118 research outputs found
Spectral components in the bright, long GRB 061007: properties of the photosphere and the nature of the outflow
We present a time-resolved spectral analysis of the bright, long GRB 061007
(z=1.261) using Swift BAT and Suzaku WAM data. We find that the prompt emission
of GRB 061007 can be equally well explained by a photospheric component
together with a power law as by a Band function, and we explore the
implications of the former model. The photospheric component, which we model
with a multicolour blackbody, dominates the emission and has a very stable
shape throughout the burst. This component provides a natural explanation for
the hardness-intensity correlation seen within the burst and also allows us to
estimate the bulk Lorentz factor and the radius of the photosphere. The
power-law component dominates the fit at high energies and has a nearly
constant slope of -1.5. We discuss the possibility that this component is of
the same origin as the high-energy power laws recently observed in some Fermi
LAT bursts.Comment: Accepted for publication in MNRA
The Two-Component Afterglow of Swift GRB 050802
This paper investigates GRB 050802, one of the best examples of a it Swift
gamma-ray burst afterglow that shows a break in the X-ray lightcurve, while the
optical counterpart decays as a single power-law. This burst has an optically
bright afterglow of 16.5 magnitude, detected throughout the 170-650nm spectral
range of the UVOT on-board Swift. Observations began with the XRT and UVOT
telescopes 286s after the initial trigger and continued for 1.2 x 10^6s. The
X-ray lightcurve consists of three power-law segments: a rise until 420s,
followed by a slow decay with alpha_2 = 0.63 +/- 0.03 until 5000s, after which,
the lightcurve decays faster with a slope of alpha_3 = 1.59 +/- 0.03. The
optical lightcurve decays as a single power-law with alpha_O = 0.82 +/- 0.03
throughout the observation. The X-ray data on their own are consistent with the
break at 5000s being due to the end of energy injection. Modelling the optical
to X-ray spectral energy distribution, we find that the optical afterglow can
not be produced by the same component as the X-ray emission at late times,
ruling out a single component afterglow. We therefore considered two-component
jet models and find that the X-ray and optical emission is best reproduced by a
model in which both components are energy injected for the duration of the
observed afterglow and the X-ray break at 5000s is due to a jet break in the
narrow component. This bright, well-observed burst is likely a guide for
interpreting the surprising finding of Swift that bursts seldom display
achromatic jet breaks.Comment: 13 pages, 5 figures, accepted MNRA
Joint spectral-timing modelling of the hard lags in GX 339-4: constraints on reflection models
The X-ray variations of hard state black hole X-ray binaries above 2 keV show
'hard lags', in that the variations at harder energies follow variations at
softer energies, with a time-lag \tau depending on frequency \nu approximately
as \tau \propto \nu^{-0.7}. Several models have so far been proposed to explain
this time delay, including fluctuations propagating through an accretion flow,
spectral variations during coronal flares, Comptonisation in the extended hot
corona or a jet, or time-delays due to large-scale reflection from the
accretion disc. In principle these models can be used to predict the shape of
the energy spectrum as well as the frequency-dependence of the time-lags,
through the construction of energy-dependent response functions which map the
emission as a function of time-delay in the system. Here we use this approach
to test a simple reflection model for the frequency-dependent lags seen in the
hard state of GX 339-4, by simultaneously fitting the model to the
frequency-dependent lags and energy spectrum measured by XMM-Newton in 2004 and
2009. Our model cannot simultaneously fit both the lag and spectral data, since
the relatively large lags require an extremely flared disc which subtends a
large solid angle to the continuum at large radii, in disagreement with the
observed Fe K\alpha emission. Therefore, we consider it more likely that the
lags > 2 keV are caused by propagation effects in the accretion flow, possibly
related to the accretion disc fluctuations which have been observed previously.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
GRB 110328A/Swift J164449.3+573451: The Tidal Obliteration of a Deeply Plunging Star?
We examine the tidal disruption event scenario to explain Sw 1644+57, a
powerful and persistent X-ray source which suddenly became active as GRB
110328A. The precise localization at the center of a z=0.35 galaxy argues for
activity of the central engine as the underlying cause. We look at the
suggestion by Bloom et al of the possibility of a tidal disruption event (TDE).
We argue that Sw 1644+57 cannot be explained by the traditional TDE model in
which the periastron distance is close to the tidal disruption radius - three
independent lines of argument indicate the orbit must be deeply plunging or
else the powerful jet we are observing could not be produced. These arguments
stem from (i) comparing the early X-ray light curve to the expected theoretical
fallback rate, (ii) looking at the time of transition to disk-dominated decay,
and (iii) considering the TDE rate. Due to the extreme excess in the tidal
force above that which would be required minimally to disrupt the star in a
deeply plunging orbit at periastron, we suggest this scenario might be referred
to more descriptively as a TOE (tidal obliteration event) rather than a TDE.Comment: 7 pages, 2 figures, accepted by the Astrophysical Journal, major
revisions since vers. [1]; corrupted file in vers. [2] replace
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
Unveiling the origin of X-ray flares in Gamma-Ray Bursts
We present an updated catalog of 113 X-ray flares detected by Swift in the
~33% of the X-ray afterglows of Gamma-Ray Bursts (GRB). 43 flares have a
measured redshift. For the first time the analysis is performed in 4 different
X-ray energy bands, allowing us to constrain the evolution of the flare
temporal properties with energy. We find that flares are narrower at higher
energies: their width follows a power-law relation w~E^{-0.5} reminiscent of
the prompt emission. Flares are asymmetric structures, with a decay time which
is twice the rise time on average. Both time scales linearly evolve with time,
giving rise to a constant rise-to-decay ratio: this implies that both time
scales are stretched by the same factor. As a consequence, the flare width
linearly evolves with time to larger values: this is a key point that clearly
distinguishes the flare from the GRB prompt emission. The flare 0.3-10 keV peak
luminosity decreases with time, following a power-law behaviour with large
scatter: L_{pk}~ t_{pk}^{-2.7}. When multiple flares are present, a global
softening trend is established: each flare is on average softer than the
previous one. The 0.3-10 keV isotropic energy distribution is a log-normal
peaked at 10^{51} erg, with a possible excess at low energies. The flare
average spectral energy distribution (SED) is found to be a power-law with
spectral energy index beta~1.1. These results confirmed that the flares are
tightly linked to the prompt emission. However, after considering various
models we conclude that no model is currently able to account for the entire
set of observations.Comment: MNRAS submitte
The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum
We report deep Chandra, HST and VLA observations of the binary neutron star
event GW170817 at d after merger. These observations show that GW170817
has been steadily brightening with time and might have now reached its peak,
and constrain the emission process as non-thermal synchrotron emission where
the cooling frequency is above the X-ray band and the synchrotron
frequency is below the radio band. The very simple power-law spectrum
extending for eight orders of magnitude in frequency enables the most precise
measurement of the index of the distribution of non-thermal relativistic
electrons accelerated by a shock launched by a
NS-NS merger to date. We find , which indicates that radiation
from ejecta with dominates the observed emission. While
constraining the nature of the emission process, these observations do
\emph{not} constrain the nature of the relativistic ejecta. We employ
simulations of explosive outflows launched in NS ejecta clouds to show that the
spectral and temporal evolution of the non-thermal emission from GW170817 is
consistent with both emission from radially stratified quasi-spherical ejecta
traveling at mildly relativistic speeds, \emph{and} emission from off-axis
collimated ejecta characterized by a narrow cone of ultra-relativistic material
with slower wings extending to larger angles. In the latter scenario, GW170817
harbored a normal SGRB directed away from our line of sight. Observations at
days are unlikely to settle the debate as in both scenarios the
observed emission is effectively dominated by radiation from mildly
relativistic material.Comment: Updated with the latest VLA and Chandra dat
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