Gamma-ray Absorption Research Articles

Gamma-ray absorption in the microquasar SS433

We discuss the gamma-ray absorption in the inner region of the microquasar SS433. Our investigation includes several contributions to the opacity of this system. They result from the ambient fields generated by the primary star, possibly an A-type supergiant, and a very extended disk around the black hole. Besides the sharp and dramatic absorption effect that occurs every time the star crosses the emission zone, we find in the UV photon field from the extended disk an important source of absorption for very high energy gamma-rays. This results in periodic gamma-ray observational signatures.

Milagro Constraints on Very High Energy Emission from Short‐Duration Gamma‐Ray Bursts

Recent rapid localizations of short, hard gamma-ray bursts (GRBs) by the Swift and HETE satellites have led to the observation of the first afterglows and the measurement of the first redshifts from this type of burst (Fox et al. 2005; Gehrels et al. 2005; Villasenor et al. 2005; Berger et al. 2005; Barthelmy et al. 2005). Detection of >100 GeV counterparts would place powerful constraints on GRB mechanisms. Seventeen short-duration (<5 s) GRBs detected by satellites occurred within the field of view of the Milagro gamma-ray observatory between 2000 January and 2006 December. We have searched the Milagro data for >100 GeV counterparts to these GRBs and find no significant emission correlated with these bursts. Due to the absorption of high-energy gamma rays by the extragalactic background light (EBL), detections are only expected for redshifts less than ~0.5. While most long-duration GRBs occur at redshifts higher than 0.5, the opposite is thought to be true of short GRBs. Lack of a detected VHE signal thus allows setting meaningful fluence limits. One GRB in the sample (050509b) has a likely association with a galaxy at a redshift of 0.225, while another (051103) has been tentatively linked to the nearby galaxy M81. Fluence limits are corrected for EBL absorption, either using the known measured redshift, or computing the corresponding absorption for a redshift of 0.1 and 0.5, as well as for the case of z = 0.

Internal absorption of gamma-rays in relativistic blobs of active galactic nuclei

We investigate the production of gamma-rays in the inverse Compton (IC) scattering process by leptons accelerated inside relativistic blobs in jets of active galactic nuclei. Leptons are injected homogeneously inside the spherical blob and initiate IC e ± pair cascade in the synchrotron radiation (produced by the same population of leptons, SSC model), provided that the optical depth for gamma-rays is larger than unity. It is shown that for likely parameters internal absorption of gamma-rays has to be important. We suggest that new type of blazars might be discovered by the future simultaneous X-ray and γ-ray observations, showing peak emissions in the hard X-rays, and in the GeV γ-rays. Moreover, the considered scenario might be also responsible for the orphan X-ray flares recently reported from BL Lac type active galaxies.

Gamma-ray absorption method (GRAM) application to the identification of EGRET unidentified sources

Recently we reported the very first detection of γ -ray resonant absorption along the line of sight toward γ -ray bright quasars (QSOs) like 3C 273, 3C 279, PKS 0528+0134, and BL Lacertae. These detections resulted from the analysis of absorption troughs in SEDs derived on the base of mostly EGRET data that were collected during monitoring campaigns of the Virgo and galactic anticenter regions by the Compton Gamma Ray Observatory (CGRO), as well as during ToO observations of QSOs flares. Among three resonant absorption mechanisms that affect the γ -ray spectrum of point-like sources, we now pinpoint the Δ -isobar resonance that has a very stable peak energy of the absorption cross-section for all elements (nuclei) and for individual nucleons. From two absorbers that are usually detected on the sight lines towards γ -ray bright QSOs, we concentrate here on the only one that is at the QSO rest frame. We discuss the advantages and drawbacks of this method distinguishing between a galactic and extragalactic origin of the EGRET unidentified sources (EUIDs). This is compared to the multiwavelength identification approach that was succesfully used in a few cases to identify galactic and extragalactic objects among EUIDs. We applied the GRAM to identifying two of the EGRET unidentified sources as blazars, and predict their respective redshifts.

A Time-dependent Leptonic Model for Microquasar Jets: Application to LS I +61 303

The Galactic high-mass X-ray binary and jet source (microquasar) LSI +61 303 has recently been detected at TeV gamma-ray energies by the MAGIC telescope. We have applied a time-dependent leptonic jet model to the broadband spectral energy distribution and suggested (though not unambiguously detected) orbital modulation of the very high energy gamma-ray emission of this source. Our model takes into account time dependent electron injection and acceleration, and the adiabatic and radiative cooling of non-thermal electrons. It includes synchrotron, synchrotron self-Compton and external inverse Compton (with seed photons from the companion star), as well as gamma-gamma absorption of gamma-rays by starlight photons. The model can successfully reproduce the available multiwavelength observational data. Our best fit to the SED indicates that a magnetic field of B_0 ~ 5 X 10^3 G at ~ 10^3 R_g is required, and electrons need to be accelerated out to TeV energies (gamma_2 = 10^6) with a nonthermal injection spectrum with a spectral index of q = 1.7, indicating the operation of acceleration mechanisms beyond the standard first-order Fermi mechanism at relativistic or non-relativistic shocks. The orbital modulation of the VHE gamma-ray emission can be explained solely by the geometrical effect of changes in the relative orientation of the stellar companion with respect to the compact object and jet as it impacts the position and depth of the gamma-gamma absorption trough. Such a scenario predicts a trend of spectral hardening during VHE gamma-ray low orbital phases.

Intergalactic Photon Spectra from the Far‐IR to the UV Lyman Limit for 0 &lt;z&lt; 6 and the Optical Depth of the Universe to High‐Energy Gamma Rays

We calculate the intergalactic photon density as a function of both energy and redshift for 0 < z < 6 for photon energies from .003 eV to the Lyman limit cutoff at 13.6 eV in a Lambda-CDM universe with $\Omega_{\Lambda} = 0.7$ and $\Omega_{m} = 0.3$. Our galaxy evolution model gives results which are consistent with Spitzer deep number counts and the spectral energy distribution of the extragalactic background radiation. We use our photon density results to extend previous work on the absorption of high energy gamma-rays in intergalactic space owing to interactions with low energy photons and the 2.7 K cosmic background radiation. We calculate the optical depth of the universe, tau, for gamma-rays having energies from 4 GeV to 100 TeV emitted by sources at redshifts from ~0 to 5. We also give an analytic fit with numerical coefficients for approximating $\tau(E_{\gamma}, z)$. As an example of the application of our results, we calculate the absorbed spectrum of the blazar PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the H.E.S.S. air Cherenkov gamma-ray telescope array.

New measurement of neutron capture resonances inBi209

The neutron capture cross section of Bi-209 has been measured at the CERN n_TOF facility by employing the pulse-height-weighting technique. Improvements over previous measurements are mainly because of an optimized detection system, which led to a practically negligible neutron sensitivity. Additional experimental sources of systematic error, such as the electronic threshold in the detectors, summing of gamma-rays, internal electron conversion, and the isomeric state in bismuth, have been taken into account. gamma-Ray absorption effects inside the sample have been corrected by employing a nonpolynomial weighting function. Because Bi-209 is the last stable isotope in the reaction path of the stellar s-process, the Maxwellian averaged capture cross section is important for the recycling of the reaction flow by alpha decays. In the relevant stellar range of thermal energies between kT=5 and 8 keV our new capture rate is about 16 presently accepted value used for nucleosynthesis calculations. At this low temperature an important part of the heavy Pb-Bi isotopes are supposed to be synthesized by the s-process in the He shells of low mass, thermally pulsing asymptotic giant branch stars. With the improved set of cross sections we obtain an s-process fraction of 19 +/- 3 residual of 81 +/- 3 is also of relevance for the design of accelerator driven systems based on a liquid metal Pb/Bi spallation target.

Catalyst concentration distribution in fluidized bed by gamma-ray absorption

The axial and radial distribution of the catalyst concentration, in the riser of an EMC - experimental cold model of a fluid catalytic cracking type unit, was measured by gamma-absorption. The solid concentration in the circulating fluidized bed was determined by measurements with a 241Am gamma-source and a NaI(Tl) detector. The operation, instrumental measurements and data acquisition system of the cold unit were automated. The catalyst concentration measurements were carried out in static experiments and in the circulating fluidized bed. A calibration of the distribution concentration was made with experimental data from gas velocity and pressure drop along the riser that gave the best multi-regression model. A spatial resolution of 5.0 . 10-3 m by scanning the riser and a catalyst density resolution of 5.0±0.8 kg/m3 were obtained. The axial catalyst concentration distribution, in the 5-13 kg/m 3 interval, showed a predicted pattern according to the literature. The radial distribution was in an annular configuration for this diluted catalyst concentration.

Reconsideration of the Theoretical Supplementation of Decay Data in Fission Product Decay Heat Summation Calculations

In the early stage of the development of the fission product (FP) decay data library for decay-heat summation calculations a serious disagreement between calculations and sample-irradiation measurements was experienced world wide. This problem was essentially circumvented by introduction of the nuclear model calculations. By applying the recent results using the total absorption gamma-ray spectrometer to the decay heat calculations we found that TAGS results seem to be free from the pandemonium problem and, in this respect, it provides a solid basis of the summation calculations without the supplementation of the nuclear model calculations. Among the typical FP nuclides which deserve the future TAGS experiment there are 98Nb, 100Nb, 105Mo and 102Tc and others from the present survey.

Gamma-ray absorption in massive X-ray binaries

Context. Gamma-ray emission in the TeV (10 12 eV) range has been detected by HESS from two X-ray binaries: PSR B1259-63 and LS 5039. In both, the early-type star provides large numbers of target photons for pair-production with TeV γ-rays. This results in a modulation of the γ-ray flux as the relative positions of the γ-ray source and companion star change with orbital phase for the observer. Aims. The extent to which this variable absorption can provide useful diagnostics for the location and nature of γ-ray emission is examined. Methods. The absorption spectrum and transmitted flux are calculated by integrating the yy cross-section along the line-of-sight, taking into account the orbit, the spectrum and the finite size of the companion star in LS 5039, PSR B1259-63 and LSI +61°303, a system similar to LS 5039 but still undetected at TeV energies. Results. In LS 5039, emission close to a black hole or a neutron star primary is considered. In both cases, the transmitted flux >250 GeV drops by an order-of-magnitude near periastron (0 = 0). A black hole yields a clear spectral signature in the average spectrum at ≈400 GeV. A neutron star yields more variability, with the spectral feature moving from 200 GeV (Φ = 0.1) to 3 TeV (Φ = 0.7). Only 20% of the flux is absorbed at Φ = 0.7, allowing for an almost direct view of the intrinsic spectrum. Low variability will require emission on large scales, more than 0.7 AU away to have <50% absorption in a jet. In LSI +61°303, significant absorption (up to 90% of the 100 GeV flux) is predicted only slightly before periastron, accompanied by a spectral hardening above 1 TeV. In PSR B1259-63, although 40% of the flux is absorbed before periastron, the large variability seen by HESS is due to the γ-ray emission process. Conclusions. The predictions made here are essential to distinguish variability in the emission of γ-rays from that due to absorption. A modulation would provide a novel way to constrain the γ-ray source. Its absence would imply that γ-ray emission occurs on large scales.

A broadband leptonic model for gamma-ray emitting microquasars

Observational and theoretical studies point to microquasars (MQs) as possible counterparts of a significant fraction of the unidentified gamma-ray sources detected so far. At present, a proper scenario to explain the emission beyond soft X-rays from these objects is not known, nor what the precise connection is between the radio and the high-energy radiation. We develop a new model where the MQ jet is dynamically dominated by cold protons and radiatively dominated by relativistic leptons. The matter content and power of the jet are both related with the accretion process. The magnetic field is assumed to be close to equipartition, although it is attached to and dominated by the jet matter. For the relativistic particles in the jet, their maximum energy depends on both the acceleration efficiency and the energy losses. The model takes into account the interaction of the relativistic jet particles with the magnetic field and all the photon and matter fields. Such interaction produces significant amounts of radiation from radio to very high energies through synchrotron, relativistic Bremsstrahlung, and inverse Compton (IC) processes. Variability of the emission produced by changes in the accretion process (e.g. via orbital eccentricity) is also expected. The effects of the gamma-ray absorption by the external photon fields on the gamma-ray spectrum have been taken into account, revealing clear spectral features that might be observed. This model is consistent to the accretion scenario, energy conservation laws, and current observational knowledge, and can provide deeper physical information of the source when tested against multiwavelength data.

GRETINA status and recent progress: The effect of neutron damage on energy and position resolution of the GRETINA detector

GRETINA is a high precision gamma-ray detector for nuclear spectroscopy experiments, which is currently being developed. It consists of a number of highly segmented coaxial germanium detectors arranged in a close-packed configuration. GRETINA uses digital signal processing electronics and pulse shape analysis to determine the energy and position of each individual interaction resulting from the gamma-ray absorption. Gamma-ray tracking is then used to reconstruct the gamma-ray scattering sequence. This results in high efficiency, peak-to-total ratio and position resolution. Particularly, the capability of reconstructing the position of the interaction with resolution at the level of a few millimeters is a fundamental requirement and is important to understand and quantify limiting factors of position resolution. This paper reports on the effects of neutron damage on pulse shape and on its impact on position resolution. Simulation results show that even a relatively heavily damaged detector maintains high position resolution. Only for neutron flux >6×109 neutron/cm2, the position resolution becomes worse than 1mm, which is the achievable limit.

Constraints on Very High Energy Gamma‐Ray Emission from Gamma‐Ray Bursts

The Milagro gamma-ray observatory employs a water Cherenkov detector to observe extensive air showers produced by high energy particles interacting in the Earth's atmosphere. Milagro has a wide field of view and high duty cycle, monitoring the northern sky almost continuously in the 100 GeV to 100 TeV energy range. Milagro is, thus, uniquely capable of searching for very high-energy emission from gamma-ray bursts (GRBs) during the prompt emission phase. Detection of >100 GeV counterparts would place powerful constraints on GRB mechanisms. Twenty-five satellite-triggered GRBs occurred within the field of view of Milagro between January 2000 and December 2001. We have searched for counterparts to these GRBs and found no significant emission from any of the burst positions. Due to the absorption of high-energy gamma rays by the extragalactic background light, detections are only expected to be possible for redshifts less than ~0.5. Three of the GRBs studied have measured redshifts. GRB 010921 has a redshift low enough (0.45) to allow an upper limit on the fluence to place an observational constraint on potential GRB models.

H.E.S.S. observations of PKS 2155-304

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been detected with high significance (~45 sigma) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of 3.32 +/- 0.06. An improved chi-square per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2 sigma). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.

Measurement the Density of Solid Materials By Gamma Rays

The increase in the production of solid materials requires a simple and easy method to measure the density of these materials. A method is suggested to measure the density which is based on the absorption of gamma ray at certain energy which helps to measure the density with error less than 4.6% and in case the error exceeds the previous percentage the absorption material may contain impurities.

Experimental search for the effect of resonant environment on the Mössbauer absorption of gamma rays by 57Fe

Due to the virtual photon exchange between atomic nuclei and the field of zero-point electromagnetic oscillations, some nuclei of a given sample are in a virtual excited state with the lifetime ∼ħ/E, where E is the energy of nuclear level. For 57Fe nuclei, whose first excited state has an energy of 14.4 keV, this time is equal to ∼4.6×10−20 s. If a thin 57Fe Mossbauer gamma-ray absorber is surrounded by a thick screen of the same atoms, the number of virtual excited nuclei in the absorber decreases and, at first glance, it should more strongly absorb Mossbauer gamma rays emitted by an external source and passing through the absorber. In this work, the ratio of the intensities of 14.4-keV gamma rays emitted by the 57Fe nuclide and passing through the thin resonant absorber is measured in the absence and presence of the resonant screen around the absorber. Comparison shows that these ratios measured for the gamma source at rest and in the oscillating state differ by 0.00123±0.00075. This value should be treated as the upper limit for the desired effect under these experimental conditions.

Limits on Very High Energy Emission from Gamma-Ray Bursts with the Milagro Observatory

The Milagro telescope monitors the northern sky for 100 GeV-100 TeV transient emission through continuous very high energy (VHE) wide-field observations. The large effective area and ~100 GeV energy threshold of Milagro allow it to detect VHE gamma-ray burst (GRB) emission with much higher sensitivity than previous instruments and a fluence sensitivity at VHE energies comparable to that of dedicated GRB satellites at keV-MeV energies. Even in the absence of a positive detection, VHE observations can place important constraints on GRB progenitor and emission models. We present limits on the VHE flux of 40 s-3 hr duration transients near Earth as well as sensitivity distributions that have been corrected for gamma-ray absorption by extragalactic background light and cosmological effects. The sensitivity distributions suggest that the typical intrinsic VHE fluence of GRBs is similar to or weaker than the keV-MeV emission, and we demonstrate how these sensitivity distributions may be used to place observational constraints on the absolute VHE luminosity of GRBs for any GRB emission and progenitor model.

FLOW-RATE MEASUREMENT IN TWO-PHASE FLOW

▪ Abstract Online, continuous, two-phase flow measurement is often necessary, particularly in the oil and gas industry. In this article, we describe some of the commercially most important techniques for gas-liquid, gas-solid, liquid-solid, and liquid-liquid flows, and provide associated illustrative sketches and regime maps. These techniques involve Venturi pressure drop, Coriolis, electromagnetic, and cross-correlation flow meters, gamma-ray absorption and gradio-manometer densitometers, and local electrical and fiber-optic sensors, for which we describe the principles of operation and interpretation. References are given to more comprehensive texts and papers; these are representative rather than exhaustive. It is emphasized that empirical calibration is the norm and that detailed fluid-mechanical analysis has so far played little part in instrument design and operation.

Implications of cosmological gamma-ray absorption

Bearing on the model for the time-dependent metagalactic radiation field developed in the first paper of this series, we compute the gamma-ray attenuation due to pair production in photon-photon scattering. Emphasis is on the effects of varying the star formation rate and the fraction of UV radiation assumed to escape from the star forming regions, the latter being important mainly for high-redshift sources. Conversely, we investigate how the metagalactic radiation field can be measured from the gamma-ray pair creation cutoff as a function of redshift, the Fazio-Stecker relation. For three observed TeV-blazars (Mkn 501, Mkn 421, H1426+428) we study the effects of gamma-ray attenuation on their spectra in detail.

On the duration of the resonance absorption and emission of nuclear gamma rays

The Krylov-Fock theorem is applicable to describing the decay of an excited quantum-mechanical system in the case where the time of excited-state formation is much shorter than the characteristic decay time. The results obtained by calculating, on the basis of this theorem, the average lifetime t av of an excited state for nuclei that resonantly scatter γ rays are in serious contradiction with available data on t av including experimental data. This disproves the assumption that the resonance absorption and emission of γ rays proceeds within times much shorter than τ=ħ/Γ (Γ is the intrinsic width of a nuclear level). A qualitative explanation of this effect is proposed.