227 research outputs found
Limit to the radio emission from a putative central compact source in SN1993J
SN1993J in M81 is the best studied young radio-luminous supernova in the
Northern Hemisphere. We recently reported results from the analysis of a
complete set of VLBI observations of this supernova at 1.7, 2.3, 5.0, and 8.4
GHz, covering a time baseline of more than one decade. Those reported results
were focused on the kinematics of the expanding shock, the particulars of its
evolving non-thermal emission, the density profile of the circumstellar medium,
and the evolving free-free opacity by the supernova ejecta. In the present
paper, we complete our analysis by performing a search for any possible signal
from a compact source (i.e., a stellar-mass black hole or a young pulsar
nebula) at the center of the expanding shell. We have performed a stacking of
all our VLBI images at each frequency, after subtraction of our best-fit shell
model at each epoch, and measured the peak intensity in the stacked residual
image. Given the large amount of available global VLBI observations, the
stacking of all the residual images allows us to put upper limits to the
eventual emission of a putative compact central source at the level of
Jy at 5 GHz (or, more conservatively, Jy, if we
make a further correction for the ejecta opacity) and somewhat larger at other
wavelengths.Comment: 4 pages, 3 figures. Accepted for publication in A&
Radio emission of SN1993J: the complete picture. I. Re-analysis of all the available VLBI data
We have performed a complete re-calibration and re-analysis of all the
available VLBI observations of supernova SN1993J, following an homogeneous and
well-defined methodology. Observations of SN1993J at 69 epochs, spanning 13
years, were performed by two teams, which used different strategies and
analysis tools. The results obtained by each group are similar, but their
conclusions on the supernova expansion and the shape and evolution of the
emitting region differ significantly. From our analysis of the combined set of
observations, we have obtained an expansion curve with unprecedented time
resolution and coverage. We find that the data from both teams are compatible
when analyzed with the same methodology. One expansion index () is enough to model the expansion observed at 1.7\,GHz, while two
expansion indices ( and ), separated
by a break time, days, are needed to model the data, at
frequencies higher than 1.7\,GHz, up to day 4000 after explosion. We thus
confirm the wavelength dependence of the size of the emitting region reported
by one of the groups. We also find that all sizes measured at epochs later than
day 4000 after explosion are systematically smaller than our model predictions.
We estimate the fractional shell width (, average of all epochs
and frequencies) and the level of opacity to the radio emission by the ejecta.
We find evidence of a spectral-index radial gradient in the supernova shell,
which is indicative of a frequency-dependent ejecta opacity. Finally, we study
the distribution and evolution of the azimuthal anisotropies (hot spots) found
around the radio shell during the expansion. These anisotropies have
intensities of % of the mean flux density of the shell, and appear to
systematically evolve during the expansion.Comment: 13 pages, 9 figures, accepted for publication in A&
Differential Astrometry over 15 degrees
We observed the pair of radio sources 1150+812 and 1803+784 in November 1993
with a VLBI array, simultaneously recording at 8.4 and 2.3 GHz. We determined
the angular separation between the two sources with submilliarcsecond accuracy
by using differential techniques. This result demonstrates the feasibility of
high precision differential astrometry for radio sources separated in the sky
by almost 15 degrees, and opens the avenue to its application to larger samples
of radio sources.Comment: 6 pages, latex2e, 2 figures, To appear in the proceedings of the
EVN/JIVE Symposium No. 4, New Astronomy Reviews (eds. Garret, M. Campbell,
R., and Gurvits, L.
Jet precession in the active nucleus of M81. Ongoing VLBI monitoring
In a recent publication, we reported results of a multi-frequency VLBI
campaign of observations of the Active Galactic Nucleus (AGN) in galaxy M\,81,
phase-referenced to the supernova SN\,1993J. We were able to extract precise
information on the relative astrometry of the AGN radio emission at different
epochs and frequencies. We found strong evidence of precession in the AGN jet
(i.e., a systematic evolution in the jet inclination at each frequency) coupled
to changes in the overall flux density at the different frequencies. In these
proceedings, we summarise the main contents of our previous publication and we
report on (preliminary) new results from our follow-up VLBI observations, now
phase-referenced to the young supernova SN2008iz. We also briefly discuss how
these results match the picture of our previously-reported precession model.Comment: 6 pages, 3 figures. Proceedings of EVN meeting 201
Multi-wavelength differential astrometry of the S5 polar cap sample
We report on the status of our S5 polar cap astrometry program. Since 1997 we
have observed all the 13 radio sources of the complete S5 polar cap sample at
the wavelengths of 3.6 cm, 2 cm and 7 mm. Images of the radio sources at 3.6
and 2 cm have already been published reporting morphological changes.
Preliminary astrometric analyses have been carried out at three frequencies
with precisions in the relative position determination ranging from 80 to 20
microarcseconds. We report also on the combination of our phase-delay global
astrometry results with the microarcsecond-precise optical astrometry that will
be provided by future space-based instruments.Comment: 2 pages. 1 figure. Proceedings of the 7th European VLBI Network
Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R.
Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico
Nacional), p. 323-324. Needs evn2004.cl
The Late-Time Radio Spectrum of SN1993J
We present VLA radio continuum measurements of SN1993J in M81 at the
frequencies of 0.32 (P-band), 1.3 and 1.7 (L-band), 4.9 (C-band), 8.5 (X-band),
and 14.9 (U-band) GHz carried out on December 17 and 21, 2000, about 2820 days
after the supernova explosion. We find that a power-law spectrum, free-free
absorbed by an homogeneous, or clumpy, distribution of ionized gas yields the
best fit to the radio data. A combined homogeneous-clumpy model is not favored,
but neither totally excluded. This result contrasts with the modeling of the
early (t \lessim 230 days) radio emission from SN1993J, where a mixture of
homogeneous and clumpy absorbers appeared to be necessary to adequately
describe the behavior of the light curves. The radio spectrum of supernova
SN1993J between 0.32 and 14.9 GHz is well characterized by (), typical of an optically thin radio
supernova. A fit to the radio spectra of SN1993J from 70 up to 2820 days
shows that the observed spectral index of SN1993J has been slowly evolving
since 1000 days on, with the observed spectral index changing from
to . This spectral evolution seems to suggest
that, in addition to the radiative (synchrotron) losses, adiabatic cooling and
ionization (Coulomb) losses at the lowest frequencies might be contributing
significantly to the integrated electron spectrum.Comment: A&A Main Journal, in press, 6 pages. Includes minor revision
The Location of the Core in M81
We report on VLBI observations of M81*, the northwest-southeast oriented
nuclear core-jet source of the spiral galaxy M81, at five different frequencies
between 1.7 and 14.8 GHz. By phase referencing to supernova 1993J we can
accurately locate the emission region of M81* in the galaxy's reference frame.
Although the emission region's size decreases with increasing frequency while
the brightness peak moves to the southwest, the emission region seems sharply
bounded to the southwest at all frequencies. We argue that the core must be
located between the brightness peak at our highest frequency (14.8 GHz) and the
sharp bound to the southwest. This narrowly constrains the location of the
core, or the purported black hole in the center of the galaxy, to be within a
region of +/-0.2 mas or +/-800 AU (at a distance of ~4 Mpc). This range
includes the core position that we determined earlier by finding the most
stationary point in the brightness distribution of M81* at only a single
frequency. This independent constraint therefore strongly confirms our earlier
core position. Our observations also confirm that M81* is a core-jet source,
with a one-sided jet that extends to the northeast from the core, on average
curved somewhat to the east, with a radio spectrum that is flat or inverted
near the core and steep at the distant end. The brightness peak is
unambiguously identified with the variable jet rather than the core, which
indicates limitations in determining the proper motion of nearby galaxies and
in refining the extragalactic reference frame.Comment: LaTeX, 10 pages with 3 figures. Typos fixed and slight rewording for
clarity from previous version. Accepted for publication in the Astrophysical
Journa
VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), sigma Oph, BM Sco, and HD 206859
We add four warmer late-type supergiants to our previous
spectro-interferometric studies of red giants and supergiants.
V766 Cen (=HR 5171 A) is found to be a high-luminosity log(L/L_sun)=5.8+-0.4
source of Teff 4290+-760 K and radius 1490+-540 Rsun located close to both the
Hayashi and Eddington limits; this source is consistent with a 40 Msun
evolutionary track without rotation and current mass 27-36 Msun. It exhibits
NaI in emission arising from a shell of radius 1.5 Rphot and a photocenter
displacement of about 0.1 Rphot. V766 Cen shows strong extended molecular (CO)
layers and a dusty circumstellar background component. This suggest an
optically thick pseudo-photosphere at about 1.5 Rphot at the onset of the wind.
V766 Cen is a red supergiant located close to the Hayashi limit instead of a
yellow hypergiant already evolving back toward warmer Teff as previously
discussed.
The stars sigma Oph, BM Sco, and HD 206859 are found to have lower
luminosities of about log(L/Lsun)=3.4-3.5 and Teff of 3900-5300 K,
corresponding to 5-9 Msun tracks. They do not show extended molecular layers as
observed for higher luminosity red supergiants of our sample. BM Sco shows an
unusually strong contribution by an over-resolved circumstellar dust component.
These stars are more likely high-mass red giants instead of red supergiants.
This leaves us with an unsampled locus in the HR diagram corresponding to
luminosities log(L/Lsun)~3.8-4.8 or masses 10-13 Msun, possibly corresponding
to the mass region where stars explode as type II-P supernovae during the RSG
stage.
Our previously found relation of increasing strength of extended molecular
layers with increasing luminosities is now confirmed to extend to double our
previous luminosities and up to the Eddington limit. This might further point
to steadily increasing radiative winds with increasing luminosity.
[Abridged]Comment: 16 pages, 14 figures, accepted for publication in Astronomy and
Astrophysics (A&A
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