Differential constraints and exact solutions of nonlinear diffusion equations

The differential constraints are applied to obtain explicit solutions of nonlinear diffusion equations. Certain linear determining equations with parameters are used to find such differential constraints. They generalize the determining equations used in the search for classical Lie symmetries

Study of fuel cell powerplant with heat recovery

It was shown that heat can be recovered from fuel cell power plants by replacing the air-cooled heat exchangers in present designs with units which transfer the heat to the integrated utility system. Energy availability for a 40-kW power plant was studied and showed that the total usable energy at rated power represents 84 percent of the fuel lower heating value. The effects of design variables on heat availability proved to be small. Design requirements were established for the heat recovery heat exchangers, including measurement of the characteristics of two candidate fuel cell coolants after exposure to fuel cell operating conditions. A heat exchanger test program was defined to assess fouling and other characteristics of fuel cell heat exchangers needed to confirm heat exchanger designs for heat recovery

New flaring of an ultraluminous X-ray source in NGC 1365

We have studied a highly variable ultraluminous X-ray source (ULX) in the Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM-Newton observations between 2002 and 2006. In 2006 April, the source peaked at a luminosity ~ 3 x 10^{40} erg/s in the 0.3-10 keV band (similar to the maximum luminosity found by ASCA in 1995), and declined on an e-folding timescale ~ 3 days. The X-ray spectrum is always dominated by a broad power-law-like component. When the source is seen at X-ray luminosities ~ 10^{40} erg/s, an additional soft thermal component (which we interpret as emission from the accretion disk) contributes ~ 1/4 of the X-ray flux; when the luminosity is higher, ~ 3 x 10^{40} erg/s, the thermal component is not detected and must contribute < 10% of the flux. At the beginning of the decline, ionized absorption is detected around 0.5-2 keV; it is a possible signature of a massive outflow. The power-law is always hard, with a photon index Gamma ~ 1.7 (and even flatter at times), as is generally the case with bright ULXs. We speculate that this source and perhaps most other bright ULXs are in a high/hard state: as the accretion rate increases well above the Eddington limit, more and more power is extracted from the inner region of the inflow through non-radiative channels, and is used to power a Comptonizing corona, jet or wind. The observed thermal component comes from the standard outer disk; the transition radius between outer standard disk and Comptonizing inner region moves further out and to lower disk temperatures as the accretion rate increases. This produces the observed appearance of a large, cool disk. Based on X-ray luminosity and spectral arguments, we suggest that this accreting black hole has a likely mass ~ 50-150 Msun (even without accounting for possible beaming).Comment: 14 pages, to appear in MNRA

Relativistic corrections to isotope shift in light ions

We calculate isotope mass shift for several light ions using Dirac wave functions and mass shift operator with relativistic corrections of the order of $(\alpha Z)^2$. Calculated relativistic corrections to the specific mass shift vary from a fraction of a percent for Carbon, to 2% for Magnesium. Relativistic corrections to the normal mass shift are typically smaller. Interestingly, the final relativistic mass shifts for the levels of one multiplet appear to be even closer than for non-relativistic operator. That can be important for the astrophysical search for possible $\alpha$-variation, where isotope shift is a source of important systematic error. Our calculations show that for levels of the same multiplet this systematics is negligible and they can be used as probes for $\alpha$-variation.Comment: 7 pages, 5 tables, revtex

Spontaneous emergence of spatial patterns ina a predator-prey model

We present studies for an individual based model of three interacting populations whose individuals are mobile in a 2D-lattice. We focus on the pattern formation in the spatial distributions of the populations. Also relevant is the relationship between pattern formation and features of the populations' time series. Our model displays travelling waves solutions, clustering and uniform distributions, all related to the parameters values. We also observed that the regeneration rate, the parameter associated to the primary level of trophic chain, the plants, regulated the presence of predators, as well as the type of spatial configuration.Comment: 17 pages and 15 figure

Omega-3 Fatty Acids Improve Recovery, whereas Omega-6 Fatty Acids Worsen Outcome, after Spinal Cord Injury in the Adult Rat

Spinal cord injury (SCI) is a cause of major neurological disability, and no satisfactory treatment is currently available. Evidence suggests that polyunsaturated fatty acids (PUFAs) could target some of the pathological mechanisms that underlie damage after SCI. We examined the effects of treatment with PUFAs after lateral spinal cord hemisection in the rat. The ω-3 PUFAs α-linolenic acid and docosahexaenoic acid (DHA) injected 30 min after injury induced significantly improved locomotor performance and neuroprotection, including decreased lesion size and apoptosis and increased neuronal and oligodendrocyte survival. Evidence showing a decrease in RNA/DNA oxidation suggests that the neuroprotective effect of ω-3 PUFAs involved a significant antioxidant function. In contrast, animals treated with arachidonic acid, an ω-6 PUFA, had a significantly worse outcome than controls. We confirmed the neuroprotective effect of ω-3 PUFAs by examining the effects of DHA treatment after spinal cord compression injury. Results indicated that DHA administered 30 min after spinal cord compression not only greatly increased survival of neurons but also resulted in significantly better locomotor performance for up to 6 weeks after injury. This report shows a striking difference in efficacy between the effects of treatment with ω-3 and ω-6 PUFAs on the outcome of SCI, with ω-3 PUFAs being neuroprotective and ω-6 PUFAs having a damaging effect. Given the proven clinical safety of ω-3 PUFAs, our observations show that these PUFAs have significant therapeutic potential in SCI. In contrast, the use of preparations enriched in ω-6 PUFAs after injury could worsen outcome after SCI

Is there further evidence for spatial variation of fundamental constants?

The detection of a spatial variation of the fine-structure constant, alpha, based on study of quasar absorption systems has recently been reported. The physics that causes this alpha-variation should have other observable manifestations, and this motivates us to look for complementary astrophysical effects. In this paper we propose a method to test whether spatial variation of fundamental constants existed during the epoch of big bang nucleosynthesis. Using existing measurements of primordial deuterium abundance we find very weak indications that such a signature might exist, but the paucity of measurements precludes any firm conclusion. We also examine existing quasar absorption spectra data that are sensitive to variation of the electron-to-proton mass ratio, mu, and x = (alpha^2 mu g_p) for spatial variation.Comment: 6 pages, 1 figur

Thermally-induced vacuum instability in a single plane wave

Ever since Schwinger published his influential paper [J. Schwinger, Phys. Rev. \textbf{82}, 664 (1951)], it has been unanimously accepted that the vacuum is stable in the presence of an electromagnetic plane wave. However, we advance an analysis that indicates this statement is not rigorously valid in a real situation, where thermal effects are present. We show that the thermal vacuum, in the presence of a single plane-wave field, even in the limit of zero frequency (a constant crossed field), decays into electron-positron pairs. Interestingly, the pair-production rate is found to depend nonperturbatively on both the amplitude of the constant crossed field and on the temperature.Comment: 5 pages, 3 figure

Assisted stellar suicide in V617 Sgr

V617 Sgr is a V Sagittae star - a group of binaries thought to be the galactic counterparts of the Compact Binary Supersoft X-ray Sources - CBSS. To check this hypothesis, we measured the time derivative of its orbital period. Observed timings of eclipse minima spanning over 30,000 orbital cycles are presented. We found that the orbital period evolves quite rapidly: P/Pdot = 1.1 x 10^{6} years. This is consistent with the idea that V617 Sgr is a wind driven accretion supersoft source. As the binary system evolves with a time-scale of about one million years, which is extremely short for a low mass evolved binary, it is likely that the system will soon end either by having its secondary completely evaporated or by the primary exploding as a supernova of type Ia.Comment: 4 pages, accepted for publication in A&A Letter