59 research outputs found
X-ray flares from propagation instabilities in long Gamma-Ray Burst jets
We present a numerical simulation of a gamma-ray burst jet from a
long-lasting engine in the core of a 16 solar mass Wolf-Rayet star. The engine
is kept active for 6000 s with a luminosity that decays in time as a power-law
with index -5/3. Even though there is no short time-scale variability in the
injected engine luminosity, we find that the jet's kinetic luminosity outside
the progenitor star is characterized by fluctuations with relatively short time
scale. We analyze the temporal characteristics of those fluctuations and we
find that they are consistent with the properties of observed flares in X-ray
afterglows. The peak to continuum flux ratio of the flares in the simulation is
consistent with some, but not all, the observed flares. We propose that
propagation instabilities, rather than variability in the engine luminosity,
are responsible for the X-ray flares with moderate contrast. Strong flares such
as the one detected in GRB 050502B, instead, cannot be reproduced by this model
and require strong variability in the engine activity.Comment: 6 pages, MNRAS in pres
Jet trails and Mach cones: The interaction of microquasars with the ISM
A sub-set of microquasars exhibit high peculiar velocity with respect to the
local standard of rest due to the kicks they receive when being born in
supernovae. The interaction between the radio plasma released by microquasar
jets from such high-velocity binaries with the ISM must lead to the production
of trails and bow shocks similar to what is observed in narrow-angle tailed
radio galaxies and pulsar wind nebulae. We present a set of numerical
simulations of this interaction that illuminate the long term dynamical
evolution and the observational properties of these microquasar bow shock
nebulae and trails. We find that this interaction always produces a structure
that consists of a bow shock, a trailing neck, and an expanding bubble. Using
our simulations to model emission, we predict that the shock surrounding the
bubble and the neck should be visible in H{\alpha} emission, the interior of
the bubble should be visible in synchrotron radio emission, and only the bow
shock is likely to be detectable in X-ray emission. We construct an analytic
model for the evolution of the neck and bubble shape and compare this model
with observations of X-ray binary SAX J1712.6-3739.Comment: 33 pages, 13 figures, 1 table; Accepted to Ap
Relativistic supernovae have shorter-lived central engines or more extended progenitors: the case of SN\,2012ap
Deep late-time X-ray observations of the relativistic, engine-driven, type Ic
SN2012ap allow us to probe the nearby environment of the explosion and reveal
the unique properties of relativistic SNe. We find that on a local scale of
~0.01 pc the environment was shaped directly by the evolution of the progenitor
star with a pre-explosion mass-loss rate <5x10^-6 Msun yr-1 in line with GRBs
and the other relativistic SN2009bb. Like sub-energetic GRBs, SN2012ap is
characterized by a bright radio emission and evidence for mildly relativistic
ejecta. However, its late time (t~20 days) X-ray emission is ~100 times fainter
than the faintest sub-energetic GRB at the same epoch, with no evidence for
late-time central engine activity. These results support theoretical proposals
that link relativistic SNe like 2009bb and 2012ap with the weakest observed
engine-driven explosions, where the jet barely fails to breakout. Furthermore,
our observations demonstrate that the difference between relativistic SNe and
sub-energetic GRBs is intrinsic and not due to line-of-sight effects. This
phenomenology can either be due to an intrinsically shorter-lived engine or to
a more extended progenitor in relativistic SNe.Comment: Version accepted to ApJ. Significantly broadened discussio
Magnetar Driven Bubbles and the Origin of Collimated Outflows in Gamma-ray Bursts
We model the interaction between the wind from a newly formed rapidly
rotating magnetar and the surrounding supernova shock and host star. The
dynamics is modeled using the two-dimensional, axisymmetric thin-shell
equations. In the first ~10-100 seconds after core collapse the magnetar
inflates a bubble of plasma and magnetic fields behind the supernova shock. The
bubble expands asymmetrically because of the pinching effect of the toroidal
magnetic field, just as in the analogous problem of the evolution of pulsar
wind nebulae. The degree of asymmetry depends on E_mag/E_tot. The correct value
of E_mag/E_tot is uncertain because of uncertainties in the conversion of
magnetic energy into kinetic energy at large radii in relativistic winds; we
argue, however, that bubbles inflated by newly formed magnetars are likely to
be significantly more magnetized than their pulsar counterparts. We show that
for a ratio of magnetic to total power supplied by the central magnetar
L_mag/L_tot ~ 0.1 the bubble expands relatively spherically. For L_mag/L_tot ~
0.3, however, most of the pressure in the bubble is exerted close to the
rotation axis, driving a collimated outflow out through the host star. This can
account for the collimation inferred from observations of long-duration
gamma-ray bursts (GRBs). Outflows from magnetars become increasingly
magnetically dominated at late times, due to the decrease in neutrino-driven
mass loss as the young neutron star cools. We thus suggest that the
magnetar-driven bubble initially expands relatively spherically, enhancing the
energy of the associated supernova, while at late times it becomes
progressively more collimated, producing the GRB.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Heavy Nuclei Synthesized in Gamma-Ray Burst Outflows as the Source of UHECRs
Recent measurements by the Pierre Auger Observatory suggest that the
composition of ultra-high energy cosmic rays (UHECRs) becomes dominated by
heavy nuclei at high energies. However, until now there has been no
astrophysical motivation for considering a source highly enriched in heavy
elements. Here we demonstrate that the outflows from Gamma-Ray Bursts (GRBs)
may indeed be composed primarily of nuclei with masses A ~ 40-200, which are
synthesized as hot material expands away from the central engine. In
particular, if the jet is magnetically-dominated (rather than a
thermally-driven fireball) its low entropy enables heavy elements to form
efficiently. Adopting the millisecond proto-magnetar model for the GRB central
engine, we show that heavy nuclei are both synthesized in proto-magnetar winds
and can in principle be accelerated to energies >1e20 eV in the shocks or
regions of magnetic reconnection that are responsible for powering the GRB.
Similar results may apply to accretion-powered GRB models if the jet originates
from a magnetized disk wind. Depending on the precise distribution of nuclei
synthesized, we predict that the average primary mass may continue to increase
beyond Fe group elements at the highest energies, possibly reaching the A ~ 90
(Zirconium), A ~ 130 (Tellurium), or even A ~ 195 (Platinum) peaks. Future
measurements of the UHECR composition at energies >~ 1e20 eV can thus confirm
or constrain our model and, potentially, probe the nature of GRB outflows. The
longer attenuation length of ultra-heavy nuclei through the extragalactic
background light greatly expands the volume of accesible sources and alleviates
the energetic constraints on GRBs as the source of UHECRs.Comment: 10 pages, 3 figures, final version now accepted to MNRA
A Giant Metrewave Radio Telescope/Chandra view of IRAS 09104+4109: A type 2 QSO in a cooling flow
IRAS 09104+4109 is a rare example of a dust enshrouded type 2 QSO in the
centre of a cool-core galaxy cluster. Previous observations of this z=0.44
system showed that as well as powering the hyper-luminous infrared emission of
the cluster-central galaxy, the QSO is associated with a double-lobed radio
source. However, the steep radio spectral index and misalignment between the
jets and ionised optical emission suggested that the orientation of the QSO had
recently changed. We use a combination of new, multi-band Giant Metrewave Radio
Telescope observations and archival radio data to confirm that the jets are no
longer powered by the QSO, and estimate their age to be 120-160 Myr. This is in
agreement with the ~70-200 Myr age previously estimated for star-formation in
the galaxy. Previously unpublished Very Long Baseline Array data reveal a 200
pc scale double radio source in the galaxy core which is more closely aligned
with the current QSO axis and may represent a more recent period of jet
activity. These results suggest that the realignment of the QSO, the cessation
of jet activity, and the onset of rapid star-formation may have been caused by
a gas-rich galaxy merger. A Chandra X-ray observation confirms the presence of
cavities associated with the radio jets, and we estimate the energy required to
inflate them to be ~7.7x10^60 erg. The mechanical power of the jets is
sufficient to balance radiative cooling in the cluster, provided they are
efficiently coupled to the intra-cluster medium (ICM). We find no evidence of
direct radiative heating and conclude that the QSO either lacks the radiative
luminosity to heat the ICM, or that it requires longer than 100-200 Myr to
significantly impact its environment. [Abridged]Comment: 23 pages, 18 figures and 7 tables. Accepted for publication in MNRA
The Proto-Magnetar Model for Gamma-Ray Bursts
Long duration Gamma-Ray Bursts (GRBs) originate from the core collapse of
massive stars, but the identity of the central engine remains elusive. Previous
work has shown that rapidly spinning, strongly magnetized proto-neutron stars
(`millisecond proto-magnetars') produce outflows with energies, timescales, and
magnetizations sigma_0 (maximum Lorentz factor) that are consistent with those
required to produce long GRBs. Here we extend this work in order to construct a
self-consistent model that directly connects the properties of the central
engine to the observed prompt emission. Just after the launch of the supernova
shock, a wind heated by neutrinos is driven from the proto-magnetar. The
outflow is collimated into a bipolar jet by its interaction with the star. As
the magnetar cools, the wind becomes ultra-relativistic and Poynting-flux
dominated (sigma_0 >> 1) on a timescale comparable to that required for the jet
to clear a cavity through the star. Although the site and mechanism of the
prompt emission are debated, we calculate the emission predicted by two models:
magnetic dissipation and internal shocks. Our results favor the magnetic
dissipation model in part because it predicts a relatively constant `Band'
spectral peak energy E_peak with time during the GRB. The jet baryon loading
decreases abruptly when the neutron star becomes transparent to neutrinos at t
~ 10-100 seconds. Jets with ultra-high magnetization cannot effectively
accelerate and dissipate their energy, suggesting this transition ends the
prompt emission and may explain the steep decay phase that follows. We assess
several phenomena potentially related to magnetar birth, including low
luminosity GRBs, thermal-rich GRBs/X-ray Flashes, very luminous supernovae, and
short duration GRBs with extended emission.Comment: 21 pages (plus 2 appendices), 21 figures, 1 table, now accepted to
MNRA
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