High-energy gamma-ray emission from the inner jet of LS I+61 303: the hadronic contribution revisited

LS I+61 303 has been detected by the Cherenkov telescope MAGIC at very high energies, presenting a variable flux along the orbital motion with a maximum clearly separated from the periastron passage. In the light of the new observational constraints, we revisit the discussion of the production of high-energy gamma rays from particle interactions in the inner jet of this system. The hadronic contribution could represent a major fraction of the TeV emission detected from this source. The spectral energy distribution resulting from p-p interactions is recalculated. Opacity effects introduced by the photon fields of the primary star and the stellar decretion disk are shown to be essential in shaping the high-energy gamma-ray light curve at energies close to 200 GeV. We also present results of Monte Carlo simulations of the electromagnetic cascades developed very close to the periastron passage. We conclude that a hadronic microquasar model for the gamma-ray emission in LS I +61 303 can reproduce the main features of its observed high-energy gamma-ray flux.Comment: 6 pages. Sligth improvements made. Accepted version by Astrophysics and Space Scienc

Antitumor Effect by Either FLASH or Conventional Dose Rate Irradiation Involves Equivalent Immune Responses.

The capability of ultrahigh dose rate FLASH radiation therapy to generate the FLASH effect has opened the possibility to enhance the therapeutic index of radiation therapy. The contribution of the immune response has frequently been hypothesized to account for a certain fraction of the antitumor efficacy and tumor kill of FLASH but has yet to be rigorously evaluated. To investigate the immune response as a potentially important mechanism of the antitumor effect of FLASH, various murine tumor models were grafted either subcutaneously or orthotopically into immunocompetent mice or in moderately and severely immunocompromised mice. Mice were locally irradiated with single dose (20 Gy) or hypofractionated regimens (3 × 8 or 2 × 6 Gy) using FLASH (≥2000 Gy/s) and conventional (CONV) dose rates (0.1 Gy/s), with/without anti-CTLA-4. Tumor growth was monitored over time and immune profiling performed. FLASH and CONV 20 Gy were isoeffective in delaying tumor growth in immunocompetent and moderately immunodeficient hosts and increased tumor doubling time to >14 days versus >7 days in control animals. Similar observations were obtained with a hypofractionated scheme, regardless of the microenvironment (subcutaneous flank vs ortho lungs). Interestingly, in profoundly immunocompromised mice, 20 Gy FLASH retained antitumor activity and significantly increased tumor doubling time to >14 days versus >8 days in control animals, suggesting a possible antitumor mechanism independent of the immune response. Analysis of the tumor microenvironment showed similar immune profiles after both irradiation modalities with significant decrease of lymphoid cells by ∼40% and a corresponding increase of myeloid cells. In addition, FLASH and CONV did not increase transforming growth factor-β1 levels in tumors compared with unirradiated control animals. Furthermore, when a complete and long-lasting antitumor response was obtained (>140 days), both modalities of irradiation were able to generate a long-term immunologic memory response. The present results clearly document that the tumor responses across multiple immunocompetent and immunodeficient mouse models are largely dose rate independent and simultaneously contradict a major role of the immune response in the antitumor efficacy of FLASH. Therefore, our study indicates that FLASH is as potent as CONV in modulating antitumor immune response and can be used as an immunomodulatory agent

A lower bound on the local extragalactic magnetic field

Assuming that the hard gamma-ray emission of Cen A is a result of synchrotron radiation of ultra-relativistic electrons, we derive a lower bound on the local extragalactic magnetic field, $B> 10^{-8}$ G. This result is consistent with (and close to) upper bounds on magnetic fields derived from consideration of cosmic microwave background distortions and Faraday rotation measurements.Comment: Includes extensive discussion of particle acceleration above 10^20 eV in the hot spot-like region of Cen

Decoherence scenarios from micro- to macroscopic superpositions

Environment induced decoherence entails the absence of quantum interference phenomena from the macroworld. The loss of coherence between superposed wave packets depends on their separation. The precise temporal course depends on the relative size of the time scales for decoherence and other processes taking place in the open system and its environment. We use the exactly solvable model of an harmonic oscillator coupled to a bath of oscillators to illustrate various decoherence scenarios: These range from exponential golden-rule decay for microscopic superpositions, system-specific decay for larger separations in a crossover regime, and finally universal interaction-dominated decoherence for ever more macroscopic superpositions.Comment: 11 pages, 7 figures, accompanying paper to quant-ph/020412

Metal-insulator transition in the one-dimensional Holstein model at half filling

We study the one-dimensional Holstein model with spin-1/2 electrons at half-filling. Ground state properties are calculated for long chains with great accuracy using the density matrix renormalization group method and extrapolated to the thermodynamic limit. We show that for small electron-phonon coupling or large phonon frequency, the insulating Peierls ground state predicted by mean-field theory is destroyed by quantum lattice fluctuations and that the system remains in a metallic phase with a non-degenerate ground state and power-law electronic and phononic correlations. When the electron-phonon coupling becomes large or the phonon frequency small, the system undergoes a transition to an insulating Peierls phase with a two-fold degenerate ground state, long-range charge-density-wave order, a dimerized lattice structure, and a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure

Anisotropy at the end of the cosmic ray spectrum?

The starburst galaxies M82 and NGC253 have been proposed as the primary sources of cosmic rays with energies above $10^{18.7}$ eV. For energies \agt 10^{20.3} eV the model predicts strong anisotropies. We calculate the probabilities that the latter can be due to chance occurrence. For the highest energy cosmic ray events in this energy region, we find that the observed directionality has less than 1% probability of occurring due to random fluctuations. Moreover, during the first 5 years of operation at Auger, the observation of even half the predicted anisotropy has a probability of less than $10^{-5}$ to occur by chance fluctuation. Thus, this model can be subject to test at very small cost to the Auger priors budget and, whatever the outcome of that test, valuable information on the Galactic magnetic field will be obtained.Comment: Final version to be published in Physical Review

Nearby quasar remnants and ultra-high energy cosmic rays

As recently suggested, nearby quasar remnants are plausible sites of black-hole based compact dynamos that could be capable of accelerating ultra-high energy cosmic rays (UHECRs). In such a model, UHECRs would originate at the nuclei of nearby dead quasars, those in which the putative underlying supermassive black holes are suitably spun-up. Based on galactic optical luminosity, morphological type, and redshift, we have compiled a small sample of nearby objects selected to be highly luminous, bulge-dominated galaxies, likely quasar remnants. The sky coordinates of these galaxies were then correlated with the arrival directions of cosmic rays detected at energies $> 40$ EeV. An apparently significant correlation appears in our data. This correlation appears at closer angular scales than those expected when taking into account the deflection caused by typically assumed IGM or galactic magnetic fields over a charged particle trajectory. Possible scenarios producing this effect are discussed, as is the astrophysics of the quasar remnant candidates. We suggest that quasar remnants be also taken into account in the forthcoming detailed search for correlations using data from the Auger Observatory.Comment: 2 figures, 4 tables, 11 pages. Final version to appear in Physical Review

Energy and Flux Measurements of Ultra-High Energy Cosmic Rays Observed During the First ANITA Flight

The first flight of the Antarctic Impulsive Transient Antenna (ANITA) experiment recorded 16 radio signals that were emitted by cosmic-ray induced air showers. For 14 of these events, this radiation was reflected from the ice. The dominant contribution to the radiation from the deflection of positrons and electrons in the geomagnetic field, which is beamed in the direction of motion of the air shower. This radiation is reflected from the ice and subsequently detected by the ANITA experiment at a flight altitude of 36km. In this paper, we estimate the energy of the 14 individual events and find that the mean energy of the cosmic-ray sample is 2.9 EeV. By simulating the ANITA flight, we calculate its exposure for ultra-high energy cosmic rays. We estimate for the first time the cosmic-ray flux derived only from radio observations. In addition, we find that the Monte Carlo simulation of the ANITA data set is in agreement with the total number of observed events and with the properties of those events.Comment: Added more explanation of the experimental setup and textual improvement

Polaron formation for a non-local electron-phonon coupling: A variational wave-function study

We introduce a variational wave-function to study the polaron formation when the electronic transfer integral depends on the relative displacement between nearest-neighbor sites giving rise to a non-local electron-phonon coupling with optical phonon modes. We analyze the ground state properties such as the energy, the electron-lattice correlation function, the phonon number and the spectral weight. Variational results are found in good agreement with analytic weak-coupling perturbative calculations and exact numerical diagonalization of small clusters. We determine the polaronic phase diagram and we find that the tendency towards strong localization is hindered from the pathological sign change of the effective next-nearest-neighbor hopping.Comment: 11 page