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astrophysics

Modeling Relativistic Pulsar Winds

J. Arons, D. C. Backer, A. Spitkovsky, and V. M. Kaspi, "Probing relativistic winds: The case of PSRJ07370-3039 A & B," in Proceedings of the 2004 Aspen Winter Conf. on Astrophysics: Binary Radio Pulsars, F. A. Rasio and I. H. Stairs, eds.; astro-ph/0404159 (2004). NP, NASA, NSF

In late 2003, radio astronomers discovered the first binary star system containing two neutron stars, both of which are observable as radio pulsars. The pulsars eclipse each other, even though the neutron stars are much too small to be the obscuring obstacles. The luminosity of pulsar A suggests that its relativistic wind is confining the magnetosphere of pulsar B to be 30% of the size it would have if B existed in isolation. Within several weeks of the discovery, Arons et al. created a simulation of A’s eclipses based on synchrotron absorption in a shocked pulsar wind colliding with the magnetosphere of pulsar B (Figure 3). So far their model is the best developed model of the system, and their predictions are continuing to be confirmed by observations.

Figure 3 (a) Relativistic 3D particle-in-cell simulation of pulsar B’s rotating magnetosphere immersed in an unmagnetized wind.

Figure 3 (b) Relativistic 3D PIC simulation of B’s rotating magnetosphere immersed in a magnetized wind — a snapshot of the equatorial plane.

Disappearing Neutrinos Reappear

The KamLAND Collaboration, T. Araki et al., “Measurement of neutrino oscillation with KamLAND: Evidence of spectral distortion,” Phys. Rev. Lett. 94, 081801 (2005). NP, JMECSST, FEPCJ, KMSC

In previous experiments at the KamLAND underground neutrino detector, it was reported that the electron neutrinos were disappearing, i.e., oscillating into the neutrino flavors that are not detectable. With more data and more precise measurements, the disappearing neutrinos have been shown to be oscillating back into the detectable electron neutrinos. This is the most direct evidence yet of neutrino oscillation. These results constitute further proof that neutrinos have mass and that the Standard Model describing fundamental particles will need to be amended.

High-Resolution Cosmology

C. L. Kuo, P. A. R. Ade, J. J. Bock, C. Cantalupo, M. D. Daub, J. Goldstein, W. L. Holzapfel, A. E. Lange, M. Lueker, M. Newcomb, J. B. Peterson, J. Ruhl, M. C. Runyan, and E. Torbet, “High-resolution observations of the cosmic microwave background power spectrum with ACBAR,” Astrophys. J. 600, 32 (2004). HEP, NASA, NSF, CARA

The Arcminute Cosmology Bolometer Array Receiver (ACBAR) is an instrument designed to produce detailed images of the cosmic microwave background (CMB) in three millimeter-wavelength bands. Kuo et al. employed new analysis techniques designed specifically for high-sensitivity ground-based CMB observations and reported the first measurements of CMB anisotropy from ACBAR — the highest signal-to-noise ratio observations to date. Overall, the resulting power spectrum appears to be consistent with the damped acoustic oscillations expected in standard cosmological models.

The Physics of Star Formation

M. R. Krumholz, C. F. McKee, and R. I. Klein, “Bondi accretion in the presence of vorticity,” Astrophys. J. (in press); astro-ph/0409454 (2004). NP, NASA, NSF

The formation of high-mass stars remains one of the most significant unsolved problems in astrophysics. The classical Bondi-Hoyle formula for the accretion of gas onto a point particle is incomplete because it does not take vorticity into account, and even a small amount of vorticity can substantially affect accretion. Using a combination of simulations and analytic treatment, Krumholz et al. have provided an approximate formula for the accretion rate of gas onto a point particle as a function of the vorticity of the surrounding gas. Their results have potential implications for models of star formation in which protostars gain mass through a process of competitive accretion.

Neutrinos in Core-Collapse Supernovae

A. Juodagalvis, K. Langanke, G. Martinez-Pinedo, W. R. Hix, D. J. Dean, and J. M. Sampaio, “Neutral-current neutrino–nucleus cross sections for A ~ 50–65 nuclei,” Nucl. Phys. A 747, 87 (2005). NP, SciDAC, NSF, DRC, MCyT, ERDF, PFST

Neutrino–nucleus reactions play an essential role in the explosions of core-collapse supernovae, but inelastic neutrino–nucleus scattering has not yet been incorporated into supernova simulations, which may be one reason why these simulations often fail to yield explosions. Juodagalvis et al. studied neutral-current neutrino–nucleus reaction cross sections for Mn, Fe, Co and Ni isotopes, presenting the cross sections as functions of initial and final neutrino energies and for a range of supernova-relevant temperatures. These cross sections will allow improved estimates of inelastic neutrino reactions on nuclei to be used in supernova simulations.

Modeling Galaxy–Mass Correlations

A. Tasitsiomi, A. V. Kravtsov, R. H. Wechsler, and J. R. Primack, “Modeling galaxy–mass correlations in dissipationless simulations,” Astrophys. J. 614, 533 (2004). HEP, NSF, NASA, KICP

Understanding the processes that shape the clustering of dark matter and galaxies is one of the main goals of observational cosmology. Tasitsiomi et al. used high-resolution, dissipationless simulations of the concordance flat LCDM model to predict galaxy–mass correlations and compare them with the recent Sloan Digital Sky Survey weak-lensing measurements. They found that assigning a luminosity to a dark-matter halo of a certain maximum circular velocity by matching the simulated subhalo velocity function to the observed luminosity function leads to good agreement with the observed galaxy–mass correlation and its dependence on luminosity, if an observationally motivated amount of scatter between luminosity and circular velocity is introduced.

High-Energy Stellar Jets

W. Zhang, S. E. Woosley, and A. Heger, “The propagation and eruption of relativistic jets from the stellar progenitors of gamma-ray bursts,” Astrophys. J. 608, 365 (2004). HEP, SciDAC, NASA

Zhang et al. have conducted 2D and 3D calculations of relativistic jet propagation and breakout in massive Wolf-Rayet stars, which are thought to be responsible for gamma-ray bursts (GRBs). As it erupts, the highly relativistic jet core is surrounded by a cocoon of less energetic but still moderately relativistic ejecta that expands and becomes visible at larger polar angles. These less energetic ejecta may be the origin of X-ray flashes and other high-energy transients. Calculations of jet stability showed that if the jet changes angle by more than 3° in several seconds, it will dissipate, producing a broad beam with inadequate Lorentz factor to make a common GRB, but possibly enough to make an X-ray flash (Figure 4).

Figure 4. Jet angle sensitivity study. Slices of the precessing jet models are shown just after breakout. For model 3P3, the jet emerges relatively intact and might produce a GRB. The jets in models 3P5 and especially 3P10 dissipate their energy before escaping and are unlikely to produce GRBs, although they may still produce hard X-ray flashes.

Holes in Type Ia Supernovae

D. Kasen, P. Nugent, R. C. Thomas, and L. Wang, “Could there be a hole in Type Ia supernovae?” Astrophys. J. 610, 876 (2004). NP, NASA

Type Ia supernovae arise from a white dwarf accreting material from a companion star. Soon after the white dwarf explodes, the ejected supernova material engulfs the companion star, which carves out a conical hole in the supernova ejecta. Kasen et al. used multidimensional Monte Carlo radiative transfer calculations to explore the observable consequences of an ejecta-hole asymmetry. They found that when one looks almost directly down the hole, the supernova is relatively brighter and has a peculiar spectrum characterized by more highly ionized species, weaker absorption features, and lower absorption velocities. Ejecta-hole asymmetry may explain the current spectropolarimetric observations of Type Ia supernovae.

Figure 5. False color image of the absolute value of the upper left (100 x 100) corner of the inverse of the coefficient matrix for a test problem. Brighter colors indicate larger elements

Improving the Efficiency of Simulations

F. D. Swesty, D. C. Smolarski, and P. E. Saylor, “A comparison of algorithms for the efficient solution of the linear systems arising from multigroup flux-limited diffusion problems,” Astrophys. J. Supp. 153, 369 (2004). NP, SciDAC, NASA, ASAP

Multi-group flux-limited diffusion (MGFLD) is a popular method for modeling the multidimensional flow of radiation in a wide variety of astrophysical phenomena, but solving the sparse linear systems that arise in this method is computationally expensive. Swesty et al. compared the effectiveness of certain iterative sparse linear system methods for the solution of implicitly differenced MGFLD equations. They found that certain combinations of algorithms and preconditioners consistently outperform others for a series of test problems (Figure 5), and that the method of preparing the linear system for solution by scaling the system has a dramatic effect on the convergence behavior of the iterative methods.