The origin of the supersoft X-ray--optical/UV flux anticorrelation in the symbiotic binary AG Draconis

AG Draconis produces a strong supersoft X-ray emission.The X-ray and optical/UV fluxes are in a strict anticorrelation throughout the active and quiescent phases. The aim of this contribution is to identify the source of the X-ray emission and reveal the nature of the observed flux anticorrelation. For this purpose we model the X-ray and UV observations with XMM-Newton, far-UV spectroscopy from FUSE, low- and high-resolution IUE spectra and optical/near-IR spectroscopic and/or photometric observations. Our analysis showed that the supersoft X-ray emission is produced by the white dwarf photosphere. The X-ray and far-UV fluxes make it possible to determine its temperature unambiguously. The supersoft X-ray--optical/UV flux anticorrelation is caused by the variable wind from the hot star. The enhanced hot star wind gives rise to the optical bursts by reprocessing high-energy photons from the Lyman continuum to the optical/UV.Comment: 9 pages, 2 figures, 3 tables. Submitted to AA on 25/11/2008, revised on 27/05/200

Chandra observation of the fast X-ray transient IGR J17544-2619: evidence for a neutron star?

IGR J17544-2619 belongs to a distinct group of at least seven fast X-ray transients that cannot readily be associated with nearby flare stars or pre-main sequence stars and most probably are X-ray binaries with wind accretion. Sofar, the nature of the accretor has been determined in only one case (SAX J1819.3-2525/V4641 Sgr). We carried out a 20 ks Chandra ACIS-S observation of IGR J17544-2619 which shows the source in quiescence going into outburst. The Chandra position confirms the previous tentative identification of the optical counterpart, a blue O9Ib supergiant at 3 to 4 kpc (Pellizza, Chaty & Negueruela, in prep.). This is the first detection of a fast X-ray transient in quiescence. The quiescent spectrum is very soft. The photon index of 5.9+/-1.2 (90% confidence error margin) is much softer than 6 quiescent black hole candidates that were observed with Chandra ACIS-S (Kong et al. 2002; Tomsick et al. 2003). Assuming that a significant fraction of the quiescent photons comes from the accretor and not the donor star, we infer that the accretor probably is a neutron star. A fit to the quiescent spectrum of the neutron star atmosphere model developed by Pavlov et al. (1992) and Zavlin et al. (1996) implies an unabsorbed quiescent 0.5--10 keV luminosity of (5.2+/-1.3) x 10^32 erg/s. We speculate on the nature of the brief outbursts.Comment: accepted for publication in Astronomy & Astrophysic

WSO/UV: World Space Observatory/Ultraviolet

We summarize the capabilities of the World Space Observatory (UV) Project (WSO/UV). An example of the importance of this project (with a planned launch date of 2007/8) for the study of Classical Novae is given.Comment: 4 pages, To appear in the proceeedings of the "Classical Nova Explosions" conference, eds. M. Hernanz and J. Jose, AI

Discovery of the 1.80 hr Spin Period of the White Dwarf of the Symbiotic System BF Cyg

We report on the discovery of a coherent periodicity in the B light curve of the symbiotic star BF Cyg. The signal was detected in some sections of the light curve of the star recorded in the year 2003 as double hump periodic variations with an amplitude of ~7 mmag. In the year 2004 the signal was also present in only a subsection of the light curve. In that year, the system was about twice as bright and the amplitude of the oscillations was about half of what it was in 2003. In 2004 the cycle structure was of a single hump, the phase of which coincided with the phase of one of the humps in the 2003 cycle. No periodic signal was detected in a third, short series of observations performed in the year 2007, when the star was three magnitudes brighter than in 2003. We interpret the periodicity as the spin period of the white dwarf component of this interacting binary system. We suggest that the signal in 2003 originated in two hot spots on or near the surface of the white dwarf, most likely around the two antipodes of an oblique dipole magnetic field of this star. Magnetic field lines funneled accreted matter from the wind of the cool component to the pole areas, where the falling material created the hot spots. This process is apparently intermittent in its nature. In 2004, the activity near only one pole was enhanced enough to raise the signal above the threshold of our detection ability.Comment: 7 pages, 5 figures, accepted for publication in MNRA

A study of the expanding envelope of Nova V1974 Cyg 1992 based on IUE high resolution spectroscopy

We have carried out a detailed analysis of the IUE archival high resolution spectra of the classical nova V1974 Cyg 1992. In addition to the P Cygni and emission lines, two shortward shifted absorption systems are present. Evidence is given that these absorptions originate in two separate expanding shells, outside the wind layers where the emission lines are formed. The outer main shell, containing most of the matter ejected at outburst, produces the so-called ``principal absorption line system'', and the inner faster moving second shell produces the so-called ``diffuse--enhanced absorption line system''. The outflow velocity of the two shells increases exponentially with time reaching a value of about 1750 km/s and 2900 km/s, respectively. We suggest that the acceleration of the shells is the result of increasing line-radiation pressure due to the UV-brightening of the star as the effective radius decreases. Around day 60 the second shell has overtaken the slower moving principal system shell, and merged with it. This explains: the sudden disappearance of the diffuse line system near that date, the upward jump of 240 km/s in velocity of the principal system and the first detection of hard X-ray emission on day 63. This velocity jump indicates that the main shell is about 4 times more massive than the second shell. The deceleration suffered by the diffuse-enhanced system after the shock provides a shock temperature of about 1.6 KeV, in fairly good agreement with the temperature of the observed hard X-ray emission.Comment: 19 pages, 11 figure

Harpagifer bispinis

This rocky, intertidal species has a relatively small range, but is common and abundant. There are no known major threats at this time; therefore, it is listed as Least Concern.Fil: Buratti, C.. Instituto Nacional de Investigaciones y Desarrollo Pesquero; ArgentinaFil: Díaz de Astarloa, Juan Martín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Hüne, M.. Universidad Austral de Chile; ChileFil: Irigoyen, A.. No especifíca;Fil: Landaeta, M.. Universidad de Valparaíso; ChileFil: Riestra, C.. Instituto Nacional de Investigaciones y Desarrollo Pesquero; ArgentinaFil: Vieira, J.P.. Universidade Federal Do Rio Grande.; Brasi

The SSS phase of RS Ophiuchi observed with Chandra and XMM-Newton I.: Data and preliminary Modeling

The phase of Super-Soft-Source (SSS) emission of the sixth recorded outburst of the recurrent nova RS Oph was observed twice with Chandra and once with XMM-Newton. The observations were taken on days 39.7, 54.0, and 66.9 after outburst. We confirm a 35-sec period on day 54.0 and found that it originates from the SSS emission and not from the shock. We discus the bound-free absorption by neutral elements in the line of sight, resonance absorption lines plus self-absorbed emission line components, collisionally excited emission lines from the shock, He-like intersystem lines, and spectral changes during an episode of high-amplitude variability. We find a decrease of the oxygen K-shell absorption edge that can be explained by photoionization of oxygen. The absorption component has average velocities of -1286+-267 km/s on day 39.7 and of -771+-65 km/s on day 66.9. The wavelengths of the emission line components are consistent with their rest wavelengths as confirmed by measurements of non-self absorbed He-like intersystem lines. We have evidence that these lines originate from the shock rather than the outer layers of the outflow and may be photoexcited in addition to collisional excitations. We found collisionally excited emission lines that are fading at wavelengths shorter than 15A that originate from the radiatively cooling shock. On day 39.5 we find a systematic blue shift of -526+-114 km/s from these lines. We found anomalous He-like f/i ratios which indicates either high densities or significant UV radiation near the plasma where the emission lines are formed. During the phase of strong variability the spectral hardness light curve overlies the total light curve when shifted by 1000sec. This can be explained by photoionization of neutral oxygen in the line of sight if the densities of order 10^{10}-10^{11} cm^{-3}.Comment: 16 pages, 10 figures, 4 tables. Accepted by ApJ; v2: Co-author Woodward adde