Self-Interacting Dark Matter Halos and the Gravothermal Catastrophe

We study the evolution of an isolated, spherical halo of self-interacting dark matter (SIDM) in the gravothermal fluid formalism. We show that the thermal relaxation time, $t_r$, of a SIDM halo with a central density and velocity dispersion of a typical dwarf galaxy is significantly shorter than its age. We find a self-similar solution for the evolution of a SIDM halo in the limit where the mean free path between collisions, $\lambda$, is everywhere longer than the gravitational scale height, $H$. Typical halos formed in this long mean free path regime relax to a quasistationary gravothermal density profile characterized by a nearly homogeneous core and a power-law halo where $\rho \propto r^{-2.19}$. We solve the more general time-dependent problem and show that the contracting core evolves to sufficiently high density that $\lambda$ inevitably becomes smaller than $H$ in the innermost region. The core undergoes secular collapse to a singular state (the ``gravothermal catastrophe'') in a time $t_{coll} \approx 290 t_r$, which is longer than the Hubble time for a typical dark matter-dominated galaxy core at the present epoch. Our model calculations are consistent with previous, more detailed, N-body simulations for SIDM, providing a simple physical interpretation of their results and extending them to higher spatial resolution and longer evolution times. At late times, mass loss from the contracting, dense inner core to the ambient halo is significantly moderated, so that the final mass of the inner core may be appreciable when it becomes relativistic and radially unstable to dynamical collapse to a black hole.Comment: ApJ in press (to appear in April), 12 pages. Extremely minor changes to agree with published versio

Print versus screen—presentation medium-dependent picture consumption

Part I: The use of digital technologies has forever changed how consumers are capturing, sharing, and storing their pictures. The long-term objective of Part I of this study is to obtain an overall picture of the current state of consumer photographers’ practices. While digital camera owners are taking more photos than ever before, most are not printing them. However, experiments conducted on the presentation mediumdependent differences in picture consumption showed that a majority of the participants prefer printed images to images viewed on a screen. Regardless of this preference, participants did not print images very often for a variety of reasons, including lack of time or money. In addition, results showed that the most commonly used printing tools included Kodak EasyShare Gallery, Shutterfly, and Flickr. Finally, participants cited Photoshop, Lightroom, and Picasa as the primary editing tools, with Facebook being mentioned as the main sharing tool. Parts II & III: The advent of digital print engines capable of achieving high image quality has opened up many new and exciting print product opportunities, including the short-run printing of magazines. However, content available for magazines is also readily available on-line. It is not immediately obvious that the capability of creating a short-run magazine translates into a viable business model; just because they can be printed does not necessarily mean that it makes sense to do so. The objective of this project was to take the first steps toward identifying and understanding the differences in how information is consumed from print on paper versus computer display and which characteristics of these media are particularly relevant in this comparison. Longer-term, it is intended to explore how such differences affect the efficacy of magazine advertising. This evaluation involved an assessment of differences for several metrics, including information retention, time taken to view images, preference for visually consuming information, and distribution of visual gaze as measured by eyetrackers. Experimentation was conducted that focused on the first three of these four factors (Part II). Experimentation in Part III generally confirmed the results of this study: specifically, that people preferred the hard copy rendition over a PDF when given the choice (for reasons of image accessibility and tangibility), and that neither the time used to view the photo books nor information retention as measured by image recognition and information recall were affected by the medium in which the photo book was seen. The results of this study also agreed with earlier research findings that observers tend to fixate first and most often in the central areas of images. This research also found, however, important differences between how the observers viewed the printed and screen versions of the photo book, with the screen group having more fixations per image for many of the images early in the book (though not spending more time with these images), while the print group switched more often between images. While the work conducted in Part II suggested that there were no important differences between the print and screen groups of observers, the results of Part III hint at differences that may be important with regard to individual images or image layouts. Further work involving image content with a more balanced mix of text and pictorial imagery might prove useful in exploring these findings further

On the relation between Unruh and Sokolov--Ternov effects

We show that the Sokolov--Ternov effect -- the depolarization of particles in storage rings coming from synchrotron radiation due to spin flip transitions -- is physically equivalent to the Unruh effect for circular acceleration if one uses a spin 1/2 particle as the Unruh--DeWitt detector. It is shown that for the electron, with gyromagnetic number $g \approx 2$, the exponential contribution to the polarization, which usually characterizes the Unruh effect, is "hidden" in the standard Sokolov-Ternov effect making it hard to observe. Thus, our conclusions are different in detail from previous work.Comment: 23 pages, no figure

Dimensionless Measures of Turbulent Magnetohydrodynamic Dissipation Rates

The magnetic Reynolds number R_M, is defined as the product of a characteristic scale and associated flow speed divided by the microphysical magnetic diffusivity. For laminar flows, R_M also approximates the ratio of advective to dissipative terms in the total magnetic energy equation, but for turbulent flows this latter ratio depends on the energy spectra and approaches unity in a steady state. To generalize for flows of arbitrary spectra we define an effective magnetic dissipation number, R_{M,e}, as the ratio of the advection to microphysical dissipation terms in the total magnetic energy equation, incorporating the full spectrum of scales, arbitrary magnetic Prandtl numbers, and distinct pairs of inner and outer scales for magnetic and kinetic spectra. As expected, for a substantial parameter range R_{M,e}\sim {O}(1) << R_M. We also distinguish R_{M,e} from {\tilde R}_{M,e} where the latter is an effective magnetic Reynolds number for the mean magnetic field equation when a turbulent diffusivity is explicitly imposed as a closure. That R_{M,e} and {\tilde R}_{M,e} approach unity even if R_M>>1 highlights that, just as in hydrodynamic turbulence,energy dissipation of large scale structures in turbulent flows via a cascade can be much faster than the dissipation of large scale structures in laminar flows. This illustrates that the rate of energy dissipation by magnetic reconnection is much faster in turbulent flows, and much less sensitive to microphysical reconnection rates compared to laminar flows.Comment: 14 pages (including 2 figs), accepted by MNRA

Phenomenological study of hadron interaction models

We present a phenomenological study of three models with different effective degrees of freedom: a Goldstone Boson Exchange (GBE) model which is based on quark-meson couplings, the quark delocalization, color screening model (QDCSM) which is based on quark-gluon couplings with delocalized quark wavefunctions, and the Fujiwara-Nijmegen (FN) mixed model which includes both quark-meson and quark-gluon couplings. We find that for roughly two-thirds of 64 states consisting of pairs of octet and decuplet baryons, the three models predict similar effective baryon-baryon interactions. This suggests that the three very different models, based on different effective degrees of freedom, are nonetheless all compatible with respect to baryon spectra and baryon-baryon interactions. We also discuss the differences between the three models and their separate characteristics.Comment: 30 pages latex, 7 tables, 12 figs; submitted to Phys. Rev.