R. Fleysher

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.

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

TeV Gamma-Ray Survey of the Northern Hemisphere Sky Using the Milagro Observatory

Milagro is a water Cerenkov extensive air shower array that continuously monitors the entire overhead sky in the TeV energy band. The results from an analysis of ~3 yr of data (2000 December-2003 November) are presented. The data have been searched for steady point sources of TeV gamma rays between declinations of 11 and 80°. Two sources are detected, the Crab Nebula and the active galaxy Mrk 421. For the remainder of the northern hemisphere, we set 95% confidence level (CL) upper limits between 275 and 600 mcrab (4.8 × 10-12 to 10.5 × 10-12 cm-2 s-1) above 1 TeV for source declinations between 5° and 70°. Since the sensitivity of Milagro depends on the spectrum of the source at the top of the atmosphere, the dependence of the limits on the spectrum of a candidate source is presented. Because high-energy gamma rays from extragalactic sources are absorbed by interactions with the extragalactic background light, the dependence of the flux limits on the redshift of a candidate source are given. The upper limits presented here are over an order of magnitude more stringent than previously published limits from TeV gamma-ray all-sky surveys.

Evidence for T[CLC]e[/CLC]V Emission from GRB 970417[CLC]a[/CLC]

Milagrito, a detector sensitive to very high energy gamma rays, monitored the northern sky from 1997 February through 1998 May. With a large field of view and a high duty cycle, this instrument was well suited to perform a search for TeV gamma-ray bursts (GRBs). We report on a search made for TeV counterparts to GRBs observed by BATSE. BATSE detected 54 GRBs within the field of view of Milagrito during this period. An excess of events coincident in time and space with one of these bursts, GRB 970417a, was observed by Milagrito. The excess has a chance probability of 2.8 × 10-5 of being a fluctuation of the background. The probability for observing an excess at least this large from any of the 54 bursts is 1.5 × 10-3. No significant correlations were detected from the other bursts.

Human brain‐structure resolved T2 relaxation times of proton metabolites at 3 tesla

The transverse relaxation times, T2, of N‐acetylaspartate (NAA), total choline (Cho), and creatine (Cr) obtained at 3T in several human brain regions of eight healthy volunteers are reported. They were obtained simultaneously in 320 voxels with three‐dimensional (3D) proton MR spectroscopy (1H‐MRS) at 1 cm3 spatial resolution. A two‐point protocol, optimized for the least error per given time by adjusting both the echo delay (TEi) and number of averages, Ni, at each point, was used. Eight healthy subjects (four males and four females, age = 26 ± 2 years) underwent the hour‐long procedure of four 15‐min, 3D acquisitions (TE1 = 35 ms, N1 = 1; and TE2 = 285 ms, N2 = 3). The results reveal that across all subjects the NAA and Cr T2s in gray matter (GM) structures (226 ± 17 and 137 ± 12 ms, respectively) were 13–17% shorter than the corresponding T2s in white matter (WM; 264 ± 10 and 155 ± 7 ms, respectively). The T2s of Cho did not differ between GM and WM (207 ± 17 and 202 ± 8, respectively). For the purpose of metabolic quantification, these values justify to within ±10% the previous use of one T2 per metabolite for 1) the entire brain and 2) all subjects. These T2 values (which to our knowledge were obtained for the first time at this field, spatial resolution, coverage, and precision) are essential for reliable absolute metabolic quantification. Magn Reson Med 57:983–989, 2007.

Evidence for TeV Gamma-Ray Emission from a Region of the Galactic Plane

Gamma-ray emission from a narrow band at the Galactic equator has previously been detected up to 30 GeV. We report evidence for a TeV gamma-ray signal from the Galactic plane by Milagro, a large field of view water Cherenkov detector for extensive air showers. An excess with a significance of 4.5 standard deviations has been observed from the region of Galactic longitude between 40 and 100 deg and latitude |b|<5 deg. Under the assumption of a simple power law spectrum, with no cutoff, in the EGRET-Milagro energy range, the measured integral flux is phi(>3.5TeV) = (6.4 +/- 1.4 +/- 2.1) 10^{-11} cm^{-2}s^{-1} str^{-1}. This flux is consistent with an extrapolation of the EGRET spectrum between 1 and 30 GeV in this Galactic region.

Age dependence of regional proton metabolites T2 relaxation times in the human brain at 3 T

Although recent studies indicate that use of a single global transverse relaxation time, T2, per metabolite is sufficient for better than ±10% quantification precision at intermediate and short echo‐time spectroscopy in young adults, the age‐dependence of this finding is unknown. Consequently, the age effect on regional brain choline (Cho), creatine (Cr), and N‐acetylaspartate (NAA) T2s was examined in four age groups using 3D (four slices, 80 voxels 1 cm3 each) proton MR spectroscopy in an optimized two‐point protocol. Metabolite T2s were estimated in each voxel and in 10 gray and white matter (GM, WM) structures in 20 healthy subjects: four adolescents (13 ± 1 years old), eight young adults (26 ± 1); two middle‐aged (51 ± 6), and six elderly (74 ± 3). The results reveal that T2s in GM (average ± standard error of the mean) of adolescents (NAA: 301 ± 30, Cr: 162 ± 7, Cho: 263 ± 7 ms), young adults (NAA: 269 ± 7, Cr: 156 ± 7, Cho: 226 ± 9 ms), and elderly (NAA: 259 ± 13, Cr: 154 ± 8, Cho: 229 ± 14 ms), were 30%, 16%, and 10% shorter than in WM, yielding mean global T2s of NAA: 343, Cr: 172, and Cho: 248 ms. The elderly NAA, Cr, and Cho T2s were 12%, 6%, and 10% shorter than the adolescents, a change of under 1 ms/year assuming a linear decline with age. Formulae for T2 age‐correction for higher quantification precision are provided. Magn Reson Med 60:790–795, 2008.