Jonathan D. Romney

To Lease or Not to Lease from Storage Clouds

In this paper, storage clouds are online services for leasing disk storage. A new modeling tool, formulated from empirical data spanning many years, lets organizations rationally evaluate the benefit of using storage clouds versus purchasing hard disk drives. The business of selling infrastructure as a service through the Internet is growing. This technology trend, also known as cloud computing, lets individuals and organizations outsource their IT requirements to remote data centers, paying for only what they use.Several online services currently lease storage infrastructure. These storage clouds let anyone with a credit card purchase storage capacity online, paying a monthly fee for the storage they use. With the significant growth of societys storage requirements, and the availability of pay-per-use online storage services, when should a consumer consider using storage clouds? The paper focuses on the problem of resolving this buy-or-lease storage decision.

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

SENSITIVE VLBI CONTINUUM AND H I ABSORPTION OBSERVATIONS OF NGC 7674: FIRST SCIENTIFIC OBSERVATIONS WITH THE COMBINED ARRAY VLBA, VLA, AND ARECIBO

We present phase-referenced VLBI observations of the radio continuum emission from, and the neutral hydrogen 21 cm absorption toward, the luminous infrared galaxy NGC 7674. The observations were carried out at 1380 MHz using the VLBA, the phased VLA, and the 305 m Arecibo radio telescope. These observations constitute the first scientific use of the Arecibo telescope in a VLBI observation with the VLBA. The high- and low-resolution radio continuum images reveal several new continuum structures in the nuclear region of this galaxy. At ~100 mas resolution, we distinguish six continuum structures extending over 14 (742 pc), with a total flux density of 138 mJy. Only three of these structures were known previously. All these structures seem to be related to active galactic nucleus (AGN) activity. The overall S-shaped pattern that the radio structures seem to form could be the result of the interstellar medium diverting the outcoming jets from the central AGN. However, we cannot rule out the possibility of a black hole merger that could result in a similar structural pattern. At the full resolution of the array (11 × 5 mas), we detect only two of the six continuum structures. Both are composed of several compact components with brightness temperatures on the order of 107 K. While it is possible that one of these compact structures could host an AGN, they could also be shocklike features formed by the interaction of the jet with compact interstellar clouds in the nuclear region of this galaxy. Complex H I absorption is detected with our VLBI array at both high and low angular resolution. Assuming that the widest H I feature is associated with a rotating H I disk or torus feeding a central AGN, we estimate an enclosed dynamical mass of ~7 × 107 M☉, comparable to the value derived from the hidden broad Hβ emission in this galaxy. The narrower H I lines could represent clumpy neutral hydrogen structures in the H I torus. The detection of H I absorption toward some of the continuum components and its absence toward others suggest an inclined H I disk or torus in the central region of NGC 7674. The overall averaged H I spectrum toward the continuum structures with H I absorption is very consistent with the Arecibo single-dish H I absorption spectrum at 33 resolution.

Further monitoring of the structure of superluminal radio sources

We have studied the sources 3C 120, 3C 273, 3C 279, and 3C 345 at six epochs between 1975.40 and 1977.13 using long baseline interferometry at 10.65 GHz, and at 1976.52 and 1977.41 at 5.01 GHz. The sources 3C 120 (usually) and 3C 345 (always) appear as double radio sources whose components are separating along constant potition angles. Assuming their redshifts are cosmological, the apparent transverse velocities of separation are v/c=6.7 +- 0.8 beginning in 1966.3 +- 0.3 yr for 3C 345 and, for 3C 120, v/c=4.1 +- 0.4 beginning in 1972.4 +- 0.5 yr and continuing into 1974, then v/c=8.5 +- 0.9 beginning in 1974.4 +- 0.2 yr and continuing until mid-1976. No models of 3C 273 are derived, but evidence for superluminal expansion is presented, showing v/c=5.2 +- 0.5 beginning in 1967.6 +- 0.4 yr. Visibility functions of 3C 279 have changed very little over the period 1976.39 +- 1977.41.

The nucleus of M81 - Simultaneous 2.3 and 8.3 GHz Mark III VLBI observations

Mark III very long baseline interferometry (VLBI) observations have been made of the nucleus in the normal galaxy M81 (NGC 3031) simultaneously at 2.3 and 8.3 GHz, and it was found that nearly 100% of the flux density of the nuclear region originates in an elongated radio core with linear dimensions 1000-4000 AU, dependent on frequency. This galactic nucleus is the most compact observed in any extragalactic source. The position coincides with that of the nucleus at optical and X-ray wavelengths within the larger uncertainties of the latter two. The position angle of the major axis of the M81 nucleus is within 3 deg of 75 deg at 2.3 GHz and within 6 deg of 50 deg at 8.3 GHz and is frequency dependent at the 4 sigma level. These values straddle the 62 deg position angle of the projection on the sky of the rotation axis of the galaxy. The spectrum of the core is slightly inverted, and the length of its major axis decreases with frequency. These results are consistent with the emissions being incoherent electron synchrotron radiation from an inhomogeneous region with an optical depth varying along the rotation axis of the galaxy.

The Celestial Reference Frame at 24 and 43?GHz. I. Astrometry

We present astrometric results for compact extragalactic objects observed with the Very Long Baseline Array at radio frequencies of 24 and 43 GHz. Data were obtained from ten 24 hr observing sessions made over a five-year period. These observations were motivated by the need to extend the International Celestial Reference Frame (ICRF) to higher radio frequencies to enable improved deep space navigation after 2016 and to improve state-of-the-art astrometry. Source coordinates for 268 sources were estimated at 24 GHz and for 131 sources at 43 GHz. The median formal uncertainties of right ascension and declination at 24 GHz are 0.08 and 0.15 mas, respectively. Median formal uncertainties at 43 GHz are 0.20 and 0.35 mas, respectively. Weighted root-mean-square differences between the 24 and 43 GHz positions and astrometric positions based on simultaneous 2.3 and 8.4 GHz Very Long Baseline Interferometry observations, such as the ICRF, are less than about 0.3 mas in both coordinates. With observations over five years we have achieved a precision at 24 GHz approaching that of the ICRF but unaccounted systematic errors limit the overall accuracy of the catalogs.

THE CELESTIAL REFERENCE FRAME AT 24 AND 43 GHz. II. IMAGING

We have measured the sub-milli-arcsecond structure of 274 extragalactic sources at 24 and 43 GHz in order to assess their astrometric suitability for use in a high frequency celestial reference frame (CRF). Ten sessions of observations with the Very Long Baseline Array have been conducted over the course of

Inner beams of SS 433

\sim

Very Long Baseline Array Continuum and H I Absorption Observations of the Ultraluminous Infrared Galaxy IRAS 17208–0014

5 years, with a total of 1339 images produced for the 274 sources. There are several quantities that can be used to characterize the impact of intrinsic source structure on astrometric observations including the source flux density, the flux density variability, the source structure index, the source compactness, and the compactness variability. A detailed analysis of these imaging quantities shows that (1) our selection of compact sources from 8.4 GHz catalogs yielded sources with flux densities, averaged over the sessions in which each source was observed, of about 1 Jy at both 24 and 43 GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25% relative to their mean values, with variations in the session-to-session flux density scale being less than 10%, (3) sources were found to be more compact with less intrinsic structure at higher frequencies, and (4) variations of the core radio emission relative to the total flux density of the source are less than 8% on average at 24 GHz. We conclude that the reduction in the effects due to source structure gained by observing at higher frequencies will result in an improved CRF and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.

Multiepoch VLBI observations of 4C 39. 25 - superluminal motion amid stationary structure

European VLBI observations of SS 433 at eight epochs in 1980 and 1981 are reported. The images derived are in excellent agreement with predictions of the kinematic model which posits ballistic motions at mildly relativistic velocity along a cone of directions swept out in a 164-day cycle. By fitting some of the results to the predicted trajectories, a distance to SS 433 of 5.0 + or - 0.5 kpc is derived. The maps exhibit numerous discrete features which appear to be ejected from the center of SS 433 in association with flares in the flux density. These features are often ejected symmetrically, but episodes of one-sided activity are also observed. There is some evidence for wide variations in the aging rate of the ejecta, and for the presence of relatively diffuse emission not associated with flares. Some of the images reveal structures at position angles beyond the cone permitted by the kinematic model. 36 references.