71 research outputs found
Cumulative light curves of gamma-ray bursts and relaxation systems
The cumulative light curves of a large sample of gamma-ray bursts (GRBs) were
obtained by summing the BATSE counts. The smoothed profiles are much simpler
than the complex and erratic running light curves that are normally used. For
most GRBs the slope of the cumulative light curve (S) is approximately constant
over a large fraction of the burst. The bursts are modelled as relaxation
systems that continuously accumulate energy in the reservoir and
discontinuously release it. The slope is a measure of the cumulative power
output of the central engine. A plot of S versus peak flux in 64ms (P64ms)
shows a very good correlation over a wide range for both short and long GRBs.
No relationship was found between S and GRBs with known redshift. The standard
slope (S'), which is representative of the power output per unit time, is
correlated separately with P64ms for both sub-classes indicating more powerful
outbursts for the short GRBs. S' is also anticorrelated with GRB duration.
These results imply that GRBs are powered by accretion into a black hole.Comment: 4 pages, 2 figures. Accepted for publication in Astronomy and
Astrophysics Letter
Temporal properties of short and long gamma-ray bursts
A temporal analysis was performed on a sample of 100 bright short GRBs with
T90 < 2s from the BATSE Current Catalog along with a similar analysis on 319
long bright GRBs with T90 > 2s from the same catalog. The short GRBs were
denoised using a median filter and the long GRBs were denoised using a wavelet
method. Both samples were subjected to an automated pulse selection algorithm
to objectively determine the effects of neighbouring pulses. The rise times,
fall times, FWHM, pulse amplitudes and areas were measured and their frequency
distributions are presented. The time intervals between pulses were also
measured. The frequency distributions of the pulse properties were found to be
similar and consistent with lognormal distributions for both the short and long
GRBs. The time intervals between the pulses and the pulse amplitudes of
neighbouring pulses were found to be correlated with each other. The same
emission mechanism can account for the two sub-classes of GRBs.Comment: 3 pages, 8 figures; Proceedings of "Gamma-Ray Burst and Afterglow
Astronomy 2001", Woods Hol
Timing diagrams and correlations in gamma-ray bursts signal jets from accretion into black holes
The temporal properties of a sample of 498 bright gamma-ray bursts (GRBs)
with durations between 0.05 s and 674 s were analysed. The large range in
duration (T90) is accompanied by a similarly large range in the median values
of the pulse timing properties including rise time, fall time, FWHM and
separation between the pulses. Four timing diagrams relating these pulse
properties to T90 are presented and show the power law relationships between
the median values of the 4 pulse timing properties and T90, but also that the
power laws depend in a consistent manner on the number of pulses per GRB. The
timing diagrams are caused by the correlated properties of the pulses in the
burst and can be explained by a combination of factors including the Doppler
boost factor Gamma, a viewing effect caused by a jet and different progenitors.
GRBs with similar values of T90 have a wide range in the number of pulses. GRBs
with the large number of short and spectrally hard pulses may occur either from
a homogeneous jet with a higher average value of Gamma or close to the axis of
an inhomogeneous jet with higher values of Gamma near the rotation axis. The
less luminous GRBs with fewer pulses may originate further from the axis of the
inhomogeneous jet. The pulses in GRBs have six distinctive statistical
properties including correlations between time intervals, correlations between
pulse amplitudes, an anticorrelation between pulse amplitudes and time
intervals, and a link to intermittency in GRS 1915+105. The timing diagrams and
correlated pulses suggest that GRBs are powered by accretion processes
signalling jets from the formation of black holes.Comment: 4 pages, 1 figur
Temporal properties of the short gamma-ray bursts
A temporal analysis has been performed on a sample of 100 bright gamma-ray
bursts (GRBs) with T90<2s from the BATSE current catalog. The GRBs were
denoised using a median filter and subjected to an automated pulse selection
algorithm as an objective way of idenitifing the effects of neighbouring
pulses. The rise times, fall times, FWHM, pulse amplitudes and areas were
measured and the frequency distributions are presented here. All are consistent
with lognormal distributions. The distribution of the time intervals between
pulses is not random but consistent with a lognormal distribution. The time
intervals between pulses and pulse amplitudes are highly correlated with each
other. These results are in excellent agreement with a similar analysis that
revealed lognormal distributions for pulse properties and correlated time
intervals between pulses in bright GRBs with T90>2s. The two sub-classes of
GRBs appear to have the same emission mechanism which is probably caused by
internal shocks. They may not have the same progenitors because of the generic
nature of the fireball model.Comment: 4 pages, 7 figure
Temporal properties of gamma-ray bursts as signatures of jets from the central engine
A comprehensive temporal analysis has been performed on the 319 brightest
GRBs with T90>2s from the BATSE current catalog. The rise times, fall times,
full-widths at half maximum (FWHM), pulse amplitudes and pulse areas were
measured and the frequency distributions are presented here. The distribution
of time intervals between pulses is not random but compatible with a lognormal
distribution when allowance was made for the 64 ms time resolution and a small
excess (5%) of long duration intervals that is often referred to as a
Pareto-Levy tail. A range of correlations are presented on pulse and burst
properties. The rise and fall times, FWHM and area of the pulses are highly
correlated with each other. The time intervals between pulses and pulse
amplitudes of neighbouring pulses are correlated with each other. It was also
found that the number of pulses, N, in GRBs is strongly correlated with the
fluence and the duration and that can explain the well known correlation
between duration and fluence. The GRBs were sorted into three catagories based
on N i.e. 3=25. The properties of pulses before and after
the stongest pulse were compared for the three catagories of bursts. This
analysis revealed that the GRBs with large numbers of pulses have narrower and
faster pulses and also larger fluences, longer durations and higher hardness
ratios than the GRBs with smaller numbers of pulses.Comment: 19 pages, 22 figures. Submitted to A&A July 200
Kerr black holes and time profiles of gamma-ray bursts
The cumulative light curves of gamma-ray bursts (GRBs) smooth the spiky
nature of the running light curve. The cumulative count increases in an
approximately linear way with time t for most bursts. In 19 out of 398 GRBs
with T90 > 2s, the cumulative light curve was found to increase with time as
\~t^2 implying a linear increase in the running light curve. The non-linear
sections last for a substantial fraction of the GRB duration, have a large
proportion of the cumulative count and many resolved pulses that usually end
with the highest pulse in the burst. The reverse behaviour was found in 11 GRBs
where the running light curve decreased with time and some bursts are good
mirror images of the increases. These GRBs are among the spectrally hardest
bursts observed by BATSE. The most likely interpretation is that these effects
are signatures of black holes that are either being spun up or down in the
accretion process. In the spin up case, the increasing Kerr parameter of the
black hole allows additional rotational and accretion energy to become
available for extraction. The process is reversed if the black hole is spun
down by magnetic field torques. The luminosity changes in GRBs are consistent
with the predictions of the BZ process and neutrino annihilation and thus
provide the link to spinning black holes. GRBs provide a new window for
studying the general relativistic effects of Kerr black holes.Comment: 5 pages, 2 figures, accepted for publication in Astronomy and
Astrophysics Letter
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
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