Modeling gamma-ray bursts

Discovered serendipitously in the late 1960s, gamma-ray bursts (GRBs) are huge explosions of energy that happen at cosmological distances. They provide a grand physical playground to those who study them, from relativistic effects such as beaming, jets, shocks and blastwaves to radiation mechanisms such as synchrotron radiation to galatic and stellar populations and history. Through the Swift and Fermi space telescopes dedicated to observing GRBs over a wide range of energies(from keV to GeV), combined with accurate pinpointing that allows ground based follow-up observations in the optical, infrared and radio, a rich tapestry of GRB observations has emerged. The general picture is of a mysterious central engine (CE) probably composed of a black hole or neutron star that ejects relativistic shells of matter into intense magnetic fields. These shells collide and combine, releasing energy in internal shocks accounting for the prompt emission and flaring we see and the external shock or plowing of the first blastwave into the ambient surrounding medium has well-explained the afterglow radiation. We have developed a shell model code to address the question of how X-ray flares are produced within the framework of the internal shock model. The shell model creates randomized GRB explosions from a central engine with multiple shells and follows those shells as they collide, merge and spread, producing prompt emission and X-ray flares. We have also included a blastwave model, which can constrain X-ray flares and explain the origin of high energy (GeV) emission seen by the Fermi telescope. Evidence suggests that gamma-ray prompt emission and X-ray flares share a common origin and that at least some flares can only be explained by long-lasting central engine activity. We pay special attention to the time history of central engine activity, internal shocks, and observed flares. We calculate the gamma-ray (Swift/BAT band) and X-ray (Swift/XRT band) lightcurves for arbitrary central engine activity and compare the model results with the observational data. We show that the observed X-ray flare phenomenology can be explained within the internal shock model. The number, width and occurring time of flares are then used to diagnose the central engine activity, putting constraints on the energy, ejection time, width and number of ejected shells. We find that the observed X-ray flare time history generally reflects the time history of the central engine, which reactivates multiple times after the prompt emission phase with progressively reduced energy. This shell model code can be used to constrain broadband observations of GRB 090926A, which showed two flares in both the Swift UVOT and XRT bands. Using the prompt emission fluence to constrain the total energy contained in the blastwave, the internal shock model requires that Lorentz factors of the shells causing flares must be less than the Lorentz factor of the blastwave when the shells are ejected. Recent observations of Gamma-Ray Bursts (GRBs) by the Fermi Large Area Telescope (LAT) revealed a power law decay feature of the high energy emission (above 100 MeV), which led to the suggestion that it originates from an external shock. We analyze four GRBs (080916C, 090510, 090902B and 090926A) jointly detected by Fermi LAT and Gamma-ray Burst Monitor (GBM), which have high quality lightcurves in both instrument energy bands. Using the MeV prompt emission (GBM) data, we can record the energy output from the central engine as a function of time. Assuming a constant radiative efficiency, we are able to track energy accumulation in the external shock using our internal/external shell model code and show that the late time lightcurves fit well within the external shock model, but the early time lightcurves are dominated by the internal shock component which has a shallow decay phase due to the initial pile-up of shells onto the blast wave

Rethinking residue, an investigation of pharyngeal residue on flexible endoscopic evaluation of swallowing: the past, present, and future directions

This dissertation investigated measures of pharyngeal residue as seen on flexible endoscopic evaluation of swallowing (FEES). Research in this area of deglutology has been stalled due to measurement problems. The particular aims of this project were to compare visual analog scale ratings to categorical ratings of residue on FEES, and to investigate various measurement aspects. METHODS: Speech language pathologists were asked to rate residue from 81 swallows on FEES that demonstrated a wide range of residue severity for thin liquid, applesauce, and cracker boluses. A total of 33 clinicians rated the amount of residue at the time point after the first swallow, twice in a randomized fashion: the first time on a visual analog scale (VAS) and the second time categorically on a five point Likert scale. The results were analyzed for (1) inter/intra-rater agreement, (2) correlations between ratings and residue severity for each rating method, and (3) clusters of ratings to better define the scales and their clinical significance. A total of 2,673 VAS ratings and 2,673 categorical ratings were collected. RESULTS: (1) Both inter- and intra-rater reliability met acceptable levels of agreement, although intra-rater reliability on VAS ratings were slightly higher (r=0.8–0.9) than categorical ratings (k=0.7–0.8). Expert ratings were not significantly different from other clinicians’ ratings for any severity of any of the 3 boluses. (2) Residue ratings fit best on a curvilinear model; a quadratic fit of the data significantly improved the r2 values for each bolus type. (3) An increased residue amount, rated on either the VAS or categorical scale, was significantly associated with worse penetration-aspiration scale scores, but no significant relationship was found between the two methods of residue ratings and measures of quality of life or diet. Novel computerized methods are proposed for future measurement pursuits. CONCLUSION: The results of this dissertation suggest that residue is best measured on a scale with unequal intervals, and clinicians can be reliable in rating overall amount of residue on FEES after the first swallow. Novel computerized measurement approaches are useful building blocks for future research. It is hoped that with better measurement will come better understanding of residue, its risks, and consequences

The efficacy of the Masako (tongue-hold) maneuver

Thesis (M.S.)--Boston UniversityPurpose: Clinicians commonly recommend the tongue-hold maneuver, also called the Masako, as an exercise to strengthen swallowing muscles. Although this exercise is widely used, limited empirical data support this maneuver as an effective exercise. The goal ofthe present study is to observe, over multiple sessions, the effects ofthe tongue- hold maneuver as a 6-week exercise in subjects with dysphagia. The results ofthis study will help to address whether the tongue-hold maneuver is beneficial and, if so, which muscle groups are strengthened by this exercise. Methods: Five subjects with dysphagia and one healthy adult performed a set oftongue- hold maneuvers 3 times a day, 5 days per week, for 6 weeks. The number o f repetitions per set was individually calculated based on 80% of the maximal repetitions until fatigue. At baseline and 6 weeks, 4 measures were observed: a subject-reported quality-of-life swallowing scale, lingual strength, the amount of residue in the valleculae, and the pressures generated by pharyngeal muscles during a normal swallow. Four healthy adults who did not perform the tongue-hold maneuver were used as controls for the lingual measures, completing the measures of lingual strength at baseline, 3 weeks, and 6 weeks. Results: No overt trends in the subject-reported swallowing scale were noted; after 6 weeks of exercise, about half ranked their swallowing as worse and half ranked their swallowing as better. The treatment group demonstrated a non-significant overall2.3% increase in anteromedian lingual strength and 8.4% increase in posteromedian lingual strength. These changes did not set the treatment group apart from the control group, who demonstrated an increase of3.8% and 6.3% in the anteromedian and posteromedian positions, respectively. Regarding pharyngeal residue, 2 subjects did not show any changes in residue scores. However, the other 3 subjects demonstrated reduced residue in the valleculae with a cracker bolus. Out ofthe 3 subjects who were measured with manometry, 2 demonstrated higher oropharyngeal pressures on normal swallows after 6 weeks of exercise, although great variability was present. These results are limited by the small sample size and heterogeneity of the treatment group, as well as high variability in instrumental measurements. Conclusion: This study investigated the tongue-hold maneuver as an exercise and provides preliminary support for its use, with caution. Specifically, clinicians should be sure to prescribe regimens that fatigue swallowing muscles and push them past normal use. When using the Iowa Oral Performance Instrument (IOPI) as a tool, clinicians should also keep in mind that a learning effect is likely to occur over the first few trials. This pilot study suggests that clinicians should continue to prescribe the tongue-hold maneuver as an exercise with caution, as some patients may benefit from it while others may not. Further investigation is required

Rethinking residue, an investigation of pharyngeal residue on flexible endoscopic evaluation of swallowing: the past, present, and future directions

This dissertation investigated measures of pharyngeal residue as seen on flexible endoscopic evaluation of swallowing (FEES). Research in this area of deglutology has been stalled due to measurement problems. The particular aims of this project were to compare visual analog scale ratings to categorical ratings of residue on FEES, and to investigate various measurement aspects. METHODS: Speech language pathologists were asked to rate residue from 81 swallows on FEES that demonstrated a wide range of residue severity for thin liquid, applesauce, and cracker boluses. A total of 33 clinicians rated the amount of residue at the time point after the first swallow, twice in a randomized fashion: the first time on a visual analog scale (VAS) and the second time categorically on a five point Likert scale. The results were analyzed for (1) inter/intra-rater agreement, (2) correlations between ratings and residue severity for each rating method, and (3) clusters of ratings to better define the scales and their clinical significance. A total of 2,673 VAS ratings and 2,673 categorical ratings were collected. RESULTS: (1) Both inter- and intra-rater reliability met acceptable levels of agreement, although intra-rater reliability on VAS ratings were slightly higher (r=0.8–0.9) than categorical ratings (k=0.7–0.8). Expert ratings were not significantly different from other clinicians’ ratings for any severity of any of the 3 boluses. (2) Residue ratings fit best on a curvilinear model; a quadratic fit of the data significantly improved the r2 values for each bolus type. (3) An increased residue amount, rated on either the VAS or categorical scale, was significantly associated with worse penetration-aspiration scale scores, but no significant relationship was found between the two methods of residue ratings and measures of quality of life or diet. Novel computerized methods are proposed for future measurement pursuits. CONCLUSION: The results of this dissertation suggest that residue is best measured on a scale with unequal intervals, and clinicians can be reliable in rating overall amount of residue on FEES after the first swallow. Novel computerized measurement approaches are useful building blocks for future research. It is hoped that with better measurement will come better understanding of residue, its risks, and consequences

Service Recovery\u27s Influence on Complainant Attitudes and Intentions: A Perceived Justice Framework.

I conducted two field studies (one in the banking industry, n = 692, and one in the new home industry, n = 339) that examined service recovery\u27s influence on complainant perceptions of perceived justice (i.e., distributive, procedural, and interactional justice), satisfaction (i.e., transaction-specific satisfaction and overall firm satisfaction), and intentions (i.e., word-of-mouth intent and purchase intent). The studies assessed these perceptions over multiple time periods (i.e., post failure, post service recovery, and two weeks subsequent to service recovery) to better capture the perceptions as they form over time. The dissertation uses an equity theory framework in a service recovery context, where the model tests the direct influence that justice may have on both transaction-specific and overall satisfaction with a failing firm. The model then posits that satisfaction perceptions directly influence one\u27s propensity to recommend a firm, as well as repurchase in the future. The model asserts that perceived justice best explains positive word-of-mouth and purchase intentions indirectly through satisfaction. Across the two studies, the model explained 38 to 45 percent of the variance in transaction-specific satisfaction; 40 to 44 percent of the variance in overall firm satisfaction; 34 to 38 percent of the variance in word-of-mouth; and 32 to 36 percent of the variance in purchase intent. The results support the assertion that distributive justice is more influential in forming transaction-specific perceptions, while procedural justice is more influential in forming overall perceptions (Lind and Tyler 1988; McFarlin and Sweeney 1992). The results also suggest that consumers may view product and service failures differently. Specifically, consumers who experience product failures may be most concerned about distributive justice (compared with procedural and interactional justice). Consumers experiencing service failures, alternatively, seem to not only expect distributive justice, but also expect higher levels (relative to those experiencing product failures) of procedural and interactional justice. Lastly, the data here suggest that transaction-specific satisfaction is the best route to positive word-of-mouth recommendations, while overall firm satisfaction is the best route to future purchase intentions

GRB 090926A and Bright Late-time Fermi LAT GRB Afterglows

GRB 090926A was detected by both the GBM and LAT instruments on-board the Fermi Gamma-Ray Space Telescope. Swift follow-up observations began ~13 hours after the initial trigger. The optical afterglow was detected for nearly 23 days post trigger, placing it in the long lived category. The afterglow is of particular interest due to its brightness at late times, as well as the presence of optical flares at T0+10^5 s and later, which may indicate late-time central engine activity. The LAT has detected a total of 16 GRBs; 9 of these bursts, including GRB 090926A, also have been observed by Swift. Of the 9 Swift observed LAT bursts, 6 were detected by UVOT, with 5 of the bursts having bright, long-lived optical afterglows. In comparison, Swift has been operating for 5 years and has detected nearly 500 bursts, but has only seen ~30% of bursts with optical afterglows that live longer than 10^5 s. We have calculated the predicted gamma-ray fluence, as would have been seen by the BAT on-board Swift, of the LAT bursts to determine whether this high percentage of long-lived optical afterglows is unique, when compared to BAT-triggered bursts. We find that, with the exception of the short burst GRB 090510A, the predicted BAT fluences indicate the LAT bursts are more energetic than 88% of all Swift bursts, and also have brighter than average X-ray and optical afterglows.Comment: 8 pages, 4 figures, submitted to ApJ Letter

Flare-less long Gamma-ray Bursts and the properties of their massive star progenitors

While there is mounting evidence that long Gamma-Ray Bursts (GRBs) are associated with the collapse of massive stars, the detailed structure of their pre-supernova stage is still debatable. Particularly uncertain is the degree of mixing among shells of different composition, and hence the role of magnetic torques and convection in transporting angular momentum. Here we show that early-time afterglow observations with the Swift satellite place constraints on the allowed GRB pre-supernova models. In particular, they argue against pre-supernova models in which different elemental shells are unmixed. These types of models would produce energy injections into the GRB engine on timescales between several hundreds of seconds to a few hours. Flaring activity has {\em not} been observed in a large fraction of well-monitored long GRBs. Therefore, if the progenitors of long GRBs have common properties, then the lack of flares indicates that the massive stars which produce GRBs are mostly well mixed, as expected in low-metallicity, rapidly rotating massive stars.Comment: Accepted to ApJLetter