Langevin Equation for the Rayleigh model with finite-ranged interactions

Both linear and nonlinear Langevin equations are derived directly from the Liouville equation for an exactly solvable model consisting of a Brownian particle of mass $M$ interacting with ideal gas molecules of mass $m$ via a quadratic repulsive potential. Explicit microscopic expressions for all kinetic coefficients appearing in these equations are presented. It is shown that the range of applicability of the Langevin equation, as well as statistical properties of random force, may depend not only on the mass ratio $m/M$ but also by the parameter $Nm/M$, involving the average number $N$ of molecules in the interaction zone around the particle. For the case of a short-ranged potential, when $N\ll 1$, analysis of the Langevin equations yields previously obtained results for a hard-wall potential in which only binary collisions are considered. For the finite-ranged potential, when multiple collisions are important ($N\gg 1$), the model describes nontrivial dynamics on time scales that are on the order of the collision time, a regime that is usually beyond the scope of more phenomenological models.Comment: 21 pages, 1 figure. To appear in Phys. Rev.

Photometric observations of the radio bright B[e]/X-ray binary CI Cam

We present multiwavelength (optical, IR, radio) observations of CI Cam, the optical counterpart to the transient X-ray source XTE J0421+560. Pre-outburst quiescent observations reveal the presence of a dusty envelope around the system. Pronounced short term variability is observed at all wavebands from U-K, but no indication of prior flaring of a similar magnitude to the 1998 April outburst is found in these data. Data obtained during the 1998 April X-ray flare reveal pronounced optical-radio flaring. The optical flux was observed to quickly return to quiescent levels, while the radio flare was of much longer duration. The optical component is likely to result from a combination of free-free/free-bound emission, emission line and thermal dust emission, caused by re-radiation of the X-ray flux, while the behaviour of the multiwavelength radio data is consistent with emission from expanding ejecta emitting via the synchrotron mechanism. Post-outburst (1998 August-1999 March) U-M broadband photometric observations reveal that while the optical (UBV) flux remains at pre-outburst quiescent levels, near IR (JHKLM) fluxes exceed the pre-outburst fluxes by ~0.5 magnitudes. Modelling the pre- and post-outburst spectral energy distribution of CI Cam reveals that the structure and/or composition of the dusty component of the circumstellar envelope appears to have changed. Due to a lack of information on the precise chemical composition of the dust within the system several explanations for this behaviour are possible, such as the production of new dust at the inner edge of the envelope, or modification of the composition of the dust due to X-ray irradiation

The system parameters of DW Ursae Majoris

We present new constraints on the system parameters of the SW Sextantis star DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the Hubble Space Telescope. Our data were obtained during a low state of the system, in which the UV light was dominated by the hot white dwarf (WD) primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set firm limits on the masses and radii of the system components and the distance between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014, M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5 the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's mass-period relation for CV secondaries, our estimates for the system parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001, M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82 \pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160 pc, from time-resolved I- and K-band photometry. Finally, we have repeated Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa in order to account for the higher surface gravity indicated by our eclipse analysis. In this way we obtained a second estimate for the distance, d = 590 \pm 100 pc, which allows us to obtain a second estimate for the spectral type of the secondary, M7 \pm 2.0. We conclude that the true value for the distance and spectral type will probably be in between the values obtained by the two methods.Comment: 23 pages including 5 figures and 3 tables. Accepted for publication in Ap

The evolutionary status of the semiregular variable QYSge

Repeated spectroscopic observations made with the 6m telescope of yielded new data on the radial-velocity variability of the anomalous yellow supergiant QYSge. The strongest and most peculiar feature in its spectrum is the complex profile of NaI D lines, which contains a narrow and a very wide emission components. The wide emission component can be seen to extend from -170 to +120 km/s, and at its central part it is cut by an absorption feature, which, in turn, is split into two subcomponents by a narrow (16km/s at r=2.5) emission peak. An analysis of all the Vr values leads us to adopt for the star a systemic velocity of Vr=-21.1 km/s, which corresponds to the position of the narrow emission component of NaI. The locations of emission-line features of NaI D lines are invariable, which point to their formation in regions that are external to the supergiant's photosphere. Differential line shifts of about 10km/s are revealed. The absorption lines in the spectrum of QYSge have a substantial width of FWHM~45 km/s. The method of model atmospheres is used to determine the following parameters: Teff=6250K, lg g=2.0, and microturbulence Vt=4.5km/s. The metallicity of the star is found to be somewhat higher than the solar one with an average overabundance of iron-peak elements of [Met/H]=+0.20. The star is found to be slightly overabundant in carbon and nitrogen, [C/Fe]=+0.25, [N/Fe]=+0.27. The alpha-process elements Mg, Si, and Ca are slightly overabundant [alpha/H]=+0.12. The strong sodium excess, [Na/Fe]=+0.75, is likely to be due to the dredge-up of the matter processed in the NeNa cycle. Heavy elements of the s-process are underabundant relative to the Sun. On the whole, the observed properties of QYSge do not give grounds for including this star into the group of RCrB or RVTau-type type objects.Comment: 29 pages, 8 figures, 4 tables; accepted by Astrophys. Bulleti

The first accurate parallax distance to a black hole

Using astrometric VLBI observations, we have determined the parallax of the black hole X-ray binary V404 Cyg to be 0.418 +/- 0.024 milliarcseconds, corresponding to a distance of 2.39 +/- 0.14 kpc, significantly lower than the previously accepted value. This model-independent estimate is the most accurate distance to a Galactic stellar-mass black hole measured to date. With this new distance, we confirm that the source was not super-Eddington during its 1989 outburst. The fitted distance and proper motion imply that the black hole in this system likely formed in a supernova, with the peculiar velocity being consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred to exist in this system is less than 1.4 AU at 22 GHz. Astrometric observations of a larger sample of such systems would provide useful insights into the formation and properties of accreting stellar-mass black holes.Comment: Accepted for publication in ApJ Letters. 6 pages, 2 figure

Optical Photometry and Spectroscopy of the Accretion-Powered Millisecond Pulsar HETE J1900.1-2455

We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1-2455. Our R-band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Halpha emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km/s, placing a lower limit of 0.05 Msun on the secondary mass. For a 1.4 Msun primary, this implies that the orbital inclination is low, < 20 degrees. Utilizing the observed relationship between the secondary mass and orbital period in short period cataclysmic variables, we estimate the secondary mass to be ~0.085 Msun, which implies an upper limit of ~2.4 Msun for the primary mass.Comment: 8 pages, 6 figures; Accepted for publication in MNRAS. Minor revisions to match final published versio

The Role of Final State Interactions in Quasielastic 56^{56}Fe(e,e′)(e,e') Reactions at large ∣q⃗∣|\vec q|

A relativistic finite nucleus calculation using a Dirac optical potential is used to investigate the importance of final state interactions [FSI] at large momentum transfers in inclusive quasielastic electronuclear reactions. The optical potential is derived from first-order multiple scattering theory and then is used to calculate the FSI in a nonspectral Green's function doorway approach. At intermediate momentum transfers excellent predictions of the quasielastic $^{56}$Fe$(e,e')$ experimental data for the longitudinal response function are obtained. In comparisons with recent measurements at $|{\vec q|}=1.14$~GeV/c the theoretical calculations of $R_L$ give good agreement for the quasielastic peak shape and amplitude, but place the position of the peak at an energy transfer of about $40$~MeV higher than the data.Comment: 13 pages typeset using revtex 3.0 with 6 postscript figures in accompanying uuencoded file; submitted to Phys. Rev.

Correlated optical, X-ray, and $-ray flaring activity seen with INTEGRAL during the 2015 outburst of V404 Cygni

Reproduced with permission from Astronomy & Astrophysics. © 2015 ESO.After 25 years of quiescence, the microquasar V404 Cyg entered a new period of activity in June 2015. This X-ray source is known to undergo extremely bright and variable outbursts seen at all wavelengths. It is therefore an object of prime interest to understand the accretion-ejection connections. These can, however, only be probed through simultaneous observations at several wavelengths. We made use of the INTEGRAL instruments to obtain long, almost uninterrupted observations from 2015 June 20th, 15:50 UTC to June 25th, 4:05 UTC, from the optical V-band, up to the soft γ-rays. V404 Cyg was extremely variable in all bands, with the detection of 18 flares with fluxes exceeding 6 Crab (20--40 keV) within 3 days. The flare recurrence can be as short as ∼ 20~min from peak to peak. A model-independent analysis shows that the >6 Crab flares have a hard spectrum. A simple 10--400 keV spectral analysis of the off-flare and flare periods shows that the variation in intensity is likely to be due to variations of a cut-off power law component only. The optical flares seem to be at least of two different types: one occurring in simultaneity with the X-ray flares, the other showing a delay greater than 10 min. The former could be associated with X-ray reprocessing by either an accretion disk or the companion star. We suggest that the latter are associated with plasma ejections that have also been seen in radio.Peer reviewe

Biomarcadores de daño genético en eritrocitos de aves silvestres como posibles bioindicadores de calidad ambiental

Los micronúcleos (MN) son considerados un biomarcador de efecto genotóxico a nivel subcelular y el incremento en su frecuencia de presentación se considera una respuesta temprana de daño cromosómico, lo que permite detectar en interface las consecuencias de los efectos clastogénicos y aneugénicos