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ABSTRACT: We have studied the statistical and polarization properties of giant pulses
(GPs) emitted by the millisecond pulsar B1937+21, with high sensitivity and
time resolution. The observations were made in June 2005 with the 100-m Robert
C. Byrd Green Bank Telescope at S-band (2052-2116 MHz) using the Mk5A VLBI
recording system, with formal time resolution of 16 ns. The total observing
time was about 4.5 hours; the rate of detection of GPs was about 130 per hour
at the average longitudes of the main pulse (MPGPs) and 60 per hour at the
interpulse (IPGPs). While the average profile shows well-defined polarization
behavior, with regular evolution of the linear polarization position angle
(PA), GPs exhibit random properties, occasionally having high linear or
circular polarization. Neither MPGPs nor IPGPs show a preferred PA. The
cumulative probability distribution (CPD) of GP pulse energy was constructed
down to the level where GPs merge with regular pulses and noise. For both MPGPs
and IPGPs, the CPD follows a power law with a break, the power index changing
from -2.4 at high energy to -1.6 for low energy. Pulse smearing due to
scattering masks the intrinsic shape and duration of the detected GPs. The
smearing time varied during the observing session within a range of a few
hundred nanoseconds. The measured polarization and statistical properties of
GPs impose strong constraints on physical models of GPs. Some of these
properties support a model in which GPs are generated by the electric discharge
caused by magnetic reconnection of field lines connecting the opposite magnetic
poles of a neutron star.
01/2013;
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D. Milisavljevic,
R. Margutti,
A. M. Soderberg,
G. Pignata,
L. Chomiuk,
R. Fesen,
F. Bufano,
N. E. Sanders,
J. T. Parrent,
S. Parker, [......],
S. Chakraborti,
E. M. Levesque,
A. MacFadyen,
C. Drescher,
G. Bock,
P. Marples,
J. P. Anderson,
S. Benetti,
D. Reichart,
K. Ivarsen
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ABSTRACT: We present X-ray, UV/optical, and radio observations of the
stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least
luminous SNe IIb or Ib observed to date. Our observations begin with a
discovery within 1 day of explosion and span several months afterward. Early
optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that
subside rapidly and are replaced by Type Ib-like He-rich features on the
timescale of one week. High-cadence monitoring of this transition suggests that
absorption attributable to a high velocity (> 12,000 km/s) H-rich shell is not
rare in Type Ib events. Radio observations imply a shock velocity of v = 0.13c
and a progenitor star mass-loss rate of 1.4 x 10^{-5} Msun yr^{-1} (assuming
wind velocity v_w=10^3 km/s). This is consistent with independent constraints
from deep X-ray observations with Swift-XRT and Chandra. Overall, the
multi-wavelength properties of SN 2011ei are consistent with the explosion of a
lower-mass (3-4 Msun), compact (R* <= 1x10^{11} cm), He core star. The star
retained a thin hydrogen envelope at the time of explosion, and was embedded in
an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. We
conclude that SN 2011ei's rapid spectral metamorphosis is indicative of
time-dependent classifications that bias estimates of explosion rates for Type
IIb and Ib objects, and that important information about a progenitor star's
evolutionary state and mass-loss immediately prior to SN explosion can be
inferred from timely multi-wavelength observations.
07/2012;
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ABSTRACT: 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.
04/2012;
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ABSTRACT: 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
04/2012;
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ABSTRACT: 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.
04/2012;
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ABSTRACT: 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.
04/2012;
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ABSTRACT: 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.
04/2012;
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[show abstract]
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ABSTRACT: 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.
04/2012;
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[show abstract]
[hide abstract]
ABSTRACT: 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.
04/2012;
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ABSTRACT: 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).
01/2012;
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ABSTRACT: We report on sensitive new 1.4-GHz Very Large Array radio observations of the pulsar wind nebula G21.5−0.9, powered by PSR J1833−1034, and its environs. Our observations were targeted at searching for the radio counterpart of the shell-like structure seen surrounding the pulsar wind nebula in X-rays. Some such radio emission might be expected as the ejecta from the ≲1000 yr old supernova expand and interact with the surrounding medium. We find, however, no radio emission from the shell, and can place a conservative 3σ upper limit on its 1-GHz surface brightness of 7 × 10−22 W m−2 Hz−1 sr−1, comparable to the lowest limits obtained for radio emission from shells around other pulsar wind nebulae. In addition, our wide-field radio image also shows the presence of two extended objects of low surface brightness. We re-examine previous 327-MHz images, on which both the new objects are visible. We identify the first, G21.64−0.84, as a new shell-type supernova remnant, with a diameter of ∼13 arcmin and an unusual double-shell structure. The second, G21.45−0.59, ∼1 arcmin in diameter, is likely an H ii region.
Monthly Notices of the Royal Astronomical Society 03/2011; 412(2):1221 - 1228. · 4.90 Impact Factor
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ABSTRACT: 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.
03/2011;
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ABSTRACT: We report on VLBI as well as Very Large Array radio observations of the Type I b/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 light-curve 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 circumstellar material 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 × 1017 cm and an average apparent expansion speed of <1.74 c. This result is consistent with the moderately relativistic ejecta speeds implied by the radio luminosity and spectrum.
The Astrophysical Journal 11/2010; 725(1):4. · 6.02 Impact Factor
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ABSTRACT: 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. Comment: 7 pages, Accepted for publication in MNRAS
07/2010;
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ABSTRACT: 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. Comment: 7 pages; 3 figures, accepted for publication in ApJ
06/2010;
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ABSTRACT: We present new Very Long Baseline Interferometry (VLBI) images of supernova (SN) 1986J, taken at 5, 8.4, and 22 GHz between t = 22 and 25 yr after the explosion. The shell expands t 0.69±0.03. We estimate the progenitor's mass-loss rate at (4-10) × 10–5 M ☉ yr–1 (for v w = 10 km s–1). 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 SN 1986J, 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 × 1016 cm at 10 Mpc). The integrated VLA spectrum of SN 1986J 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–1 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 (~103× denser than the average corresponding CSM) and fortuitously close to the center of the projected shell. We include a movie of the evolution of SN 1986J at 5 GHz from t = 0 to 25 yr.
The Astrophysical Journal 03/2010; 712(2):1057. · 6.02 Impact Factor
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ABSTRACT: 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. Comment: Accepted for publication in the Astrophysical Journal. 17 pages, 7 figures
02/2010;
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ABSTRACT: 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 star's 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.
The Astrophysical Journal 12/2008; 517(1):L43. · 6.02 Impact Factor
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ABSTRACT: We report on VLBI observations of the Type I b/c supernova SN 2001em, 3 years after the explosion. It has been suggested that SN 2001em might be a jet-driven gamma-ray burst (GRB), with the jet oriented far from the line of sight so that the GRB would not be visible from Earth. To test this conjecture, we determined the size of SN 2001em. It is only marginally resolved at our resolution of ~0.9 mas. The 3 σ upper limit on the major axis angular size of the radio source was 0.59 mas (FWHM of an elliptical Gaussian), corresponding to a one-sided apparent expansion velocity of 70,000 km s-1 for a distance of 80 Mpc. No low-brightness jet was seen in our image to a level of 4% of the peak brightness. If we assume instead a spherical shell geometry typical of a supernova, we find the angular radius of SN 2001em was 0.17 mas, implying an isotropic expansion velocity of 20,000 km s-1, which is comparable to the expansion velocities of supernova shells. Our observations are therefore not consistent with a relativistically expanding radio source in SN 2001em but are instead consistent with a supernova shell origin of SN 2001's radio emission.
The Astrophysical Journal 12/2008; 625(2):L99. · 6.02 Impact Factor
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ABSTRACT: 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°, or 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).
The Astrophysical Journal 12/2008; 572(1):487. · 6.02 Impact Factor
