The origins of Causality Violations in Force Free Simulations of Black Hole Magnetospheres

Recent simulations of force-free, degenerate (ffde) black hole magnetospheres indicate that the fast mode radiated from (or near) the event horizon can modify the global potential difference in the poloidal direction orthogonal to the magnetic field, V, in a black hole magnetosphere. There is a fundamental contradiction in a wave that alters V coming from near the horizon. The background fields in ffde satisfy the ``ingoing wave condition'' near the horizon (that arises from the requirement that all matter is ingoing at the event horizon), yet outgoing waves are radiated from this region in the simulation. Studying the properties of the waves in the simulations are useful tools to this end. It is shown that regularity of the stress-energy tensor in a freely falling frame requires that the outgoing (as viewed globally) waves near the event horizon are redshifted away and are ineffectual at changing V. It is also concluded that waves in massless MHD (ffde) are extremely inaccurate depictions of waves in a tenuous MHD plasma, near the event horizon, as a consequence black hole gravity. Any analysis based on ffde near the event horizon is seriously flawed.Comment: 9 pages to appear in ApJ Letter

Test particle motion in a gravitational plane wave collision background

Test particle geodesic motion is analysed in detail for the background spacetimes of the degenerate Ferrari-Ibanez colliding gravitational wave solutions. Killing vectors have been used to reduce the equations of motion to a first order system of differential equations which have been integrated numerically. The associated constants of the motion have also been used to match the geodesics as they cross over the boundary between the single plane wave and interaction zones.Comment: 11 pages, 6 Postscript figure

In Situ Scanning Tunneling Microscopy of DNA-Modified Gold Surfaces: Bias and Mismatch Dependence

In situ scanning tunneling microscopy has been performed on DNA-modified gold surfaces under physiological conditions. The STM images of DNA-modified gold surfaces are strongly dependent on the applied potential and percentage of DNA duplexes containing a single base mismatch. At negative surface potentials we observe reproducible features that are attributed to DNA agglomerates where the DNA duplexes are in the upright orientation; at positive potentials, when DNA molecules lie down on the surface, the film is transparent, and only the gold surface is distinguishable. These observations indicate that DNA possesses a non-negligible local density of states which can be probed when the DNA duplex is in the upright orientation. By varying the percentage of DNA duplexes containing a single base mismatch, we have observed a dramatic change in the image contrast as a result of the perturbation induced by the mismatch on the electronic pathway inside the DNA. These results emphasize the central role of the integrity of the π-stack for DNA charge transport. Duplex DNA is a promising candidate in molecular electronics, but only in arrangements where the orbitals can efficiently overlap with the electronic states of the electrodes and the environment does not constrain the DNA in non-native, poorly stacked conformations

Limitations of Radar Coordinates

The construction of a radar coordinate system about the world line of an observer is discussed. Radar coordinates for a hyperbolic observer as well as a uniformly rotating observer are described in detail. The utility of the notion of radar distance and the admissibility of radar coordinates are investigated. Our results provide a critical assessment of the physical significance of radar coordinates.Comment: 12 pages, revtex and pictex macros, 3 pictex figures, 1 eps figure. Expanded versio

A constant dark matter halo surface density in galaxies

We confirm and extend the recent finding that the central surface density r_0*rho_0 galaxy dark matter halos, where r_0 and rho_0 are the halo core radius and central density, is nearly constant and independent of galaxy luminosity. Based on the co-added rotation curves of about 1000 spiral galaxies, mass models of individual dwarf irregular and spiral galaxies of late and early types with high-quality rotation curves and, galaxy-galaxy weak lensing signals from a sample of spiral and elliptical galaxies, we find that log(r_0*rho_0) = 2.15 +- 0.2, in units of log(Msol/pc^2). We also show that the observed kinematics of Local Group dwarf spheroidal galaxies are consistent with this value. Our results are obtained for galactic systems spanning over 14 magnitudes, belonging to different Hubble Types, and whose mass profiles have been determined by several independent methods. In the same objects, the approximate constancy of rho_0*r_0 is in sharp contrast to the systematical variations, by several orders of magnitude, of galaxy properties, including rho_0 and central stellar surface density.Comment: Accepted for publication in MNRAS. 9 pages, 4 figure

Long-Term Multiwavelength Studies of High-Redshift Blazar 0836+710

Aims. The observation of gamma -ray flares from blazar 0836+710 in 2011, following a period of quiescence, offered an opportunity to study correlated activity at different wavelengths for a high-redshift (z=2.218) active galactic nucleus. Methods. Optical and radio monitoring, plus Fermi-LAT gamma-ray monitoring provided 2008-2012 coverage, while Swift offered auxiliary optical, ultraviolet, and X-ray information. Other contemporaneous observations were used to construct a broad-band spectral energy distribution. Results. There is evidence of correlation but not a measurable lag between the optical and gamma-ray flaring emission. On the contrary, there is no clear correlation between radio and gamma-ray activity, indicating radio emission regions that are unrelated to the parts of the jet that produce the gamma-rays. The gamma-ray energy spectrum is unusual in showing a change of shape from a power law to a curved spectrum when going from the quiescent state to the active state.Comment: 11 pages, 10 figures, Accepted for publication in A&

Implication of the PAMELA antiproton data for dark matter indirect detection at LHC

Since the PAMELA results on the "anomalously" high positron fraction and the lack of antiproton excess in our Galaxy, there has been a tremendous number of studies advocating new types of dark matter, with larger couplings to electrons than to quarks. This raises the question of the production of dark matter particles (and heavy associated coloured states) at LHC. Here, we explore a very simple benchmark dark matter model and show that, in spite of the agreement between the PAMELA antiproton measurements and the expected astrophysical secondary background, there is room for large couplings of a WIMP candidate to heavy quarks. Contrary to what could have been naively anticipated, the PAMELA pbar/p measurements do not challenge dark matter model building, as far as the quark sector is concerned. A quarkophillic species is therefore not forbidden.Owing to these large couplings, one would expect that a new production channel opens up at the LHC, through quark--quark and quark--gluon interactions. Alas, when the PDF of the quark is taken into account, prospects for a copious production fade away.Comment: 7 pages, 2 figures, captions of some figures modified, main conclusion unchange