Energetic Gamma Ray Experiment Telescope Research Articles

Supersymmetric interpretation of the Egret GeV anomaly, Xenon-10 dark matter search limits, and the CERN LHC

The observation of the Egret experiment of an excess of diffuse gamma rays with energies above ${E}_{\ensuremath{\gamma}}=1\text{ }\text{ }\mathrm{GeV}$ has previously been interpreted in the context of the minimal supergravity model (mSUGRA) as coming from neutralino annihilation into mainly $b$-quarks in the galactic halo, with neutralino mass in the vicinity of 50--70 GeV. We observe that, in order to obtain the correct relic abundance of neutralinos in accord with WMAP measurements, the corresponding neutralino-proton direct detection (DD) rates should be in excess of recent limits from the Xenon-10 collaboration. While it does not appear possible to satisfy the Egret, WMAP, and Xenon-10 constraints simultaneously within the mSUGRA model, we find that it is easily possible in models with nonuniversal Higgs soft masses (NUHM). In either case, gluino pair production from ${m}_{\stackrel{\texttildelow{}}{g}}\ensuremath{\sim}400--500\text{ }\text{ }\mathrm{GeV}$ should occur at large rates at the CERN LHC, and a gluino pair production signal should be visible with just $0.1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of integrated luminosity. The NUHM interpretation predicts a rather light spectrum of heavy Higgs bosons with ${m}_{A}\ensuremath{\sim}140--200\text{ }\text{ }\mathrm{GeV}$ over the whole parameter space which would interpret Egret data. Spin-independent DD rates are predicted to be just above ${10}^{\ensuremath{-}8}\text{ }\text{ }\mathrm{pb}$, within range of the next round of direct dark matter detection experiments.

Radio detections towards unidentified variable EGRET sources

Context. A considerable fraction of theγ-ray sources discovered with the Energetic Gamma-Ray Experiment Telescope (EGRET) remain unidentified. The EGRET sources that have been proper ly identified are either pulsars or variable sources at both r adio and gamma-ray wavelengths. Most of the variable sources are strong radio blazars. However, some low galactic-latitude EGRET sources, with highly variableγ-ray emission, lack any evident counterpart according to the radio data available until now. Aims. The primary goal of this paper is to identify and characterise the potential radio counterparts of four highly variable γ-ray sources in the galactic plane through mapping the radio surroundings of the EGRET confidence contours and determining th e variable radio sources in the field whenever possible. Methods. We have carried out a radio exploration of the fields of the sel ected EGRET sources using the Giant Metrewave Radio Telescope (GMRT) interferometer at 21 cm wavelength, with pointings being separated by months. Results. We detected a total of 151 radio sources. Among them, we identified a few radio sources whose flux density has apparently changed on timescales of months. Despite the limitations of our search, their possible variability makes these objects a top-priority target for multiwavelength studies of the potential counte rparts of highly variable, unidentified gamma-ray sources.

Search with EGRET for a gamma ray line from the Galactic center

We search data from the Energetic Gamma Ray Experiment Telescope (EGRET) for a gamma-ray line in the energy range 0.1–10 GeV from the 10°×10° region around the Galactic center. Our null results lead to upper limits to the line flux from the Galactic center. Such lines may have appeared if the dark matter in the Galactic halo is composed of weakly interacting massive particles (WIMPs) in the mass range 0.1–10 GeV. For a given dark-matter-halo model, our null search translates to upper limits to the WIMP two-photon annihilation cross section as a function of WIMP mass. We show that, for a toy model in which Majorana WIMPs in this mass range annihilate only to electron-positron pairs, these upper limits supersede those derived from measurements of the 511-keV line and continuum photons from internal bremsstrahlung at the Galactic center.

Transparency of the Sun to Gamma Rays due to Axionlike Particles

It is shown that the Sun can become partially transparent to high energy photons in the presence of a pseudoscalar. In particular, if the axion interpretation of the PVLAS result were true, then up to 2% of GeV energy gamma rays might pass through the Sun, while an even stronger effect is expected for some axion parameters. We discuss the possibilities of observing this effect. Present data are limited to the observation of the solar occultation of 3C 279 by the Energetic Gamma-Ray Experiment Telescope in 1991; 98% C.L. detection of a nonzero flux of gamma rays passing through the Sun is not yet conclusive. Since the same occultation happens every October, future experiments, e.g., the Gamma-Ray Large Area Space Telescope, are expected to have better sensitivity.

Does the Blazar Gamma‐Ray Spectrum Harden with Increasing Flux? Analysis of 9 Years of EGRET Data

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) discovered gamma-ray emission from more than 67 blazars during its nine-year lifetime. We conducted an exhaustive search of the EGRET archives and selected all the blazars that were observed multiple times and were bright enough to enable a spectral analysis using standard powerlaw models. The sample consists of 18 flat-spectrum radio quasars (FSRQs), 6 low-frequency-peaked BL Lacs (LBLs) and 2 high-frequency-peaked BL Lacs (HBLs). We do not detect any clear pattern in'the variation of spectral index with flux. Some of the blazars do not show any statistical evidence for spectral variability. The spectrum hardens with increasing flux in a few cases. There is also evidence for a flux-hardness anticorrelation at lo\v fluxes in five blazars. The well observed blazars (3C 279,3C 273, PKS 0528-i-134, PKS 1622-297, PKS 0208- 512) do not show any overall trend in the long-term spectral dependence on flux, but the sample shows a mixture of hard and soft states. We observed spectral hysteresis at weekly timescales in all the three FSRQs for which data from flares lasting for 3 approx. 4 weeks were available. All three sources show a counterclockwise rotation despite the widely different flux profiles. Hysteresis in the spectral index vs. flux space has never been observed in FSRQs in gamma-rays at weekly timescales. itre analyze the observed spectral behavior in the context of various inverse-Compton mechanisms believed to be responsible for emission in the EGRET energy range. Our analysis uses the EGRET skymaps that were regenerated to include the changes in performance during the mission.

GLAST and Suzaku: Study on Cosmic-Ray Acceleration and Interaction in the Cosmos

The Gamma-Ray Large Area Space Telescope (GLAST) is an international and multi-agency mission scheduled for launch in the fall of 2007. The Large Area Telescope (LAT), the primary instrument of the mission, will survey the high energy sky found to be very dynamic and surprisingly diverse by its predecessor the Energetic Gamma Ray Experiment Telescope (EGRET). GLAST-LAT will have a much improved sensitivity when compared with EGRET and extend the higher energy coverage to ∼300 GeV. The insrument is now mounted on the spacecraft and undergoing a suite of pre-flight tests. Data analysis software has been tried out by collaborators in two rounds of “Data Challenges” using simulated observations including backgrounds. The instrument performance and observational data on selected sources presented here have been obtained through the Data Challenges in the collaborative efforts. There are features in the GLAST-LAT observation possibly unfamiliar to X-ray astronomers: 1) GLAST will operate mostly in the survey mode; 2) the foreground objects (gas, dust, and star-light) become gamma-ray sources; 3) multiple sources will be “confused” because of the wide point-spread-function. The last two features will pose a challenge for analysis on extended Galactic sources such as supernova remnants and pulsar wind nebulae: multi-wavelength study with X-ray instruments like Suzaku and atmospheric Chrenkov telescopes will become essential to dig out the underlying physics.

Temporal Evolution of Pair Attenuation Signatures in Gamma‐Ray Burst Spectra

The spectra obtained above 100 MeV by the EGRET experiment aboard the Compton Gamma-Ray Observatory for a handful of gamma-ray bursts has given no indication of any spectral attenuation that might preclude detection of bursts at higher energies. With the discovery of optical afterglows and counterparts to bursts in the last few years, enabling the determination of significant redshifts for these sources, it is anticipated that profound spectral attenuation will arise in the Gamma-Ray Large Area Space Telescope (GLAST) energy band of 30 MeV-300 GeV for many if not most bursts. This paper explores time-dependent expectations for burst spectral properties in the EGRET/GLAST band, focusing on how attenuation of photons by pair creation internal to the source generates distinctive spectral signatures. The energy of spectral breaks and the associated spectral indices provide valuable information that constrains the bulk Lorentz factor of the GRB outflow at a given time. Moreover, the distinct temporal behavior that is present for internal attenuation is easily distinguished from extrinsic absorption due to intervening cosmic background fields. These characteristics define palpable observational goals for both spaced-based hard gamma-ray experiments such as GLAST, and ground-based Cherenkov telescopes, and strongly impact the observability of bursts above 300 MeV.

A possible EGRET source with a two-sided radio jet structure on a parsec scale - 3EG J0808+5114

We report on the European VLBI Network observations of the radio source 0803+5126 in the field of the Energetic Gamma Ray Experiment Telescope (EGRET) source 3EG J0808+5114. High-resolution images of 0803+5126 at 6 cm showed a core emission with an extremely low brightness temperature plus an asymmetric two-sided jet structure on both parsec and tens of parsec scales, suggesting a relatively weak beaming effect in radio emission. The radio structures on different scales exhibit quite different orientations of extension, indicating the occurrence of jet bending. Based on the positional uncertainty of 3EG J0808+5114, its separations to various possible radio counterparts, and very low gamma-ray variability, it is reasonable to believe that 0803+5126 is the most probable radio counterpart of 3EG J0808+5114, making it the first EGRET-detected active galactic nucleus to be associated with a two-sided jet structure on a parsec scale. The central engine is estimated to have a mass of similar to 5.0 x 10(8) M-circle dot by using Mg II emission line features, for which the Eddington limit is approximately two times lower than the integrated gamma-ray luminosity. The minimum Doppler factor in gamma-ray emission is similar to 2.5, but the true one should not be much larger than this value on account of its quiet stable gamma-ray emission. This is consistent with our very long baseline interferometry results of the two-sided radio jet structure. The relativistic beaming effect and downstream jet bending may play an important role in the paradox between the large gamma-ray luminosity far above its Eddington limit and the observed asymmetric two-sided radio structure.

Is the dark matter interpretation of the EGRET gamma excess compatible with antiprotonmeasurements?

We investigate the internal consistency of the halo dark matter model which has beenproposed by de Boer et al for explaining the excess of diffuse galactic gamma raysobserved by the EGRET experiment. Any model based on dark matter annihilationinto quark jets, such as the supersymmetric model proposed by de Boer et al,inevitably also predicts a primary flux of antiprotons from the same jets. Sincepropagation of the antiprotons in the unconventional, disc-dominated type ofhalo model used by de Boer et al is strongly constrained by the measured ratio ofboron to carbon nuclei in cosmic rays, we investigate the viability of the modelusing the DarkSUSY package to compute the gamma-ray and antiproton fluxes.We are able to show that their model is excluded by a wide margin from themeasured flux of antiprotons. We therefore find that a model of the type suggested byMoskalenko et al, where the intensities of protons and electrons in the cosmic rays varywith galactic position, is far more plausible for explaining the gamma excess.

EGRET Unidentified Source Radio Observations and Performance of Receiver Gain Calibration

Last year, we have developed the receiver gain calibration system by using Johnson-Nyquist noise, for accuracy flux measurement, because we have been starting radio identification program of transient radio sources, blazars and radio counterpart of The Energetic Gamma Ray Experiment Telescope (EGRET) unidentified γ-ray sources in Waseda Nasu Pulsar Observatory.It is shown that there are a few low correlation data between receiver gain and ambient temperature around receiver for anything troubles of receiver, because we can detect gain and ambient temperature through a day by developed system. Estimated fluctuations of daily data of steady sources decrease by removing low correlation data before analysing.As the result of our analysis by using above system, radio counterpart of EGRET identified source showed fading light-curve for a week.

A Cross-Correlation Analysis of WMAP and EGRET Data in Wavelet Space

We cross-correlate the first-year Wilkinson Microwave Anisotropy Probe (WMAP) data and the diffuse gamma-ray intensity maps from the Energetic Gamma-Ray Experiment Telescope (EGRET) using spherical-wavelet approaches. Correlations at the 99.7% significance level have been detected, at scales around 15 degrees in the WMAP foreground-cleaned W-band and Q-band maps, based on data from regions that are outside the most conservative WMAP foreground mask; no significant correlation is found with the Tegmark cleaned map. The detected correlation is most likely of Galactic origin and thus can help probe the origins of possible Galactic foreground residuals and ultimately remove them from measured microwave sky maps.

The Search for pulsed TeV gamma-ray sources

The EGRET experiment detected pulsed emission from several pulsars in the HE energy range 20 MeV 130 GeV and detecting the signature periodicity of pulsars if it exists. Data recorded on the Crab pulsar with the Whipple Imaging Atmospheric Cerenkov Telescope has been analysed with this software and upper limits on pulsed emission have been obtained.

A Systematic and Quantitative Approach to the Identification of High-Energy γ-Ray Source Populations

A large fraction of the detections to be made by the Gamma-Ray Large Area Space Telescope (GLAST) will initially be unidentified. We argue that traditional methodological approaches to identifying individuals and/or populations of γ-ray sources will encounter procedural limitations. These limitations will hamper our ability to classify source populations lying in the anticipated data set with the required degree of confidence, particularly those for which no member has yet been convincingly detected in the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO). Here we suggest a new paradigm for achieving the classification of γ-ray source populations based on the implementation of an a priori protocol to search for theoretically motivated candidate sources. In order to protect the discovery potential of the sample, it is essential that such a paradigm be defined before the data are revealed. Key to the new procedure is a statistical assessment by which the discovery of a new population can be claimed. Although we explicitly refer here to the case of GLAST, the scheme we present may be adapted to other experiments that are confronted with a similar problematic.

Comptel Constraints on Unidentified EGRET Sources

A major legacy of the EGRET experiment aboard the Compton Gamma-Ray Observatory (CGRO) is the detection of a large number of unidentified Gamma-ray sources. Out of the 271 EGRET sources are 170 not identified yet [Hartman et al. ApJS (123) (1999) 79]. To provide additional information on these unidentified EGRET sources, we derived simultaneous MeV fluxes or flux limits for each source by analyzing the contemporaneous COMPTEL data. Then we compare these MeV fluxes to the extrapolations of the published EGRET spectra. Our results can be grouped into 4 categories [Zhang et al. A&A (421) (2004) 983]: (1) non-constraining upper limits (∼120 sources), (2) hints (>2 sigma) or detections with consistent MeV fluxes (∼16 sources), (3) enhanced MeV emission (2 sources), and (4) constraining MeV flux limits, requiring a spectral break (∼22 sources). In summary, for about 40 of the unidentified EGRET sources we can provide supplementary spectral information in the neighboring gamma-ray band, which — together with other source parameters — might provide clues to the underlying source nature.

The Highest‐Energy Photons Seen by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory

During its nine-year lifetime, the Energetic Gamma Ray Experiment Telescope (EGBET) on the Compton Gamma Ray Observatory (CGRO) detected 1506 cosmic photons with measured energy E>10 GeV. Of this number, 187 are found within a 1 deg of sources that are listed in the Third EGRET Catalog and were included in determining the detection likelihood, flux, and spectra of those sources. In particular, five detected EGRET pulsars are found to have events above 10 GeV, and together they account for 37 events. A pulsar not included in the Third EGRET Catalog has 2 events, both with the same phase and in one peak of the lower-energy gamma-ray light-curve. Most of the remaining 1319 events appear to be diffuse Galactic and extragalactic radiation based on the similarity of the their spatial and energy distributions with the diffuse model and in the E>100, MeV emission. No significant time clustering which would suggest a burst was detected.

Possible TeV source candidates among the unidentified EGRET sources

We study the γ -ray emission from the pulsar magnetosphere based on outer gap models, and the TeV radiation from pulsar wind nebulae through inverse Compton scattering using a onezone model. We have shown previously that GeV radiation from the magnetosphere of mature pulsars with ages of ∼10 5 ‐10 6 yr can contribute to the high-latitude unidentified Energetic Gamma Ray Experiment Telescope (EGRET) sources. We carry out Monte Carlo simulations of γ -ray pulsars in the Galaxy and the Gould Belt, assuming values for the pulsar birth rate, initial position, proper motion velocity, period, and magnetic field distribution and evolution based on observational statistics. We select from the simulation a sample of mature pulsars in the Galactic plane (|b| 5 ◦ ) and a sample at high latitudes (|b| > 5 ◦ ) which could be detected by EGRET. We calculate the TeV fluxes from the pulsar wind nebulae of our simulated sample produced through inverse Compton scattering by relativistic electrons on the cosmic microwave background and synchrotron seed photons. The predicted fluxes are consistent with the present observational constraints. We suggest that strong EGRET sources may be potential TeV source candidates for present and future ground-based TeV telescopes.

Gamma rays from the galactic center

The EGRET telescope has found evidence for a γ-ray source at the Galactic Center (GC). We investigate whether the spectral features of this source are compatible with the γ-ray flux induced by pair annihilations of dark matter (DM), weakly interacting massive particles (WIMPs). We show that the discrimination between this interpretation and other viable explanations will be possible with GLAST, the next major γ-ray telescope in space. On the other hand, we also show that, if the data will point to an alternative explanation, there will still be the possibility for GLAST to single out a weaker DM source at the GC. The talk is based on [A. Cesarini, F. Fucito, A. Lionetto, A. Morselli, and P. Ullio, preprint astro-ph/0305075 ].

Limits on supersymmetric dark matter from EGRET observations of the Galactic center region

In most supersymmetic models, neutralino dark matter particles are predicted to accumulate in the Galactic center and annihilate generating, among other products, gamma rays. The Energetic Gamma Ray Experiment Telescope has made observations in this region, and is sensitive to gamma rays from 30 MeV to {approx}30 GeV. We have used an improved point source analysis including an energy dependent point spread function and an unbinned maximum likelihood technique, which has allowed us to lower the limits on gamma ray flux from the Galactic center by more than 1 order of magnitude. We find that the present Energetic Gamma Ray Experiment Telescope data can limit many supersymmetric models if the density of the Galactic dark matter halo is cuspy or spiked toward the Galactic center. We also discuss the ability of the Gamma ray Large Area Space Telescope to test these models.

On the duty-cycle of γ-ray blazars

We study several properties of blazars detected in the γ-ray energy range by comparing the Energetic Gamma Ray Experiment Telescope (EGRET) sources with a sample of radio blazars which can be considered possible γ-ray candidates. We define three classes: non-γ-ray blazars, blazars with quasi-steady γ-ray emission and γ-ray blazars with substantial activity level. We find that, on average, BL Lacertae objects show a relatively steady γ-ray emission, when detected. On the other hand, most flat-spectrum radio quasars (FSRQs) show substantial γ-ray variability. We attribute a γ-ray activity index ψ = ψ -7 x 10 -7 cm -2 s -1 to all EGRET blazars, and show that FSRQs dominate the sample with non-zero ψ in the range 0 < ψ -7 < 0.035. By combining the information of detected and candidate active galactic nuclei, we characterize the blazar activity, including the discovery of a region of consistency between the γ-ray flaring duty-cycle and the recurrence time between flares. We also find a possible relation between the activity index of FSRQs and their black hole mass. More optical and γ-ray data are crucially important to test this relation.

The Galactic center as a dark matter gamma-ray source

The EGRET telescope has identified a gamma-ray source at the Galactic center. We point out here that the spectral features of this source are compatible with the gamma-ray flux induced by pair annihilations of dark matter weakly interacting massive particles (WIMPs). We show that the discrimination between this interpretation and other viable explanations will be possible with GLAST, the next major gamma-ray telescope in space, on the basis of both the spectral and the angular signature of the WIMP-induced component. If, on the other hand, the data will point to an alternative explanation, we prove that there will still be the possibility for GLAST to single out a weaker dark matter source at the Galactic center. The potential of GLAST has been explored both in the context of a generic simplified toy-model for WIMP dark matter, and in a more specific setup, the case of dark matter neutralinos in the minimal supergravity framework. In the latter, we find that even in the case of moderate dark matter densities in the Galactic center region, there are portions of the parameter space which will be probed by GLAST.