51 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
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
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
Quenching Massive Galaxies with On-the-fly Feedback in Cosmological Hydrodynamic Simulations
Massive galaxies today typically are not forming stars despite being
surrounded by hot gaseous halos with short central cooling times. This likely
owes to some form of "quenching feedback" such as merger-driven quasar activity
or radio jets emerging from central black holes. Here we implement heuristic
prescriptions for these phenomena on-the-fly within cosmological hydrodynamic
simulations. We constrain them by comparing to observed luminosity functions
and color-magnitude diagrams from SDSS. We find that quenching from mergers
alone does not produce a realistic red sequence, because 1 - 2 Gyr after a
merger the remnant accretes new fuel and star formation reignites. In contrast,
quenching by continuously adding thermal energy to hot gaseous halos
quantitatively matches the red galaxy luminosity function and produces a
reasonable red sequence. Small discrepancies remain - a shallow red sequence
slope suggests that our models underestimate metal production or retention in
massive red galaxies, while a deficit of massive blue galaxies may reflect the
fact that observed heating is intermittent rather than continuous. Overall,
injection of energy into hot halo gas appears to be a necessary and sufficient
condition to broadly produce red and dead massive galaxies as observed.Comment: 23 pages, 14 figures. MNRAS accepted. Added Sec. 4.4 and
significantly modified the Discussion at the suggestion of the refere
Residual Cooling and Persistent Star Formation amid AGN Feedback in Abell 2597
New Chandra X-ray and Herschel FIR observations enable a multiwavelength
study of active galactic nucleus (AGN) heating and intracluster medium (ICM)
cooling in the brightest cluster galaxy of Abell 2597. The new Chandra
observations reveal the central < 30 kiloparsec X-ray cavity network to be more
extensive than previously thought, and associated with enough enthalpy to
theoretically inhibit the inferred classical cooling flow. Nevertheless, we
present new evidence, consistent with previous results, that a moderately
strong residual cooling flow is persisting at 4%-8% of the classically
predicted rates in a spatially structured manner amid the feedback-driven
excavation of the X-ray cavity network. New Herschel observations are used to
estimate warm and cold dust masses, a lower-limit gas-to-dust ratio, and a star
formation rate consistent with previous measurements. The cooling time profile
of the ambient X-ray atmosphere is used to map the locations of the
observational star formation entropy threshold as well as the theoretical
thermal instability threshold. Both lie just outside the < 30 kpc central
region permeated by X-ray cavities, and star formation as well as ionized and
molecular gas lie interior to both. The young stars are distributed in an
elongated region that is aligned with the radio lobes, and their estimated ages
are both younger and older than the X-ray cavity network, suggesting both
jet-triggered as well as persistent star formation over the current AGN
feedback episode. Bright X-ray knots that are coincident with extended Ly-alpha
and FUV continuum filaments motivate a discussion of structured cooling from
the ambient hot atmosphere along a projected axis that is perpendicular to
X-ray cavity and radio axis. We conclude that the cooling ICM is the dominant
contributor of the cold gas reservoir fueling star formation and AGN activity
in the Abell 2597 BCG.Comment: 20 pages, 7 figures; Accepted for publication in MNRA
The X-ray counterpart to the gravitational wave event GW 170817
A long-standing paradigm in astrophysics is that collisions- or mergers- of
two neutron stars (NSs) form highly relativistic and collimated outflows (jets)
powering gamma-ray bursts (GRBs) of short (< 2 s) duration. However, the
observational support for this model is only indirect. A hitherto outstanding
prediction is that gravitational wave (GW) events from such mergers should be
associated with GRBs, and that a majority of these GRBs should be off-axis,
that is, they should point away from the Earth. Here we report the discovery of
the X-ray counterpart associated with the GW event GW170817. While the
electromagnetic counterpart at optical and infrared frequencies is dominated by
the radioactive glow from freshly synthesized r-process material in the merger
ejecta, known as kilonova, observations at X-ray and, later, radio frequencies
exhibit the behavior of a short GRB viewed off-axis. Our detection of X-ray
emission at a location coincident with the kilonova transient provides the
missing observational link between short GRBs and GWs from NS mergers, and
gives independent confirmation of the collimated nature of the GRB emission.Comment: 38 pages, 10 figures, Nature, in pres
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