Origin of 10(15)-10(16) G magnetic fields in the central engine of gamma ray bursts

Various authors have suggested that the gamma-ray burst (GRB) central engine is a rapidly rotating, strongly magnetized, (similar to 10(15)-10(16) G) compact object. The strong magnetic field can accelerate and collimate the relativistic flow and the rotation of the compact object can be the energy source of the GRB. The major problem in this scenario is the difficulty of finding an astrophysical mechanism for obtaining such intense fields. Whereas, in principle, a neutron star could maintain such strong fields, it is difficult to justify a scenario for their creation. If the compact object is a black hole, the problem is more difficult since, according to general relativity it has ""no hair"" (i.e., no magnetic field). Schuster, Blackett, Pauli, and others have suggested that a rotating neutral body can create a magnetic field by non-minimal gravitational-electromagnetic coupling (NMGEC). The Schuster-Blackett form of NMGEC was obtained from the Mikhail and Wanas`s tetrad theory of gravitation (MW). We call the general theory NMGEC-MW. We investigate here the possible origin of the intense magnetic fields similar to 10(15)-10(16) G in GRBs by NMGEC-MW. Whereas these fields are difficult to explain astrophysically, we find that they are easily explained by NMGEC-MW. It not only explains the origin of the similar to 10(15)-10(16) G fields when the compact object is a neutron star, but also when it is a black hole.

Effects of gamma ray bursts in Earth`s biosphere

We continue former work on the modeling of potential effects of Gamma Ray Bursts on Phanerozoic Earth. We focus on global biospheric effects of ozone depletion and model the spectral reduction of light by NO(2) formed in the stratosphere. We also illustrate the current complexities involved in the prediction of how terrestrial ecosystems would respond to this kind of burst. We conclude that more biological field and laboratory data are needed to reach even moderate accuracy in this modeling.

Methods and results of a search for gravitational waves associated with gamma-ray bursts using the GEO 600, LIGO, and Virgo detectors

In this paper we report on a search for short-duration gravitational wave bursts in the frequency range 64 Hz-1792 Hz associated with gamma-ray bursts (GRBs), using data from GEO 600 and one of the LIGO or Virgo detectors. We introduce the method of a linear search grid to analyze GRB events with large sky localization uncertainties, for example the localizations provided by the Fermi Gamma-ray Burst Monitor (GBM). Coherent searches for gravitational waves (GWs) can be computationally intensive when the GRB sky position is not well localized, due to the corrections required for the difference in arrival time between detectors. Using a linear search grid we are able to reduce the computational cost of the analysis by a factor of O(10) for GBM events. Furthermore, we demonstrate that our analysis pipeline can improve upon the sky localization of GRBs detected by the GBM, if a high-frequency GW signal is observed in coincidence. We use the method of the linear grid in a search for GWs associated with 129 GRBs observed satellite-based gamma-ray experiments between 2006 and 2011. The GRBs in our sample had not been previously analyzed for GW counterparts. A fraction of our GRB events are analyzed using data from GEO 600 while the detector was using squeezed-light states to improve its sensitivity; this is the first search for GWs using data from a squeezed-light interferometric observatory. We find no evidence for GW signals, either with any individual GRB in this sample or with the population as a whole. For each GRB we place lower bounds on the distance to the progenitor, under an assumption of a fixed GW emission energy of 10(-2)M circle dot c(2), with a median exclusion distance of 0.8 Mpc for emission at 500 Hz and 0.3 Mpc at 1 kHz. The reduced computational cost associated with a linear search grid will enable rapid searches for GWs associated with Fermi GBM events once the advanced LIGO and Virgo detectors begin operation.

Lightning mapping observation of a terrestrial gamma-ray flash

We report the observation with the North Alabama Lightning Mapping Array (LMA) related to a terrestrial gamma-ray flash (TGF) detected by RHESSI on 26 July 2008. The LMA data explicitly show the TGF was produced during the initial development of a compact intracloud (IC) lightning flash between a negative charge region centered at about 8.5 km above sea level (-22C temperature level) a higher positive region centered at 13 km, both confined to the convective core of an isolated storm in close proximity to the RHESSI footprint. After the occurrence of an LMA source with a high peak power (26 kW), the initial lightning evolution caused an unusually large IC current moment that became detectable 2 ms after the first LMA source and increased for another 2 ms, during which the burst of gamma-rays was produced. This slowly building current moment was most likely associated with the upward leader progression, which produced an uncommonly large IC charge moment change (+90 Ckm) in 3 ms while being punctuated by a sequence of fast discharge. These observations suggest that the leader development may be involved in the TGF production. Copyright © 2010 by the American Geophysical Union.

Radio follow-up of the gamma-ray flaring gravitational lens B0218+357

B0218+357 è un blazar soggetto al lensing che si trova a z=0.944. Questo sistema consiste in due componenti compatte (A e B) e un anello di Einstein. Recentemente è stato associato ad una sorgente gamma soggetta a burst osservata con il satellite Fermi-LAT. Questo blazar ha mostrato una forte variabilità in banda γ da agosto a settembre del 2012. Gli episodi di variabilità osservati hanno consentito di misurare per la prima volta in banda gamma il ritardo temporale previsto dalla teoria del lensing gravitazionale. Le osservazioni in banda gamma sono state seguite da un programma di monitoring con il Very Long Baseline Array (VLBA) in banda radio con lo scopo di verificare l’esistenza di una correlazione tra l’emissione nelle due bande. In questa Tesi tali osservazioni radio sono state analizzate con lo scopo di studiare la variabilità di B0218+357 e, quindi, attestare la connessione tra l’emissione alle alte energie e quella in banda radio. L’obiettivo principale di questo lavoro di Tesi è quello di studiare l’evoluzione della densità di flusso, dell’indice spettrale e della morfologia delle immagini A e B e delle loro sottocomponenti. I dati analizzati sono stati ottenuti con l’interferometro VLBA a tre frequenze di osser- vazione: 2.3, 8.4 GHz (4 epoche con osservazioni simultanee alle due frequenze) e 22 GHz (16 epoche). Le osservazioni hanno coperto un periodo di circa due mesi, subito successivo al flare in banda gamma. La riduzione dei dati è stata effettuata con il pacchetto AIPS. Dall’analisi delle immagini, nella componente B è possibile riconoscere la tipica struttura nucleo-getto chiaramente a tutte e tre le frequenze, invece nella componente A questa struttura è identificabile solo a 22 GHz. A 2.3 e 8.4 GHz la risoluzione non è sufficiente a risolvere nucleo e getto della componente A e l’emissione diffusa risulta dominante. Utilizzando il metodo dello stacking sulle immagini a 2.3 GHz, è stato possibile rivelare le parti più brillanti dell’Einstein ring associato a questa sorgente. Questo è stato possibile poiché la sorgente non ha mostrato alcun segno di variabilità significativa né di struttura né di flusso nelle componenti. Quindi dall’analisi delle curve di luce delle due componenti A e B non è emersa una variabilità significativa chiaramente associabile al flare osservato in banda gamma. Per verificare questo risultato, le curve di luce ottenute sono state confrontate con le osservazioni del radio telescopio OVRO (15 GHz) nel periodo corrispondente alle nostre osservazioni. La curva di luce OVRO è risultata in pieno accordo con le curve di luce ottenute durante questo lavoro di tesi e ha confermato che B0218+257 non ha mostrato un’importante attività radio nel periodo delle osservazioni VLBA. In definitiva, la mancanza di variabilità radio associata a quella osservata nei raggi gamma può essere dovuta al fatto che la regione in cui si è originato il flare gamma è otticamente spessa alle lunghezze d’onda radio, oppure non esiste una precisa correlazione tra le due emissioni, rimanendo quindi un problema aperto da investigare.

Gamma-ray luminosity function of gamma-ray bright AGNs

Detection of gamma-ray emissions from a class of active galactic nuclei (viz blazars), has been one of the important findings from the Compton Gamma-Ray Observatory (CGRO). However, their gamma-ray luminosity function has not-been well determined. Few attempts have been made in earlier works, where BL Lacs and Flat Spectrum Radio Quasars (FSRQs) have been considered as a single source class. In this paper, we investigated the evolution and gamma-ray luminosity function of FSRQs and BL Lacs separately. Our investigation indicates no evolution for BL Lacs, however FSRQs show significant evolution. Pure luminosity evolution is assumed for FSRQs and exponential and power law evolution models are examined. Due to the small number of sources, the low luminosity end index of the luminosity function for FSRQs is constrained with an upper limit. BL Lac luminosity function shows no signature of break. As a consistency check, the model source distributions derived from these luminosity functions show no significant departure from the observed source distributions.

Contribution from normal and starburst galaxies to the extragalactic gamma-ray background (EGRB)

The extragalactic diffuse emission at gamma-ray energies has interesting cosmological implications since these photons suffer little or no attenuation during their propagation from the site of origin. The emission could originate from either truly diffuse processes or from unresolved point sources such as AGNs, normal galaxies and starburst galaxies. Here, we examine the unresolved point source origin of the extragalactic gamma-ray background emission from normal galaxies and starburst galaxies. gamma-ray emission from normal galaxies is primarily coming from cosmic-ray interactions with interstellar matter and radiation (similar to 90%) along with a small contribution from discrete point sources (similar to 10%). Starburst galaxies are expected to have enhanced supernovae activity which leads to higher cosmic-ray densities, making starburst galaxies sufficiently luminous at gamma-ray energies to be detected by the current gamma-ray mission(Fermi Gamma-ray Space Telescope).

TlBr raw material purification, crystal growth, annealing, detector fabrication and characterisation for gamma-ray detector applications

The research reported in this thesis dealt with single crystals of thallium bromide grown for gamma-ray detector applications. The crystals were used to fabricate room temperature gamma-ray detectors. Routinely produced TlBr detectors often are poor quality. Therefore, this study concentrated on developing the manufacturing processes for TlBr detectors and methods of characterisation that can be used for optimisation of TlBr purity and crystal quality. The processes under concern were TlBr raw material purification, crystal growth, annealing and detector fabrication. The study focused on single crystals of TlBr grown from material purified by a hydrothermal recrystallisation method. In addition, hydrothermal conditions for synthesis, recrystallisation, crystal growth and annealing of TlBr crystals were examined. The final manufacturing process presented in this thesis deals with TlBr material purified by the Bridgman method. Then, material is hydrothermally recrystallised in pure water. A travelling molten zone (TMZ) method is used for additional purification of the recrystallised product and then for the final crystal growth. Subsequent processing is similar to that described in the literature. In this thesis, literature on improving quality of TlBr material/crystal and detector performance is reviewed. Aging aspects as well as the influence of different factors (temperature, time, electrode material and so on) on detector stability are considered and examined. The results of the process development are summarised and discussed. This thesis shows the considerable improvement in the charge carrier properties of a detector due to additional purification by hydrothermal recrystallisation. As an example, a thick (4 mm) TlBr detector produced by the process was fabricated and found to operate successfully in gamma-ray detection, confirming the validity of the proposed purification and technological steps. However, for the complete improvement of detector performance, further developments in crystal growth are required. The detector manufacturing process was optimized by characterisation of material and crystals using methods such as X-ray diffraction (XRD), polarisation microscopy, high-resolution inductively coupled plasma mass (HR-ICPM), Fourier transform infrared (FTIR), ultraviolet and visual (UV-Vis) spectroscopy, field emission scanning electron microscope (FESEM) and energy-dispersive X-ray spectroscopy (EDS), current-voltage (I-V) and capacity voltage (CV) characterisation, and photoconductivity, as well direct detector examination.

Contribution from unresolved discrete sources to the extragalactic gamma-ray background (EGRB)

The origin of the extragalactic gamma-ray background (EGRB) is still an open question, even nearly forty years after its discovery. The emission could originate either from truly diffuse processes or from unresolved point sources. Although the majority of the 271 point sources detected by EGRET (Energetic Gamma Ray Experiment Telescope) are unidentified, of the identified sources, blazars are the dominant candidates. Therefore, unresolved blazars may be considered the main contributor to the EGRB, and many studies have been carried out to understand their distribution, evolution and contribution to the EGRB. Considering that gamma-ray emission comes mostly from jets of blazars and that the jet emission decreases rapidly with increasing jet to line-of-sight angle, it is not surprising that EGRET was not able to detect many large inclination angle active galactic nuclei (AGNs). Though Fermi could only detect a few large inclination angle AGNs during the first three months of its survey, it is expected to detect many such sources in the near future. Since non-blazar AGNs are expected to have higher density as compared to blazars, these could also contribute significantly to the EGRB. In this paper, we discuss contributions from unresolved discrete sources including normal galaxies, starburst galaxies, blazars and off-axis AGNs to the EGRB.

Deep searches for decametre-wavelength pulsed emission from radio-quiet gamma-ray pulsars

We report the results of extensive follow-up observations of the gamma-ray pulsar J1732-3131, which has recently been detected at decametre wavelengths, and the results of deep searches for the counterparts of nine other radio-quiet gamma-ray pulsars at 34 MHz, using the Gauribidanur radio telescope. No periodic signal from J1732-3131 could be detected above a detection threshold of 8 sigma, even with an effective integration time of more than 40 h. However, the average profile obtained by combining data from several epochs, at a dispersion measure of 15.44 pc cm(-3), is found to be consistent with that from the earlier detection of this pulsar at a confidence level of 99.2 per cent. We present this consistency between the two profiles as evidence that J1732-3131 is a faint radio pulsar with an average flux density of 200-400 mJy at 34 MHz. Despite the extremely bright sky background at such low frequencies, the detection sensitivity of our deep searches is generally comparable to that of higher frequency searches for these pulsars, when scaled using reasonable assumptions about the underlying pulsar spectrum. We provide details of our deep searches, and put stringent upper limits on the decametre-wavelength flux densities of several radio-quiet gamma-ray pulsars.

Gamma-Ray Attenuation Technique For Measuring Void Fraction In Horizontal Gas-Liquid Two-Phase Flow

The measurement of void fraction is of importance to the oil industry and chemical industry. In this article, the principle and mathematical method of determining the void fraction of horizontal gas-liquid flow by using a single-energy gamma-ray system is described. The gamma-ray source is the radioactive isotope of Am-241 with gamma-ray energy of 59.5 keV. The time-averaged value of the void fraction in a 50.0-mm i.d. transparent horizontal pipeline is measured under various combinations of the liquid flow and gas flow. It is found that increasing the gas flow rate at a fixed liquid flow rate would increase the void fraction. Test data are compared with the predictions of the correlations and a good agreement is found. The result shows that the designed gamma-ray system can be used for measuring the void fraction in a horizontal gas-liquid two-phase flow with high accuracy.

The relationship between the radio and gamma-ray emission of blazars

Blazars are active galaxies with a jet closely oriented to our line of sight. They are powerful, variable emitters from radio to gamma-ray wavelengths. Although the general picture of synchrotron emission at low energies and inverse Compton at high energies is well established, important aspects of blazars are not well understood. In particular, the location of the gamma-ray emission region is not clearly established, with some theories favoring a location close to the central engine, while others place it at parsec scales in the radio jet.

We developed a program to locate the gamma-ray emission site in blazars, through the study of correlated variations between their gamma-ray and radio-wave emission. Correlated variations are expected when there is a relation between emission processes at both bands, while delays tell us about the relative location of their energy generation zones. Monitoring at 15 GHz using the Owens Valley Radio Observatory 40 meter telescope started in mid-2007. The program monitors 1593 blazars twice per week, including all blazars detected by the Fermi Gamma-ray Space Telescope (Fermi) north of -20 degrees declination. This program complements the continuous monitoring of gamma-rays by Fermi.

Three year long gamma-ray light curves for bright Fermi blazars are cross-correlated with four years of radio monitoring. The significance of cross-correlation peaks is investigated using simulations that account for the uneven sampling and noise properties of the light curves, which are modeled as red-noise processes with a simple power-law power spectral density. We found that out of 86 sources with high quality data, only three show significant correlations (AO 0235+164, B2 2308+34 and PKS 1502+106). Additionally, we find a significant correlation for Mrk 421 when including the strong gamma-ray/radio flare of late 2012. In all four cases radio variations lag gamma-ray variations, suggesting that the gamma-ray emission originates upstream of the radio emission. For PKS 1502+106 we locate the gamma-ray emission site parsecs away from the central engine, thus disfavoring the model of Blandford and Levinson (1995), while other cases are inconclusive. These findings show that continuous monitoring over long time periods is required to understand the cross-correlation between gamma-ray and radio-wave variability in most blazars.

Gamma-ray induced color centers in Yb : YAG crystals grown by Czochralski method

20 at.% Yb:YAG single crystals have been grown by the CZ method and gamma-ray irradiation induced color centers and valence change of Fe3+ and Yb3+ ions in Yb:YAG have been studied. One significant 255 nm absorption band was observed in as-grown crystals and was attributed to Fe3+ ions. Two additional absorption (AA) bands located at 255 nm and 345 nm, respectively, were produced after gamma irradiation. The changes in the AA spectra after gamma irradiation and air annealing are mainly related to the charge exchange of the Fe3+, Fe2+, oxygen vacancies and F-type color centers. Analysis shows that the broad AA band is associated with Fe2+ ions and F-type color centers. The transition Yb3+ Yb2+ takes place as an effect of recharging of one of the Yb3+ ions from a pair in the process of gamma irradiation. (C) 2006 Elsevier Ltd. All rights reserved.

Gamma-ray spectroscopy at TRIUMF-ISAC

The 8π spectrometer at TRIUMF-ISAC consists of 20 Compton-suppressed germanium detectors and various auxiliary devices. The Ge array, once used for studies of nuclei at high angular momentum, has been transformed into the world's most powerful device dedicated to radioactive-decay studies. Many improvements in the spectrometer have been made, including a high-throughput data acquisition system, installation of a moving tape collector, incorporation of an array of 20 plastic scintillators for β-particle tagging, 5 Si(Li) detectors for conversion electrons, and 10 BaF2 detectors for fast-lifetime measurements. Experiments can be performed where data from all detectors are collected simultaneously, resulting in a very detailed view of the nucleus through radioactive decay. A number of experimental programmes have been launched that take advantage of the versatility of the spectrometer, and the intense beams available at TRIUMF-ISAC. © 2006 American Institute of Physics.