C-start: optimal start of larval fish

We investigate the C-start escape response of larval fish by combining flow simulations using remeshed vortex methods with an evolutionary optimization. We test the hypothesis of the optimality of C-start of larval fish by simulations of larval-shaped, two- and three-dimensional self-propelled swimmers. We optimize for the distance travelled by the swimmer during its initial bout, bounding the shape deformation based on the larval mid-line curvature values observed experimentally. The best motions identified within these bounds are in good agreement with in vivo experiments and show that C-starts do indeed maximize escape distances. Furthermore we found that motions with curvatures beyond the ones experimentally observed for larval fish may result in even larger escape distances. We analyse the flow field and find that the effectiveness of the C-start escape relies on the ability of pronounced C-bent body configurations to trap and accelerate large volumes of fluid, which in turn correlates with large accelerations of the swimme

Flash-Heating of Circumstellar Clouds by Gamma Ray Bursts

The blast-wave model for gamma-ray bursts (GRBs) has been called into question by observations of spectra from GRBs that are harder than can be produced through optically thin synchrotron emission. If GRBs originate from the collapse of massive stars, then circumstellar clouds near burst sources will be illuminated by intense gamma radiation, and the electrons in these clouds will be rapidly scattered to energies as large as several hundred keV. Low-energy photons that subsequently pass through the hot plasma will be scattered to higher energies, hardening the intrisic spectrum. This effect resolves the "line-of-death" objection to the synchrotron shock model. Illuminated clouds near GRBs will form relativistic plasmas containing large numbers of electron-positron pairs that can be detected within ~ 1-2 days of the explosion before expanding and dissipating. Localized regions of pair annihilation radiation in the Galaxy would reveal past GRB explosions.Comment: 9 pages, 1 figure, submitted to ApJ Letter

Tumour rejection in rats sensitized to embryonic tissue. I. Rejection of tumour cells implanted s.c. and detection of cytotoxic lymphoid cells.

Wistar rats were sensitized to rat embryonic tissue by immunization with irradiated (5000 rad) rat embryo cells (2 X 10(6) s.c. + 1 X 10(6) i.p.) derived from embryos aged 14-15 days, or by implantation of irradiated (5000 rad) tissue grafts from these embryos. Three to five immunizations were given at weekly intervals, and the rats were then challenged subcutaneously 7-10 days after the final inoculum with minimal tumour-producing tumour cell doses. Immunization with irradiated rat embryo cells failed to influence the growth and development of tumour cells prepared from hepatoma D23 and D30, sarcoma Mc57, mammary carcinoma AAF57 or cells prepared from spontaneously arising mammary carcinomata Sp4 and Sp15. Using adoptive transfer techniques, lymphoid cells from embryo-sensitized rats, when used in a 3000 : 1 ratio (lymphoid cells : tumour cells), were shown effectively to retard the growth of hepatoma D23 in 3 out of 7 experiments performed. Similar adoptive transfer procedures proved ineffective in preventing the growth of mammary carcinoma AAF57. Using in vitro cytotoxicity tests, lymph node cells and spleen cells from embryo-immunized rats were shown to be cytotoxic for several rat tumour cell targets : hepatoma D23 (7/10 tests), sarcoma Mc7 (8/12 tests), mammary carcinoma AAF57 (2/2 tests) and Sp4 (3/4 tests), and for 14-15-day-old rat embryo cells (5/10 tests). In comparative tests lymphoid cells were relatively non-cytotoxic for 20-day-old rat embryo cells (1/6 tests) or cells prepared from adult rat lung or kidney (1/10 tests). The role of embryonic antigen(s) in tumour rejection is discussed

Probing the Pulsar Wind Nebula of PSR B0355+54

We present XMM-Newton and Chandra X-ray observations of the middle-aged radio pulsar PSR B0355+54. Our X-ray observations reveal emission not only from the pulsar itself, but also from a compact diffuse component extending ~50'' in the opposite direction to the pulsar's proper motion. There is also evidence for the presence of fainter diffuse emission extending ~5' from the point source. The compact diffuse feature is well-fitted with a power-law, the index of which is consistent with the values found for other pulsar wind nebulae. The morphology of the diffuse component is similar to the ram-pressure confined pulsar wind nebulae detected for other sources. The X-ray emission from the pulsar itself is described well by a thermal plus power-law fit, with the thermal emission most likely originating in a hot polar cap.Comment: 9 pages (uses emulateapj.cls), 8 figures, 2 tables, accepted for publication in Ap

Secure Distributed Virtual Conferencing: Multicast or Bust

We describe a secure distributed virtual conferencing application (SDVC) that provides high quality streaming video and audio using IP multicast for efficient distribution, using strong authentication via cryptographic means and optionally providing fully encrypted communication without sacrificing quality of the medium or the user experience. We summarize our experiences with SDVC in a recent live demonstration and conclude with a discussion of future plans.http://deepblue.lib.umich.edu/bitstream/2027.42/107912/1/citi-tr-99-1.pd

VLA Limits for Intermediate Mass Black Holes in Three Globular Clusters

The observational evidence for central black holes in globular clusters has been argued extensively, and their existence has important consequences for both the formation and evolution of the cluster. Most of the evidence comes from dynamical arguments, but the interpretation is difficult, given the short relaxation times and old ages of the clusters. One of the most robust signatures for the existence of a black hole is radio and/or X-ray emission. We observed three globular clusters, NGC6093 (M80), NGC6266 (M62), and NGC7078 (M15), with the VLA in the A and C configuration with a 3-sigma noise of 36, 36 and 25 microJy, respectively. We find no statistically-significant evidence for radio emission from the central region for any of the three clusters. NGC6266 shows a 2-sigma detection. It is difficult to infer a mass from these upper limits due to uncertainty about the central gas density, accretion rate, and accretion model.Comment: 5 pages, accepted for publication in the Astronomical Journa

The Expected Rate of Gamma-Ray Burst Afterglows In Supernova Searches

We predict the rate at which Gamma-Ray Burst (GRB) afterglows should be detected in supernova searches as a function of limiting flux. Although GRB afterglows are rarer than supernovae, they are detectable at greater distances because of their higher intrinsic luminosity. Assuming that GRBs trace the cosmic star formation history and that every GRB gives rise to a bright afterglow, we find that the average detection rate of supernovae and afterglows should be comparable at limiting magnitudes brighter than K=18. The actual rate of afterglows is expected to be somewhat lower since only a fraction of all gamma-ray selected GRBs were observed to have associated afterglows. However, the rate could also be higher if the initial gamma-ray emission from GRB sources is more beamed than their late afterglow emission. Hence, current and future supernova searches can place strong constraints on the afterglow appearance fraction and the initial beaming angle of GRB sources.Comment: 13 pages, submitted to ApJ

Radiative Shock-Induced Collapse of Intergalactic Clouds

Accumulating observational evidence for a number of radio galaxies suggests an association between their jets and regions of active star formation. The standard picture is that shocks generated by the jet propagate through an inhomogeneous medium and trigger the collapse of overdense clouds, which then become active star-forming regions. In this contribution, we report on recent hydrodynamic simulations of radiative shock-cloud interactions using two different cooling models: an equilibrium cooling-curve model assuming solar metallicities and a non-equilibrium chemistry model appropriate for primordial gas clouds. We consider a range of initial cloud densities and shock speeds in order to quantify the role of cooling in the evolution. Our results indicate that for moderate cloud densities (>1 cm^{-3}) and shock Mach numbers (<20), cooling processes can be highly efficient and result in more than 50% of the initial cloud mass cooling to below 100 K. We also use our results to estimate the final H_2 mass fraction for the simulations that use the non-equilibrium chemistry package. This is an important measurement, since H_2 is the dominant coolant for a primordial gas cloud. We find peak H_2 mass fractions of >0.01 and total H_2 mass fractions of >10^{-5} for the cloud gas. Finally, we compare our results with the observations of jet-induced star formation in ``Minkowski's Object.'' We conclude that its morphology, star formation rate (~ 0.3M_solar/yr) and stellar mass (~ 1.2 x 10^7 M_solar) can be explained by the interaction of a 90,000 km/s jet with an ensemble of moderately dense (~ 10 cm^{-3}), warm (10^4 K) intergalactic clouds in the vicinity of its associated radio galaxy at the center of the galaxy cluster.Comment: 30 pages, 7 figures, submitted to Astrophysical Journa

An analysis of gamma-ray burst spectral break models

Typical gamma-ray burst spectra are characterized by a spectral break, Ep, which for bright BATSE bursts is found to be narrowly clustered around 300 keV. Recently identified X-ray flashes, which may account for a significant portion of the whole GRB population, seem to extend the Ep distribution to a broader range below 40 keV. Positive correlations among Ep and some other observed parameters have been noticed. On the other hand, within the cosmological fireball model, the issues concerning the dominant energy ingredient of the fireball as well as the location of the GRB emission site are still unsettled, leading to several variants of the fireball model. Here we analyze these models within a unified framework, and critically reexamine the Ep predictions in the various model variants. Attention is focused on the predictions of the narrowness of the Ep distribution in different models, and the correlations among Ep and some other measurable observables. These model properties may be tested against the current and upcoming GRB data, through which the nature of the fireball as well as the mechanism and site of the GRB emission will be identified. In view of the current data, various models are appraised through a simple Monte-Carlo simulation, and a tentative discussion about the possible nature of X-ray flashes is presented.Comment: 14 pages, 3 figures, emulateapj style, accepted for publication in ApJ. Monte Carlo simulation added to allow a clearer comparison among the model