2,040 research outputs found
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
Constraints on Off-Axis GRB Jets in Type Ibc Supernovae From Late-Time Radio Observations
It has been suggested that the peculiar properties of the luminous Type Ic
supernova SN 1998bw and its low-energy gamma-ray burst GRB 980425 may be
understood if they originated in a standard gamma-ray burst explosion viewed
far from the axis of the relativistic jet. In this scenario, strong radio
emission is predicted from the jet on a timescale 1 to 10 years after the
explosion as it decelerates and spreads into our line of sight. To test this
hypothesis we have carried out late-time radio observations of SN 1998bw at
years, yielding upper limits which are consistent with the continued
fading of the supernova. We find these limits to be consistent with an off-axis
jet only if the progenitor mass loss rate is
M yr (for a wind velocity km s) or the
fraction of the shock energy in magnetic fields is . These values are low relative to those inferred for cosmological
GRBs. We combine the SN 1998bw measurements with existing observations for a
sample of 15 local Type Ibc supernovae to estimate that at most 6% produce
collimated, relativistic outflows.Comment: Revised version, as it appears in ApJ
Preliminary Results from the Caltech Core-Collapse Project (CCCP)
We present preliminary results from the Caltech Core-Collapse Project (CCCP),
a large observational program focused on the study of core-collapse SNe.
Uniform, high-quality NIR and optical photometry and multi-epoch optical
spectroscopy have been obtained using the 200'' Hale and robotic 60''
telescopes at Palomar, for a sample of 50 nearby core-collapse SNe. The
combination of both well-sampled optical light curves and multi-epoch
spectroscopy will enable spectroscopically and photometrically based subtype
definitions to be disentangled from each other. Multi-epoch spectroscopy is
crucial to identify transition events that evolve among subtypes with time. The
CCCP SN sample includes every core-collapse SN discovered between July 2004 and
September 2005 that was visible from Palomar, found shortly (< 30 days) after
explosion (based on available pre-explosion photometry), and closer than ~120
Mpc. This complete sample allows, for the first time, a study of core-collapse
SNe as a population, rather than as individual events. Here, we present the
full CCCP SN sample and show exemplary data collected. We analyze available
data for the first ~1/3 of the sample and determine the subtypes of 13 SNe II
based on both light curve shapes and spectroscopy. We discuss the relative SN
II subtype fractions in the context of associating SN subtypes with specific
progenitor stars.Comment: To appear in the proceedings of the meeting "The Multicoloured
Landscape of Compact Objects and their Explosive Origins", Cefalu, Italy,
June 2006, to be published by AIP, Eds. L. Burderi et a
Dust in the wind: the role of recent mass loss in long gamma-ray bursts
We study the late-time (t>0.5 days) X-ray afterglows of nearby (z<0.5) long
Gamma-Ray Bursts (GRB) with Swift and identify a population of explosions with
slowly decaying, super-soft (photon index Gamma_x>3) X-ray emission that is
inconsistent with forward shock synchrotron radiation associated with the
afterglow. These explosions also show larger-than-average intrinsic absorption
(NH_x,i >6d21 cm-2) and prompt gamma-ray emission with extremely long duration
(T_90>1000 s). Chance association of these three rare properties (i.e. large
NH_x,i, super-soft Gamma_x and extreme duration) in the same class of
explosions is statistically unlikely. We associate these properties with the
turbulent mass-loss history of the progenitor star that enriched and shaped the
circum-burst medium. We identify a natural connection between NH_x,i Gamma_x
and T_90 in these sources by suggesting that the late-time super-soft X-rays
originate from radiation reprocessed by material lost to the environment by the
stellar progenitor before exploding, (either in the form of a dust echo or as
reprocessed radiation from a long-lived GRB remnant), and that the interaction
of the explosion's shock/jet with the complex medium is the source of the
extremely long prompt emission. However, current observations do not allow us
to exclude the possibility that super-soft X-ray emitters originate from
peculiar stellar progenitors with large radii that only form in very dusty
environments.Comment: 6 pages, Submitted to Ap
Short-Baseline Electron Neutrino Disappearance, Tritium Beta Decay and Neutrinoless Double-Beta Decay
We consider the interpretation of the MiniBooNE low-energy anomaly and the
Gallium radioactive source experiments anomaly in terms of short-baseline
electron neutrino disappearance in the framework of 3+1 four-neutrino mixing
schemes. The separate fits of MiniBooNE and Gallium data are highly compatible,
with close best-fit values of the effective oscillation parameters Delta m^2
and sin^2 2 theta. The combined fit gives Delta m^2 >~ 0.1 eV^2 and 0.11 <
sin^2 2 theta < 0.48 at 2 sigma. We consider also the data of the Bugey and
Chooz reactor antineutrino oscillation experiments and the limits on the
effective electron antineutrino mass in beta-decay obtained in the Mainz and
Troitsk Tritium experiments. The fit of the data of these experiments limits
the value of sin^2 2 theta below 0.10 at 2 sigma. Considering the tension
between the neutrino MiniBooNE and Gallium data and the antineutrino reactor
and Tritium data as a statistical fluctuation, we perform a combined fit which
gives Delta m^2 \simeq 2 eV and 0.01 < sin^2 2 theta < 0.13 at 2 sigma.
Assuming a hierarchy of masses m_1, m_2, m_3 << m_4, the predicted
contributions of m_4 to the effective neutrino masses in beta-decay and
neutrinoless double-beta-decay are, respectively, between about 0.06 and 0.49
and between about 0.003 and 0.07 eV at 2 sigma. We also consider the
possibility of reconciling the tension between the neutrino MiniBooNE and
Gallium data and the antineutrino reactor and Tritium data with different
mixings in the neutrino and antineutrino sectors. We find a 2.6 sigma
indication of a mixing angle asymmetry.Comment: 14 pages; final version published in Phys.Rev.D82:053005,201
Science with the EXTraS Project: Exploring the X-ray Transient and variable Sky
The EXTraS project (Exploring the X-ray Transient and variable Sky) will
characterise the temporal behaviour of the largest ever sample of objects in
the soft X-ray range (0.1-12 keV) with a complex, systematic and consistent
analysis of all data collected by the European Photon Imaging Camera (EPIC)
instrument onboard the ESA XMM-Newton X-ray observatory since its launch. We
will search for, and characterize variability (both periodic and aperiodic) in
hundreds of thousands of sources spanning more than nine orders of magnitude in
time scale and six orders of magnitude in flux. We will also search for fast
transients, missed by standard image analysis. Our analysis will be completed
by multiwavelength characterization of new discoveries and phenomenological
classification of variable sources. All results and products will be made
available to the community in a public archive, serving as a reference for a
broad range of astrophysical investigations.Comment: 4 pages, 1 figure. Refereed Proceeding of "The Universe of Digital
Sky Surveys" conference held at the INAF - Observatory of Capodimonte,
Naples, on 25th-28th November 2014, to be published in the Astrophysics and
Space Science Proceedings, edited by Longo, Napolitano, Marconi, Paolillo,
Iodic
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
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
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