5,533 research outputs found
SN 2001em: No Jet-Driven Gamma Ray Burst Event
We report on our second-epoch VLBI and VLA observations of the Type Ib/c
supernova 2001em, five years after the explosion. It was suggested that SN
2001em might be a jet-driven gamma ray burst (GRB), with the jet oriented near
the plane of the sky, which would entail relativistic expansion or motion. Our
VLBI image shows that SN 2001em is still unresolved five years after the
explosion. For a distance of 83 Mpc (H_0 = 70 km/s/Mpc), the nominal expansion
velocity is 5800 +/- 10,000 km/s, and the proper motion is 33,000 +/- 34,000
km/s. Our values are inconsistent with either relativistic expansion or motion,
but are consistent with the non-relativistic expansion speeds and small proper
motions seen in other supernovae. In particular these values are consistent
with radio emission from SN 2001em being due to normal, non-relativistic
supernova ejecta interacting with the circumstellar medium. Our VLA
observations show a power-law decay in flux density since the time of the peak
in the 8.4 GHz radio lightcurve in ~2003.Comment: 5 pages, 2 figs, accepted for publication in ApJ Letters; added
reference
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.Comment: 7 pages, Accepted for publication in MNRA
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