resolution, good angular resolution, and high sensitivity is required. In this paper we present the design and simulation of a Compton telescope consisting of cubic-centimeter Cadmium Zinc Telluride (CdZnTe) detectors as absorbers behind a silicon tracker with the addition of a passive coded mask. The goal of the design was to create a very sensitive instrument that is capable of high angular resolution. The simulated telescope showed achievable energy resolutions of 1.68$\%$ FWHM at 511 keV and 1.11$\%$ at 1809 keV, on-axis angular resolutions in Compton mode of 2.63$^{\circ}$ FWHM at 511 keV and 1.30$^{\circ}$ FWHM at 1809 keV, and is capable of resolving sources to at least 0.2$^{\circ}$ at lower energies with the use of the coded mask. An initial assessment of the instrument in Compton imaging mode yields an effective area of 183 cm$^{2}$ at 511 keV and an anticipated all-sky sensitivity of 3.6 x 10$^{-6}$ photons cm$^{-2}$ s$^{-1}$ for a broadened 511 keV source over a 2-year observation time. Additionally, combining a coded mask with a Compton imager to improve point source localization for positron detection has been demonstrated

Search for polarization from the prompt gamma-ray emission of GRB 041219a with SPI on INTEGRAL

Measuring the polarization of the prompt γ-ray emission from gamma-ray bursts (GRBs) can significantly improve our understanding of both the GRB emission mechanisms as well as the underlying engine driving the explosion. We searched for polarization in the prompt γ-ray emission of GRB 041219a with the SPI instrument on INTEGRAL. Using multiple-detector coincidence events in the 100-350 keV energy band, our analysis yields a polarization fraction from this GRB of 98%+/-33%. Statistically, we cannot claim a polarization detection from this source. Moreover, different event selection criteria lead to even less significant polarization fractions, e.g., lower polarization fractions are obtained when higher energies are included in the analysis. We cannot strongly rule out the possibility that the measured modulation is dominated by instrumental systematics. Therefore, SPI observations of GRB 041219a do not significantly constrain GRB models. However, this measurement demonstrates the capability of SPI to measure polarization, as well as the techniques developed for this analysis

Analytical Fitting of Gamma-ray Photopeaks in Germanium Cross Strip Detectors

In an ideal germanium detector, fully-absorbed monoenergetic gamma-rays will appear in the measured spectrum as a narrow peak, broadened into a Gaussian of width determined only by the statistical properties of charge cloud generation and the electronic noise of the readout electronics. Multielectrode detectors complicate this picture. Broadening of the charge clouds as they drift through the detector will lead to charge sharing between neighboring electrodes and, inevitably, low-energy tails on the photopeak spectra. We simulate charge sharing in our germanium cross strip detectors in order to reproduce the low-energy tails due to charge sharing. Our goal is to utilize these simulated spectra to develop an analytical fit (shape function) for the spectral lines that provides a robust and high-quality fit to the spectral profile, reliably reproduces the interaction energy, noise width, and the number of counts in both the true photopeak and the low-energy tail, and minimizes the number of additional parameters. Accurate modeling of the detailed line profiles is crucial for both calibration of the detectors as well as scientific interpretation of measured spectra.Comment: Submitted to NIM

Search for Redshifted 2.2 MeV Neutron Capture Line From A0535+262 in Outburst

The Be/X-ray binary system A0535+262 underwent a giant outburst in May-June 2005, followed by a dimmer outburst in August-September 2005. This increased intensity provided an opportunity to search for redshifted neutron-capture lines from the surface of the neutron star. If discovered, such lines would constrain the neutron star equation of state, providing the motivation of this search. The spectrometer (SPI) on board the INTEGRAL satellite observed the dimmer outburst and provided the data for this research. We have not detected a line with enough significance, with the width-dependent upper limits on the broadened and redshifted neutron capture line in the range of (2 - 11) x 10^(-4) photons cm^(-2) s^(-1). To our knowledge, these are the strongest upper limits on the redshifted 2.2 MeV emission from an accreting neutron star. Our analysis of the transparency of the neutron star surface for 2.2 MeV photons shows that photons have a small but finite chance of leaving the atmosphere unscattered, which diminishes the possibility of detection.Comment: 5 pages, 6 figures, 2 tables. Accepted to the Astrophysical Journa

New Pedagogical Models Facilitated by Technology

This paper discusses the outcome of research conducted to assess the effectiveness of Work-Based Learning (WBL) from the perspective of stakeholders. WBL has increasingly become an area of interest for the higher education (HE) sector. It can support the personal and professional development of students who are already in work. The focus of the learning and development tends to be on the student’s workplace activities. Previous research has mainly considered only two stakeholder contexts, namely the learner and the academic institution. The significance of the study stems from extending the stakeholder contexts to include the employer and the professional body. The aim of this study was to assess the effectiveness of delivery of WBL from the perspective of a range of stakeholders including students, programme leaders (PLs), tutors, university support services, employers and representatives of professional bodies. Case study research methodology was adapted with mixed method research techniques for data capture and analysis using both qualitative and quantitative approaches. The study examined five (5) WBL programmes at Northumbria University in the UK. The three most influential factors in the effectiveness of WBL were found to be: quality, access and support. The contribution to new knowledge in WBL research is through a “Four-Pillar model” which has been developed to reflect the stakeholder contexts. Consideration of this model helps ensure WBL programmes cater for the current demands from the labour market. The findings of this study include factors which facilitate and/or obstruct the effective implementation of WBL programmes whilst identifying feasible strategies to overcome those challenges and share them with all stakeholders of WBL. Recommendations are made on resolving the identified issues and to extend and improve the effectiveness of WBL. Finally this paper looks at how these results could apply to encourage WBL uptake in a third world developing country like Sri Lanka where you are starting from a zero base. Sri Lanka is yet to embark on WBL formally although online distance learning is more of a reality