What is learned from high energy bursts and flares

The Energetic Gamma Ray Experiment Telescope (EGRET) with its large Nal Total Absorption Shower Counter (TASC) has the scientific capability of performing spectroscopy of high energy cosmic gamma ray bursts and solar flares. EGRET, with a spectroscopy energy range from 0.6 to 140 MeV, provides an opportunity to increase the understanding of the high energy mechanisms of gamma ray bursts and solar flares. A likely interpretation of gamma ray burst sources is that they are rotating, magnetized neutron stars. High magnetic fields can influence the emission of high energy gamma rays, so observational spectroscopic data at high energies can provide information on the upper limits of the magnetic fields in the GRB regions of magnetized neutron stars. Likewise, spectroscopy of high energy gamma rays can provide information useful for deriving the flare proton spectrum which in turn can lead to an understanding of high energy solar flare particle acceleration mechanisms

GeV Emission from Prompt and Afterglow Phases of Gamma-Ray Bursts

We investigate the GeV emission from gamma-ray bursts (GRBs), using the results from the Energetic Gamma Ray Experimental Telescope (EGRET), and in view of the Gamma-ray Large Area Space Telescope (GLAST). Assuming that the conventional prompt and afterglow photons originate from synchrotron radiation, we compare an accompanying inverse-Compton component with EGRET measurements and upper limits on GeV fluence, taking Klein-Nishina feedback into account. We find that EGRET constraints are consistent with the theoretical framework of the synchrotron self-Compton model for both prompt and afterglow phases, and discuss constraints on microphysical parameters in both phases. Based on the inverse-Compton model and using EGRET results, we predict that GLAST would detect GRBs with GeV photons at a rate >~20 yr^{-1} from each of the prompt and afterglow phases. This rate applies to the high-energy tail of the prompt synchrotron emission and to the inverse-Compton component of the afterglow. Theory predicts that in a large fraction of the cases where synchrotron GeV prompt emission would be detected by GLAST, inverse-Compton photons should be detected as well at high energies >~10 GeV. Therefore GLAST will enable a more precise test of the high-energy emission mechanism. Finally, we show that the contribution of GRBs to the flux of the extragalactic gamma-ray background measured with EGRET is at least 0.01% and likely around 0.1%.Comment: 11 pages, 7 figures; accepted by Ap

Economia da produção de pimenta vermelha no município de Turuçu-RS.

bitstream/item/30264/1/boletim-19.pd

Evidence for Post-Quiescent, High-Energy Emission from Gamma-Ray Burst 990104

It is well known that high-energy emission (MeV-GeV) has been observed in a number of gamma-ray bursts, and temporally-extended emission from lower energy gamma rays through radio wavelengths is well established. An important observed characteristic of some bursts at low energy is quiescence: an initial emission followed by a quiet period before a second (postquiescent) emission. Evidence for significant high-energy, postquiescent emission has been lacking. Here we present evidence for high-energy emission, coincident with lower energy emission, from the postquiescent emission episode of the very bright and long burst, GRB 990104. We show light curves and spectra that confirm emission above 50 MeV, approximately 152 seconds after the BATSE trigger and initial emission episode. Between the initial emission episode and the main peak, seen at both low and high energy, there was a quiescent period of ~100 s during which the burst was relatively quiet. This burst was found as part of an ongoing search for high-energy emission in gamma-ray bursts using the EGRET fixed interval (32 s) accumulation spectra, which provide sensitivity to later, high-energy emission that is otherwise missed by the standard EGRET BATSE-triggered burst spectra.Comment: 5 pages, including 5 figures. Missing citation added to introduction. Accepted for publication in ApJ

Small group activities within academic communities improve the connectedness of students and faculty.

BackgroundThe University of California, San Diego, School of Medicine implemented a curriculum change that included reduction of lectures, incorporation of problem-based learning and other small group activities. Six academic communities were introduced for teaching longitudinal curricular content and organizing extracurricular activities.MethodsSurveys were collected from 904 first- and second-year medical students over 6 years. Student satisfaction data with their sense of connectedness and community support were collected before and after the implementation of the new curriculum. In a follow-up survey, medical students rated factors that contributed to their sense of connectedness with faculty and students (n = 134).ResultsStudents' perception of connectedness to faculty significantly increased following implementation of a curriculum change that included academic communities. Students ranked small group clinical skills activities within academic communities significantly higher than other activities concerning their sense of connectedness with faculty. Students' perception of connectedness among each other was high at baseline and did not significantly change. Small group activities scored higher than extracurricular activities regarding students' connectedness among themselves.ConclusionsThe implementation of a new curriculum with more small group educational activities including academic communities enhanced connectedness between students and faculty and resulted in an increased sense of community

A summer prematriculation program to help students succeed in medical school.

Medical schools with a diverse student body face the challenge of ensuring that all students succeed academically. Many medical schools have implemented prematriculation programs to prepare students from diverse backgrounds; however, evidence on their impact is largely lacking. In this study, we analyzed participants' demographics as well as the impact of the prematriculation program on Year 1 performance. Predictive validity of the program was assessed and compared to other traditional predictors, including grade point average (GPA) and Medical College Admission Test (MCAT) scores and subscores. Linear mixed effect models determined the impact of the prematriculation program, and linear regression analysis assessed the predictive value of the overall score in the prematriculation program and other traditional predictors. Demographics of students participating in the prematriculation program from 2013 to 2015 (n = 75) revealed a significantly higher prevalence of academically disadvantaged students including older students, students with lower GPA and MCAT scores and students of racial and ethnic populations that are underrepresented in medicine, compared to non-participants (n = 293). Participants performed significantly better in Year 1 courses that were covered in the prematriculation program compared to courses that were not covered. The overall performance in the prematriculation program correlated significantly with Year 1 performance and was found to be a strong predictor for Year 1 performance. This study suggests that a prematriculation program can help students to succeed in the first year of medical school. The results have implications for medical schools seeking to implement or evaluate the effectiveness of their prematriculation program

Pair Production Absorption Troughs in Gamma-Ray Burst Spectra: A Potential Distance Discriminator

Relativistic bulk motion with large Lorentz factors has recently been inferred for gamma-ray bursts regardless of whether they are of galactic or cosmological origin. This conclusion results from calculations of internal pair production transparency in bursts that usually assume an infinite power-law source spectrum for simplicity, an approximation that is quite adequate for some bursts detected by EGRET. However, for a given bulk Lorentz factor \teq{\Gamma}, photons above the EGRET range can potentially interact with sub-MeV photons in such calculations. Hence it is essential to accurately address the spectral curvature in bursts seen by BATSE. In this paper we present the major properties induced in photon-photon opacity considerations by such spectral curvature. The observed spectral breaks around 1 MeV turn out to be irrelevant to opacity in cosmological bursts, but are crucial to estimates of source transparency in the 1 GeV -- 1 TeV range for sources located in the galactic halo. We find that broad absorption troughs can arise at these energies for suitable bulk motion parameters \teq{\Gamma}. Such troughs are probably an unambiguous signature of a galactic halo population, and if observed by experiments such as Whipple, MILAGRO and GLAST, would provide powerful evidence that such bursts are not at cosmological distances.Comment: 10 pages, AASTeX format, including 2 eps figures, ApJLett in pres

Search for Sub-TeV Gamma Rays Coincident with BATSE Gamma Ray Bursts

Project GRAND is a 100m x 100m air shower array of proportional wire chambers (PWCs). There are 64 stations each with eight 1.29 m^2 PWC planes arranged in four orthogonal pairs placed vertically above one another to geometrically measure the angles of charged secondaries. A steel plate above the bottom pair of PWCs differentiates muons (which pass undeflected through the steel) from non-penetrating particles. FLUKA Monte Carlo studies show that a TeV gamma ray striking the atmosphere at normal incidence produces 0.23 muons which reach ground level where their angles and identities are measured. Thus, paradoxically, secondary muons are used as a signature for gamma ray primaries. The data are examined for possible angular and time coincidences with eight gamma ray bursts (GRBs) detected by BATSE. Seven of the GRBs were selected because of their good acceptance by GRAND and high BATSE Fluence. The eighth GRB was added due to its possible coincident detection by Milagrito. For each of the eight candidate GRBs, the number of excess counts during the BATSE T90 time interval and within plus or minus five degrees of BATSE's direction was obtained. The highest statistical significance reported in this paper (2.7 sigma) is for the event that was predicted to be the most likely to be observed (GRB 971110).Comment: To be presented at the XXVIII International Cosmic Ray Conference, Tsukuba, Japa

EGRET Spectral Index and the Low-Energy Peak Position in the Spectral Energy Distribution of EGRET-Detected Blazars

In current theoretical models of the blazar subclass of active galaxies, the broadband emission consists of two components: a low-frequency synchrotron component with a peak in the IR to X-ray band, and a high-frequency inverse Compton component with a peak in the gamma-ray band. In such models, the gamma-ray spectral index should be correlated with the location of the low-energy peak, with flatter gamma-ray spectra expected for blazars with synchrotron peaks at higher photon energies and vice versa. Using the EGRET-detected blazars as a sample, we examine this correlation and possible uncertainties in its construction.Comment: 17 pages including 1 figure, accepted for publication in The Astrophysical Journa