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Parameterizing the gamma-ray emission of short GRB jets
No full textThe goal of this phase of our research project was to compare current competing models of the structure of the prompt gamma-ray emission that results from the merging of two neutron stars in a binary system, and to see how the differences in proposed parameters affect the detectability of these events from Earth at off-axis angles. A series of programs were written in Python to reproduce two models: a simulation based on the jointly detected GRB170817 by Kathirgamaraju et. al 2017, and a piecewise analytical approximation by Beniamini and Nakar 2018. The simulated model was tested singularly in how the properties of the luminosity profile changed as the parameter gammamin - the minimum allowed speed of the jet that defined its boundary - was raised and lowered, effectively widening and narrowing the jet structure and allowing for more or less successful detection events at large angles. We then attempted to find a broken-power-law approximation of this structure by visually fitting a modified equation from Beniamini and Nakar and generating a new luminosity profile from it. It was found that the modified equation-based structure was able to accurately produce profiles that matched the behavior of both the original simulation and the fully analytical model, though a discrepancy between the two becomes apparent in their comparison that the analytical model predicts significantly higher luminosities at large observation angles than the simulation. Continuing work with this project involves statistically testing the impact of this discrepancy on the rate of successful detection of events
From black hole to afterglow
No full textJets, launched by compact objects, collide with the interstellar medium and emit across the electromagnetic spectrum. We refer to this emission as the "afterglow". We use the HARMPI code to simulate a jet and extract the jet energy as a function of angle from the jet axis. We are currently writing a C++ code to calculate the afterglow as a function of arbitrary observer angle. The jet properties from HARMPI will be used as an input for the afterglow calculation in our C++ code and will allow us to predict several observable features. This project is motivated by the recent detection of the GW170817 afterglow and it is a necessary step towards being able to analyze future detections
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High-energy emission and recent afterglow studies of gamma-ray bursts
Get PDFtextGamma-ray Bursts (GRBs) are powerful explosions that emit
most of their energy, as their name suggests, in gamma-rays of typical energies
of about 1 MeV. This emission lasts for about two minutes or less and it is called the prompt emission. The isotropic energy radiated in GRBs is equivalent to the energy that the
Sun will radiate in its entire lifetime. After decades of studying
this cosmological phenomenon, we have come to learn that it involves
a collimated and relativistic jet. Also, we know that they
radiate energy in the X-ray, optical and radio bands for days, weeks and years, respectively, which is called the
afterglow. Recently, NASA's Fermi Satellite was launched and,
in addition to MeV photons, it detected GeV photons from these astrophysical sources.
We show that these GeV photons are produced when the GRB jet interacts with the medium
that surrounds it: the external forward shock model. We arrive at this conclusion
not only by studying the GeV emission, but also by studying the afterglow observations
(Chapter 2). We corroborate this model by studying the electron acceleration
in the external forward shock model and find that electrons can radiate at the maximum observed
energy of ~ 10 GeV (Chapter 3). We also provide an extensive analysis of the most recent
afterglow observations of GRB 090902B within the same
framework of an external forward shock origin. We find
that the data for this burst requires a small deviation from the
traditionally used power-law electron energy distribution, however,
our previous results remain unchanged (Chapter 4). To conclude,
we use the end of the prompt emission phase, which exhibits a
steep X-ray temporal decay, to constrain the behavior of the
central engine responsible for launching the relativistic jet (Chapter 5).Physic
A Millimeter Rebrightening in GRB 210702A
Get PDFWe present X-ray to radio frequency observations of the bright long gamma-ray burst GRB 210702A. Our Atacama Large Millimeter/submillimeter Array 97.5 GHz observations show a significant rebrightening by a factor of ≈2 beginning at 8.2 days post-burst and rising to peak brightness at 18.1 days before declining again. This is the first such rebrightening seen in a millimeter afterglow light curve. A standard forward shock model in a stellar wind circumburst medium can explain most of our X-ray, optical, and millimeter observations prior to the rebrightening, but significantly overpredicts the self-absorbed radio emission, and cannot explain the millimeter rebrightening. We investigate possible explanations for the millimeter rebrightening, and find that energy injection or a reverse shock from a late-time shell collision are plausible causes. Similar to other bursts, our radio data may require alternative scenarios such as a thermal electron population or a structured jet to explain the data. Our observations demonstrate that millimeter light curves can exhibit some of the rich features more commonly seen in optical and X-ray afterglow light curves, motivating further millimeter wavelength studies of GRB afterglows
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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