Evidence for the Galactic X-ray Bulge II

A mosaic of 5 \ros~PSPC pointed observations in the Galactic plane ($l\sim25^{\circ}$) reveals X-ray shadows in the $0.5-2.0$ keV band cast by distant molecular clouds. The observed on-cloud and off-cloud X-ray fluxes indicate that $\sim15$% and $\sim37$% of the diffuse X-ray background in this direction in the \tq~keV and 1.5 keV bands, respectively, originates behind the molecular gas which is located at $\sim$3 kpc from the Sun. The implication of the derived background X-ray flux beyond the absorbing molecular cloud is consistent with, and lends further support to recent observations of a Galactic X-ray bulge.Comment: 19 pages, 5 figures, 2 table

Absorption of femtosecond laser pulses in high-density plasma.

The absorption of 250-fs KrF laser pulses incident on solid targets of aluminum and gold has been measured as a function of polarization and angle of incidence for the intensity range of 1014–2.5×1015 W cm−2. Maximum absorption of over 60% occurs for p-polarized radiation at angles of incidence in the range of 48°–57°. The measured results are in agreement with absorption on a steep density gradient

Small fish eat smaller fish: A model of interaction strength in early life stages of two tuna species

Fish larvae are rarely a major driver of fish mortality, but tunas can produce large batches of larvae that rapidly develop the capacity to kill other fish. We combine a model for the killing potential from Atlantic bluefin tuna (BFT) larvae on larval albacore (ALB) with field observations at a major spawning ground. Both species spawn from June to August, but BFT has a narrow spawning peak at the beginning of the season that results in priority effects. Our model shows that, following a recent stock recovery, BFT larvae have increased their killing pressure, leaving areas of up to 1000 km2 with < 1% chance of ALB daily survival. Such increase in killing pressure suggests larval ALB has reduced chances to survive; yet in large areas with few BFT, other drivers of early survival prevail over BFT predation. This shows that strong predatory interactions can occur during larval stages in some fishes.En prensa3,38

Spawning site distribution of a bluefin tuna reduces jellyfish predation on early life stages

Bluefin tunas across the world migrate long distances to spawn in particularly warm and oligotrophic areas constrained by oceanographic fronts. The low abundance of predators in these areas increases survival chances of their early life stages, but its importance for choice of spawning habitat is unknown. Here, we use estimated clearance rates and data on spatial distributions of Atlantic bluefin tuna larvae Thunnus thynnus and metaephyrae of the jellyfish Pelagia noctiluca to quantify predation at a major spawning ground in the Mediterranean Sea. We found that high densities of P. noctiluca can rapidly deplete tuna eggs and preflexion larvae, but their patchy distribution and low spatial and temporal overlap results in overall low predation. The specific distribution of the spawning sites suggests that bluefin tunas may use local oceanography as cues to spawn outside areas with high predator densities.Postprint3,38

Stellar X-ray sources in the Chandra COSMOS survey

We present an analysis of the X-ray properties of a sample of solar- and late-type field stars identified in the Chandra Cosmic Evolution Survey (COSMOS), a deep (160ks) and wide (0.9 deg2) extragalactic survey. The sample of 60 sources was identified using both morphological and photometric star/galaxy separation methods. We determine X-ray count rates, extract spectra and light curves and perform spectral fits to determine fluxes and plasma temperatures. Complementary optical and near-IR photometry is also presented and combined with spectroscopy for 48 of the sources to determine spectral types and distances for the sample. We find distances ranging from 30pc to ~12kpc, including a number of the most distant and highly active stellar X-ray sources ever detected. This stellar sample extends the known coverage of the L_X-distance plane to greater distances and higher luminosities, but we do not detect as many intrinsically faint X-ray sources compared to previous surveys. Overall the sample is typically more luminous than the active Sun, representing the high-luminosity end of the disk and halo X-ray luminosity functions. The halo population appears to include both low-activity spectrally hard sources that may be emitting through thermal bremsstrahlung, as well as a number of highly active sources in close binaries.Comment: 13 pages, 5 figures, accepted for publication in Ap

Abundance of Pelagia noctiluca early life stages in the western Mediterranean Sea scales with surface chlorophyll

Pelagia noctiluca is the most successful and well-studied jellyfish in the Mediterranean Sea. This species tolerates a wide range of water temperatures and succeeds in low to medium food regimes, but factors driving its distribution and population dynamics remain poorly understood. Here we applied a multiscale analytical approach using survey data and a physical-biochemical coupled model to assess how environmental factors affect the 3-dimensional distribution and seasonal abundance of P. noctiluca early life stages. The surveys took place after the spring bloom, when warm water favors fecundity and growth, but food shortage limits the reproductive investment and early survival. We found that most early life stages of P. noctiluca remained above the shallow thermocline and upper mixed layer where temperature is warm. Their spatial distribution was positively correlated with surface chlorophyll concentration, and over 90% of the variation in interannual abundance was explained by basin-scale productivity in June. Warmer water during winter and spring seasons coupled with protracted spring blooms increase the population of P. noctiluca, and this explains the trend of increasing outbreaks observed in the western Mediterranean Sea over the past decades.Postprin

Radio Astrometry Of The Triple Systems Algol And UX Arietis

We have used multi-epoch long-baseline radio interferometry to determine the proper motion and orbital elements of Algol and UX Arietis, two radio-bright, close binary stellar systems with distant tertiary components. For Algol, we refine the proper motion and outer orbit solutions, confirming the recent result of Zavala et al. (2010) that the inner orbit is retrograde. The radio centroid closely tracks the motion of the KIV secondary. In addition, the radio morphology varies from double-lobed at low flux level to crescent-shaped during active periods. These results are most easily interpreted as synchrotron emission from a large, co-rotating meridional loop centered on the K-star. If this is correct, it provides a radio-optical frame tie candidate with an uncertainty {\pm}0.5 mas. For UX Arietis, we find a outer orbit solution that accounts for previous VLBI observations of an acceleration term in the proper motion fit. The outer orbit solution is also consistent with previously published radial velocity curves and speckle observations of a third body. The derived tertiary mass, 0.75 solar masses, is consistent with the K1 main-sequence star detected spectroscopically. The inner orbit solution favors radio emission from the active K0IV primary only. The radio morphology, consisting of a single, partially resolved emission region, may be associated with the persistent polar spot observed using Doppler imaging

Continuous heating of a giant X-ray flare on Algol

Giant flares can release large amounts of energy within a few days: X-ray emission alone can be up to ten percent of the star's bolometric luminosity. These flares exceed the luminosities of the largest solar flares by many orders of magnitude, which suggests that the underlying physical mechanisms supplying the energy are different from those on the Sun. Magnetic coupling between the components in a binary system or between a young star and an accretion disk has been proposed as a prerequisite for giant flares. Here we report X-ray observations of a giant flare on Algol B, a giant star in an eclipsing binary system. We observed a total X-ray eclipse of the flare, which demonstrates that the plasma was confined to Algol B, and reached a maximum height of 0.6 stellar radii above its surface. The flare occurred around the south pole of Algol B, and energy must have been released continously throughout its life. We conclude that a specific extrastellar environment is not required for the presence of a flare, and that the processes at work are therefore similar to those on the Sun.Comment: Nature, Sept. 2 199

The Sun as an X-ray Star: III. Flares

In previous works we have developed a method to convert solar X-ray data, collected with the Yohkoh/SXT, into templates of stellar coronal observations. Here we apply the method to several solar flares, for comparison with stellar X-ray flares. Eight flares, from weak (GOES class C5.8) to very intense ones (X9) are selected as representative of the flaring Sun. The emission measure distribution vs. temperature, EM(T), of the flaring regions is derived from Yohkoh/SXT observations in the rise, peak and decay of the flares. The EM(T) is rather peaked and centered around $T \approx 10^7$ K for most of the time. Typically, it grows during the rise phase of the flare, and then it decreases and shifts toward lower temperatures during the decay, more slowly if there is sustained heating. The most intense flare we studied shows emission measure even at very high temperature ($T \approx 10^8$ K). Time-resolved X-ray spectra both unfiltered and filtered through the instrumental responses of the non-solar instruments ASCA/SIS and ROSAT/PSPC are then derived. Synthesized ASCA/SIS and ROSAT/PSPC spectra are generally well fitted with single thermal components at temperatures close to that of the EM(T) maximum, albeit two thermal components are needed to fit some flare decays. ROSAT/PSPC spectra show that solar flares are in a two-orders of magnitude flux range ($10^6 - 10^8$ erg cm$^{-2}$ s$^{-1}$) and a narrow PSPC hardness ratio range, however higher than that of typical non-flaring solar-like stars.Comment: 32 pages, 8 figures, 3 table

From the Heart of The Ghoul: C and N Abundances in the Corona of Algol B

Chandra Low Energy Transmission Grating Spectrograph observations of Algol have been used to determine the abundances of C and N in the secondary star for the first time. The analysis was performed relative to similar observations of an adopted "standard" star HR 1099. It is demonstrated that HR 1099 and Algol are coronal twins in many respects and that their X-ray spectra are very similar in nearly all details, except for the observed strengths of C and N lines. The H-like transitions of C and N in the coronae of Algol and HR 1099 demonstrate that the surface abundances of Algol B have been strongly modified by CN-processing, as shown earlier by Schmitt & Ness (2002). It is found that N is enhanced in Algol B by a factor of 3 compared to HR 1099. No C lines are detected in the Algol spectrum, indicating a C depletion relative to HR 1099 by a factor of 10 or more. These C and N abundances indicate that Algol B must have lost at least half of its initial mass, and are consistent with predictions of evolutionary models that include non-conservative mass transfer and angular momentum loss through magnetic activity. Little or no dredge-up of material subjected to CN-processing has occurred on the subgiant component of HR 1099. It is concluded that Fe is very likely depleted in the coronae of both Algol and HR 1099 relative to their photospheres by 0.5 dex, and C, N and O by 0.3 dex. Instead, Ne is enhanced by up to 0.5 dex.Comment: 17 pages, 4 figures, ApJ accepte