Front dynamics in turbulent media

A study of a stable front propagating in a turbulent medium is presented. The front is generated through a reaction-diffusion equation, and the turbulent medium is statistically modeled using a Langevin equation. Numerical simulations indicate the presence of two different dynamical regimes. These regimes appear when the turbulent flow either wrinkles a still rather sharp propagating interfase or broadens it. Specific dependences of the propagating velocities on stirring intensities appropriate to each case are found and fitted when possible according to theoretically predicted laws. Different turbulent spectra are considered.Comment: 8 pages, REVTEX, 6 postscript figures included. To appear in Phys. Fluids (1997

Orbital X-Ray Variability of the Microquasar LS 5039

The properties of the orbit and the donor star in the high mass X-ray binary microquasar LS 5039 indicate that accretion processes should mainly occur via a radiatively driven wind. In such a scenario, significant X-ray variability would be expected due to the eccentricity of the orbit. The source has been observed at X-rays by several missions, although with a poor coverage that prevents to reach any conclusion about orbital variability. Therefore, we conducted RossiXTE observations of the microquasar system LS 5039 covering a full orbital period of 4 days. Individual observations are well fitted with an absorbed power-law plus a Gaussian at 6.7 keV, to account for iron line emission that is probably a diffuse background feature. In addition, we have taken into account that the continuum is also affected by significant diffuse background contamination. Our results show moderate power-law flux variations on timescales of days, as well as the presence of miniflares on shorter timescales. The new orbital ephemeris of the system recently obtained by Casares et al. have allowed us to show, for the first time, that an increase of emission is seen close to the periastron passage, as expected in an accretion scenario. Moreover, the detected orbital variability is a factor of ~4 smaller than the one expected by using a simple wind accretion model, and we suggest that an accretion disk around the compact object could be responsible for this discrepancy. On the other hand, significant changes in the photon index are also observed clearly anti-correlated with the flux variations. We interpret the overall X-ray spectral characteristics of LS 5039 in the context of X-ray radiation produced by inverse Compton and/or synchrotron processes in the jet of this microquasar.Comment: published in Astrophysical Journal, submission format (real number of pages: 7, 4 figures

Langevin approach to generate synthetic turbulence

We present an analytical scheme, easily implemented numerically, to generate synthetic Gaussian turbulent flows by using a linear Langevin equation, where the noise term acts as a stochastic stirring force. The characteristic parameters of the velocity field are well introduced, in particular the kinematic viscosity and the spectrum of energy. As an application, the diffusion of a passive scalar is studied for two different energy spectra. Numerical results are compared favorably with analytical calculations.Comment: 7 pages, REVTEX, 6 figures. To appear in Physics of Fluids (April 1997

Multi-wavelength differential astrometry of the S5 polar cap sample

We report on the status of our S5 polar cap astrometry program. Since 1997 we have observed all the 13 radio sources of the complete S5 polar cap sample at the wavelengths of 3.6 cm, 2 cm and 7 mm. Images of the radio sources at 3.6 and 2 cm have already been published reporting morphological changes. Preliminary astrometric analyses have been carried out at three frequencies with precisions in the relative position determination ranging from 80 to 20 microarcseconds. We report also on the combination of our phase-delay global astrometry results with the microarcsecond-precise optical astrometry that will be provided by future space-based instruments.Comment: 2 pages. 1 figure. Proceedings of the 7th European VLBI Network Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R. Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico Nacional), p. 323-324. Needs evn2004.cl

Deep Infrared Imaging of the Microquasars 1E1740-2942 and GRS 1758-258

We present deep infrared ($2.2 \mu$m) imaging of the Galactic microquasars 1E1740-2942 and GRS 1758-258 using the Keck-I 10-meter telescope in June 1998. The observations were taken under excellent seeing conditions (\sim 0.45 \arcsec full-width half-maximum), making them exceptionally deep for these crowded fields. We used the USNO-A2.0 catalog to astrometrically calibrate the infrared images (along with an optical CCD image in the case of GRS 1758-258), providing independent frame ties to the known radio positions of the objects. For 1E1740-2942, we confirm potential candidates for the microquasar previously identified by Marti et al., and show that none of the objects near the microquasar have varied significantly from 1998 to 1999. For GRS 1758-258, our astrometry indicates a position shifted from previous reports of candidates for the microquasar. We find no candidates inside our 90% confidence radius to a $2 \sigma$ limiting magnitude of $K_s = 20.3$ mag. We discuss the implications of these results for the nature of the microquasar binary systems.Comment: To appear in the Astrophysical Journal; 15 pages, including 4 figure

Radio detection of the young binary HD 160934

Precise determination of dynamical masses of pre-main-sequence (PMS) stars is essential to calibrate stellar evolution models that are widely used to derive theoretical masses of young low-mass objects. Binary stars in young, nearby loose associations are particularly good candidates for this calibration since all members share a common age. Interestingly, some of these young binaries present a persistent and compact radio emission, which makes them excellent targets for astrometric VLBI studies. We aim to monitor the orbital motion of the binary system HD 160934, a member of the AB Doradus moving group. We observed HD 160934 with the Very Large Array and the European VLBI Network at 8.4 and 5 GHz, respectively. The orbital information derived from these observations was analyzed along with previously reported orbital measurements. We show that the two components of the binary, HD 160934 A and HD 160934 c, display compact radio emission at VLBI scales, providing precise information on the relative orbit. Revised orbital elements were estimated. Future VLBI monitoring of this pair should determine precise model-independent mass estimates for the A and c components, which will serve as calibration tests for PMS evolutionary models.Comment: 5 pages, 5 figures, accepted for publication in A&

Near-infrared synchrotron emission from the compact jet of GX339-4

We have compiled contemporaneous broadband observations of the black hole candidate X-ray binary GX 339-4 when in the low/hard X-ray state in 1981 and 1997. The data clearly reveal the presence of two spectral components, with thermal and non-thermal spectra, overlapping in the optical -- near-infrared bands. The non-thermal component lies on an extrapolation of the radio spectrum of the source, and we interpret it as optically thin synchrotron emission from the powerful, compact jet in the system. Detection of this break from self-absorbed to optically thin synchrotron emission from the jet allows us to place a firm lower limit on the ratio of jet (synchrotron) to X-ray luminosities of $\geq 5$%. We further note that extrapolation of the optically thin synchrotron component from the near-infrared to higher frequencies coincides with the observed X-ray spectrum, supporting models in which the X-rays could originate via optically thin synchrotron emission from the jet (possibly instead of Comptonisation).Comment: Accepted for publication in ApJ Lette

Significant postmortem diffusion of ethanol: A case report.

In the realm of forensic pathology, ethanol is one of the most frequently encountered xenobiotics. The determination of ethanol concentration in blood after death is of great interest in forensic settings. It is important to be able to determine the level of intoxication of the deceased at the time of death, which is directly correlated to the ability to act prior to death, especially when a suicide is suspected. This estimation is not always easy to establish owing to various artifacts that are important to know for a proper ethanol blood level interpretation, among them postmortem (PM) diffusion. We describe here a case of unusual ethanol distribution in body compartments and discuss the importance of PM diffusion and redistribution while performing complementary toxicological analysis, especially when the blood and urine samples seemed to be inconsistent after the first results