Smithsonian Astrophysical Observatory

About: Smithsonian Astrophysical Observatory is a facility organization based out in Cambridge, Massachusetts, United States. It is known for research contribution in the topics: Galaxy & Stars. The organization has 1665 authors who have published 3622 publications receiving 132183 citations. The organization is also known as: SAO.

Radar imaging of the lunar poles.

Abstract: Long-wavelength measurements reveal a paucity of ice in the Moon's polar craters. We have used a radio telescope at Arecibo Observatory, Puerto Rico, to map features of the lunar poles — some as small as 300 metres across — by collecting long-wavelength radar images that can penetrate several metres of lunar dust. We find that areas of the crater floors at the poles that are in permanent shadow from the Sun, which are potential cold traps for water or other volatiles, do not give rise to strong radar echoes like those associated with thick ice deposits in the polar craters on Mercury. Any lunar ice present within regions visible to the Arecibo radar must therefore be in the form of distributed grains or thin layers.

The Timing of Flares Associated with the Two Dynamical Types of Solar Coronal Mass Ejections

Abstract: In this Letter, we consider a sample of Transition Region and Coronal Explorer flare-associated solar coronal mass ejections (CMEs) and study the timing behavior of the flares associated with fast or slow CMEs. We find that flares associated with fast CMEs tend to happen within half an hour of the CME onsets, while the timing of flares associated with slow CMEs is only loosely related to the CME onsets. This suggests that the occurrence of flares may be integral to the early development of fast CMEs but is not crucial for slow CMEs. This observational result supports a recent qualitative theory of the initiation and expulsion mechanism of the two dynamical types of CMEs.

Unveiling the Nature of Unidentified γ-Ray Sources. VI. γ-Ray Blazar Candidates in the WISH Survey and their Radio Properties

Abstract: According to the Fermi Large Area Telescope Second Source Catalog (2FGL), about one-third of the γ-ray sources listed have no assigned counterparts at lower energies. Many statistical methods have been developed to find proper counterparts for these sources. We explore the sky area covered at low radio frequency by the Westerbork in the Southern Hemisphere (WISH) survey to search for blazar-like associations among the unidentified γ-ray sources (UGSs) listed in the 2FGL. Searching the WISH and NRAO Very Large Array Sky Survey radio surveys within the positional uncertainty regions of the 2FGL UGSs, we select as γ-ray blazar candidates the radio sources characterized by flat radio spectra between 352 MHz and 1400 MHz. We propose new γ-ray blazar associations for eight UGSs and we also discuss their spectral properties at low radio frequencies. We compare the radio flux density distribution of the low radio frequency γ-ray blazar candidates with that of γ-ray blazars associated with other methods. We find significant differences between these distributions. Finally, we discuss the results of this association method and its possible applicability to other regions of the sky and future radio surveys.

Hydrostatic and caustic mass profiles of galaxy clusters

Abstract: We compare X-ray and caustic mass profiles for a sample of 16 massive galaxy clusters. We assume hydrostatic equilibrium in interpreting the X-ray data, and use large samples of cluster members with redshifts as a basis for applying the caustic technique. The hydrostatic and caustic masses agree to better than $\approx20\%$ on average across the radial range covered by both techniques $(\sim[0.2-1.25]R_{500})$. The mass profiles were measured independently and do not assume a common functional form. Previous studies suggest that, at $R_{500}$, the hydrostatic and caustic masses are biased low and high respectively. We find that the ratio of hydrostatic to caustic mass at $R_{500}$ is $1.20^{+0.13}_{-0.11}$; thus it is larger than 0.9 at $\approx3\sigma$ and the combination of under- and over-estimation of the mass by these two techniques is $\approx10\%$ at most. There is no indication of any dependence of the mass ratio on the X-ray morphology of the clusters, indicating that the hydrostatic masses are not strongly systematically affected by the dynamical state of the clusters. Overall, our results favour a small value of the so-called hydrostatic bias due to non-thermal pressure sources.