Searching for annihilation radiation from SN 1006 with SPI on INTEGRAL

Historical Type Ia supernovae are a leading candidate for the source of positrons observed through their diffuse annihilation emission in the Galaxy. However, search for annihilation emission from individual Type Ia supernovae has not been possible before the improved sensitivity of \integral. The total 511 keV annihilation flux from individual SNe Ia, as well as their contribution to the overall diffuse emission, depends critically on the escape fraction of positrons produced in $^{56}$Co decays. Late optical light curves suggest that this fraction may be as high as 5%. We searched for positron annihilation radiation from the historical Type Ia supernova SN 1006 using the SPI instrument on \integral. We did not detect significant 511 keV line emission, with a 3$\sigma$ flux upper limit of 0.59 x 10$^{-4}$ ergs cm^-2 s^-1 for \wsim 1 Msec exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and rules out the possibility that Type Ia supernovae produce all of the positrons in the Galaxy (~ 12% escape fraction), if the mean positron lifetime is less than 10$^{5}$ years. Future observations with \integral will provide stronger limits on the escape fraction of positrons, the mean positron lifetime, and the contribution of Type Ia supernovae to the overall positron content of the Galaxy.Comment: 3 pages, 2 figures, accepted for publication in ApJ

Radiative Transfer on Perturbations in Protoplanetary Disks

We present a method for calculating the radiative tranfer on a protoplanetary disk perturbed by a protoplanet. We apply this method to determine the effect on the temperature structure within the photosphere of a passive circumstellar disk in the vicinity of a small protoplanet of up to 20 Earth masses. The gravitational potential of a protoplanet induces a compression of the disk material near it, resulting in a decrement in the density at the disk's surface. Thus, an isodensity contour at the height of the photosphere takes on the shape of a well. When such a well is illuminated by stellar irradiation at grazing incidence, it results in cooling in a shadowed region and heating in an exposed region. For typical stellar and disk parameters relevant to the epoch of planet formation, we find that the temperature variation due to a protoplanet at 1 AU separation from its parent star is about 4% (5 K) for a planet of 1 Earth mass, about 14% (19 K) for planet of 10 Earth masses, and about 18% (25 K) for planet of 20 Earth masses, We conclude that even such relatively small protoplanets can induce temperature variations in a passive disk. Therefore, many of the processes involved in planet formation should not be modeled with a locally isothermal equation of state.Comment: 23 pages, 8 figures (including 3 color figs). Submitted to Ap

New Constraints on the Energetics, Progenitor Mass, and Age of the Supernova Remnant G292.0+1.8 Containing PSR J1124-5916

We present spatially resolved spectroscopy of the supernova remnant (SNR) G292.0+1.8 with the Chandra X-ray observatory. This SNR contains the 135 ms pulsar, J1124-5916. We apply non-equilibrium ionization (NEI) models to the data. By comparing the derived abundances with those predicted from nucleosynthesis models, we estimate a progenitor mass of 30-40 solar masses. We also derive the intrinsic parameters of the supernova explosion such as its energy, the age of the SNR, the blast wave velocity, and the swept-up mass. In the Sedov interpretation, our estimated SNR age of 2,600 years is close to the pulsar's characteristic age of 2,900 years. This confirms the pulsar/SNR association and relaxes the need for the pulsar to have a non-canonical value for the braking index, a large period at birth or a large transverse velocity. We discuss the properties of the pulsar wind nebula (PWN) in the light of the Kennel and Coroniti model and estimate the pulsar wind magnetization parameter. We also report the first evidence for steepening of the power law spectral index with increasing radius from the pulsar.Comment: 5 pages, 3 figures. To appear in ApJL, Feb 1 2003 (submitted Oct 9 2002, accepted Dec 19 2002

Mining Missing Hyperlinks from Human Navigation Traces: A Case Study of Wikipedia

Hyperlinks are an essential feature of the World Wide Web. They are especially important for online encyclopedias such as Wikipedia: an article can often only be understood in the context of related articles, and hyperlinks make it easy to explore this context. But important links are often missing, and several methods have been proposed to alleviate this problem by learning a linking model based on the structure of the existing links. Here we propose a novel approach to identifying missing links in Wikipedia. We build on the fact that the ultimate purpose of Wikipedia links is to aid navigation. Rather than merely suggesting new links that are in tune with the structure of existing links, our method finds missing links that would immediately enhance Wikipedia's navigability. We leverage data sets of navigation paths collected through a Wikipedia-based human-computation game in which users must find a short path from a start to a target article by only clicking links encountered along the way. We harness human navigational traces to identify a set of candidates for missing links and then rank these candidates. Experiments show that our procedure identifies missing links of high quality

Impact of birth weight and gender on early postnatal hypothalamic energy balance regulatory gene expression in the young lamb

Peer reviewedPreprin

The functional integral with unconditional Wiener measure for anharmonic oscillator

In this article we propose the calculation of the unconditional Wiener measure functional integral with a term of the fourth order in the exponent by an alternative method as in the conventional perturbative approach. In contrast to the conventional perturbation theory, we expand into power series the term linear in the integration variable in the exponent. In such a case we can profit from the representation of the integral in question by the parabolic cylinder functions. We show that in such a case the series expansions are uniformly convergent and we find recurrence relations for the Wiener functional integral in the $N$ - dimensional approximation. In continuum limit we find that the generalized Gelfand - Yaglom differential equation with solution yields the desired functional integral (similarly as the standard Gelfand - Yaglom differential equation yields the functional integral for linear harmonic oscillator).Comment: Source file which we sent to journa