1,743 research outputs found
The X-ray light curve of Gamma-ray bursts: clues to the central engine
We present the analysis of a large sample of gamma-ray burst (GRB) X-ray
light curves in the rest frame to characterise their intrinsic properties in
the context of different theoretical scenarios. We determine the morphology,
time scales, and energetics of 64 long GRBs observed by \emph{Swift}/XRT
\emph{without} flaring activity. We furthermore provide a one-to-one comparison
to the properties of GRBs \emph{with} X-ray flares. We find that the steep
decay morphology and its connection with X-ray flares favour a scenario in
which a central engine origin. We show that this scenario can also account for
the shallow decay phase, provided that the GRB progenitor star has a
self-similar structure with a constant envelope-to-core mass ratio . However, difficulties arise for very long duration
( s) shallow phases. Alternatively, a spinning-down magnetar
whose emitted power refreshes the forward shock can quantitatively account for
the shallow decay properties. In particular we demonstrate that this model can
account for the plateau luminosity vs. end time anticorrelation.Comment: 12 pages, 8 figures, accepted for publication in A&
On the average Gamma-Ray Burst X-ray flaring activity
Gamma-ray burst X-ray flares are believed to mark the late time activity of
the central engine. We compute the temporal evolution of the average flare
luminosity in the common rest frame energy band of 44 GRBs taken from
the large \emph{Swift} 5-years data base. Our work highlights the importance of
a proper consideration of the threshold of detection of flares against the
contemporaneous continuous X-ray emission. In the time interval ; this implies
that the flare isotropic energy scaling is . The decay of the continuum underlying the flare emission closely
tracks the average flare luminosity evolution, with a typical flare to
steep-decay luminosity ratio which is : this
suggests that flares and continuum emission are deeply related to one another.
We infer on the progenitor properties considering different models. According
to the hyper-accreting black hole scenario, the average flare luminosity
scaling can be obtained in the case of rapid accretion () or
when the last \sim 0.5 M_{\sun} of the original 14 M_{\sun} progenitor star
are accreted. Alternatively, the steep behaviour could be
triggered by a rapid outward expansion of an accretion shock in the material
feeding a convective disk. If instead we assume the engine to be a rapidly
spinning magnetar, then its rotational energy can be extracted to power a jet
whose luminosity is likely to be between the monopole () and
dipole () cases. In both scenarios we suggest the variability,
which is the main signature of the flaring activity, to be established as a
consequence of different kinds of instabilities.Comment: MNRAS accepte
Gamma-Ray Burst long lasting X-ray flaring activity
In this paper we shed light on late time (i.e. with peak time t_{pk} \gtrsim
1000 s) flaring activity. We address the morphology and energetic of flares in
the window \sim 10^3-10^6 s to put constraints on the temporal evolution of the
flare properties and to identify possible differences in the mechanism
producing the early and late time flaring emission, if any. This requires the
complete understanding of the observational biases affecting the detection of
X-ray flares superimposed on a fading continuum at t > 1000 s. We consider all
the Swift GRBs that exhibit late time flares. Our sample consists of 36 flares,
14 with redshift measurements. We inherit the strategy of data analysis from
Chincarini et al. (2010) in order to make a direct comparison with the early
time flare properties. The morphology of the flare light curve is the same for
both early time and late time flares, while they differ energetically. The
width of late time flares increases with time similarly to the early time
flares. Simulations confirmed that the increase of the width with time is not
due to the decaying statistics, at least up to 10^4 s. The energy output of
late time flares is one order of magnitude lower than the early time flare one,
being \sim 1% E_{prompt}. The evolution of the peak luminosity as well as the
distribution of the peak flux-to-continuum ratio for late time flares indicate
that the flaring emission is decoupled from the underlying continuum,
differently from early time flares/steep decay. A sizable fraction of late time
flares are compatible with afterglow variability. The internal shock origin
seems the most promising explanation for flares. However, some differences that
emerge between late and early time flares suggest that there could be no unique
explanation about the nature of late time flares.Comment: 8 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Lag-luminosity relation in gamma-ray burst X-ray flares: a direct link to the prompt emission
The temporal and spectral analysis of 9 bright X-ray flares out of a sample
of 113 flares observed by Swift reveals that the flare phenomenology is
strictly analogous to the prompt gamma-ray emission: high energy flare profiles
rise faster, decay faster and peak before the low energy emission. However,
flares and prompt pulses differ in one crucial aspect: flares evolve with time.
As time proceeds flares become wider, with larger peak lag, lower luminosities
and softer emission. The flare spectral peak energy E_{p,i} evolves to lower
values following an exponential decay which tracks the decay of the flare flux.
The two flares with best statistics show higher than expected isotropic energy
E_{iso} and peak luminosity L_{p,iso} when compared to the E_{p,i}-E_{iso} and
E_{p,i}-L_{iso} prompt correlations. E_{p,i} is found to correlate with L_{iso}
within single flares, giving rise to a time resolved E_{p,i}(t)-L_{iso}(t).
Like prompt pulses, flares define a lag-luminosity relation: L_{p,iso}^{0.3-10
keV} t_{lag}^{-0.95+/-0.23}. The lag-luminosity is proven to be a fundamental
law extending 5 decades in time and 5 in energy. Moreover, this is direct
evidence that GRB X-ray flares and prompt gamma-ray pulses are produced by the
same mechanism. Finally we establish a flare-afterglow morphology connection:
flares are preferentially detected superimposed to one-break or canonical X-ray
afterglows.Comment: MNRAS accepte
Survival of the Fittest: Numerical Modeling of Supernova 2014C
Initially classified as a supernova (SN) type Ib, 100 days after the
explosion SN\,2014C made a transition to a SN type II, presenting a gradual
increase in the H emission. This has been interpreted as evidence of
interaction between the supernova shock wave and a massive shell previously
ejected from the progenitor star. In this paper, we present numerical
simulations of the propagation of the SN shock through the progenitor star and
its wind, as well as the interaction of the SN ejecta with the massive shell.
To determine with high precision the structure and location of the shell, we
couple a genetic algorithm to a hydrodynamic and a bremsstrahlung radiation
transfer code. We iteratively modify the density stratification and location of
the shell by minimizing the variance between X-ray observations and synthetic
predictions computed from the numerical model. By assuming spherical symmetry,
we found that the shell has a mass of 2.6 M, extends from 1.6 cm to cm, implying that it was ejected yrs before the SN explosion, and has a
density stratification decaying as . We found that the product of
metallicity by the ionization fraction (due to photo-ionization by the
post-shock X-ray emission) %and/or the SN UV radiation is 0.5. Finally,
we predict that, if the density stratification follows the same power-law
behaviour, the SN will break out from the shell by mid 2022, i.e. 8.5 years
after explosion
Spitzer Space Telescope Infrared Observations of the Binary Neutron Star Merger GW170817
We present Spitzer Space Telescope 3.6 and 4.5 micron observations of the
binary neutron star merger GW170817 at 43, 74, and 264 days post-merger. Using
the final observation as a template, we uncover a source at the position of
GW170817 at 4.5 micron with a brightness of 22.9+/-0.3 AB mag at 43 days and
23.8+/-0.3 AB mag at 74 days (the uncertainty is dominated by systematics from
the image subtraction); no obvious source is detected at 3.6 micron to a
3-sigma limit of >23.3 AB mag in both epochs. The measured brightness is dimmer
by a factor of about 2-3 times compared to our previously published kilonova
model, which is based on UV, optical, and near-IR data at <30 days. However,
the observed fading rate and color (m_{3.6}-m_{4.5}> 0 AB mag) are consistent
with our model. We suggest that the discrepancy is likely due to a transition
to the nebular phase, or a reduced thermalization efficiency at such late time.
Using the Spitzer data as a guide, we briefly discuss the prospects of
observing future binary neutron star mergers with Spitzer (in LIGO/Virgo
Observing Run 3) and the James Webb Space Telescope (in LIGO/Virgo Observing
Run 4 and beyond).Comment: 6 pages, 2 figures, submitted to ApJ
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