2,073 research outputs found
The Unique Signature of Shell Curvature in Gamma-Ray Bursts
As a result of spherical kinematics, temporal evolution of received gamma-ray
emission should demonstrate signatures of curvature from the emitting shell.
Specifically, the shape of the pulse decay must bear a strict dependence on the
degree of curvature of the gamma-ray emitting surface. We compare the spectral
evolution of the decay of individual GRB pulses to the evolution as expected
from curvature. In particular, we examine the relationship between photon flux
intensity (I) and the peak of the \nu F\nu distribution (E_{peak}) as predicted
by colliding shells. Kinematics necessitate that E_{peak} demonstrate a
power-law relationship with I described roughly as: I=E_{peak}^{(1-\zeta)}
where \zeta represents a weighted average of the low and high energy spectral
indices. Data analyses of 24 BATSE gamma-ray burst pulses provide evidence that
there exists a robust relationship between E_{peak} and I in the decay phase.
Simulation results, however, show that a sizable fraction of observed pulses
evolve faster than kinematics allow. Regardless of kinematic parameters, we
found that the existence of curvature demands that the I - E_{peak} function
decay be defined by \sim (1-\zeta). Efforts were employed to break this
curvature dependency within simulations through a number of scenarios such as
anisotropic emission (jets) with angular dependencies, thickness values for the
colliding shells, and various cooling mechanisms. Of these, the only method
successful in dominating curvature effects was a slow cooling model. As a
result, GRB models must confront the fact that observed pulses do not evolve in
the manner which curvature demands.Comment: 3 pages, To appear in Proc. from the 2nd Workshop on Gamma-Ray Bursts
in the Afterglow Er
Radio and X-ray Observations of the Type Ic SN 2007gr Reveal an Ordinary, Non-relativistic Explosion
We present extensive radio and X-ray observations of the nearby Type Ic SN
2007gr in NGC 1058 obtained with the Very Large Array and the Chandra X-ray
Observatory and spanning 5 to 150 days after explosion. Through our detailed
modeling of these data, we estimate the properties of the blastwave and the
circumstellar environment. We find evidence for a freely-expanding and
non-relativistic explosion with an average blastwave velocity, v~0.2c, and a
total internal energy for the radio emitting material of E ~ 2 x 10^46 erg
assuming equipartition of energy between electrons and magnetic fields
(epsilon_e=epsilon_B=0.1). The temporal and spectral evolution of the radio
emission points to a stellar wind-blown environment shaped by a steady
progenitor mass loss rate of Mdot ~ 6 x 10^-7 solar masses per year (wind
velocity, v_w=10^3 km/s). These parameters are fully consistent with those
inferred for other SNe Ibc and are in line with the expectations for an
ordinary, homologous SN explosion. Our results are at odds with those of Paragi
et al. (2010) who recently reported evidence for a relativistic blastwave in SN
2007gr based on their claim that the radio emission was resolved away in a low
signal-to-noise Very Long Baseline Interferometry (VLBI) observation. Here we
show that the exotic physical scenarios required to explain the claimed
relativistic velocity -- extreme departures from equipartition and/or a highly
collimated outflow -- are excluded by our detailed Very Large Array radio
observations. Moreover, we present an independent analysis of the VLBI data and
propose that a modest loss of phase coherence provides a more natural
explanation for the apparent flux density loss which is evident on both short
and long baselines. We conclude that SN 2007gr is an ordinary Type Ibc
supernova.Comment: 14 pages, 6 figures, submitted to Ap
SN 2007bg: The Complex Circumstellar Environment Around One of the Most Radio-Luminous Broad-Lined Type Ic Supernovae
In this paper we present the results of the radio light curve and X-ray
observations of broad-lined Type Ic SN 2007bg. The light curve shows three
distinct phases of spectral and temporal evolution, implying that the SNe shock
likely encountered at least 3 different circumstellar medium regimes. We
interpret this as the progenitor of SN 2007bg having at least two distinct
mass-loss episodes (i.e., phases 1 and 3) during its final stages of evolution,
yielding a highly-stratified circumstellar medium. Modelling the phase 1 light
curve as a freely-expanding, synchrotron-emitting shell, self-absorbed by its
own radiating electrons, requires a progenitor mass-loss rate of
\dot{M}~1.9x10^{-6}(v_{w}/1000 km s^{-1}) Solar masses per year for the last
t~20(v_{w}/1000 km s^{-1}) yr before explosion, and a total energy of the radio
emitting ejecta of E\sim1x10^{48} erg after 10 days from explosion. This places
SN 2007bg among the most energetic Type Ib/c events. We interpret the second
phase as a sparser "gap" region between the two winds stages. Phase 3 shows a
second absorption turn-on before rising to a peak luminosity 2.6 times higher
than in phase 1. Assuming this luminosity jump is due to a circumstellar medium
density enhancement from a faster previous mass-loss episode, we estimate that
the phase 3 mass-loss rate could be as high as \dot{M}<~4.3x10^{-4}(v_{w}/1000
km s^{-1}) Solar masses per year. The phase 3 wind would have transitioned
directly into the phase 1 wind for a wind speed difference of ~2. In summary,
the radio light curve provides robust evidence for dramatic global changes in
at least some Ic-BL progenitors just prior (~10-1000 yr) to explosion. The
observed luminosity of this SN is the highest observed for a
non-gamma-ray-burst broad-lined Type Ic SN, reaching L_{8.46 GHz}~1x10^{29} erg
Hz^{-1} s^{-1}, ~567 days after explosion.Comment: 11 pages, 5 figures, accepted for publication in MNRA
1945-11-23, Herman to Irene
https://digitalcommons.chapman.edu/wvasos_collection/1055/thumbnail.jp
A Radio Flare from GRB 020405: Evidence for a Uniform Medium Around a Massive Stellar Progenitor
We present radio observations of GRB 020405 starting 1.2 days after the
burst, which reveal a rapidly-fading ``radio flare''. Based on its temporal and
spectral properties, we interpret the radio flare as emission from the reverse
shock. This scenario rules out a circumburst medium with a radial density
profile \rho ~ r^{-2} expected around a mass-losing massive star, since in that
case the reverse shock emission decays on the timescale of the burst duration
t~100 s. Using published optical and X-ray data, along with the radio data
presented here, we further show that a self-consistent model requires
collimated ejecta with an opening angle of 6 degrees (t_j~0.95 days). As a
consequence of the early jet break, the late-time (t>10 days) emission measured
with the Hubble Space Telescope significantly deviates from an extrapolation of
the early, ground-based data. This, along with an unusually red spectrum, F_\nu
\~ \nu^{-3.9}, strengthens the case for a supernova that exploded at about the
same time as GRB 020405, thus pointing to a massive stellar progenitor for this
burst. This is the first clear association of a massive progenitor with a
uniform medium, indicating that a \rho ~ r^{-2} profile is not a required
signature, and in fact may not be present on the lengthscales probed by the
afterglow in the majority of bursts.Comment: Submitted to ApJ; 14 pages, 2 tables, 3 figure
Relativistic ejecta from XRF 060218 and the rate of cosmic explosions
Over the last decade, long-duration gamma-ray bursts (GRBs) including the
subclass of X-ray flashes (XRFs) have been revealed to be a rare variety of
Type Ibc supernova (SN). While all these events result from the death of
massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those
of ordinary Type Ibc SNe by many orders of magnitude. The essential physical
process that causes a dying star to produce a GRB or XRF, and not just an SN,
remains the crucial open question. Here we present radio and X-ray observations
of XRF 060218 (associated with SN 2006aj), the second nearest GRB identified
to-date, which allow us to measure its total energy and place it in the larger
context of cosmic explosions. We show that this event is 100 times less
energetic but ten times more common than cosmological GRBs. Moreover, it is
distinguished from ordinary Type Ibc SNe by the presence of 10^48 erg coupled
to mildly-relativistic ejecta, along with a central engine (an accretion-fed,
rapidly rotating compact source) which produces X-rays for weeks after the
explosion. This suggests that the production of relativistic ejecta is the key
physical distinction between GRBs/XRFs and ordinary SNe, while the nature of
the central engine (black hole or magnetar) may distinguish typical bursts from
low-luminosity, spherical events like XRF 060218.Comment: To appear in Nature on August 31 2006 (15 pages, 3 figures, 1 table,
including Supplementary Information
A Relativistic Type Ibc Supernova Without a Detected Gamma-ray Burst
Long duration gamma-ray bursts (GRBs) mark the explosive death of some
massive stars and are a rare sub-class of Type Ibc supernovae (SNe Ibc). They
are distinguished by the production of an energetic and collimated relativistic
outflow powered by a central engine (an accreting black hole or neutron star).
Observationally, this outflow is manifested in the pulse of gamma-rays and a
long-lived radio afterglow. To date, central engine-driven SNe have been
discovered exclusively through their gamma-ray emission, yet it is expected
that a larger population goes undetected due to limited satellite sensitivity
or beaming of the collimated emission away from our line-of-sight. In this
framework, the recovery of undetected GRBs may be possible through radio
searches for SNe Ibc with relativistic outflows. Here we report the discovery
of luminous radio emission from the seemingly ordinary Type Ibc SN 2009bb,
which requires a substantial relativistic outflow powered by a central engine.
The lack of a coincident GRB makes SN 2009bb the first engine-driven SN
discovered without a detected gamma-ray signal. A comparison with our extensive
radio survey of SNe Ibc reveals that the fraction harboring central engines is
low, ~1 percent, measured independently from, but consistent with, the inferred
rate of nearby GRBs. Our study demonstrates that upcoming optical and radio
surveys will soon rival gamma-ray satellites in pinpointing the nearest
engine-driven SNe. A similar result for a different supernova is reported
independently.Comment: To appear in Nature on Jan 28 2010. Embargoed for discussion in the
press until 13:00 US Eastern Time on Jan 27 (Accepted version, 27 pages,
Manuscript and Suppl. Info.
The sub-energetic GRB 031203 as a cosmic analogue to GRB 980425
Over the six years since the discovery of the gamma-ray burst GRB 980425,
associated with the nearby (distance, ~40 Mpc) supernova 1998bw, astronomers
have fiercely debated the nature of this event. Relative to bursts located at
cosmological distances, (redshift, z~1), GRB 980425 was under-luminous in
gamma-rays by three orders of magnitude. Radio calorimetry showed the explosion
was sub-energetic by a factor of 10. Here, we report observations of the radio
and X-ray afterglow of the recent z=0.105 GRB 031203 and demonstrate that it
too is sub-energetic. Our result, when taken together with the low gamma-ray
luminosity, suggest that GRB 031203 is the first cosmic analogue to GRB 980425.
We find no evidence that this event was a highly collimated explosion viewed
off-axis. Like GRB 980425, GRB 031203 appears to be an intrinsically
sub-energetic gamma-ray burst. Such sub-energetic events have faint afterglows.
Intensive follow-up of faint bursts with smooth gamma-ray light curves (common
to both GRBs 031203 and 980425) may enable us to reveal their expected large
population.Comment: To Appear in Nature, August 5, 200
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