Showing papers by "Brian D. Fields published in 2002"

The Guaranteed Gamma-Ray Background

Abstract: The diffuse extragalactic gamma-ray background (EGRB) above 100 MeV encodes unique information about high-energy processes in the universe. Numerous sources for the EGRB have been proposed, but the two systems that are certain to make some contribution are active galaxies (blazars) as well as normal galaxies. In this Letter we evaluate the contribution to the background from both sources. The active galaxy contribution arises from unresolved blazars. We compute this contribution using the Stecker-Salamon model. For normal galaxies, the emission is due to cosmic-ray interactions with diffuse gas. Our key assumption here is that the cosmic-ray flux in a galaxy is proportional to the supernova rate and thus the massive star formation rate, quantified observationally by the cosmic star formation rate (CSFR). In addition, the existence of stars today requires a considerably higher interstellar medium mass in the past. Using the CSFR to compute both these effects, we find that normal galaxies are responsible for a significant portion (~) of the EGRB near 1 GeV but make a smaller contribution at other energies. Finally, we present a "minimal" two-component model that includes contributions from both normal galaxies and blazars. We show that the spectrum of the diffuse radiation is a key constraint on this model: while neither the blazar spectra nor the galactic spectra are separately optimal fits to the observed spectrum, the combined emission provides an excellent fit. We close by noting key observational tests of this two-component model, which can be probed by future gamma-ray observatories, such as the Gamma-ray Large Area Space Telescope.

Constraining strong baryon–dark-matter interactions with primordial nucleosynthesis and cosmic rays

Abstract: Self-interacting dark matter (SIDM) was introduced by Spergel & Steinhardt to address possible discrepancies between collisionless dark matter simulations and observations on scales of less than 1 Mpc. We examine the case in which dark matter particles not only have strong self-interactions but also have strong interactions with baryons. The presence of such interactions will have direct implications for nuclear and particle astrophysics. Among these are a change in the predicted abundances from big bang nucleosynthesis (BBN) and the flux of gamma-rays produced by the decay of neutral pions which originate in collisions between dark matter and Galactic cosmic rays (CR). From these effects we constrain the strength of the baryon--dark matter interactions through the ratio of baryon - dark matter interaction cross section to dark matter mass, $s$. We find that BBN places a weak upper limit to this ratio $< 10^8 cm^2/g$. CR-SIDM interactions, however, limit the possible DM-baryon cross section to $< 5 \times 10^{-3} cm^2/g$; this rules out an energy-independent interaction, but not one which falls with center-of-mass velocity as $s \propto 1/v$ or steeper.

On the baryometric status of He3

Abstract: Recent observations by Bania et al. (2002) measure He3 versus oxygen in Galactic HII regions, finding that He3/H is within a factor of 2 of the solar abundance for [O/H] > -0.6. These results are consistent with a flat behavior in this metallicity range, tempting one to deduce from these observations a primordial value for the He3 abundance, which could join D and Li7 as an indicator of the cosmic baryon density. However, using the same data, we show that it is not possible to obtain a strong constraint on the baryon density range. This is due to (i) the intrinsically weak sensitivity of the primordial He3 abundance to the baryon density; (ii) the limited range in metallicity of the sample; (iii) the intrinsic scatter in the data; and (iv) our limited understanding of the chemical and stellar evolution of this isotope. Consequently, the He3 observations correspond to an extended range of baryon-to-photon ratio, eta = (2.2 - 6.5) x 10^{-10}, which diminishes the role of He3 as a precision baryometer. On the other hand, once the baryon-to-photon ratio is determined by the CMB, D/H, or Li7/H, the primordial value of He3/H can be inferred. Henceforth new observations of Galactic He3, can in principle greatly improve our understanding of stellar and/or chemical evolution and reconcile the observations of the HII regions and those of the planetary nebulae.

A Simple Model for r-Process Scatter and Halo Evolution

Abstract: Recent halo-star observations of heavy elements produced by rapid neutron capture (r-process) show a striking behavior: within a single star, the relative abundances of r-process elements heavier than Ba are the same as those of solar system matter, while across stars with similar metallicity Fe/H, the r/Fe ratio varies by more than 2 orders of magnitude. In this paper we present a simple analytic model that describes a star's abundances in terms of its "ancestry," i.e., the number of nucleosynthesis events (e.g., supernova explosions) that contributed to the star's composition. This model leads to a very simple analytic expression for the abundance scatter versus Fe/H, which is in good agreement with the data and with more sophisticated numerical models. We investigate two classes of scenarios for r-process nucleosynthesis, one in which r-process synthesis events occur in only ~4% of supernovae but iron synthesis is ubiquitous, and one in which iron nucleosynthesis occurs in only about 9% of supernovae (the Wasserburg-Qian model). We find that the predictions in these scenarios are similar for [Fe/H] -2.5, but that these models can be readily distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] -3.

Production of Lithium, Beryllium, and Boron by Hypernovae and the Possible Hypernova–Gamma‐Ray Burst Connection

Abstract: We investigate a possible nucleosynthetic signature of highly energetic explosions of C-O cores ("hypernovae" [HNe]) that might be associated with gamma-ray bursts (GRBs). We note that the direct impact of C- and O-enriched hypernova ejecta on the ambient hydrogen and helium leads to spallation reactions that can produce large amounts of the light nuclides lithium, beryllium, and boron (LiBeB). Using analytic velocity spectra of the hypernova ejecta, we calculate the LiBeB yields of different exploding C-O cores associated with observed hypernovae. The deduced yields are much higher than those produced by similar (direct) means in normal Type II supernovae (SNe) and are higher than the commonly used ones arising from shock wave acceleration induced by Type II supernova explosions. To avoid overproduction of these elements in our Galaxy, hypernovae should be rare events, with 3 × 10-2 hypernovae per supernova, assuming a constant HN/SN ratio over time; this result also implies that the HN production of Be is only a fraction of other sources, e.g., superbubbles. Our limit to the HN/SN ratio is in good agreement with that of long-duration GRBs if we assume that the gamma-ray emission is focussed in a solid angle Ω so that (Ω/4π)-1 3 × 104. This encouraging result supports the possible HN-GRB association. Thus, Galactic LiBeB abundance measurements offer a promising way to probe the HN rate history and the possible HN-GRB correlation. On the other hand, if hypernovae are associated with massive pregalactic stars (Population III), they would produce an LiBeB pre-enrichment in protogalactic gas, which could show up as a plateau in the lowest metallicities of the Be-Fe relation in halo stars.

Discovery of an Old, Nearby, and Overlooked Supernova Remnant Centered on the Southern Constellation Antlia Pneumatica

Abstract: We report the discovery of a supernova remnant (SNR) with an angular diameter of 24°, centered on the southern constellation Antlia Pneumatica. The SNR is detected well in Hα and X-rays. Within the Antlia SNR's outline, a marginally significant feature is detected in the 1.8 MeV gamma-ray line of the radioisotope 26Al. At an estimated distance dA 60-340 pc, the Antlia SNR is perhaps the nearest SNR except for the Local Bubble. Consequently, any associated neutron star or black hole is expected to have a large proper motion. Of the trajectories of nearby pulsars with well-determined proper motions, only B0950+08's passes within the SNR outline. If the SNR and the pulsar B0950+08 indeed both originated from the same supernova, then their age t = 1.8(dA/100 pc) Myr.

A peculiar linear radio feature in the supernova remnant N206

Abstract: We present images of the supernova remnant N206 in the Large Magellanic Cloud taken with the Australia Telescope Compact Array at wavelengths of 3 and 6 cm. Based on our data and previously published flux densities, the spectral index of N206 is -0.20 ± 0.07. The 6 cm radio morphology shows a filled center. Most interesting is the discovery of a peculiar linear feature previously undetected at any wavelength. The feature lies to the east of the center of the remnant, stretching from about to of the remnant's radius. It is wedge-shaped, with a steady opening angle from an apex on the eastern side. The feature resembles the disturbance expected from an object moving through the material supersonically at about 800 km s-1. We present arguments suggesting that the linear feature might have been produced by a low-mass star or compact object ejected from a binary system that may have led to a Type Ic supernova.

The Guaranteed Gamma-Ray Background

Abstract: The diffuse extragalactic gamma-ray background (EGRB) above 100 MeV encodes unique information about high-energy processes in the universe. Numerous sources for the EGRB have been proposed, but the two systems which are certain to make some contribution are active galaxies (blazars) as well as normal galaxies. In this paper, we evaluate the contribution to the background from both sources. The active galaxy contribution arises from unresolved blazars. We compute this contribution using the Stecker-Salamon model. For normal galaxies, the emission is due to cosmic-ray interactions with diffuse gas. Our key assumption here is that the cosmic-ray flux in a galaxy is proportional to the supernova rate and thus the massive star formation rate, quantified observationally by the cosmic star formation rate (CSFR). In addition, the existence of stars today requires a considerably higher ISM mass in the past. Using the CSFR to compute both these effects, we find that normal galaxies are responsible for a significant portion (\sim 1/3) of the EGRB near 1 GeV, but make a smaller contribution at other energies. Finally, we present a "minimal" two-component model which includes contributions from both normal galaxies and blazars. We show that the spectrum of the diffuse radiation is a key constraint on this model: while neither the blazar spectra, nor the galactic spectra, are separately optimal fits to the observed spectrum, the combined emission provides an excellent fit. We close by noting key observational tests of this two-component model, which can be probed by future gamma-ray observatories such as GLAST.

A Peculiar Linear Radio Feature in the Supernova Remnant N206

Abstract: We present images of the supernova remnant N206 in the LMC, taken with ATCA at wavelengths of 3 and 6 cm. Based on our data and previously published flux densities, the spectral index of N206 is -0.20 +- 0.07. The 6-cm radio morphology shows a filled center. Most interesting is the discovery of a peculiar linear feature previously undetected at any wavelength. The feature lies to the east of the center of the remnant, stretching from about one-fourth to three-fourths of the remnant's radius. It is wedge-shaped, with a steady opening angle from an apex on the eastern side. The feature resembles the disturbance expected from an object moving through the material supersonically at about 800 km/s. We present arguments suggesting that the linear feature might have been produced by a low-mass star or compact object ejected from a binary system which may have led to a Type Ic supernova.