arXiv subjects

M. F. Bietenholz

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

At least 37 records · Page 2Linked to original sources

arXiv2012

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 VLBI 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 which 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 that 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

astro-ph.IM↗

arXiv2012

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

We made VLBI 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. 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 1-sigma upper limits of 15 and 30 micro-arcsec/yr in RA and decl., 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 1-sigma upper limits on its proper motion of 39 micro-arcsec/yr (1.0c) and 30 micro-arcsec/yr (0.8c) in RA and decl., 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 micro-arcsec/yr in RA and decl., respectively. Some of 3C 454.3's jet components show significantly superluminal motion with speeds of up to ~200 micro-arcsec/yr or 5c in the CRF. The core of 3C 454.3 thus provides for Gravity Probe B a sufficiently stable reference in the distant universe.

astro-ph.IM↗

arXiv2012

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

When 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: (i) phase-referenced maps; or (ii) 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 model fits. 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 superior astrometric results 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.

astro-ph.IM↗

arXiv2012

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 in right ascension and -27.27 +- 0.03 +- 0.09 mas/yr 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 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.

astro-ph.IM↗

arXiv2012

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 VLBI 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) deg, which is closely aligned with the sky projections of the orbit normal (p.a. = -49.5 +- 8.6 deg) 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| >= 70 deg) than near the equator (|latitude| <= 20 deg), 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.

astro-ph.IM↗

arXiv2012

VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi

We present measurements of the total radio flux density as well as very-long-baseline interferometry (VLBI) images of the star, IM Pegasi, which was used as the guide star for the NASA/Stanford relativity mission Gravity Probe B. We obtained flux densities and images from 35 sessions of observations at 8.4 GHz (wavelength = 3.6 cm) between 1997 January and 2005 July. The observations were accurately phase-referenced to several extragalactic reference sources, and we present the images in a star-centered frame, aligned by the position of the star as derived from our fits to its orbital motion, parallax, and proper motion. Both the flux density and the morphology of IM Peg are variable. For most sessions, the emission region has a single-peaked structure, but 25% of the time, we observed a two-peaked (and on one occasion perhaps a three-peaked) structure. On average, the emission region is elongated by 1.4 +- 0.4 mas (FWHM), with the average direction of elongation being close to that of the sky projection of the orbit normal. The average length of the emission region is approximately equal to the diameter of the primary star. No significant correlation with the orbital phase is found for either the flux density or the direction of elongation, and no preference for any particular longitude on the star is shown by the emission region.

astro-ph.IM↗

arXiv2012

VLBI Observations of the Nearby Type IIb Supernova 2011dh

We report on phase-referenced VLBI radio observations of the Type IIb supernova 2011dh, at times t = 83 days and 179 days after the explosion and at frequencies, respectively, of 22.2 and 8.4 GHz. We detected SN 2011dh at both epochs. At the first epoch only an upper limit on SN 2011dh's angular size was obtained, but at the second epoch, we determine the angular radius SN 2011dh's radio emission to be 0.25 +- 0.08 mas by fitting a spherical shell model directly to the visibility measurements. At a distance of 8.4 Mpc this angular radius corresponds to a time-averaged (since t=0) expansion velocity of the forward shock of 21000 +- 7000 km/s. Our measured values of the radius of the emission region are in excellent agreement with those derived from fitting synchrotron self-absorbed models to the radio spectral energy distribution, providing strong confirmation for the latter method of estimating the radius. We find that SN 2011dh's radius evolves in a power-law fashion, with R proportional to t^(0.92 +- 0.10).

astro-ph.HE↗

arXiv2012

EVLA Observations of the Radio Evolution of SN 2011dh

We report on Expanded Very Large Array (EVLA) observations of the Type IIb supernova 2011dh, performed over the first 100 days of its evolution and spanning 1-40 GHz in frequency. The radio emission is well-described by the self-similar propagation of a spherical shockwave, generated as the supernova ejecta interact with the local circumstellar environment. Modeling this emission with a standard synchrotron self-absorption (SSA) model gives an average expansion velocity of v \approx 0.1c, supporting the classification of the progenitor as a compact star (R_* \approx 10^11 cm). We find that the circumstellar density is consistent with a ρ \propto r^-2 profile. We determine that the progenitor shed mass at a constant rate of \approx 3 \times 10^-5 M_\odot / yr, assuming a wind velocity of 1000 km / s (values appropriate for a Wolf-Rayet star), or \approx 7 \times 10^-7 M_\odot / yr assuming 20 km / s (appropriate for a yellow supergiant [YSG] star). Both values of the mass-loss rate assume a converted fraction of kinetic to magnetic energy density of ε_B = 0.1. Although optical imaging shows the presence of a YSG, the rapid optical evolution and fast expansion argue that the progenitor is a more compact star - perhaps a companion to the YSG. Furthermore, the excellent agreement of the radio properties of SN 2011dh with the SSA model implies that any YSG companion is likely in a wide, non-interacting orbit.

astro-ph.HE↗

arXiv2011

Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. I. Jet Energetics and the Pristine Parsec-Scale Environment of a Supermassive Black Hole

We present continued radio observations of the tidal disruption event SwiftJ164449.3+573451 extending to \sim216 days after discovery. The data are part of a long-term program to monitor the expansion and energy scale of the relativistic outflow, and to trace the parsec-scale environment around a previously-dormant supermassive black hole (SMBH). The new observations reveal a significant change in the radio evolution starting at \sim1 month, with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude, and an overall density profile around the SMBH of rho \propto r^{-3/2} (0.1-1.2 pc) with a significant flattening at r\sim0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an L \propto t^{-5/3} tail, which is observed in the X-rays. Instead, we conclude that the relativistic jet was launched with a wide range of Lorentz factors, obeying E(>Gamma) \propto Gamma^{-2.5}. The similar ratio of duration to dynamical timescale for Sw1644+57 and GRBs suggests that this result may be applicable to GRBs as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at r\sim0.5 pc in good agreement with the Bondi radius for a \sim10^6 M_sun black hole. The density at \sim0.5 pc is about a factor of 30 times lower than inferred for the Milky Way galactic center, potentially due to a smaller number of mass-shedding massive stars. From our latest observations (\sim216 d) we find that the jet energy is E_{iso}\sim5x10^{53} erg (E_j\sim2.4x10^{51} erg for theta_j=0.1), the radius is r\sim1.2 pc, the Lorentz factor is Gamma\sim2.2, the ambient density is n\sim0.2 cm^{-3}, and the projected size is r_{proj}\sim25 microarcsec. Assuming no future changes in the observed evolution we predict that the radio emission from Sw1644+57 should be detectable with the EVLA for several decades, and will be resolvable with VLBI in a few years.

astro-ph.HE↗

arXiv2011

The Birth of a Relativistic Outflow in the Unusual γ-ray Transient Swift J164449.3+573451

Active galactic nuclei (AGN), powered by long-term accretion onto central supermassive black holes, produce relativistic jets with lifetimes of greater than one million yr that preclude observations at birth. Transient accretion onto a supermassive black hole, for example through the tidal disruption of a stray star, may therefore offer a unique opportunity to observe and study the birth of a relativistic jet. On 2011 March 25, the Swift γ-ray satellite discovered an unusual transient source (Swift J164449.3+573451) potentially representing such an event. Here we present the discovery of a luminous radio transient associated with Swift J164449.3+573451, and an extensive set of observations spanning centimeter to millimeter wavelengths and covering the first month of evolution. These observations lead to a positional coincidence with the nucleus of an inactive galaxy, and provide direct evidence for a newly-formed relativistic outflow, launched by transient accretion onto a million solar mass black hole. While a relativistic outflow was not predicted in this scenario, we show that the tidal disruption of a star naturally explains the high-energy properties, radio luminosity, and the inferred rate of such events. The weaker beaming in the radio compared to γ-rays/X-rays, suggests that radio searches may uncover similar events out to redshifts of z ~ 6.

astro-ph.HE↗

arXiv2011

Characterisation of Long Baseline Calibrators at 2.3 GHz

We present a detailed multi-epoch analysis of 31 potential southern hemisphere radio calibrators that were originally observed as part of a program to maintain the International Celestial Reference Frame (ICRF). At radio wavelengths, the primary calibrators are Active Galactic Nuclei (AGN), powerful radio emitters which exist at the centre of most galaxies. These are known to vary at all wavelengths at which they have been observed. By determining the amount of radio source structure and variability of these AGN, we determine their suitability as phase calibrators for long baseline radio interferometry at 2.3 GHz. For this purpose, we have used a set of complementary metrics to classify these 31 southern sources into five categories pertaining to their suitability as VLBI calibrators. We find that all of the sources in our sample would be good interferometric calibrators and almost ninety per cent would be very good calibrators.

astro-ph.CO↗

arXiv2011

The High-Metallicity Explosion Environment of the Relativistic Supernova 2009bb

We investigate the environment of the nearby (d ~ 40Mpc) broad-lined Type Ic supernova SN 2009bb. This event was observed to produce a relativistic outflow likely powered by a central accreting compact object. While such a phenomenon was previously observed only in long-duration gamma-ray bursts (LGRBs), no LGRB was detected in association with SN 2009bb. Using an optical spectrum of the SN 2009bb explosion site, we determine a variety of ISM properties for the host environment, including metallicity, young stellar population age, and star formation rate. We compare the SN explosion site properties to observations of LGRB and broad-lined SN Ic host environments on optical emission line ratio diagnostic diagrams. Based on these analyses, we find that the SN 2009bb explosion site has a very high metallicity of ~2x solar, in agreement with other broad-lined SN Ic host environments and at odds with the low-redshift LGRB host environments and recently proposed maximum metallicity limits for relativistic explosions. We consider the implications of these findings and the impact that SN 2009bb's unusual explosive properties and environment have on our understanding of the key physical ingredient that enables some SNe to produce a relativistic outflow.

astro-ph.HE↗

arXiv2011

Radio Imaging of SN 1993J: The Story Continues

We present the most recent VLBI images of SN 1993J, taken at 1.7 GHz on 2010 March 5-6, along with a discussion of its evolution with time. The new image is the latest in a sequence covering almost the entire lifetime of the supernova. For these latest observations we used an "in beam calibrator" technique, and obtained a background rms brightness of 3.7 micro-Jy/beam. The supernova shell remains quite circular in outline. Modulations in brightness are seen around the rim which evolve relatively slowly, having remained generally similar over the last several years of observation. We determine the outer radius of the supernova using visibility-plane model-fitting. The supernova has slowed down to around 30% of its original expansion velocity, and continues to expand with radius approximately proportional to t^0.8, however, deviations from a strict power-law evolution are seen. We do not find any clear-cut evidence for systematically frequency-dependent evolution, suggesting that the radii as determined from visibility-plane model-fitting continue to provide reasonable estimates of the physical outer shock-front radius.

astro-ph.CO↗

arXiv2010

VLBI Observations of SN 2009bb

We report on VLBI, as well as VLA radio observations of the Type Ib/c supernova 2009bb. The high radio luminosity of this supernova seems to require relativistic outflow, implying that the early radio emission was "engine-driven", that is driven by collimated outflow from a compact object, even though no gamma-ray emission was seen. The radio light curve shows a general decline, with a "bump" near t = 52 d, seen most prominently at 5 GHz. The lightcurve bump could be either engine-driven, or it might represent the turn-on of the normal radio emission from a supernova, driven by interaction with the CSM rather than by the engine. We undertook VLBI observations to resolve SN 2009bb's relativistic outflow. Our observations constrain the angular outer radius at an age of 85 d to be <0.64 mas, corresponding to <4 x 10^17 cm and an average apparent expansion speed of <1.74c. This result is consistent with the moderately relativistic ejecta speeds implied by the radio luminosity and spectrum.

astro-ph.HE↗

arXiv2010

The First VLBI Image of the Young, Oxygen-Rich Supernova Remnant in NGC 4449

We report on sensitive 1.4-GHz VLBI radio observations of the unusually luminous supernova remnant SNR 4449-1 in the galaxy NGC 4449, which gave us the first well-resolved image of this object. The remnant's radio morphology consists of two approximately parallel bright ridges, suggesting similarities to the barrel shape seen for many older Galactic supernova remnants or possibly to SN 1987A. The angular extent of the remnant is 65 x 40 mas, corresponding to (3.7 x 2.3) x 10^{18} (D/3.8 Mpc) cm. We also present a new, high signal-to-noise optical spectrum. By comparing the remnant's linear size to the maximum velocities measured from optical lines, as well as using constraints from historical images, we conclude that the supernova explosion occurred between ~1905 and 1961, likely around 1940. The age of the remnant is therefore likely ~70 yr. We find that SNR 4449-1's shock wave is likely still interacting with the circumstellar rather than interstellar medium.

astro-ph.HE↗

arXiv2010

SN 1986J VLBI: II. The Evolution of the Shell and the Central Source

We present new VLBI images of supernova 1986J, taken at 5, 8.4 and 22 GHz between t=22 to 25 yr after the explosion. The shell expands as t^(0.69+-0.03). We estimate the progenitor's mass-loss rate at (4 ~ 10) * 10^-5 Msol/yr (for v_w = 10 km/s). Two bright spots are seen in the images. The first, in the northeast, is now fading. The second, very near the center of the projected shell and unique to SN1986J, is still brightening relative to the shell, and now dominates the VLBI images. It is marginally resolved at 22 GHz (diameter ~0.3 mas; ~5 * 10^16 cm at 10 Mpc). The integrated VLA spectrum of SN1986J shows an inversion point and a high-frequency turnover, both progressing downward in frequency and due to the central bright spot. The optically-thin spectral index of the central bright spot is indistinguishable from that of the shell. The small proper motion of 1500+-1500 km/s of the central bright spot is consistent with our previous interpretation of it as being associated with the expected black-hole or neutron-star remnant. Now, an alternate scenario seems also plausible, where the central bright spot, like the northeast one, results when the shock front impacts on a condensation within the circumstellar medium (CSM). The condensation would have to be so dense as to be opaque at cm wavelengths (~1000x denser than the average corresponding CSM) and fortuitously close to the center of the projected shell. We include a movie of the evolution of SN1986J at 5 GHz from t=0 to 25 yr.

astro-ph.HE↗

arXiv2010

A Relativistic Type Ibc Supernova Without a Detected Gamma-ray Burst

Long duration gamma-ray bursts (GRBs) mark the explosive death of some massive stars and are a rare sub-class of Type Ibc supernovae (SNe Ibc). They are distinguished by the production of an energetic and collimated relativistic outflow powered by a central engine (an accreting black hole or neutron star). Observationally, this outflow is manifested in the pulse of gamma-rays and a long-lived radio afterglow. To date, central engine-driven SNe have been discovered exclusively through their gamma-ray emission, yet it is expected that a larger population goes undetected due to limited satellite sensitivity or beaming of the collimated emission away from our line-of-sight. In this framework, the recovery of undetected GRBs may be possible through radio searches for SNe Ibc with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary Type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. The lack of a coincident GRB makes SN 2009bb the first engine-driven SN discovered without a detected gamma-ray signal. A comparison with our extensive radio survey of SNe Ibc reveals that the fraction harboring central engines is low, ~1 percent, measured independently from, but consistent with, the inferred rate of nearby GRBs. Our study demonstrates that upcoming optical and radio surveys will soon rival gamma-ray satellites in pinpointing the nearest engine-driven SNe. A similar result for a different supernova is reported independently.

astro-ph.HE↗

arXiv2009

VLBI Observations of SN 2008D

We report on two epochs of very-long-baseline interferometry (VLBI) observations of the Type Ib/c supernova SN 2008D, which was associated with the X-ray outburst XRF 080109. At our first epoch, at t = 30 days after the explosion, we observed at 22 and 8.4 GHz, and at our second, at t = 133 days, at 8.4 and 5.0 GHz. The VLBI observations allow us to accurately measure the source's size and position at each epoch, and thus constrain its expansion velocity and proper motion. We find the source at best marginally resolved at both epochs, allowing us to place a 3sigma upper limit of ~0.75c on the expansion velocity of a circular source. For an elongated source, our measurements are compatible with mildly relativistic expansion. However, our 3sigma upper limit on the proper motion is 4 micro-arcsec/day, corresponding to an apparent velocity of <0.6c, and is consistent with a stationary flux centroid. This limit rules out a relativistic jet such as an gamma-ray burst jet away from the line of sight, which would be expected to show apparent proper motion of >c. Taken together, our measurements argue against the presence of any long-lived relativistic outflow in SN 2008D. On the other hand, our measurements are consistent with the nonrelativistic expansion velocities of <30,000 km/s and small proper motions (<500 km/s) seen in typical supernovae.

astro-ph↗