Comment on the formation of black holes in nonsymmetric gravity

We critically examine the claim made by Burko and Ori that black holes are expected to form in nonsymmetric gravity and find their analysis to be inconclusive. Their conclusion is a result of the approximations they make, and not a consequence of the true dynamics of the theory. The approximation they use fails to capture the crucial equivalence principle violations which enable the full nonsymmetric field equations to detect and tame would-be horizons. An examination of the dynamics of the full theory reveals no indication that black holes should form. For these reasons, one cannot conclude from their analysis that nonsymmetric gravity has black holes. A definitive answers awaits a comprehensive study of gravitational collapse, using the full field equations.Comment: 6 pages, RevTe

Non-Singular Gravity Without Black Holes

A non-singular, static spherically symmetric solution to the nonsymmetric gravitational and electromagnetic theory field equations is derived, which depends on the four parameters m, l^2, Q and s, where m is the mass, Q is the electric charge, l^2 is the NGT charge of a body and s is a dimensionless constant. The electromagnetic field invariants are also singularity-free, so that it is possible to construct regular particle-like solutions in the theory. All the curvature invariants are finite, there are no null surfaces in the spacetime and there are no black holes. A new stable, superdense object (SDO) replaces black holes.Comment: 26 pages, UTPT-94-0

VLT/SINFONI time-resolved spectroscopy of the central, luminous, H-rich WN stars of R136

Using the Very Large Telescope's Spectrograph for INtegral Field Observation in the Near-Infrared (VLT/SINFONI), we have obtained repeated AO-assisted, NIR spectroscopy of the six central luminous, Wolf-Rayet (WR) stars in the core of the very young (~1 Myr), massive and dense cluster R136, in the Large Magellanic Cloud (LMC). We also de-archived available images that were obtained with the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS), and extracted high-quality, differential photometry of our target stars to check for any variability related to binary motion. Previous studies, relying on spatially unresolved, integrated, optical spectroscopy, had reported that one of these stars was likely to be a 4.377-day binary. Our study set out to identify the culprit and any other short-period system among our targets. However, none displays significant photometric variability, and only one star, BAT99-112 (R136c), located on the outer fringe of R136, displays a marginal variability in its radial velocities; we tentatively report an 8.2-day period. The binary status of BAT99-112 is supported by the fact that it is one of the brightest X-ray sources among all known WR stars in the LMC, consistent with it being a colliding-wind system. Follow-up observations have been proposed to confirm the orbital period of this potentially very massive system.Comment: 9 pages, 6 figures; accepted for publication in MNRA

Gravitational Waves in the Nonsymmetric Gravitational Theory

We prove that the flux of gravitational radiation from an isolated source in the Nonsymmetric Gravitational Theory is identical to that found in Einstein's General Theory of Relativity.Comment: 10 Page

Probing Brownstein-Moffat Gravity via Numerical Simulations

In the standard scenario of the Newtonian gravity, a late-type galaxy (i.e., a spiral galaxy) is well described by a disk and a bulge embedded in a halo mainly composed by dark matter. In Brownstein-Moffat gravity, there is a claim that late-type galaxy systems would not need to have halos, avoiding as a result the dark matter problem, i.e., a modified gravity (non-Newtonian) would account for the galactic structure with no need of dark matter. In the present paper, we probe this claim via numerical simulations. Instead of using a "static galaxy," where the centrifugal equilibrium is usually adopted, we probe the Brownstein-Moffat gravity dynamically via numerical $N$-body simulations.Comment: 33 pages and 14 figures - To appear in The Astrophysical Journa

A 10-hour period revealed in optical spectra of the highly variable WN8 Wolf-Rayet star WR 123

Aims. What is the origin of the large-amplitude variability in Wolf-Rayet WN8 stars in general and WR123 in particular? A dedicated spectroscopic campaign targets the ten-hour period previously found in the high-precision photometric data obtained by the MOST satellite. Methods. In June-August 2003 we obtained a series of high signal-to-noise, mid-resolution spectra from several sites in the {\lambda}{\lambda} 4000 - 6940 A^{\circ} domain. We also followed the star with occasional broadband (Johnson V) photometry. The acquired spectroscopy allowed a detailed study of spectral variability on timescales from \sim 5 minutes to months. Results. We find that all observed spectral lines of a given chemical element tend to show similar variations and that there is a good correlation between the lines of different elements, without any significant time delays, save the strong absorption components of the Hei lines, which tend to vary differently from the emission parts. We find a single sustained periodicity, P \sim 9.8 h, which is likely related to the relatively stable pulsations found in MOST photometry obtained one year later. In addition, seemingly stochastic, large-amplitude variations are also seen in all spectral lines on timescales of several hours to several days.Comment: 6 pages, 4 figures, 2 tables, data available on-line, accepted in A&A Research Note

Elimination of IR/UV via Gravity in Noncommutative Field Theory

Models of particle physics with Noncommutative Geometry (NCG) generally suffer from a manifestly non-Wilsonian coupling of infrared and ultraviolet degrees of freedom known as the "IR/UV Problem" which would tend to compromise their phenomenological relevance. In this Letter we explicitly show how one may remedy this by coupling NCG to gravity. In the simplest scenario the Lagrangian gets multiplied by a nonconstant background metric; in $\phi-4$ theory the theorem that $\int d^4 x \phi \star \phi = \int d^4 x \phi^2$ is no longer true and the field propagator gets modified by a factor which depends on both NCG and the variation of the metric. A suitable limit of this factor as the propagating momentum gets asymptotically large then eradicates the IR/UV problem. With gravity and NCG coupled to each other, one might expect anti-symmetric components to arise in the metric. Cosmological implications of such are subsequently discussed.Comment: 6 pages; MPLA versio

The spin rates and spin evolution of the O components in WR+O binaries

Despite 50 years of extensive binary research we have to conclude that the Roche lobe overflow/mass transfer process that governs close binary evolution is still poorly understood. It is the scope of the present paper to lift a tip of the veil by studying the spin-up and spin-down processes of the O-type components of WR+O binaries. We critically analyze the available observational data of rotation speeds of the O-type components in WR+O binaries. By combining a binary evolutionary code and a formalism that describes the effects of tides in massive stars with an envelope in radiative equilibrium, we compute the corresponding rotational velocities during the Roche lobe overflow of the progenitor binaries. In all the studied WR+O binaries, we find that the O-type stars were affected by accretion of matter during the RLOF of the progenitor. This means that common envelope evolution which excludes any accretion onto the secondary O-star, has not played an important role to explain the WR+O binaries. Moreover, although it is very likely that the O-type star progenitors were spun-up by the mass transfer, many ended the RLOF/mass transfer phase with a rotational velocity that is significantly smaller than the critical rotation speed. This may indicate that during the mass transfer phase there is a spin-down process which is of the same order as, although significantly less than that of the spin-up process. We propose a Spruit-Tayler type dynamo spin-down suggested in the past to explain the rotation speeds of the mass gainers in long-period Algols.Comment: 6 pages, accepted for publication in A&A; accepted versio

Low Gain Avalanche Detectors (LGAD) for particle physics and synchrotron applications

A new avalanche silicon detector concept is introduced with a low gain in the region of ten, known as a Low Gain Avalanche Detector, LGAD. The detector's characteristics are simulated via a full process simulation to obtain the required doping profiles which demonstrate the desired operational characteristics of high breakdown voltage (500 V) and a gain of 10 at 200 V reverse bias for X-ray detection. The first low gain avalanche detectors fabricated by Micron Semiconductor Ltd are presented. The doping profiles of the multiplication junctions were measured with SIMS and reproduced by simulating the full fabrication process which enabled further development of the manufacturing process. The detectors are 300 μm thick p-type silicon with a resistivity of 8.5 kΩcm, which fully depletes at 116 V. The current characteristics are presented and demonstrate breakdown voltages in excess of 500 V and a current density of 40 to 100 nAcm−2 before breakdown measured at 20oC. The gain of the LGAD has been measured with a red laser (660 nm) and shown to be between 9 and 12 for an external bias voltage range from 150 V to 300 V