M. I. Ratner

VLBI limits on the proper motion of the ‘core’ of the superluminal quasar 3C345

VLBI (very-long-baseline interferometry) observations between 1971 and 1983 have been used to determine the positions of the ‘core’ of the quasar 3C345 relative to the more distant compact quasar NRAO512 with a fractional uncertainty as small as 2 parts in 108. The core of 3C345 appears stationary in right ascension to within 20 arc μs yr−1, a subluminal bound corresponding to 0.7c. The apparent velocities of the jets are superluminal, up to 14c in magnitude.

VLBI observations of 23 hot spots in the starburst galaxy M82

The simultaneous 2.3 and 8.4 GHz VLBI observations of 23 hot spots in the nuclear region of M82 presented indicate the presence of six hot spots at 2.3 GHz, but only one at 8.4 GHz. Attention is given to a mapping of the brightest hot spot, 41.9+58, at 2.3 GHz, which exhibits a complex brightness distribution whose angular width in the NE-SW direction is 45 mas at the 10-percent contour. These data are consistent with the hot spots in M82 being powerful SNRs, with ages between about 10 and 300 yr. 34 references.

Proper Motion of Components in 4C 39.25

From a series of simultaneous 8.4 and 2.3 GHz VLBI observations of the quasar 4C 39.25 phase referenced to the radio source 0920+390, carried out in 1990-1992, we have measured the proper motion of component b in 4C 39.25: mu(sub alpha) = 90 +/- 43 (mu)as/yr, mu(sub beta) = 7 +/- 68 (mu)as/yr, where the quoted uncertainties account for the contribution of the statistical standard deviation and the errors assumed for the parameters related to the geometry of the interferometric array, the atmosphere, and the source structure. This proper motion is consistent with earlier interpretations of VLBI hybrid mapping results, which showed an internal motion of this component with respect to other structural components. Our differential astrometry analyses show component b to be the one in motion. Our results thus further constrain models of this quasar.

MILLIARCSECOND CHANGE OF IM PEGASI RADIO POSITION IN 1 HOUR COINCIDENT WITH SHARP RISE IN FLUX DENSITY

Continuum VLBI observations at 3.6 cm of the RS CVn binary star IM Pegasi (HR 8703) for ~16 hr beginning on 1997 January 16 revealed an apparent motion of the stars radio position that coincided temporally with a large relative change in its flux density. Specifically, a rise in flux density from 18 to 46 mJy in 1.4 hr coincided with a detected position change over that interval of (Δα, Δδ)=(-0.68±0.15, 0.55±0.20) mas. The magnitude of this position change is much larger than can be explained by parallax, proper motion, and orbital motion and is about two-thirds the estimated angular diameter of the primary component of the binary.

Possible Corotation of the Milliarcsecond Radio Structure of the Close Binary HR 1099

We present the first VLBI images of the RS CVn binary star HR 1099 (=V711 Tauri, HD 22468) obtained from observations at 8.4 GHz in 1996 May and September made in support of the NASA/Stanford Gravity Probe B project. The first set of observations was made during a decay stage of a flare event. The second set of observations was made during a quiescent period. The detected emission region for the active epoch had an estimated major-axis length (FWHM) of 2:7 � 0:2 mas. This region consisted of a halo and two superimposed compact condensations that were oriented approximately east-west. The centers of the compact condensations were separated by 1:7 � 0:1 mas. Compared to the � 1.3 mas separation on the sky at this epoch of the centers of the stellar components of the binary, the observed separation of the condensations differs by only 0.4 mas, which is less than the � 0.6 mas angular radius of the larger component. During the observations, the compact western condensation rotated north-northwestward by 24 � � 4 � ,o r by 0:7 � 0:1 mas, relative to the eastern condensation. We speculate that (1) either both condensations originate from the corona of the larger stellar component or one condensation is close to the surface of each of the two stellar components of the binary and (2) the relative rotation of the two condensations is a consequence of the rotation of the binary system. We speculate further that the halo is a consequence of flare-energized electrons confined by the magnetosphere of the larger stellar component or by the combined magnetospheres of the two stellar components. Our quiescent epoch, in contrast, was characterized by a single emission region with a major axis estimated to be 1:7 � 0:1 mas (FWHM). Subject headings: binaries: close — radio continuum: stars — stars: activity — stars: imaging — stars: individual (V711 Tauri) — techniques: interferometric

VLBI for Gravity Probe B. V. Proper Motion and Parallax of the Guide Star, IM Pegasi

We present the principal astrometric results of the very long baseline interferometry (VLBI) program undertaken in support of the Gravity Probe B (GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVn binary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and 2005. We discuss the statistical assumptions behind these results and our methods for estimating the systematic errors. We find the proper motion of IM Peg in an extragalactic reference frame closely related to the International Celestial Reference Frame 2 (ICRF2) to be −20.83 ± 0.03 ± 0.09 mas yr −1 in right ascension and −27.27 ± 0.03 ± 0.09 mas yr −1 in declination. For each component, the first uncertainty is the statistical standard error and the second is the total standard error (SE) including plausible systematic errors. We also obtain a parallax of 10.37 ± 0.07 mas (distance: 96.4 ± 0.7 pc), for which there is no evidence of any significant contribution of systematic error. Our parameter estimates for the ∼25 day period orbital motion of the stellar radio emission have SEs corresponding to ∼0.10 mas on the sky in each coordinate. The total SE of our estimate of IM Peg’s proper motion is ∼30% smaller than the accuracy goal set by the GP-B project before launch: 0.14 mas yr −1 for each coordinate of IM Peg’s proper motion. Our results ensure that the uncertainty in IM Peg’s proper motion makes only a very small contribution to the uncertainty of the GP-B relativity tests.

VLBI for Gravity Probe B. III. A Limit on the Proper Motion of the "Core" of the Quasar 3C 454.3

We made very long baseline interferometry observations at 8.4 GHz between 1997 and 2005 to estimate the coordinates of the “core” component of the superluminal quasar, 3C 454.3, the ultimate reference point in the distant universe for the NASA/Stanford Gyroscope Relativity Mission, Gravity Probe B (GP-B). These coordinates are determined relative to those of the brightness peaks of two other compact extragalactic sources, B2250+194 and B2252+172, nearby on the sky, and within a celestial reference frame (CRF), defined by a large suite of compact extragalactic radio sources, and nearly identical to the International Celestial Reference Frame 2 (ICRF2). We find that B2250+194 and B2252+172 are stationary relative to each other, and also in the CRF, to within 1σ upper limits of 15 and 30 μas yr −1 in α and δ, respectively. The core of 3C 454.3 appears to jitter in its position along the jet direction over ∼0.2 mas, likely due to activity close to the putative supermassive black hole nearby, but on average is stationary in the CRF within 1σ upper limits on its proper motion of 39 μas yr −1 (1.0c) and 30 μas yr −1 (0.8c) in α and δ, respectively, for the period 2002–2005. Our corresponding limit over the longer interval, 1998–2005, of more importance to GP-B, is 46 and 56 μas yr −1 in α and δ, respectively. Some of 3C 454.3’s jet components show significantly superluminal motion with speeds of up to ∼200 μas yr −1 or 5c in the CRF. The core of 3C 454.3 thus

VLBI for Gravity Probe B. IV. A New Astrometric Analysis Technique and a Comparison with Results from Other Techniques

When very long baseline interferometry (VLBI) observations are used to determine the position or motion of a radio source relative to reference sources nearby on the sky, the astrometric information is usually obtained via (1) phase-referenced maps or (2) parametric model fits to measured fringe phases or multiband delays. In this paper, we describe a “merged” analysis technique which combines some of the most important advantages of these other two approaches. In particular, our merged technique combines the superior model-correction capabilities of parametric model fits with the ability of phase-referenced maps to yield astrometric measurements of sources that are too weak to be used in parametric modelfits. We compare the results from this merged technique with the results from phase-referenced maps and from parametric model fits in the analysis of astrometric VLBI observations of the radio-bright star IM Pegasi (HR 8703) and the radio source B2252+172 nearby on the sky. In these studies we use central-core components of radio sources 3C 454.3 and B2250+194 as our positional references. We obtain astrometric results for IM Peg with our merged technique even when the source is too weak to be used in parametric model fits, and we find that our merged technique yields astrometric results superior to the phase-referenced mapping technique. We used our merged technique to estimate the proper motion and other astrometric parameters of IM Peg in support of the NASA/Stanford Gravity Probe B mission.

VLBI FOR GRAVITY PROBE B. II. MONITORING OF THE STRUCTURE OF THE REFERENCE SOURCES 3C 454.3, B2250+194, AND B2252+172

We used 8.4 GHz very long baseline interferometry images obtained at up to 35 epochs between 1997 and 2005 to examine the radio structures of the main reference source, 3C 454.3, and two secondary reference sources, B2250+194 and B2252+172, for the guide star for the NASA/Stanford relativity mission Gravity Probe B (GP-B). For one epoch in 2004 May, we also obtained images at 5.0 and 15.4 GHz. The 35 8.4 GHz images for quasar 3C 454.3 confirm a complex, evolving, core-jet structure. We identified at each epoch a component, C1, near the easternmost edge of the core region. Simulations of the core region showed that C1 is located, on average, 0.18 ± 0.06 mas west of the unresolved “core” identified in 43 GHz images. We also identified in 3C 454.3 at 8.4 GHz several additional components that moved away from C1 with proper motions ranging in magnitude between 0.9c and 5c. The detailed motions of the components exhibit two distinct bends in the jet axis located ∼3 and ∼5.5 mas west of C1. The spectra between 5.0 and 15.4 GHz for the “moving” components are steeper than those for C1. The 8.4 GHz images of B2250+194 and B2252+172, in contrast to those of 3C 454.3, reveal compact structures. The spectrum between 5.0 and 15.4 GHz for B2250+194 is inverted while that for B2252+172 is flat. Based on its position near the easternmost edge of the 8.4 GHz radio structure, close spatial association with the 43 GHz core, and relatively flat spectrum, we believe 3C 454.3 component C1 to be the best choice for the ultimate reference point for the GP-B guide star. The compact structures and inverted-to-flat spectra of B2250+194 and B2252+172 make these objects valuable secondary reference sources.

VLBI for Gravity Probe B. VI. The Orbit of IM Pegasi and the Location of the Source of Radio Emission

We present a physical interpretation for the locations of the sources of radio emission in IM Pegasi (IM Peg, HR 8703), the guide star for the NASA/Stanford relativity mission Gravity Probe B. This emission is seen in each of our 35 epochs of 8.4 GHz very long baseline interferometry observations taken from 1997 to 2005. We found that the mean position of the radio emission is at or near the projected center of the primary to within about 27% of its radius, identifying this active star as the radio emitter. The positions of the radio brightness peaks are scattered across the disk of the primary and slightly beyond, preferentially along an axis with position angle, P.A. =− 38 ◦ ± 8 ◦ , which is closely aligned with the sky projections of the orbit normal (P.A. =− 49. 5 ± 8. ◦ 6) and the expected spin axis of the primary. Comparison with simulations suggests that brightness peaks are 3.6 +0.4 −0.7 times more likely to occur (per unit surface area) near the pole regions of the primary (latitude, |λ| 70 ◦ ) than near the equator (|λ| 20 ◦ ), and to also occur close to the surface with ∼2/3 of them at altitudes not higher than 25% of the radius of the primary.