J. Ryan

Instrument description and performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory

The imaging Compton telescope COMPTEL is one of the four instruments on board the Compton Gamma-Ray Observatory (GRO), which was launched on 1991 April 5 by the space shuttle Atlantis into an Earth orbit of 450 km altitude. COMPTEL is exploring the 1-30 MeV energy range with an angular resolution (1σ) between 1° and 2° within a large field of view of about 1 steradian. Its energy resolution (8.8% FWHM at 1.27 MeV) makes it a powerful gamma-ray line spectrometer. Its effective area (for on-axis incidence) varies between 10 and 50 cm 2 depending on energy and event selections. Within a 14 day observation period COMPTEL is able to detect sources which are about 20 times weaker than the Crab. The measurement principle of COMPTEL also allows the measurements of solar neutrons

The Soft Gamma-Ray Spectral Variability of Cygnus X-1

We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV γ-ray emission between the hard and soft spectral states, using spectra that cover the energy range from 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder γ-ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft γ-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of ~4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.

TeV gamma-ray sources from a survey of the Galactic plane with Milagro

A survey of Galactic gamma-ray sources at a median energy of ~20 TeV has been performed using the Milagro Gamma-Ray Observatory. Eight candidate sources of TeV emission are detected with pretrial significances >4.5 σ in the region of Galactic longitude l [30°, 220°] and latitude b [-10°, 10°]. Four of these sources, including the Crab Nebula and the recently published MGRO J2019+37, are observed with significances >4 σ after accounting for the trials. All four of these sources are also coincident with EGRET sources. Two of the lower significance sources are coincident with EGRET sources, and one of these sources is Geminga. The other two candidates are in the Cygnus region of the Galaxy. Several of the sources appear to be spatially extended. The fluxes of the sources at 20 TeV range from ~25% of the Crab flux to nearly as bright as the Crab.

The Large-Scale Cosmic-Ray Anisotropy as Observed with Milagro

Results are presented of a harmonic analysis of the large-scale cosmic-ray (CR) anisotropy as observed by the Milagro observatory. We show a two-dimensional display of the sidereal anisotropy projections in right ascension (R.A.) generated by the fitting of three harmonics to 18 separate declination bands. The Milagro observatory is a water Cherenkov detector located in the Jemez mountains near Los Alamos, New Mexico. With a high duty cycle and large field of view, Milagro is an excellent instrument for measuring this anisotropy with high sensitivity at TeV energies. The analysis is conducted using a seven-year data sample consisting of more than 95 billion events, the largest such data set in existence. We observe an anisotropy with a magnitude around 0.1% for CRs with a median energy of 6 TeV. The dominant feature is a deficit region of depth (2.49 ± 0.02 stat. ± 0.09 sys.) ×10–3 in the direction of the Galactic north pole centered at 189 deg R.A. We observe a steady increase in the magnitude of the signal over seven years.

Discovery of TeV gamma-ray emission from the Cygnus region of the galaxy

The diffuse gamma radiation arising from the interaction of cosmic ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Milagro is a water Cherenkov detector that continuously views the entire overhead sky. The large field-of-view combined with the long observation time makes Milagro the most sensitive instrument available for the study of large, low surface brightness sources such as the diffuse gamma radiation arising from interactions of cosmic radiation with interstellar matter. In this paper we present spatial and flux measurements of TeV gamma-ray emission from the Cygnus Region. The TeV image shows at least one new source MGRO J2019+37 as well as correlations with the matter density in the region as would be expected from cosmic-ray proton interactions. However, the TeV gamma-ray flux as measured at {approx}12 TeV from the Cygnus region (after excluding MGRO J2019+37) exceeds that predicted from a conventional model of cosmic ray production and propagation. This observation indicates the existence of either hard-spectrum cosmic-ray sources and/or other sources of TeV gamma rays in the region.

Discovery of Localized Regions of Excess 10-TeV Cosmic Rays

The 7 year data set of the Milagro TeV observatory contains 2.2 x 10(11) events of which most are due to hadronic cosmic rays. These data are searched for evidence of intermediate scale structure. Excess emission on angular scales of approximately 10 degrees has been found in two localized regions of unknown origin with greater than 12sigma significance. Both regions are inconsistent with pure gamma-ray emission with high confidence. One of the regions has a different energy spectrum than the isotropic cosmic-ray flux at a level of 4.6sigma, and it is consistent with hard spectrum protons with an exponential cutoff, with the most significant excess at approximately 10 TeV. Potential causes of these excesses are explored, but no compelling explanations are found.

MILAGRO OBSERVATIONS OF MULTI-TeV EMISSION FROM GALACTIC SOURCES IN THE FERMI BRIGHT SOURCE LIST

We present the result of a search of the Milagro sky map for spatial correlations with sources from a subset of the recent Fermi Bright Source List (BSL). The BSL consists of the 205 most significant sources detected above 100 MeV by the Fermi Large Area Telescope. We select sources based on their categorization in the BSL, taking all confirmed or possible Galactic sources in the field of view of Milagro. Of the 34 Fermi sources selected, 14 are observed by Milagro at a significance of 3 standard deviations or more. We conduct this search with a new analysis which employs newly optimized gamma-hadron separation and utilizes the full eight-year Milagro data set. Milagro is sensitive to gamma rays with energy from 1 to 100 TeV with a peak sensitivity from 10 to 50 TeV depending on the source spectrum and declination. These results extend the observation of these sources far above the Fermi energy band. With the new analysis and additional data, multi-TeV emission is definitively observed associated with the Fermi pulsar, J2229.0+6114, in the Boomerang pulsar wind nebula (PWN). Furthermore, an extended region of multi-TeV emission is associated with the Fermi pulsar, J0634.0+1745, the Geminga pulsar.

A direct observation of solar neutrons following the 0118 UT flare on 1980 June 21

The Gamma Ray Spectrometer on the Solar Maximum Mission satellite has observed energetic solar neutrons (greater than 50 MeV) at the earth following a solar flare that occurred on the west limb on June 21, 1980 at 01:18:20 UT. Impulsive photon emission from 10 keV to greater than 65 MeV lasting over a period of about 66 s was followed by a transient flux of 50-600 MeV neutrons incident over a 17 minute period. The peak counting rate corresponds to an average flux at the earth of (3.8 + or - 0.6) x 10 to the -2nd neutrons/sq cm s at 130 MeV. These observations indicate the emission of 3 x 10 to the 28th neutrons/sr with energies greater than 50 MeV, requiring the rapid acceleration (much less than 60 s) of protons to GeV energies during the impulsive phase of the flare.

Observations of GRB 990123 by the Compton gamma ray observatory

GRB 990123 was the first burst from which simultaneous optical, X-ray, and gamma-ray emission was detected; its afterglow has been followed by an extensive set of radio, optical, and X-ray observations. We have studied the gamma-ray burst itself as observed by the Compton Gamma Ray Observatory detectors. We find that gamma-ray fluxes are not correlated with the simultaneous optical observations and that the gamma-ray spectra cannot be extrapolated simply to the optical fluxes. The burst is well fitted by the standard four-parameter GRB function, with the exception that excess emission compared with this function is observed below ~15 keV during some time intervals. The burst is characterized by the typical hard-to-soft and hardness-intensity correlation spectral evolution patterns. The energy of the peak of the νfν spectrum, Ep, reaches an unusually high value during the first intensity spike, 1470 ± 110 keV, and then falls to ~300 keV during the tail of the burst. The high-energy spectrum above ~1 MeV is consistent with a power law with a photon index of about -3. By fluence, GRB 990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly placing it on the - power-law portion of the intensity distribution. However, the redshift measured for the afterglow is inconsistent with the Euclidean interpretation of the - power law. Using the redshift value of ≥1.61 and assuming isotropic emission, the gamma-ray energy exceeds 1054 ergs.

A MEASUREMENT OF THE SPATIAL DISTRIBUTION OF DIFFUSE TeV GAMMA-RAY EMISSION FROM THE GALACTIC PLANE WITH MILAGRO

Diffuse � -ray emission produced by the interaction of cosmic-ray particles with matter and radiation in the Galaxy can be used to probe the distribution of cosmic rays and their sources in different regions of the Galaxy. With its large field of view and long observation time, the Milagro Gamma Ray Observatory is an ideal instrument for surveying large regions of the northern hemisphere sky and for detecting diffuse � -ray emission at very high energies. Here the spatial distributionand thefluxof thediffuse � -rayemission inthe TeVenergyrange withamedian energyof 15TeV for Galactic longitude between 30 � and 110 � and between 136 � and 216 � and for Galactic latitude between � 10 � and 10 � aredetermined.Themeasuredfluxesareconsistentwithpredictionsof theGALPROPmodeleverywhere,except for the Cygnus region (l 2½ 65 � ;85 � � ). For the Cygnus region, the flux is twice the predicted value. This excess can be explained by the presence of active cosmic-ray sources accelerating hadrons, which interact with the local dense interstellar medium and produce gamma rays through pion decay. Subject headingg gamma rays: observations