5,472 research outputs found
Radio emission of SN1993J. The complete picture: II. Simultaneous fit of expansion and radio light curves
We report on a simultaneous modelling of the expansion and radio light curves
of SN1993J. We have developed a simulation code capable of generating synthetic
expansion and radio light curves of supernovae by taking into consideration the
evolution of the expanding shock, magnetic fields, and relativistic electrons,
as well as the finite sensitivity of the interferometric arrays used in the
observations. Our software successfully fits all the available radio data of SN
1993J with an standard emission model for supernovae extended with some
physical considerations, as an evolution in the opacity of the ejecta material,
a radial drop of the magnetic fields inside the radiating region, and a
changing radial density profile of the circumstellar medium beyond day 3100
after explosion.Comment: 12 pages, 12 figures, accepted for publication in A&
Serendipitous discovery of the long-sought AGN in Arp 299-A
Context: The dusty nuclear regions of luminous infra-red galaxies (LIRGs) are
heated by either an intense burst of massive star formation, an active galactic
nucleus (AGN), or a combination of both. Disentangling the contribution of each
of those putative dust-heating agents is a challenging task, and direct imaging
of the innermost few pc can only be accomplished at radio wavelengths, using
very high-angular resolution observations. Aims: We observed the nucleus A of
the interacting starburst galaxy Arp 299, using very long baseline
interferometry (VLBI) radio observations at 1.7 and 5.0 GHz. Our aim was to
characterize the compact sources in the innermost few pc region of Arp 299-A,
as well as to detect recently exploded core-collapse supernovae. Methods: We
used the European VLBI Network (EVN) to image the 1.7 and 5.0 GHz compact radio
emission of the parsec-scale structure in the nucleus of Arp 299-A with
milliarcsecond resolution. Results: Our EVN observations show that one of the
compact VLBI sources, A1, previously detected at 5.0 GHz, has a flat spectrum
between 1.7 and 5.0 GHz and is the brightest source at both frequencies. Our
1.7 GHz EVN image shows also diffuse, low-surface brightness emission extending
westwards from A1 and displays a prominent core-jet structure. Conclusions: The
morphology, radio luminosity, spectral index and ratio of radio-to-X-ray
emission of the A1-A5 region is consistent with a low-luminosity AGN (LLAGN),
and rules out the possibility that it is a chain of young radio supernovae
(RSNe) and supernova remnants (SNRs). We therefore conclude that A1-A5 is the
long-sought AGN in Arp 299-A. This finding may suggest that both starburst and
AGN are frequently associated phenomena in mergers.Comment: Accepted for publication in Letters to Astronomy and Astrophysics on
12 August 2010. 4 pages, 1 figur
Absolute kinematics of radio source components in the complete S5 polar cap sample. III. First wide-field high-precision astrometry at 15.4 GHz
We report on the first wide-field, high-precision astrometric analysis of the
13 extragalactic radio sources of the complete S5 polar cap sample at 15.4 GHz.
We describe new algorithms developed to enable the use of differenced phase
delays in wide-field astrometric observations and discuss the impact of using
differenced phase delays on the precision of the wide-field astrometric
analysis. From this global fit, we obtained estimates of the relative source
positions with precisions ranging from 14 to 200 as at 15.4 GHz, depending
on the angular separation of the sources (from 1.6 to 20.8
degrees). These precisions are 10 times higher than the achievable
precisions using the phase-reference mapping technique.Comment: 9 pages, 7 figure
Radio continuum and X-ray emission from the most extreme FIR-excess galaxy NGC 1377: An extremely obscured AGN revealed
Galaxies which strongly deviate from the radio-far IR correlation are of
great importance for studies of galaxy evolution as they may be tracing early,
short-lived stages of starbursts and active galactic nuclei (AGNs). The most
extreme FIR-excess galaxy NGC1377 has long been interpreted as a young dusty
starburst, but millimeter observations of CO lines revealed a powerful
collimated molecular outflow which cannot be explained by star formation alone.
We present new radio observations at 1.5 and 10 GHz obtained with the Jansky
Very Large Array (JVLA) and Chandra X-ray observations towards NGC1377. The
observations are compared to synthetic starburst models to constrain the
properties of the central energy source. We obtained the first detection of the
cm radio continuum and X-ray emission in NGC1377. We find that the radio
emission is distributed in two components, one on the nucleus and another
offset by 4.5 to the South-West. We confirm the extreme FIR-excess of the
galaxy, with a 4.2, which deviates by more than
7- from the radio-FIR correlation. Soft X-ray emission is detected on
the off-nucleus component. From the radio emission we estimate for a young
( Myr) starburst a star formation rate SFR0.1 M yr. Such
a SFR is not sufficient to power the observed IR luminosity and to drive the CO
outflow. We find that a young starburst cannot reproduce all the observed
properties of the nucleus of NGC1377. We suggest that the galaxy may be
harboring a radio-quiet, obscured AGN of 10M, accreting at
near-Eddington rates. We speculate that the off-nucleus component may be
tracing an hot-spot in the AGN jet.Comment: 14 pages, accepted for publication on Astronomy and Astrophysics on
08/07/201
8.4GHz VLBI observations of SN2004et in NGC6946
We report on 8.4GHz Very Long Baseline Interferometry (VLBI) observations of
the type II-P supernova SN2004et in the spiral galaxy NGC 6946, made on 20
February 2005 (151 days after explosion). The Very Large Array (VLA) flux
density was 1.230.07 mJy, corresponding to an isotropic luminosity at
8.4GHz of (4.450.3) erg s Hz and a brightness
temperature of (1.30.3) K. We also provide an improved
source position, accurate to about 0.5 mas in each coordinate. The VLBI image
shows a clear asymmetry. From model fitting of the size of the radio emission,
we estimate a minimum expansion velocity of 15,7002,000 km s. This
velocity is more than twice the expected mean expansion velocity estimated from
a synchrotron self-absorbed emission model, thus suggesting that synchrotron
self-absorption is not relevant for this supernova. With the benefit of an
optical spectrum obtained 12 days after explosion, we favor an emission model
which consists of two hot spots on an underlying expanding shell of width
comparable to that of SN 1993J.Comment: Accepted for publication in A&A (22/05/07
The Core-Collapse Supernova Rate in Arp299 Revisited
We present a study of the CCSN rate in nuclei A and B1 of the luminous
infrared galaxy Arp299, based on 11 years of Very Large Array monitoring of
their radio emission at 8.4 GHz. Significant variations in the nuclear radio
flux density can be used to identify the CCSN activity in the absence of
high-resolution very long baseline interferometry observations. In the case of
the B1-nucleus, the small variations in its measured diffuse radio emission are
below the fluxes expected from radio supernovae, thus making it well-suited to
detect RSNe through flux density variability. In fact, we find strong evidence
for at least three RSNe this way, which results in a lower limit for the CCSN
rate of 0.28 +/- 0.16 per year. In the A-nucleus, we did not detect any
significant variability and found a SN detection threshold luminosity which
allows only the detection of the most luminous RSNe known. Our method is
basically blind to normal CCSN explosions occurring within the A-nucleus, which
result in too small variations in the nuclear flux density, remaining diluted
by the strong diffuse emission of the nucleus itself. Additionally, we have
attempted to find near-infrared counterparts for the earlier reported RSNe in
the Arp299 nucleus A, by comparing NIR adaptive optics images from the Gemini-N
telescope with contemporaneous observations from the European VLBI Network.
However, we were not able to detect NIR counterparts for the reported radio SNe
within the innermost regions of nucleus A. While our NIR observations were
sensitive to typical CCSNe at 300 mas from the centre of the nucleus A,
suffering from extinction up to A_v~15 mag, they were not sensitive to such
highly obscured SNe within the innermost nuclear regions where most of the EVN
sources were detected. (abridged)Comment: 12 pages, 4 figures and 7 tables. Accepted for publication in MNRA
Evidence of nuclear disks in starburst galaxies from their radial distribution of supernovae
Galaxy-galaxy interactions are expected to be responsible for triggering
massive star formation and possibly accretion onto a supermassive black hole,
by providing large amounts of dense molecular gas down to the central
kiloparsec region. Several scenarios to drive the gas further down to the
central ~100 pc, have been proposed, including the formation of a nuclear disk
around the black hole, where massive stars would produce supernovae. Here, we
probe the radial distribution of supernovae and supernova remnants in the
nuclear regions of the starburst galaxies M82, Arp 299-A, and Arp 220, by using
high-angular resolution (< 0."1) radio observations published in the literature
(for M82 and Arp 220), or obtained by ourselves from the European VLBI Network
(Arp 299-A). Our main goal was to characterize the nuclear starbursts in those
galaxies and thus test scenarios that propose that nuclear disks of sizes ~100
pc form in the central regions of starburst galaxies. We obtained the radial
distribution of supernovae (SNe) in the nuclear starbursts of M82, Arp 299-A,
and Arp 220, and derived scale-length values for the putative nuclear disks
powering the bursts in those central regions. The scale lengths for the
(exponential) disks range from ~20-30 pc for Arp 299-A and Arp 220, up to ~140
pc for M82. The radial distribution of SNe for the nuclear disks in Arp 299-A
and Arp 220 is also consistent with a power-law surface density profile of
exponent gamma=1, as expected from detailed hydrodynamical simulations of
nuclear disks. Our results support scenarios where a nuclear disk of size ~100
pc is formed in (U)LIRGs, and sustained by gas pressure, in which case the
accretion onto the black hole could be lowered by supernova feedback.Comment: Accepted for publication in Astronomy and Astrophysic
Radio monitoring of NGC 7469: Late time radio evolution of SN 2000ft and the circumnuclear starburst in NGC 7469
We present the results of an eight-year long monitoring of the radio emission
from the Luminous Infrared Galaxy (LIRG) NGC 7469, using 8.4 GHz Very Large
Array (VLA) observations at 0.3'' resolution. Our monitoring shows that the
late time evolution of the radio supernova SN 2000ft follows a decline very
similar to that displayed at earlier times of its optically thin phase. The
late time radio emission of SN 2000ft is therefore still being powered by its
interaction with the presupernova stellar wind, and not with the interstellar
medium (ISM). Indeed, the ram pressure of the presupernova wind is \rho_w v_w^2
\approx 7.6E-9 dyn/cm^2, at a supernova age of approximately 2127 days, which
is significantly larger than the expected pressure of the ISM around SN 2000ft.
At this age, the SN shock has reached a distance r_{sh \approx 0.06 pc, and our
observations are probing the interaction of the SN with dense material that was
ejected by the presupernova star about 5820 years prior to its explosion. From
our VLA monitoring, we estimate that the swept-up mass by the supernova shock
after about six years of expansion is \approx 0.29 M_sun, assuming an average
expansion speed of the supernova of 10000 km/s.
We also searched for recently exploded core-collapse supernovae in our VLA
images. Apart from SN 2000ft (S_\nu \approx 1760 microJy at its peak,
corresponding to 1.1E28 erg/s/Hz, we found no evidence for any other radio
supernova (RSN) more luminous than \approx 6.0E26 erg/s/Hz, which suggests that
no other Type IIn SN has exploded since 2000 in the circumnuclear starburst of
NGC 7469.Comment: 10 pages, 5 figures, accepted for publication in MNRA
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