212 research outputs found
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.Comment: Accepted for publication in the Astrophysical Journal Letter
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
Area-preserving diffeomorphisms in gauge theory on a non-commutative plane: a lattice study
We consider Yang-Mills theory with the U(1) gauge group on a non-commutative
plane. Perturbatively it was observed that the invariance of this theory under
area-preserving diffeomorphisms (APDs) breaks down to a rigid subgroup SL(2,R).
Here we present explicit results for the APD symmetry breaking at finite gauge
coupling and finite non-commutativity. They are based on lattice simulations
and measurements of Wilson loops with the same area but with a variety of
different shapes. Our results are consistent with the expected loss of
invariance under APDs. Moreover, they strongly suggest that non-perturbatively
the SL(2,R) symmetry does not persist either.Comment: 28 pages, 15 figures, published versio
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