221 research outputs found
A VLA Study of High-redshift GRBs I - Multi-wavelength Observations and Modeling of GRB 140311A
We present the first results from a recently concluded study of GRBs at
with the Karl G. Jansky Very Large Array (VLA). Spanning to
GHz and 7 epochs from 1.5 to 82.3 d, our observations of GRB 140311A are
the most detailed joint radio and millimeter observations of a GRB afterglow at
to date. In conjunction with optical/near-IR and X-ray data, the
observations can be understood in the framework of radiation from a single
blast wave shock with energy erg
expanding into a constant density environment with density, . The X-ray and radio observations require a jet break at d, yielding an opening angle of and a beaming-corrected blast wave kinetic energy of
erg. The results from our radio follow-up
and multi-wavelength modeling lend credence to the hypothesis that detected
high-redshift GRBs may be more tightly beamed than events at lower redshift. We
do not find compelling evidence for reverse shock emission, which may be
related to fast cooling driven by the moderately high circumburst density.Comment: 16 pages, 13 figures, submitted to Ap
The superluminous supernova SN 2017egm in the nearby galaxy NGC 3191: a metal-rich environment can support a typical SLSN evolution
At redshift z=0.03, the recently-discovered SN 2017egm is the nearest Type I
superluminous supernova (SLSN) to date, and first near the center of a massive
spiral galaxy (NGC 3191). Using SDSS spectra of NGC 3191, we find a metallicity
~2 Z at the nucleus and ~1.3 Z for a star forming region at a
radial offset similar to SN 2017egm. Archival radio-to-UV photometry reveals a
star-formation rate ~15 M yr (with ~70% dust-obscured), which
can account for a Swift X-ray detection, and stellar mass ~
M. We model the early UV-optical light curves with a magnetar
central-engine model, using the Bayesian light curve fitting tool MOSFiT. The
fits indicate ejecta mass 2-4 M, spin period 4-6 ms, magnetic field
(0.7-1.7)G, and kinetic energy 1-2 erg. These
parameters are consistent with the overall distributions for SLSNe, modeled by
Nicholl et al (2017), although the derived mass and spin are towards the low
end, possibly indicating enhanced loss of mass and angular momentum before
explosion. This has two implications: (i) SLSNe can occur at solar metallicity,
although with a low fraction ~10%; and (ii) metallicity has at most a modest
effect on their properties. Both conclusions are in line with results for long
gamma-ray bursts. Assuming a monotonic rise gives an explosion date MJD
. However, a short-lived excess in the data relative to the
best-fitting models may indicate an early-time `bump'. If confirmed, SN 2017egm
would be the first SLSN with a spectrum during the bump-phase; this shows the
same O II lines seen at maximum light, which may be an important clue for
explaining these bumps.Comment: Accepted for publication in ApJ
Nebular-phase spectra of superluminous supernovae: physical insights from observational and statistical properties
We study the spectroscopic evolution of superluminous supernovae (SLSNe)
later than 100 days after maximum light. We present new data for Gaia16apd and
SN2017egm, and analyse these with a larger sample comprising 41 spectra of 12
events. The spectra become nebular within 2-4 -folding times after light
curve peak, with the rate of spectroscopic evolution correlated to the light
curve timescale. Emission lines are identified with well-known transitions of
oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from
other Type Ic SNe by a prominent O I 7774 line and higher-ionisation
states of oxygen. The iron-dominated region around 5000 \AA\ is more similar to
broad-lined SNe Ic than to normal SNe Ic. Principal Component Analysis shows
that 5 `eigenspectra' capture 75% of the variance, while a clustering analysis
shows no clear evidence for multiple SLSN sub-classes. Line velocities are
5000--8000 km/s, and show stratification of the ejecta. O I 7774
likely arises in a dense inner region that also produces calcium emission,
while [O I] 6300 comes from further out until 300--400 days. The
luminosities of O I 7774 and Ca II suggest significant clumping, in
agreement with previous studies. Ratios of [Ca II]7300/[O
I]6300 favour progenitors with relatively massive helium cores, likely
M, though more modelling is required here. SLSNe with broad
light curves show the strongest [O I] 6300, suggesting larger ejecta
masses. We show how the inferred velocity, density and ionisation structure
point to a central power source.Comment: Accepted in ApJ, updated to match accepted versio
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