1. 
 Abdo, A. A., et al.
(författare)

Fermi Observations of HighEnergy GammaRay Emission from GRB 080916C
 2009

Ingår i: Science.  00368075 . 10959203. ; 323:5922, s. 16881693

Tidskriftsartikel (refereegranskat)abstract
 <p>Gammaray bursts (GRBs) are highly energetic explosions signaling the death of massive stars in distant galaxies. The Gammaray Burst Monitor and Large Area Telescope onboard the Fermi Observatory together record GRBs over a broad energy range spanning about 7 decades of gammaray energy. In September 2008, Fermi observed the exceptionally luminous GRB 080916C, with the largest apparent energy release yet measured. The highenergy gamma rays are observed to start later and persist longer than the lower energy photons. A simple spectral form fits the entire GRB spectrum, providing strong constraints on emission models. The known distance of the burst enables placing lower limits on the bulk Lorentz factor of the outflow and on the quantum gravity mass.</p>


2. 
 Aktas, A, et al.
(författare)

A direct search for stable magnetic monopoles produced in positronproton collisions at HERA
 2005

Ingår i: European Physical Journal C. Particles and Fields.  Springer.  14346044. ; 41, s. 133141

Tidskriftsartikel (refereegranskat)abstract
 A direct search has been made for magnetic monopoles produced in e(+)p collisions at a centre of mass energy of 300 GeV at HERA. The beam pipe surrounding the interaction region in 19951997 was investigated using a SQUID magnetometer to look for stopped magnetic monopoles. During this time an integrated luminosity of 62 pb(1) was delivered. No magnetic monopoles were observed and charge and mass dependent upper limits on the e(+)p production cross section are set.


3. 
 Abdo, A. A., et al.
(författare)

Fermi Large Area Telescope constraints on the gammaray opacity of the universe
 2010

Ingår i: Astrophysical Journal.  Institute of Physics (IOP).  0004637X . 15384357. ; 723:2, s. 10821096

Tidskriftsartikel (refereegranskat)abstract
 <p>The extragalactic background light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above similar to 10 GeV during propagation from sources at cosmological distances. This results in a redshift and energydependent attenuation of the gammaray flux of extragalactic sources such as blazars and gammaray bursts (GRBs). The Large Area Telescope on board Fermi detects a sample of gammaray blazars with redshift up to z similar to 3, and GRBs with redshift up to z similar to 4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of gammaray flux attenuation by the EBL. We place upper limits on the gammaray opacity of the universe at various energies and redshifts and compare this with predictions from wellknown EBL models. We find that an EBL intensity in the opticalultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. can be ruled out with high confidence.</p>


4. 
 Aktas, A., et al.
(författare)

Measurement of event shape variables in deepinelastic scattering at HERA
 2006

Ingår i: European Physical Journal C. Particles and Fields.  Springer.  14346044. ; 46:2, s. 343356

Tidskriftsartikel (refereegranskat)abstract
 Deepinelastic ep scattering data taken with the H1 detector at HERA and corresponding to an integrated luminosity of 106 pb(1) are used to study the differential distributions of event shape variables. These include thrust, jet broadening, jet mass and the Cparameter. The fourmomentum transfer Q is taken to be the relevant energy scale and ranges between 14 GeV and 200 GeV. The event shape distributions are compared with perturbative QCD predictions, which include resummed contributions and analytical power law corrections, the latter accounting for nonperturbative hadronisation effects. The data clearly exhibit the running of the strong coupling alpha(s)(Q) and are consistent with a universal power correction parameter alpha(0) for all event shape variables. A combined QCD fit using all event shape variables yields alpha(s)(m(Z)) = 0.1198 +/ 0.0013 (+0.0056)(0.0043) and alpha(0) = 0.476 +/ 0.008 (+0.018)(0.059).


5. 
 Aktas, A., et al.
(författare)

Photoproduction of dijets with high transverse momenta at HERA
 2006

Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and HighEnergy Physics.  Elsevier.  03702693. ; 639:1, s. 2131

Tidskriftsartikel (refereegranskat)abstract
 Differential dijet cross sections are measured in photoproduction in the region of photon virtualities Q(2) < 1 GeV2 with the H1 detector at the HERA ep collider using an integrated luminosity of 66.6 pb(1). Jets are defined with the inclusive k perpendicular to algorithm and a minimum transverse momentum of the leading jet of 25 GeV is required. Dijet cross sections are measured indirect and resolved photon enhanced regions separately. Longitudinal proton momentum fractions up to 0.7 are reached. The data compare well with predictions from Monte Carlo event generators based on leading order QCD and parton showers and with nexttoleading order QCD calculations corrected for hadronisation effects.


6. 
 Aktas, A., et al.
(författare)

Elastic J/psi production at HERA
 2006

Ingår i: European Physical Journal C. Particles and Fields.  Springer.  14346044. ; 46:3, s. 585603

Tidskriftsartikel (refereegranskat)abstract
 Cross sections for elastic production of J/psi mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb(1). Results are presented for photon virtualities Q(2) up to 80 GeV2. The dependence on the photonproton centre of mass energy Wgamma p is analysed in the range 40 <= Wgamma p <= 305 GeV in photoproduction and 40 <= Wgamma p <= 160 GeV in electroproduction. The Wgamma p dependences of the cross sections do not change significantly with Q(2) and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q(2). Differential cross sections d sigma/dt, where t is the squared fourmomentum transfer at the proton vertex, are measured in the range vertical bar t vertical bar < 1.2 GeV2 as functions of Wgamma p and Q(2). Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/psi production and decay angular distributions are consistent with schannel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q(2) and is found to be described by perturbative QCD based models.


7. 
 Aktas, A., et al.
(författare)

Forward jet production in deep inelastic scattering at HERA
 2006

Ingår i: European Physical Journal C. Particles and Fields.  Springer.  14346044. ; 46:1, s. 2742

Tidskriftsartikel (refereegranskat)abstract
 The production of forward jets has been measured in deep inelastic ep collisions at HERA. The results are presented in terms of single differential cross sections as a function of the Bjorken scaling variable (xB(j)) and as triple differential cross sections d3 sigma/dx(Bj)dQ(2)dp(t)(,jet)(2), where Q(2) is the four momentum transfer squared and p(t)(,iet)(2) is the squared transverse momentum of the forward jet. Also cross sections for events with a dijet system in addition to the forward jet are measured as a function of the rapidity separation between the forward jet and the two additional jets. The measurements are compared with nexttoleading order QCD calculations and with the predictions of various QCDbased models.


8. 
 Abdo, A. A., et al.
(författare)

A limit on the variation of the speed of light arising from quantum gravity effects
 2009

Ingår i: Nature.  00280836 . 14764687. ; 462:7271, s. 331334

Tidskriftsartikel (refereegranskat)abstract
 <p>A cornerstone of Einstein's special relativity is Lorentz invariancethe postulate that all observers measure exactly the same speed of light in vacuum, independent of photonenergy. While special relativity assumes that there is no fundamental lengthscale associated with such invariance, there is a fundamental scale (the Planck scale, l(Planck) approximate to 1.62 x 10(33) cm or EPlanck = M(Planck)c(2) approximate to 1.22 x 10(19) GeV), at which quantum effects are expected to strongly affect the nature of spacetime. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy(17). Even a tiny variation in photon speed, when accumulated over cosmological lighttravel times, may be revealed by observing sharp features in gammaray burst (GRB) lightcurves(2). Here we report the detection of emission up to similar to 31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E(Planck) on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l(Planck)/1.2 on the length scale of the effect). Our results disfavour quantumgravity theories(3,6,7) in which the quantum nature of spacetime on a very small scale linearly alters the speed of light.</p>


9. 
 Abdo, A. A., et al.
(författare)

A limit on the variation of the speed of light arising from quantum gravity effects
 2009

Ingår i: Nature.  00280836 . 14764687. ; 462:7271, s. 331334

Tidskriftsartikel (refereegranskat)abstract
 <p>A cornerstone of Einstein’s special relativity is Lorentz invariance—the postulate that all observers measure exactly the same speed of light in vacuum, independent of photonenergy. While special relativity assumes that there is no fundamental lengthscale associated with such invariance, there is a fundamental scale (the Planck scale, lPlanck~1.62×1033cm or EPlanck = MPlanckc2~1.22×1019GeV), at which quantum effects are expected to strongly affect the nature of space–time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological lighttravel times, may be revealed by observing sharp features in γray burst (GRB) lightcurves. Here we report the detection of emission up to ~31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2EPlanck on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of lPlanck/1.2 on the length scale of the effect). Our results disfavour quantumgravity theories in which the quantum nature of space–time on a very small scale linearly alters the speed of light.</p>


10. 
 Abdo, A. A., et al.
(författare)

FERMI LARGE AREA TELESCOPE VIEW OF THE CORE OF THE RADIO GALAXY CENTAURUS A
 2010

Ingår i: Astrophysical Journal.  0004637X . 15384357. ; 719:2, s. 14331444

Tidskriftsartikel (refereegranskat)abstract
 <p>We present gammaray observations with the Large Area Telescope (LAT) on board the Fermi GammaRay Space Telescope of the nearby radio galaxy Centaurus A (Cen A). The previous EGRET detection is confirmed, and the localization is improved using data from the first 10 months of Fermi science operation. In previous work, we presented the detection of the lobes by the LAT; in this work, we concentrate on the gammaray core of Cen A. Flux levels as seen by the LAT are not significantly different from that found by EGRET, nor is the extremely soft LAT spectrum (Gamma = 2.67 +/ 0.10(stat) +/ 0.08(sys) where the photon flux is Phi alpha EGamma). The LAT core spectrum, extrapolated to higher energies, is marginally consistent with the nonsimultaneous HESS spectrum of the source. The LAT observations are complemented by simultaneous observations from Suzaku, the Swift Burst Alert Telescope and Xray Telescope, and radio observations with the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry program, along with a variety of nonsimultaneous archival data from a variety of instruments and wavelengths to produce a spectral energy distribution (SED). We fit this broadband data set with a singlezone synchrotron/synchrotron selfCompton model, which describes the radio through GeV emission well, but fails to account for the nonsimultaneous higher energy TeV emission observed by HESS from 2004 to 2008. The fit requires a low Doppler factor, in contrast to BL Lac objects which generally require larger values to fit their broadband SEDs. This indicates that the gammaray emission originates from a slower region than that from BL Lac objects, consistent with previous modeling results from Cen A. This slower region could be a slower moving layer around a fast spine, or a slower region farther out from the black hole in a decelerating flow. The fit parameters are also consistent with Cen A being able to accelerate ultrahigh energy cosmicrays, as hinted at by results from the Auger observatory.</p>

