1,651 research outputs found
New insights on hadron acceleration at supernova remnant shocks
We outline the main features of nuclei acceleration at supernova remnant
forward shocks, stressing the crucial role played by self-amplified magnetic
fields in determining the energy spectrum observed in this class of sources. In
particular, we show how the standard predictions of the non-linear theory of
diffusive shock acceleration has to be completed with an additional ingredient,
which we propose to be the enhanced velocity of the magnetic irregularities
particles scatter against, to reconcile the theory of efficient particle
acceleration with recent observations of gamma-ray bright supernova remnants.Comment: 7 pages, 2 figures. To apper in "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012), Olaf Reimer and Diego F. Torres
(eds.
High Energy Cosmic Rays From Supernovae
Cosmic rays are charged relativistic particles that reach the Earth with
extremely high energies, providing striking evidence of the existence of
effective accelerators in the Universe. Below an energy around
eV cosmic rays are believed to be produced in the Milky Way while above that
energy their origin is probably extragalactic. In the early '30s supernovae
were already identified as possible sources for the Galactic component of
cosmic rays. After the '70s this idea has gained more and more credibility
thanks to the the development of the diffusive shock acceleration theory, which
provides a robust theoretical framework for particle energization in
astrophysical environments. Afterwards, mostly in recent years, much
observational evidence has been gathered in support of this framework,
converting a speculative idea in a real paradigm. In this Chapter the basic
pillars of this paradigm will be illustrated. This includes the acceleration
mechanism, the non linear effects produced by accelerated particles onto the
shock dynamics needed to reach the highest energies, the escape process from
the sources and the transportation of cosmic rays through the Galaxy. The
theoretical picture will be corroborated by discussing several observations
which support the idea that supernova remnants are effective cosmic ray
factories.Comment: Final draft of a chapter in "Handbook of Supernovae" edited by Athem
W. Alsabti and Paul Murdi
The Extremes of Thermonuclear Supernovae
The majority of thermonuclear explosions in the Universe seem to proceed in a
rather standardised way, as explosions of carbon-oxygen (CO) white dwarfs in
binary systems, leading to 'normal' Type Ia supernovae (SNe Ia). However, over
the years a number of objects have been found which deviate from normal SNe Ia
in their observational properties, and which require different and not seldom
more extreme progenitor systems. While the 'traditional' classes of peculiar
SNe Ia - luminous '91T-like' and faint '91bg-like' objects - have been known
since the early 1990s, other classes of even more unusual transients have only
been established 20 years later, fostered by the advent of new wide-field SN
surveys such as the Palomar Transient Factory. These include the faint but
slowly declining '02es-like' SNe, 'Ca-rich' transients residing in the
luminosity gap between classical novae and supernovae, extremely short-lived,
fast-declining transients, and the very luminous so-called
'super-Chandrasekhar' SNe Ia. Not all of them are necessarily thermonuclear
explosions, but there are good arguments in favour of a thermonuclear origin
for most of them. The aim of this chapter is to provide an overview of the zoo
of potentially thermonuclear transients, reviewing their observational
characteristics and discussing possible explosion scenarios.Comment: Author version of a chapter for the 'Handbook of Supernovae', edited
by A. Alsabti and P. Murdin, Springer. 50 pages, 7 figure
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.
Multiwavelength Observations of Pulsar Wind Nebulae
The extended nebulae formed as pulsar winds expand into their surroundings
provide information about the composition of the winds, the injection history
from the host pulsar, and the material into which the nebulae are expanding.
Observations from across the electromagnetic spectrum provide constraints on
the evolution of the nebulae, the density and composition of the surrounding
ejecta, the geometry of the central engines, and the long-term fate of the
energetic particles produced in these systems. Such observations reveal the
presence of jets and wind termination shocks, time-varying compact emission
structures, shocked supernova ejecta, and newly formed dust. Here I provide a
broad overview of the structure of pulsar wind nebulae, with specific examples
from observations extending from the radio band to very-high-energy gamma-rays
that demonstrate our ability to constrain the history and ultimate fate of the
energy released in the spin-down of young pulsars.Comment: 20 pages, 11 figures. Invited review to appear in Proc. of the
inaugural ICREA Workshop on "The High-Energy Emission from Pulsars and their
Systems" (2010), eds. N. Rea and D. Torres, (Springer Astrophysics and Space
Science series
Hydrogen-poor superluminous stellar explosions
Supernovae (SNe) are stellar explosions driven by gravitational or
thermonuclear energy, observed as electromagnetic radiation emitted over weeks
or more. In all known SNe, this radiation comes from internal energy deposited
in the outflowing ejecta by either radioactive decay of freshly-synthesized
elements (typically 56Ni), stored heat deposited by the explosion shock in the
envelope of a supergiant star, or interaction between the SN debris and
slowly-moving, hydrogen-rich circumstellar material. Here we report on a new
class of luminous SNe whose observed properties cannot be explained by any of
these known processes. These include four new SNe we have discovered, and two
previously unexplained events (SN 2005ap; SCP 06F6) that we can now identify as
members. These SNe are all ~10 times brighter than SNe Ia, do not show any
trace of hydrogen, emit significant ultra-violet (UV) flux for extended periods
of time, and have late-time decay rates which are inconsistent with
radioactivity. Our data require that the observed radiation is emitted by
hydrogen-free material distributed over a large radius (~10^15 cm) and
expanding at high velocities (>10^4 km s^-1). These long-lived, UV-luminous
events can be observed out to redshifts z>4 and offer an excellent opportunity
to study star formation in, and the interstellar medium of, primitive distant
galaxies.Comment: Accepted to Nature. Press embargoed until 2011 June 8, 18:00 U
Supernova Remnants as Clues to Their Progenitors
Supernovae shape the interstellar medium, chemically enrich their host
galaxies, and generate powerful interstellar shocks that drive future
generations of star formation. The shock produced by a supernova event acts as
a type of time machine, probing the mass loss history of the progenitor system
back to ages of 10 000 years before the explosion, whereas supernova
remnants probe a much earlier stage of stellar evolution, interacting with
material expelled during the progenitor's much earlier evolution. In this
chapter we will review how observations of supernova remnants allow us to infer
fundamental properties of the progenitor system. We will provide detailed
examples of how bulk characteristics of a remnant, such as its chemical
composition and dynamics, allow us to infer properties of the progenitor
evolution. In the latter half of this chapter, we will show how this exercise
may be extended from individual objects to SNR as classes of objects, and how
there are clear bifurcations in the dynamics and spectral characteristics of
core collapse and thermonuclear supernova remnants. We will finish the chapter
by touching on recent advances in the modeling of massive stars, and the
implications for observable properties of supernovae and their remnants.Comment: A chapter in "Handbook of Supernovae" edited by Athem W. Alsabti and
Paul Murdin (18 pages, 6 figures
The bright optical afterglow of the nearby gamma-ray burst of 29 March 2003
Many past studies of cosmological gamma-ray bursts (GRBs) have been limited
because of the large distance to typical GRBs, resulting in faint afterglows.
There has long been a recognition that a nearby GRB would shed light on the
origin of these mysterious cosmic explosions, as well as the physics of their
fireballs. However, GRBs nearer than z=0.2 are extremely rare, with an
estimated rate of localisation of one every decade. Here, we report the
discovery of bright optical afterglow emission from GRB 030329. Our prompt
dissemination and the brilliance of the afterglow resulted in extensive
followup (more than 65 telescopes) from radio through X-ray bands, as well as
measurement of the redshift, z=0.169. The gamma-ray and afterglow properties of
GRB 030329 are similar to those of cosmological GRBs (after accounting for the
small distance), making this the nearest known cosmological GRB. Observations
have already securely identified the progenitor as a massive star that exploded
as a supernova, and we anticipate futher revelations of the GRB phenomenon from
studies of this source.Comment: 13 pages, 4 figures. Original tex
Radio Remnants of Compact Binary Mergers - the Electromagnetic Signal that will follow the Gravitational Waves
The question "what is the observable electromagnetic (EM) signature of a
compact binary merger?" is an intriguing one with crucial consequences to the
quest for gravitational waves (GW). Compact binary mergers are prime sources of
GW, targeted by current and next generation detectors. Numerical simulations
have demonstrated that these mergers eject energetic sub-relativistic (or even
relativistic) outflows. This is certainly the case if the mergers produce short
GRBs, but even if not, significant outflows are expected. The interaction of
such outflows with the surround matter inevitably leads to a long lasting radio
signal. We calculate the expected signal from these outflows (our calculations
are also applicable to short GRB orphan afterglows) and we discuss their
detectability. We show that the optimal search for such signal should,
conveniently, take place around 1.4 GHz. Realistic estimates of the outflow
parameters yield signals of a few hundred Jy, lasting a few weeks, from
sources at the detection horizon of advanced GW detectors. Followup radio
observations, triggered by GW detection, could reveal the radio remnant even
under unfavorable conditions. Upcoming all sky surveys can detect a few dozen,
and possibly even thousands, merger remnants at any give time, thereby
providing robust merger rate estimates even before the advanced GW detectors
become operational. In fact, the radio transient RT 19870422 fits well the
overall properties predicted by our model and we suggest that its most probable
origin is a compact binary merger radio remnant
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