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.

Blazar Spectral Properties at 74 MHz

Abstract: Blazars are the most extreme class of active galactic nuclei. Despite a previous investigation at 102 MHz for a small sample of BL Lac objects and our recent analysis of blazars detected in the Westerbork Northern Sky Survey, a systematic study of the blazar spectral properties at frequencies below 100 MHz has been never carried out. In this paper, we present the first analysis of the radio spectral behavior of blazars based on the recent Very Large Array Low-frequency Sky Survey (VLSS) at 74 MHz. We search for blazar counterparts in the VLSS catalog, confirming that they are detected at 74 MHz. We then show that blazars present radio-flat spectra (i.e., radio spectral indices of ~0.5) when evaluated, which also about an order of magnitude in frequency lower than previous analyses. Finally, we discuss the implications of our findings in the context of the blazars-radio galaxies connection since the low-frequency radio data provide a new diagnostic tool to verify the expectations of the unification scenario for radio-loud active galaxies.

X-Ray Halos and Large Grains in the Diffuse Interstellar Medium

Abstract: Recent observations with dust detectors on board the interplanetary spacecrafts Ulysses and Galileo have recorded a substantial flux of large interstellar grains with radii between 0.25 and 2.0 μm entering the solar system from the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by an a-3.5 power law in grain radii a and extends to a maximum grain radius of 0.25 μm. The extension of the interstellar grain size distribution to such large radii will have a major effect on the median grain size and on the amount of mass needed to be tied up in dust for a given visual optical depth. It is therefore important to investigate whether this population of larger dust particles prevails in the general interstellar medium or whether it is merely a local phenomenon. The presence of large interstellar grains can be inferred mainly from their effect on the intensity and radial profile of scattering halos around X-ray sources. In this Letter, we examine the grain size distribution that gives rise to the X-ray halo around Nova Cygni 1992. The results of our study confirm the need to extend the interstellar grain size distribution in the direction of this source to and possibly beyond 2 μm. The model that gives the best fit to the halo data is characterized by (1) a grain size distribution that follows an a-3.5 power law up to 0.50 μm, followed by an a-4.0 extension from 0.50 to 2.0 μm, and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044 and 0.0022, respectively, consistent with solar abundance constraints. Additional observations of X-ray halos probing other spatial directions are badly needed to test the general validity of this result.

The «Skyhook»: A shuttle-borne tool for low-orbital-altitude research

Abstract: A Shuttle-borne tethered satellite has the potential of contributing significantly to the scientific versatility of the Shuttle Orbiter, that will be launched by NASA in the early '80s. If the Shuttle is in a 200 km orbit, the use of a 100 km tether connected to the Shuttle at one end and to the satellite at the other makes the performance of various relevant electromagnetic measurements in the magnetosphere possible. The tethered satellite (a subsatellite) can also be lowered from the Shuttle down to heights in the vicinity of 100 km, to an altitude band that has great importance from the standpoint of both upper atmospheric experimentation and gravity-gradient measurements. From a preliminary analysis, we find that one of the major problem areas of the system — the heating of the wire and of the subsatellite due to air drag — can be solved within the present state of the art in wire technology and heat shielding. System masses are also reasonable: In fact, a system with a 100 km wire and a Shuttle-borne reel, tethering a 100 kg subsatellite, would have a mass no larger than 120 kg.

The infrared–gamma-ray connection: a wise view of the extragalactic gamma-ray sky

Abstract: Using data from the WISE all-sky survey we discovered that the non-thermal infrared (IR) emission of blazars, the largest known population of extragalactic gamma-ray sources, has peculiar spectral properties. In this work, we confirm and strengthen our previous analyses using the latest available releases of both the WISE and the Fermi source catalogs. We also show that there is a tight correlation between the mid-IR colors and the gamma-ray spectral index of Fermi blazars. We name this correlation "the infrared--gamma-ray connection". We discuss how this connection links both the emitted powers and the spectral shapes of particles accelerated in jets arising from blazars over ten decades in energy. Based on this evidence, we argue that the infrared--gamma-ray connection is stronger than the well known radio--gamma-ray connection.