Detection Of Gamma-ray Emission Research Articles

Detection of gamma-ray emission from the Coma cluster with Fermi Large Area Telescope and tentative evidence for an extended spatial structure

Many galaxy clusters have giant halos of non-thermal radio emission, indicating the presence of relativistic electrons in the clusters. Relativistic protons may also be accelerated by merger and/or accretion shocks in galaxy clusters. These cosmic-ray (CR) electrons and/or protons are expected to produce gamma-rays through inverse-Compton scatterings or inelastic $pp$ collisions respectively. Despite of intense efforts in searching for high-energy gamma-ray emission from galaxy clusters, conclusive evidence is still missing so far. Here we report the discovery of $\ge 200$ MeV gamma-ray emission from the Coma cluster direction with an unbinned likelihood analysis of the 9 years of {\it Fermi}-LAT Pass 8 data. The gamma-ray emission shows a spatial morphology roughly coincident with the giant radio halo, with an apparent excess at the southwest of the cluster. Using the test statistic analysis, we further find tentative evidence that the gamma-ray emission at the Coma center is spatially extended. The extended component has an integral energy flux of $\sim 2\times 10^{-12}{\rm \ erg\ cm^{-2}\ s^{-1}}$ in the energy range of 0.2 - 300 GeV and the spectrum is soft with a photon index of $\simeq-2.7$. Interpreting the gamma-ray emission as arising from CR proton interaction, we find that the volume-averaged value of the CR to thermal pressure ratio in the Coma cluster is about $\sim 2\%$. Our results show that galaxy clusters are likely a new type of GeV gamma-ray sources, and they are probably also giant reservoirs of CR protons.

Gamma-ray Observations of Tycho’s Supernova Remnant with VERITAS and Fermi

Abstract High-energy gamma-ray emission from supernova remnants (SNRs) has provided a unique perspective for studies of Galactic cosmic-ray acceleration. Tycho’s SNR is a particularly good target because it is a young, type Ia SNR that has been well-studied over a wide range of energies and located in a relatively clean environment. Since the detection of gamma-ray emission from Tycho’s SNR by VERITAS and Fermi-LAT, there have been several theoretical models proposed to explain its broadband emission and high-energy morphology. We report on an update to the gamma-ray measurements of Tycho’s SNR with 147 hr of VERITAS and 84 months of Fermi-LAT observations, which represent about a factor of two increase in exposure over previously published data. About half of the VERITAS data benefited from a camera upgrade, which has made it possible to extend the TeV measurements toward lower energies. The TeV spectral index measured by VERITAS is consistent with previous results, but the expanded energy range softens a straight power-law fit. At energies higher than 400 GeV, the power-law index is 2.92 ± 0.42stat ± 0.20sys. It is also softer than the spectral index in the GeV energy range, 2.14 ± 0.09stat ± 0.02sys, measured in this study using Fermi-LAT data. The centroid position of the gamma-ray emission is coincident with the center of the remnant, as well as with the centroid measurement of Fermi-LAT above 1 GeV. The results are consistent with an SNR shell origin of the emission, as many models assume. The updated spectrum points to a lower maximum particle energy than has been suggested previously.

SUPERNOVA REMNANT KESTEVEN 27: INTERACTION WITH A NEIGHBOR Hi CLOUD VIEWED BYFERMI

We report on the likely detection of gamma-ray emission from the supernova remnant (SNR) Kesteven 27 (Kes 27). We analyze 5.7 yr Fermi Large Area Telescope data of the SNR region and find an unresolved source at a position consistent with the radio brightness peak and the X-ray knot of Kes 27, which is located in the eastern region of the SNR and caused by the interaction with a nearby HI cloud. The source's emission is best fit with a power-law spectrum with a photon index of 2.5 +/- 0.1 and a >0.2 GeV luminosity of 5.8*10^34 erg s^-1 assuming a distance of 4.3 kpc, as derived from radio observations of the nearby HI cloud. Comparing the properties of the source with that of other SNRs that are known to be interacting with nearby high-density clouds, we discuss the origin of the source's emission. The spectral energy distribution of the source can be described by a hadronic model that considers the interaction of energetic protons, escaping from the shock front of Kes 27, with a high-density cloud.

DISCOVERY OF GeV EMISSION FROM THE DIRECTION OF THE LUMINOUS INFRARED GALAXY NGC 2146

Recent detections of high-energy gamma-ray emission from starburst galaxies M82 and NGC 253 suggest that starburst galaxies are huge reservoirs of cosmic rays and these cosmic rays convert a significant fraction of their energy into gamma-rays by colliding with the dense interstellar medium. In this paper, we report the search for high-energy gamma-ray emission from several nearby star-forming and starburst galaxies using the 68 month data obtained with the Fermi Large Area Telescope. We found a ∼5.5σ detection of gamma-ray emission above 200 MeV from a source spatially coincident with the location of the luminous infrared galaxy NGC 2146. Also taking into account the temporal and spectral properties of the gamma-ray emission, we suggest that the gamma-ray source is likely to be the counterpart of NGC 2146. The gamma-ray luminosity suggests that cosmic rays in NGC 2146 convert most of their energy into secondary pions, so NGC 2146 is a "proton calorimeter." It is also found that NGC 2146 obeys the quasi-linear scaling relation between gamma-ray luminosity and total infrared luminosity for star-forming galaxies, strengthening the connection between massive star formation and gamma-ray emission of star-forming galaxies. Possible TeV emission from NGC 2146 is predicted and the implications for high-energy neutrino emission from starburst galaxies are discussed.

Detection of gamma-ray emission in the region of the supernova remnants G296.5+10.0 and G166.0+4.3

52 months of accumulated observations by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope in the region of the supernova remnants G296.5+10.0 (PKS 1209-51/52) and G166.0+4.3 (VRO 42.05.01) are analysed. GeV emission is detected coincident with the position of the sources at the ≃5σ and 11σ levels above the background, respectively, for the best-fitting spectral and spatial scenarios. The gamma-ray spectrum of the sources can be described with a power law in energy. G166.0+4.3 shows a soft GeV spectrum while that of G296.5+10.0 is flat (in the νFν representation). The origin of the gamma-ray emission from the sources is explored. Both leptonic and hadronic mechanisms can account for the high-energy emission from G296.5+10.0, while a leptonic scenario is preferred for G166.0+4.3.

Do NLS1s have a beamed outflow? An unusual X-ray perspective for Mrk 766

Recently, the detection of Gamma-ray emission from Narrow Line Seyfert 1s has suggested that these sources may emit beamed jets towards us. I report the results of a time-resolved spectral analysis of an XMM-Newton long observation of Mrk 766, showing that the X-ray source has been eclipsed several times by clouds with a cold, dense core, and a less dense, highly ionized tail. These clouds have blueshift velocities v > 104 km s−1 (as measured through He-like iron absorption lines), and are therefore part of a strong outflow. These results provide new important constraints on the geometry and structure of this source, and may be relevant to understand the peculiarity of NLS1s as a class of AGNs.

Analysis of MESSENGER Gamma-Ray Spectrometer data from the Mercury flybys

During its three flybys of Mercury, the MESSENGER spacecraft made the first detection of gamma-ray emission from the planet's surface. With a closest approach distance of ∼200 km, the flybys provided an opportunity to measure elemental abundances of Mercury's near-equatorial regions, which will not be visited at low altitude during MESSENGER's orbital mission phase. Despite being limited by low planetary photon flux, sufficient counts were accumulated during the first two flybys to estimate bounds on abundances for some elements having relatively strong gamma-ray spectral peaks, including Si, Fe, Ti, K, and Th. Only for Si is the standard deviation σ sufficiently small to conclude that this element was detected with 99% confidence. Iron and potassium are detected at the 2− σ (95% confidence) level, whereas only upper bounds on Ti and Th can be determined. Relative to a Si abundance assumed to be 18 weight percent (wt%), 2− σ upper bounds have been estimated as 9.7 wt% for Fe, 7.0 wt% for Ti, 0.087 wt% for K, and 2.2 ppm for Th. The relatively low upper bound on K rules out some previously suggested models for surface composition for the regions sampled. Upper bounds on Fe/Si and Ti/Si ratios are generally consistent with Ti and Fe abundances estimated from the analysis of measurements by the MESSENGER Neutron Spectrometer during the flybys but are also permissive of much lower concentrations.

Recent MAGIC results

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes (IACTs) observing high energy gamma rays from violent sources in the Universe. MAGIC-I, the first telescope, started scientific operation in 2004. MAGIC-II was completed in 2009 and after commissioning, the two telescopes are operated as a single instrument performing stereo observations. The telescopes, with a diameter of 17 m each, are the largest IACTs in operation in the world. This system gives us a unique opportunity to explore the high energy Universe with a low energy threshold. Many scientific results have been achieved with MAGIC-I on galactic and extra-galactic sources. Among them, the first detection of the pulsed gamma ray emission from the Crab pulsar at 25 GeV, and the detection of gamma ray emissions from very distant Active Galactic Nuclei, demonstrate the strong advantage and unique features of MAGIC with its low threshold energy setting.

Simulating the γ-ray emission from galaxy clusters: a universal cosmic ray spectrum and spatial distribution

Entering a new era of high-energy gamma-ray experiments, there is an exciting quest for the first detection of gamma-ray emission from clusters of galaxies. To complement these observational efforts, we use high-resolution simulations of a broad sample of galaxy clusters, and follow self-consistent cosmic ray (CR) physics using an improved spectral description. We study CR proton spectra as well as the different contributions of the pion decay and inverse Compton emission to the total flux and present spectral index maps. We find a universal spectrum of the CR component in clusters with surprisingly little scatter across our cluster sample. The spatial CR distribution also shows approximate universality; it depends however on the cluster mass. This enables us to derive a semi-analytic model for both, the distribution of CRs as well as the pion-decay gamma-ray emission that results from hadronic CR interactions with ambient gas protons. In addition, we provide an analytic framework for the inverse Compton emission that is produced by shock-accelerated CR electrons and valid in the full gamma-ray energy range. Combining the complete sample of the brightest X-ray clusters observed by ROSAT with our gamma-ray scaling relations, we identify the brightest clusters for the gamma-ray space telescope Fermi and current imaging air Cherenkov telescopes (MAGIC, HESS, VERITAS). We reproduce the result in Pfrommer (2008), but provide somewhat more conservative predictions for the fluxes in the energy regimes of Fermi and imaging air Cherenkov telescopes when accounting for the bias of `artificial galaxies' in cosmological simulations. We find that it will be challenging to detect cluster gamma-ray emission with Fermi after the second year but this mission has the potential of constraining interesting values of the shock acceleration efficiency after several years of surveying.

Detection of Gamma-Ray Emission from the Vela Pulsar Wind Nebula with AGILE

Pulsars are known to power winds of relativistic particles that can produce bright nebulae by interacting with the surrounding medium. These pulsar wind nebulae are observed by their radio, optical, and x-ray emissions, and in some cases also at TeV (teraelectron volt) energies, but the lack of information in the gamma-ray band precludes drawing a comprehensive multiwavelength picture of their phenomenology and emission mechanisms. Using data from the AGILE satellite, we detected the Vela pulsar wind nebula in the energy range from 100 MeV to 3 GeV. This result constrains the particle population responsible for the GeV emission and establishes a class of gamma-ray emitters that could account for a fraction of the unidentified galactic gamma-ray sources.

Recent results on galactic sources with MAGIC telescope

Located at the Canary island of La Palma, the single-dish MAGIC telescope currently has the lowest energy threshold achieved by any Cherenkov telescope, which can be as low as 25 GeV. In the last two years, the MAGIC telescope has detected a significant amount of galactic sources that emit at very high energies (up to several TeV). Here we present the most recent results that have yielded important scientific highlights in astrophysics, which include the first detection of gamma-ray emission from a pulsar, an X-ray binary system and a stellar-mass black hole. We also make a review of the latest results of the MAGIC observations on galactic sources, which will include also γ-ray unidentified sources (TeV J2032+4130), the Galactic Centre, X-ray binaries (LSI +61 303), pulsars (Crab pulsar) and SNRs (IC443).

Computed radiography as a gamma ray detector—dose response and applications

Computed radiography (CR) can be used for imaging the spatial distribution of photon emissions from radionuclides. Its wide dynamic range and good response to medium energy gamma rays reduces the need for long exposure times. Measurements of small doses can be performed without having to pre-sensitize the computed radiography plates via an x-ray exposure, as required with screen-film systems. Cassette-based Agfa MD30 and Kodak GP25 CR plates were used in applications involving the detection of gamma ray emissions from technetium-99m and iodine-131. Cassette entrance doses as small as 1 µGy (140 keV gamma rays) produce noisy images, but the images are suitable for applications such as the detection of breaks in radiation protection barriers. A consequence of the gamma ray sensitivity of CR plates is the possibility that some nuclear medicine patients may fog their x-rays if the x-ray is taken soon after their radiopharmaceutical injection. The investigation showed that such fogging is likely to be diffuse.

Supernovae as the Site of ther‐Process: Implications for Gamma‐Ray Astronomy

We discuss how detection of gamma-ray emission from the decay of r-process nuclei can improve our understanding of r-process nucleosynthesis. We find that a gamma-ray detector with a sensitivity of ~10^(-7)γ cm^(-2) s^(-1) at E_γ ≈ 100-700 keV may detect the emission from the decay of ^(125)Sb,^(137)Cs,^(144)Ce,^(155)Eu, and ^(194)Os produced in a future Galactic supernova. In addition, such a detector may determine the emission from the decay of ^(126)Sn in the Vela supernova remnant and the diffuse emission from the decay of ^(126)Sn produced by past supernovae in our Galaxy. The required detector sensitivity is similar to what is projected for the proposed Advanced Telescope for High Energy Nuclear Astrophysics (ATHENA). Both the detection of gamma-ray emission from the decay of several r-process nuclei (e.g.,^(125)Sb and ^(194)Os) produced in future Galactic supernovae and the detection of emission from the decay of ^(126)Sn in the Vela supernova remnant would prove that supernovae are a site of the r-process. Furthermore, the former detection would allow us to determine whether or not the r-process nuclei are produced in relative proportions specified by the solar r-process abundance pattern in supernova r-process events. Finally, detection of diffuse emission from the decay of ^(126)Sn in our Galaxy would eliminate neutron star-neutron star mergers as the main source for the r-process nuclei near mass number A ~ 126.

Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei

We have used the Very Long Baseline Array at 15 GHz to image the structure of 132 strong compact active galactic nuclei (AGNs) and quasars with a resolution better than 1 mas and a dynamic range typically exceeding 1000:1. These observations were made as part of a program to investigate the subparsec structure of quasars and AGNs and to study the changes in their structure with time. Many of the sources included in our study, particularly those located south of declination +35?, have not been previously imaged with milliarcsecond resolution. Each of the sources has been observed at multiple epochs. In this paper, we show images of each of the 132 sources that we have observed. For each source we present data at the epoch that had the best quality data. In a future paper we will discuss the kinematics derived from the observations at all epochs. Most of the sources we have observed show the canonical core-jet morphology with structure somewhat characteristic of the jets seen on arcsecond scales with the Very Large Array and Westerbork telescopes. The milliarcsecond jets generally appear one-sided, but two-sided structure is often found in lower luminosity radio galaxies and in high-luminosity quasars with gigahertz-peaked spectra. In many cases there is significant curvature, sometimes up to 90? or more, particularly close to the core. In other cases the jets have a more gradual curvature. In some sources there are multiple bends or twists along the jet, suggestive of a three-dimensional curved structure. Many of the jets may be described by a small number of apparently discrete components, but in other cases there appears to be a monotonically decreasing distribution of radio emission. Usually the structure is unresolved along the direction perpendicular to the jet, but a few sources have broad plumes. Much of the visible parsec-scale structure in compact radio sources can probably be explained as the projection of a relativistic beamed, twisted jet, which appears bright at those positions where it approaches the viewer. In some low-luminosity radio galaxies, the structure appears more symmetric at 2 cm than at longer wavelengths. The apparent long-wavelength asymmetry in these sources is probably due to absorption by intervening ionized material. A few sources contain only a single component, with any secondary feature at least 1000 times weaker. Peak rest-frame brightness temperatures are typically of the order of 1011?1012 K, with no evidence for any excess over the limit of 1012 K expected from inverse Compton cooling. We find no obvious correlation of radio morphology and the detection of gamma-ray emission by EGRET.

High-Energy Gamma Rays from the BL Lacertae Object PKS 2155−304

We present the first detection of gamma-ray emission from the extragalactic source PKS 2155-304, which is often considered to be a prototype of X-ray-selected BL Lacertae objects. The 30 MeV-10 GeV photon spectrum derived from measurements taken with EGRET on the Compton Observatory during 1994 November 15-29 indicates a very hard spectrum with a power-law spectral index of 1.71 ± 0.24. This spectral hardness when combined with the proximity of the source (z = 0.116) makes it an excellent candidate for detection at TeV gamma-ray energies. We briefly discuss the implications of these gamma-ray measurements for models of BL Lac objects.

A survey of COMPTEL results

During the first three years of operation, the imaging Compton telescope COMPTEL on board of the Compton Gamma-Ray Observatory has observed MeV-gamma radiation from a variety of objects including pulsars, stellar black-hole candidates, supernova remnants, extended regions of interstellar space, nuclei of active galaxies, gamma-ray bursters and solar flares. In the field of gamma-ray line spectroscopy, COMPTEL has achieved a major break-through with the establishment of the first map of the galactic plane in the light of the 1.8 MeV line from radioactive 26Al, with the discovery of the 1.15 MeV 44Ti-line from Cas A, and with the first detection of gamma-ray emission from the Orion complex in the 3 to 7 MeV range that can be identified with the 4.44 MeV and 6.13 MeV nuclear deexcitation lines of 12C ∗ and 16O ∗, respectively. An overview about the COMPTEL highlight results is given.

Detection of high-energy gamma-ray emission from the BL Lacertae object Markarian 421 by the EGRET telescope on the Compton Observatory

Gamma radiation in the energy range from 50 MeV to well over 1 GeV has been observed from the direction of the BL Lac object Markarian 421 by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Observatory during the period 1991 June 27-July 11. The source flux is weak, but still statistically significant at the level of 10^-5^; the integrated photon flux above 100 MeV is (1.4 +/- 0.3) x 10^-7^ cm^-2^ s^- 1^. The differential photon energy spectrum can be represented by a power law with exponent 1.96 +/- 0.14. This is the first detection of gamma-ray emission from a BL Lac object.