arXiv subjects

M. F. Bietenholz

Publications and source records attributed to M. F. Bietenholz.

At least 19 recordsLinked to original sources

arXiv2023

Gravitational redshift test of EEP with RA from near Earth to the distance of the Moon

The Einstein Equivalence Principle (EEP) is a cornerstone of general relativity and predicts the existence of gravitational redshift. We report on new results of measuring this shift with RadioAstron (RA), a space VLBI spacecraft launched into an evolving high eccentricity orbit around Earth with geocentric distances reaching 353,000 km. The spacecraft and ground tracking stations at Pushchino, Russia, and Green Bank, USA, were each equipped with a hydrogen maser frequency standard allowing a possible violation of the predicted gravitational redshift, in the form of a violation parameter $\varepsilon$, to be measured. By alternating between RadioAstron's frequency referencing modes during dedicated sessions between 2015 and 2017, the recorded downlink frequencies can essentially be corrected for the non-relativistic Doppler shift. We report on an analysis using the Doppler-tracking frequency measurements made during these sessions and find $\varepsilon = (2.1 \pm 3.3)\times10^{-4}$. We also discuss prospects for measuring $\varepsilon$ with a significantly smaller uncertainty using instead the time-domain recordings of the spacecraft signals and envision how $10^{-7}$ might be possible for a future space VLBI mission.

gr-qc↗

arXiv2023

A radio-emitting outflow produced by the tidal disruption event AT2020vwl

A tidal disruption event (TDE) occurs when a star is destroyed by a supermassive black hole. Broadband radio spectral observations of TDEs trace the emission from any outflows or jets that are ejected from the vicinity of the supermassive black hole. However, radio detections of TDEs are rare, with less than 20 published to date, and only 11 with multi-epoch broadband coverage. Here we present the radio detection of the TDE AT2020vwl and our subsequent radio monitoring campaign of the outflow that was produced, spanning 1.5 years post-optical flare. We tracked the outflow evolution as it expanded between $10^{16}$ cm to $10^{17}$ cm from the supermassive black hole, deducing it was non-relativistic and launched quasi-simultaneously with the initial optical detection through modelling the evolving synchrotron spectra of the event. We deduce that the outflow is likely to have been launched by material ejected from stream-stream collisions (more likely), the unbound debris stream, or an accretion-induced wind or jet from the supermassive black hole (less likely). AT2020vwl joins a growing number of TDEs with well-characterised prompt radio emission, with future timely radio observations of TDEs required to fully understand the mechanism that produces this type of radio emission in TDEs.

astro-ph.HE↗

arXiv2022

AT2019azh: an unusually long-lived, radio-bright thermal tidal disruption event

Tidal disruption events (TDEs) occur when a star is destroyed by a supermassive black hole at the center of a galaxy, temporarily increasing the accretion rate onto the black hole and producing a bright flare across the electromagnetic spectrum. Radio observations of TDEs trace outflows and jets that may be produced. Radio detections of the outflows from TDEs are uncommon, with only about one third of TDEs discovered to date having published radio detections. Here we present over two years of comprehensive, multi-radio frequency monitoring observations of the tidal disruption event AT2019azh taken with the Very Large Array (VLA) and MeerKAT radio telescopes from approximately 10 days pre-optical peak to 810 days post-optical peak. AT2019azh shows unusual radio emission for a thermal TDE, as it brightened very slowly over two years, and showed fluctuations in the synchrotron energy index of the optically thin synchrotron emission from 450 days post-disruption. Based on the radio properties, we deduce that the outflow in this event is likely non-relativistic and could be explained by a spherical outflow arising from self-stream intersections, or a mildly collimated outflow from accretion onto the supermassive black hole. This data-set provides a significant contribution to the observational database of outflows from TDEs, including the earliest radio detection of a non-relativistic TDE to date, relative to the optical discovery.

astro-ph.HE↗

arXiv2021

Luminous Late-time Radio Emission from Supernovae Detected by the Karl G. Jansky Very Large Array Sky Survey (VLASS)

We present a population of 19 radio-luminous supernovae (SNe) with emission reaching $L_ν{\sim}10^{26}-10^{29}\,\rm{erg\,s^{-1}Hz^{-1}}$ in the first epoch of the Very Large Array Sky Survey (VLASS) at $2-4$GHz. Our sample includes one long Gamma-Ray Burst, SN 2017iuk/GRB171205A, and 18 core-collapse SNe detected at $\approx (1-60)$years after explosion. No thermonuclear explosion shows evidence for bright radio emission, and hydrogen-poor progenitors dominate the sub-sample of core-collapse events with spectroscopic classification at the time of explosion (79\%). We interpret these findings into the context of the expected radio emission from the forward shock interaction with the circumstellar medium (CSM). We conclude that these observations require a departure from the single wind-like density profile (i.e., $ρ_{\rm{CSM}}\propto r^{-2}$) that is expected around massive stars and/or a departure from a spherical Newtonian shock. Viable alternatives include the shock interaction with a detached, dense shell of CSM formed by a large effective progenitor mass-loss rate $\dot M \sim (10^{-4}-10^{-1})$ M$_{\odot}$ yr$^{-1}$ (for an assumed wind velocity of $1000\,\rm{km\,s^{-1}}$); emission from an off-axis relativistic jet entering our line of sight; or the emergence of emission from a newly-born pulsar-wind nebula. The relativistic SN 2012ap that is detected 5.7 and 8.5 years after explosion with $L_ν{\sim}10^{28}$ erg s$^{-1}$ Hz$^{-1}$ might constitute the first detections of an off-axis jet+cocoon system in a massive star. However, none of the VLASS-SNe with archival data points are consistent with our model off-axis jet light curves. Future multi-wavelength observations will distinguish among these scenarios.Our VLASS source catalogs, which were used to perform the VLASS cross matching, are publicly available at https://doi.org/10.5281/zenodo.4895112.

astro-ph.HE↗

arXiv2021

Late-Time Evolution and Modeling of the Off-Axis Gamma-ray Burst Candidate FIRST J141918.9+394036

We present new radio and optical data, including very long baseline interferometry, as well as archival data analysis, for the luminous decades-long radio transient FIRST J141918.9+394036. The radio data reveal a synchrotron self-absorption peak around 0.3 GHz and a radius of around 1.3 mas (0.5 pc) 26 years post-discovery, indicating a blastwave energy $\sim5 \times 10^{50}$ erg. The optical spectrum shows a broad [OIII]$λ$4959,5007 emission-line that may indicate collisional-excitation in the host galaxy, but its association with the transient cannot be ruled out. The properties of the host galaxy are suggestive of a massive stellar progenitor that formed at low metallicity. Based on the radio light curve, blastwave velocity, energetics, nature of the host galaxy and transient rates we find that the properties of FIRST J1419+39 are most consistent with long gamma-ray burst (LGRB) afterglows. Other classes of (optically-discovered) stellar explosions as well as neutron star mergers are disfavored, and invoking any exotic scenario may not be necessary. It is therefore likely that FIRST J1419+39 is an off-axis LGRB afterglow (as suggested by Law et al. and Marcote et al.), and under this premise the inverse beaming fraction is found to be $f_b^{-1}\simeq280^{+700}_{-200}$, corresponding to an average jet half-opening angle $<θ_j>\simeq5^{+4}_{-2}$ degrees (68% confidence), consistent with previous estimates. From the volumetric rate we predict that surveys with the VLA, ASKAP and MeerKAT will find a handful of FIRST J1419+39-like events over the coming years.

astro-ph.HE↗

arXiv2021

The MeerKAT Galaxy Cluster Legacy Survey I. Survey Overview and Highlights

MeerKAT's large number of antennas, spanning 8 km with a densely packed 1 km core, create a powerful instrument for wide-area surveys, with high sensitivity over a wide range of angular scales. The MeerKAT Galaxy Cluster Legacy Survey (MGCLS) is a programme of long-track MeerKAT L-band (900-1670 MHz) observations of 115 galaxy clusters, observed for $\sim$6-10 hours each in full polarisation. The first legacy product data release (DR1), made available with this paper, includes the MeerKAT visibilities, basic image cubes at $\sim$8" resolution, and enhanced spectral and polarisation image cubes at $\sim$8" and 15" resolutions. Typical sensitivities for the full-resolution MGCLS image products are $\sim$3-5 μJy/beam. The basic cubes are full-field and span 4 deg^2. The enhanced products consist of the inner 1.44 deg^2 field of view, corrected for the primary beam. The survey is fully sensitive to structures up to $\sim$10' scales and the wide bandwidth allows spectral and Faraday rotation mapping. HI mapping at 209 kHz resolution can be done at $0<z<0.09$ and $0.19<z<0.48$. In this paper, we provide an overview of the survey and DR1 products, including caveats for usage. We present some initial results from the survey, both for their intrinsic scientific value and to highlight the capabilities for further exploration with these data. These include a primary beam-corrected compact source catalogue of $\sim$626,000 sources for the full survey, and an optical/infrared cross-matched catalogue for compact sources in Abell 209 and Abell S295. We examine dust unbiased star-formation rates as a function of clustercentric radius in Abell 209 and present a catalogue of 99 diffuse cluster sources (56 are new), some of which have no suitable characterisation. We also highlight some of the radio galaxies which challenge current paradigms and present first results from HI studies of four targets.

astro-ph.GA↗

arXiv2020

Radio Afterglows of Very High Energy Gamma-Ray Bursts 190829A and 180720B

We present high cadence multi-frequency radio observations of the long Gamma-Ray Burst (GRB) 190829A, which was detected at photon energies above 100 GeV by the High Energy Stereoscopic System (H.E.S.S.). Observations with the Meer Karoo Array Telescope (MeerKAT, 1.3 GHz), and Arcminute Microkelvin Imager - Large Array (AMI-LA, 15.5 GHz) began one day post-burst and lasted nearly 200 days. We used complementary data from Swift X-Ray Telescope (XRT), which ran to 100 days post-burst. We detected a likely forward shock component with both MeerKAT and XRT up to over 100 days post-burst. Conversely, the AMI-LA light curve appears to be dominated by reverse shock emission until around 70 days post-burst when the afterglow flux drops below the level of the host galaxy. We also present previously unpublished observations of the other H.E.S.S.-detected GRB, GRB 180720B from AMI-LA, which shows likely forward shock emission that fades in less than 10 days. We present a comparison between the radio emission from the three GRBs with detected very high energy (VHE) gamma-ray emission and a sensitivity-limited radio afterglow sample. GRB 190829A has the lowest isotropic radio luminosity of any GRB in our sample, but the distribution of luminosities is otherwise consistent, as expected, with the VHE GRBs being drawn from the same parent distribution as the other radio-detected long GRBs.

astro-ph.HE↗

arXiv2019

The dust content of the Crab Nebula

We have modelled the near-infrared to radio images of the Crab Nebula with a Bayesian SED model to simultaneously fit its synchrotron, interstellar and supernova dust emission. We infer an interstellar dust extinction map with an average $A_{\text{V}}$=1.08$\pm$0.38 mag, consistent with a small contribution (<22%) to the Crab's overall infrared emission. The Crab's supernova dust mass is estimated to be between 0.032 and 0.049 M$_{\odot}$ (for amorphous carbon grains) with an average dust temperature $T_{\text{dust}}$=41$\pm$3K, corresponding to a dust condensation efficiency of 8-12%. This revised dust mass is up to an order of magnitude lower than some previous estimates, which can be attributed to our different interstellar dust corrections, lower SPIRE flux densities, and higher dust temperature than were used in previous studies. The dust within the Crab is predominantly found in dense filaments south of the pulsar, with an average V-band dust extinction of $A_{\text{V}}$=0.20-0.39 mag, consistent with recent optical dust extinction studies. The modelled synchrotron power-law spectrum is consistent with a radio spectral index $α_{\text{radio}}$=0.297$\pm$0.009 and an infrared spectral index $α_{\text{IR}}$=0.429$\pm$0.021. We have identified a millimetre excess emission in the Crab's central regions, and argue that it most likely results from two distinct populations of synchrotron emitting particles. We conclude that the Crab's efficient dust condensation (8-12%) provides further evidence for a scenario where supernovae can provide substantial contributions to the interstellar dust budgets in galaxies.

astro-ph.HE↗

arXiv2019

The gravitational redshift monitored with RadioAstron from near Earth up to 350,000 km

We report on our efforts to test the Einstein Equivalence Principle by measuring the gravitational redshift with the VLBI spacecraft RadioAstron, in an eccentric orbit around Earth with geocentric distances as small as $\sim$ 7,000 km and up to 350,000 km. The spacecraft and its ground stations are each equipped with stable hydrogen maser frequency standards, and measurements of the redshifted downlink carrier frequencies were obtained at both 8.4 and 15 GHz between 2012 and 2017. Over the course of the $\sim$ 9 d orbit, the gravitational redshift between the spacecraft and the ground stations varies between $6.8 \times 10^{-10}$ and $0.6 \times 10^{-10}$. Since the clock offset between the masers is difficult to estimate independently of the gravitational redshift, only the variation of the gravitational redshift is considered for this analysis. We obtain a preliminary estimate of the fractional deviation of the gravitational redshift from prediction of $ε= -0.016 \pm 0.003_{\rm stat} \pm 0.030_{\rm syst}$ with the systematic uncertainty likely being dominated by unmodelled effects including the error in accounting for the non-relativistic Doppler shift. This result is consistent with zero within the uncertainties. For the first time, the gravitational redshift has been probed over such large distances in the vicinity of Earth. About three orders of magnitude more accurate estimates may be possible with RadioAstron using existing data from dedicated interleaved observations combining uplink and downlink modes of operation.

gr-qc↗

arXiv2018

Jets in Hydrogen-poor Super-luminous Supernovae: Constraints from a Comprehensive Analysis of Radio Observations

The energy source powering the extreme optical luminosity of hydrogen-stripped Superluminous Supernovae (SLSNe-I) is not known, but recent studies have highlighted the case for a central engine. Radio and/or X-ray observations are best placed to track the fastest ejecta and probe the presence of outflows from a central engine. We compile all the published radio observations of SLSNe-I to date and present three new observations of two new SLSNe-I. None were detected. Through modeling the radio emission, we constrain the sub-parsec environments and possible outflows in SLSNe-I. In this sample we rule out on-axis collimated relativistic jets of the kind detected in Gamma-Ray Bursts (GRBs). We constrain off-axis jets with opening angles of 5\arcdeg\ (30\arcdeg) to energies of $\rm{E_k<4\times10^{50}\,erg}$ ($\rm{E_k<10^{50}\,erg}$) in environments shaped by progenitors with mass-loss rates of $\dot{M}<10^{-4}\,M_{\odot}\,{\rm yr}^{-1}$ ($\dot{M}<10^{-5}\,M_{\odot}\,{\rm yr}^{-1}$) for all off-axis angles, assuming fiducial values $ε_e=0.1$ and $ε_B=0.01$. The deepest limits rule out emission of the kind seen in faint un-collimated GRBs (with the exception of GRB\,060218), and from relativistic supernovae. Finally, for the closest SLSN-I SN 2017egm we constrained the energy of an uncollimated non-relativistic outflow like those observed in normal SNe to $E_{\rm k}\lesssim10^{48}$ erg.

astro-ph.HE↗

arXiv2015

Imaging the Expanding Shell of SN 2011dh

We report on third epoch VLBI observations of the radio-bright supernova SN 2011dh located in the nearby (7.8 Mpc) galaxy M51. The observations took place at $t=453$ d after the explosion and at a frequency of 8.4 GHz. We obtained a fairly well resolved image of the shell of SN 2011dh, making it one of only six recent supernovae for which resolved images of the ejecta are available. SN 2011dh has a relatively clear shell morphology, being almost circular in outline, although there may be some asymmetry in brightness around the ridge. By fitting a spherical shell model directly to the visibility measurements we determine the angular radius of SN 2011dh's radio emission to be $636 \pm 29$ $μ$as. At a distance of 7.8 Mpc, this angular radius corresponds to a linear radius of $(7.4 \pm 0.3) \times 10^{16}$ cm and an average expansion velocity since the explosion of $19000^{+2800}_{-2400}$ kms$^{-1}$. We combine our VLBI measurements of SN 2011dh's radius with values determined from the radio spectral energy distribution under the assumption of a synchrotron-self-absorbed spectrum, and find all the radii are consistent with a power-law evolution, with $R \sim t^{0.97\pm0.01}$, implying almost free expansion over the period $t=4$ d to 453 d.

astro-ph.SR↗

arXiv2015

VLBI for Gravity Probe B: The Guide Star IM Pegasi

We review the radio very long baseline interferometry (VLBI) observations of the guide star, IM Peg, and three compact extragalactic reference sources, 3C 454.3, B2250+194, and B2252+172, made in support of the NASA/Stanford gyroscope relativity mission, GP-B. The main goal of the observations was the determination of the proper motion of IM Peg relative to the distant universe. VLBI observations made between 1997 and 2005 yield a proper motion of IM Peg of -20.83 $\pm$ 0.09 mas yr$^{-1}$ in RA and -27.27 $\pm$ 0.09 mas yr$^{-1}$ in dec, in a celestial reference frame of extragalactic radio galaxies and quasars virtually identical to the International Celestial Reference Frame 2 (ICRF2). They also yield a parallax for IM Peg of 10.37 $\pm$ 0.07 mas, corresponding to a distance of 96.4 $\pm$ 0.7 pc. The uncertainties are standard errors with statistical and estimated systematic contributions added in quadrature. These results met the pre-launch requirements of the GP-B mission to not discernibly degrade the estimates of the geodetic and frame-dragging effects. The paper also reports on a 1$σ$ upper limit on the magnitude of the components of the proper motion of the 'core' of 3C 454.3 relative to the ICRF2 of 46 and 56 $μ$as yr$^{-1}$ in RA and dec, respectively, and presents densely sampled ellipses of the parallax and the orbit of the giant of the binary system. It further gives a sequence of images of the flickering radio emission relative to the disk of the giant. For a 'movie of a star,' see http://www.yorku.ca/bartel/impeg.mpg .

astro-ph.SR↗

arXiv2014

An African VLBI network of radio telescopes

The advent of international wideband communication by optical fibre has produced a revolution in communications and the use of the internet. Many African countries are now connected to undersea fibre linking them to other African countries and to other continents. Previously international communication was by microwave links through geostationary satellites. These are becoming redundant in some countries as optical fibre takes over, as this provides 1000 times the bandwidth of the satellite links. In the 1970's and 1980's some two dozen large (30 m diameter class) antennas were built in various African countries to provide the satellite links. Twenty six are currently known in 19 countries. As these antennas become redundant, the possibility exists to convert them for radio astronomy at a cost of roughly one tenth that of a new antenna of similar size. HartRAO, SKA Africa and the South African Department of Science and Technology (DST) have started exploring this possibility with some of the African countries.

astro-ph.IM↗

arXiv2014

Radio limits on off-axis GRB afterglows and VLBI observations of SN 2003gk

We report on a VLA survey for late-time radio emission from 59 supernovae (SNe) of Type I b/c, which have been associated with long-duration gamma-ray bursts (GRBs). An "off-axis" GRB burst (i.e. whose relativistic jet points away from us) is expected to have late-time radio emission even in the absence of significant prompt gamma-ray emission. From our sample, we detected only SN 2003gk with an 8.4-GHz flux density of $2260 \pm 130 \,μ$Jy. Our subsequent VLBI observations of SN 2003gk, at an age of $\sim$8 yr, allowed us to determine its radius to be $(2.4 \pm 0.4) \times 10^{17}$ cm, or $94 \pm 15$ light days. This radius rules out relativistic expansion as expected for an off-axis GRB jet, and instead suggests an expansion speed of $\sim 10\:000$ km s$^{-1}$ typical for non-relativistic core-collapse supernovae. We attribute the late-onset radio emission to interaction of the ejecta with a dense shell caused by episodic mass-loss from the progenitor. In addition, we present new calculations for the expected radio lightcurves from GRB jets at various angles to the line of sight, and compare these to our observed limits on the flux densities of the remainder of our SN sample. From this comparison we can say that only a fraction of broadlined Type I b/c SNe have a radio-bright jet similar to those seen for GRB afterglows at cosmological distances. However, we also find that for a reasonable range of parameters, as might be representative of the actual population of GRB events rather than the detected bright ones, the radio emission from the GRB jets can be quite faint, and that at present, radio observations do not place strong constraints on off-axis GRB jets.

astro-ph.HE↗

arXiv2013

The Proper Motion of PSR J0205+6449 in 3C 58

We report on sensitive phase-referenced and gated 1.4-GHz VLBI radio observations of the pulsar PSR J0205+6449 in the young pulsar-wind nebula 3C 58, made in 2007 and 2010. We employed a novel technique where the ~105-m Green Bank telescope is used simultaneously to obtain single-dish data used to determine the pulsar's period as well as to obtain the VLBI data, allowing the VLBI correlation to be gated synchronously with the pulse to increase the signal-to-noise. The high timing noise of this young pulsar precludes the determination of the proper motion from the pulsar timing. We derive the position of the pulsar accurate at the milliarcsecond level, which is consistent with a re-determined position from the Chandra X-ray observations. We reject the original tentative optical identification of the pulsar by Shearer and Neustroev (2008), but rather identify a different optical counterpart on their images, with R-band magnitude ~24. We also determine an accurate proper motion for PSR J0205+6449 of (2.3 +- 0.3) mas/yr, corresponding to a projected velocity of only (35 +- 6) km/s for a distance of 3.2 kpc, at p.a. -38 deg. This projected velocity is quite low compared to the velocity dispersion of known pulsars of ~200 km/s. Our measured proper motion does not suggest any particular kinematic age for the pulsar.

astro-ph.GA↗

arXiv2012

Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. II. The Relativistic Jet Shuts Off and a Transition to Forward Shock X-ray/Radio Emission

We present continued multi-frequency radio observations of the relativistic tidal disruption event Sw1644+57 extending to dt~600 d. The data were obtained with the JVLA and AMI Large Array. We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at ~500 d, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of dt/t<0.2. The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at <500 d, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at ~610 d is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of 30-60 eV and inner radius of 2-10 R_s cannot accommodate the observed flux level or the detected emission at >1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below Mdot_Edd~0.006 Msun/yr (for a 3x10^6 Msun black hole and order unity efficiency) indicating that the peak accretion rate was about 330 Mdot_Edd, and the total accreted mass by 500 d is about 0.15 Msun. From the radio data we further find significant flattening in the integrated energy of the forward shock at >250 d with E_j,iso~2x10^54 erg (E_j~10^52$ erg for a jet opening angle, theta_j=0.1) following a rise by about a factor of 15 at 30-250 d. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.

astro-ph.HE↗

arXiv2012

Panchromatic Observations of SN 2011dh Point to a Compact Progenitor Star

We report the discovery and detailed monitoring of X-ray emission associated with the Type IIb SN 2011dh using data from the Swift and Chandra satellites, placing it among the best studied X-ray supernovae to date. We further present millimeter and radio data obtained with the SMA, CARMA, and EVLA during the first three weeks after explosion. Combining these observations with early optical photometry, we show that the panchromatic dataset is well-described by non-thermal synchrotron emission (radio/mm) with inverse Compton scattering (X-ray) of a thermal population of optical photons. In this scenario, the shock partition fractions deviate from equipartition by a factor, (e_e/e_B) ~ 30. We derive the properties of the shockwave and the circumstellar environment and find a shock velocity, v~0.1c, and a progenitor mass loss rate of ~6e-5 M_sun/yr. These properties are consistent with the sub-class of Type IIb SNe characterized by compact progenitors (Type cIIb) and dissimilar from those with extended progenitors (Type eIIb). Furthermore, we consider the early optical emission in the context of a cooling envelope model to estimate a progenitor radius of ~1e+11 cm, in line with the expectations for a Type cIIb SN. Together, these diagnostics are difficult to reconcile with the extended radius of the putative yellow supergiant progenitor star identified in archival HST observations, unless the stellar density profile is unusual. Finally, we searched for the high energy shock breakout pulse using X-ray and gamma-ray observations obtained during the purported explosion date range. Based on the compact radius of the progenitor, we estimate that the breakout pulse was detectable with current instruments but likely missed due to their limited temporal/spatial coverage. [Abridged]

astro-ph.HE↗

arXiv2012

VLBI for Gravity Probe B. I. Overview

We describe the NASA/Stanford gyroscope relativity mission, Gravity Probe B (GP-B), and provide an overview of the following series of six astrometric and astrophysical papers that report on our radio observations and analyses made in support of this mission. The main goal of this 8.5 year program of differential VLBI astrometry was to determine the proper motion of the guide star of the GP-B mission, the RS CVn binary IM Pegasi (IM Peg; HR 8703). This proper motion is determined with respect to compact, extragalactic reference sources. The results are: -20.833 +- 0.090 mas/yr and -27.267 +- 0.095 mas/yr for, respectively, the right ascension and declination, in local Cartesian coordinates, of IM Peg's proper motion, and 10.370 +- 0.074 mas (i.e., 96.43 +- 0.69 pc) for its parallax (and distance). Each quoted uncertainty is meant to represent an ~70% confidence interval that includes the estimated contribution from systematic error. These results are accurate enough not to discernibly degrade the GP-B estimates of its gyroscopes' relativistic precessions: the frame-dragging and geodetic effects.

astro-ph.IM↗