Radio Dataset Research Articles

ORION’S BAR: PHYSICAL CONDITIONS ACROSS THE DEFINITIVE H+/H0/H2INTERFACE

Previous work has shown the Orion Bar to be an interface between ionized and molecular gas, viewed roughly edge-on, which is excited by the light from the Trapezium cluster. Much of the emission from any star-forming region will originate from such interfaces, so the Bar serves as a foundation test of any emission model. Here we combine X-ray, optical, infrared (IR), and radio data sets to derive emission spectra along the transition from H+ to H0 to H2 regions. We then reproduce the spectra of these layers with a simulation that simultaneously accounts for the detailed microphysics of the gas, the grains, and molecules, especially H2 and CO. The magnetic field, observed to be the dominant pressure in another region of the Orion Nebula, is treated as a free parameter, along with the density of cosmic rays. Our model successfully accounts for the optical, IR, and radio observations across the Bar by including a significant magnetic pressure and also heating by an excess density of cosmic rays, which we suggest is due to cosmic rays being trapped in the compressed magnetic field. In the Orion Bar, as we had previously found in M17, momentum carried by radiation and winds from the newly formed stars pushes back and compresses the surrounding gas. There is a rough balance between outward momentum in starlight and the total pressure in atomic and molecular gas surrounding the H+ region. If the gas starts out with a weak magnetic field, the starlight from a newly formed cluster will push back the gas and compress the gas, magnetic field, and cosmic rays until magnetic pressure becomes an important factor.

Exploring the Jet/Accretion Flow Relationship in low Disk Luminosity Sources

Astrophysical jets seem to gain strength disproportionate to the power of their associated accretion flow, making low-luminosity sources ideal targets for studies of the role of outflows. Radio/X-ray correlations have supported the case for a strong relationship between the jets and the hard X-ray emitting regions, and here we explore the strongest scenario where the base of the jets subsumes the role of the corona. The properties of coronae, as inferred from spectral models, are very similar to what is empirically required at jet bases assuming conservation laws hold. We present a few preliminary fits to simultaneous radio and X-ray data sets from our GX 339-4 and Cyg X-1 campaigns. The fits are performed in detector space, and include a jet plus thermal disk continuum model, with added Gaussian line and non-relativistic reflection features similar to the approach of other X-ray models. We find that we can fit the entire radio through X-ray spectrum quite well, with any deviation occurring in the line/reflection region. The results suggest that a jet/corona unification can provide a reasonable description of the data. Future work will benefit from a more complex approach to the disk feedback features.

The Broadband Afterglow of GRB 980703

We present radio observations of the afterglow of the bright gamma-ray burst GRB980703 made between one day and one year after the burst. These data are combined with published late-time radio measurements and existing optical, near-infrared (NIR) and X-ray observations to create a comprehensive broadband dataset for modeling the physical parameters of the outflow. While a wind-stratified medium cannot be ruled out statistically, it requires a high fraction of the shock energy in the electrons, and so is not favored on theoretical grounds. Instead, the data are consistent with a fireball model in which the ejecta are collimated and expanding into a constant density medium. The radio data cannot be fit with an isotropic shock but instead require a jet break at ~ 3.5 days, not seen at optical wavelengths due to the presence of a a bright host galaxy. The addition of the full radio dataset constrains the self-absorption frequency, giving an estimate of the circumburst density of n ~ 30 cm^-3, a value which differs substantially from previous estimates. This result is consistent with the growing number of GRB afterglows for which broadband modeling yields n ~= 0.1-100 cm^-3, with a typical value ~ 10 cm^-3.