Images of Bursting Sources of High-Energy Cosmic Rays. I: Effects of Magnetic Fields

It has recently been shown that the highest energy cosmic rays (CRs) may originate in the same cosmological objects producing $\gamma$-ray bursts. This model requires the presence of intergalactic magnetic fields (IGMF) to delay the arrival times of $\sim 10^{20}$ eV CRs by 50 years or longer relative to the $\gamma$-rays, of an amplitude that is consistent with other observational constraints. Sources of CRs coming from individual bursts should be resolved with the planned ``Auger'' experiment, with as many as hundreds of CRs for the brightest sources. We analyze here the apparent angular and energy distribution of CRs from bright sources below the pion production threshold (in the energy range $10^{19}{\rm eV} < E < 4\times10^{19}{\rm eV}$) expected in this model. This observable distribution depends on the structure of the IGMF: the apparent spectral width $\Delta E$ is small, $\Delta E/E\lesssim1\%$, if the intergalactic field correlation length $\lambda$ is much larger than $1{\rm Mpc}$, and large, $\Delta E/E=0.3$, in the opposite limit $\lambda\ll 1{\rm Mpc}$. The apparent angular size is also larger for smaller $\lambda$. If the sources of CRs we predict are found, they will corroborate the bursting model and they will provide us with a technique to investigate the structure of the IGMF.Comment: Submitted to the ApJL; 10 pages AASTeX, including 2 PostScript figure

Gravitational Lensing in Clusters of Galaxies: New Clues Regarding the Dynamics of Intracluster Gas

Long arcs in clusters of galaxies, produced by gravitational lensing, can be used to estimate the mass interior to the arcs and therefore, constrain the cluster mass distribution. The radial density distribution of the intracluster gas (ICM) can be extracted from the X-ray surface brightness observations. If the gas temperature is also known, it is then possible to probe the dynamical state of the gas and test whether the ICM is in hydro- static equilibrium within the gravitational potential of the cluster as a result of thermal pressure support. We analyze three clusters that exhibit large arcs, whose X-ray surface brightness profiles have been observed, and whose gas temperatures have been determined. In two of the clusters, A2218 and A1689, the central mass implied by lensing is a factor of $2$--$2.5$ too large for the gas at the observed temperature to be in hydrostatic equilibrium solely due to thermal pressure support. In other words, if we accept the mass estimate derived from the lensing analysis and demand that the X-ray surface brightness profile be consistent with the observations, the required gas temperature is a factor of $2$--$2.5$ higher than observed. The results for the third cluster, A2163 (the most luminous and the hottest cluster known), are more ambiguous. The discrepancy between the X-ray and the lensing mass estimates arise because the presence of arcs imply a highly concentrated cluster mass distribution whereas the observed X-ray profiles imply a more extended mass distribution. The large X-ray core radii are not the result of the limited resolution of the X-ray detectors. We consider various possibilities that could account for the discrepancy.Comment: 20 pages, uuencoded compressed postscript, CITA/93/3

Impact of Cosmic Rays on Population III Star Formation

We explore the implications of a possible cosmic ray (CR) background generated during the first supernova explosions that end the brief lives of massive Population III stars. We show that such a CR background could have significantly influenced the cooling and collapse of primordial gas clouds in minihaloes around redshifts of z ~ 15 - 20, provided the CR flux was sufficient to yield an ionization rate greater than about 10^-19 s^-1 near the center of the minihalo. The presence of CRs with energies less than approximately 10^7 eV would indirectly enhance the molecular cooling in these regions, and we estimate that the resulting lower temperatures in these minihaloes would yield a characteristic stellar mass as low as ~ 10 M_sun. CRs have a less pronounced effect on the cooling and collapse of primordial gas clouds inside more massive dark matter haloes with virial masses greater than approximately 10^8 M_sun at the later stages of cosmological structure formation around z ~ 10 - 15. In these clouds, even without CR flux the molecular abundance is already sufficient to allow cooling to the floor set by the temperature of the cosmic microwave background.Comment: MNRAS in press, 12 pages, 6 figure

How cells feel: stochastic model for a molecular mechanosensor

Understanding mechanosensitivity, i.e. how cells sense the stiffness of their environment is very important, yet there is a fundamental difficulty in understanding its mechanism: to measure an elastic modulus one requires two points of application of force - a measuring and a reference point. The cell in contact with substrate has only one (adhesion) point to work with, and thus a new method of measurement needs to be invented. The aim of this theoretical work is to develop a self-consistent physical model for mechanosensitivity, a process by which a cell detects the mechanical stiffness of its environment (e.g. a substrate it is attached to via adhesion points) and generates an appropriate chemical signaling to remodel itself in response to this environment. The model uses the molecular mechanosensing complex of latent TGF-$\beta$ attached to the adhesion point as the biomarker. We show that the underlying Brownian motion in the substrate is the reference element in the measuring process. The model produces the closed expression for the rate of release of active TGF-$\beta$, which depends on the substrate stiffness and the pulling force coming from the cell in a subtle and non-trivial way. It is consistent with basic experimental data showing an increase in signal for stiffer substrates and higher pulling forces. In addition, we find that for each cell there is a range of stiffness where a homeostatic configuration of the cell can be achieved, outside of which the cell either relaxes its cytoskeletal forces and detaches from the very weak substrate, or generates an increasingly strong pulling force through stress fibers with a positive feedback loop on very stiff substrates. In this way, the theory offers the underlying mechanism for the myofibroblast conversion in wound healing and smooth muscle cell dysfunction in cardiac disease

Gravitational Collapse of Small-Scale Structure as the Origin of the Lyman Alpha Forest

If gravitational clustering is a hierarchical process, the present large-scale structure of the galaxy distribution implies that structures on smaller scales must have formed at high redshift. We simulate the formation of small-scale structure (average cell mass: $\Delta \bar m_b=10^{4.2}$M$_\odot$) and the evolution of photoionized gas, in the specific case of a CDM model with a cosmological constant. The photoionized gas has a natural minimal scale of collapse, the Jeans scale ($m_{b,J}\simeq 10^{9}$M$_\odot$). We find that low column density (\nhi \le 10^{14}\cm^{-2}) lines originate in regions resembling Zel'dovich pancakes, where gas with overdensities in the range $3 - 30$ is enclosed by two shocks but is typically re-expanding at approximately the Hubble velocity. However, higher column density (\nhi \ge 10^{15}\cm^{-2}) lines stem from more overdense regions where the shocked gas is cooling. We show that this model can probably account for the observed number of lines, their distribution in column density and b-parameters, as well as the cloud physical sizes as observed in gravitationally lensed quasars. We find a redshift evolution that isComment: 15p postscript file to appear in The Astrophysical Journal Letters (1994