It is widely believed that the maximum energy of synchrotron photons when electrons are accelerated in shocks via the Fermi process is about 50 MeV (in plasma comoving frame). We show that under certain conditions, which are expected to be realized in relativistic shocks of gamma-ray bursts, synchrotron photons of energy much larger than 50 MeV (comoving frame) can be produced. The requirement is that magnetic field should decay downstream of the shock front on a length scale that is small compared with the distance traveled by the highest energy electrons before they lose half their energy; photons of energy much larger than 50 MeV are produced close to the shock front whereas the highest Lorentz factor that electrons can attain is controlled by the much weaker field that occupies most of the volume of the shocked plasma.Comment: 5 pages, 1 figure; MNRAS in pres

We present a Monte Carlo (MC) code we wrote to simulate the photospheric process and to study the photospheric spectrum above the peak energy. Our simulations were performed with a photon to electron ratio N γ /N e = 10 5 , as determined by observations of the GRB prompt emission. We searched an exhaustive parameter space to determine if the photospheric process can match the observed high-energy spectrum of the prompt emission. If we do not consider electron re-heating, we determined that the best conditions to produce the observed high-energy spectrum are low photon temperatures and high optical depths. However, for these simulations, the spectrum peaks at an energy below 300 keV by a factor ∼ 10. For the cases we consider with higher photon temperatures and lower optical depths, we demonstrate that additional energy in the electrons is required to produce a power-law spectrum above the peak-energy. By considering electron re-heating near the photosphere, the spectrum for these simulations have a peak-energy ∼ 300 keV and a power-law spectrum extending to at least 10 MeV with a spectral index consistent with the prompt emission observations. We also performed simulations for different values of N γ /N e and determined that the simulation results are very sensitive to N γ /N e . Lastly, in addition to Comptonizing a Blackbody spectrum, we also simulate the Comptonization of a f ν ∝ ν −1/2 fast cooled synchrotron spectrum. The spectrum for these simulations peaks at ∼ 10 4 keV, with a flat spectrum f ν ∝ ν 0 below the peak energy.

Bacterial swarming is a type of motility characterized by a rapid and collective migration of bacteria on surfaces. Most swarming species form densely packed dynamic clusters in the form of whirls and jets, in which hundreds of rod-shaped rigid cells move in circular and straight patterns, respectively. Recent studies have suggested that short-range steric interactions may dominate hydrodynamic interactions and that geometrical factors, such as a cell's aspect ratio, play an important role in bacterial swarming. Typically, the aspect ratio for most swarming species is only up to 5, and a detailed understanding of the role of much larger aspect ratios remains an open challenge. Here we study the dynamics of Paenibacillus dendritiformis C morphotype, a very long, hyperflagellated, straight (rigid), rod-shaped bacterium with an aspect ratio of ϳ20. We find that instead of swarming in whirls and jets as observed in most species, including the shorter T morphotype of P. dendritiformis, the C morphotype moves in densely packed straight but thin long lines. Within these lines, all bacteria show periodic reversals, with a typical reversal time of 20 s, which is independent of their neighbors, the initial nutrient level, agar rigidity, surfactant addition, humidity level, temperature, nutrient chemotaxis, oxygen level, illumination intensity or gradient, and cell length. The evolutionary advantage of this unique back-and-forth surface translocation remains unclear. Motile bacteria are able to colonize surfaces using various motility mechanisms (1). One efficient method includes flagellation-based cell motion in conjunction with collective lubrication (typically by secretion of surfactants) to enable fast expansion on hard surfaces. This mode of "bacterial swarming" that has been studied extensively for many species (1-15) enables rapid colony expansion (up to centimeters per hour). Swarming is often marked by hundreds of cells moving in a coordinated fashion while generating whirl and jet patterns.Studies of the collective dynamics of swarming have examined multiple aspects of motility. On the macroscopic level, it was discovered that swarming colonies show an advantage over liquid cultures in that they exhibit an increased resistance to antimicrobials (1,4,5,(15)(16)(17)(18)(19)(20). Studies of collective secretions of signaling and quorum-sensing molecules have shown how interactions between cells in swarming colonies are controlled (11) and exposed the identification of associated genetic manipulations and upregulated proteins that control biosurfactant secretions and flagellar behavior. On the single-cell level, attention was given to swarm cell trajectories and the ways in which these trajectories are determined by flagellar motion (8,(21)(22)(23). A combination of experiments (2, 3, 14, 15, 24-38) and theory (39-43) suggests that hydrodynamic interactions play a significant role in this social form of migration.Hydrodynamic interactions may not always be the dominant physical mechanism controlling bacterial motion....

Within the first 10 days after Swift discovered the jetted tidal disruption event (TDE) Sw J1644+57, simultaneous observations in the radio, near-infrared, optical, X-ray and gammaray bands were carried out. These multiwavelength data provide a unique opportunity to constrain the emission mechanism and make-up of a relativistic super-Eddington jet. We consider an exhaustive variety of radiation mechanisms for the generation of X-rays in this TDE, and rule out many processes such as SSC, photospheric and proton synchrotron. The infrared to gamma-ray data for Sw J1644+57 are consistent with synchrotron and external-inverse-Compton (EIC) processes provided that electrons in the jet are continuously accelerated on a time scale shorter than ∼ 1% of the dynamical time to maintain a power-law distribution. The requirement of continuous electron acceleration points to magnetic reconnection in a Poynting flux dominated jet. The EIC process may require fine tuning to explain the observed temporal decay of the X-ray lightcurve, whereas the synchrotron process in a magnetic jet needs no fine tuning for this TDE.

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