103 research outputs found
On the Shape of Pulse Spectra in Gamma-Ray Bursts
The discovery (Liang & Kargatis 1996), that the peak energy of time-resolved
spectra of gamma-ray burst (GRB) pulses decays exponentially with fluence, is
analytically shown to imply that the time-integrated photon number spectrum of
a pulse should have a unique shape, given by an underlying E^-1 behavior. We
also show that the asymptotic low energy normalization of the time-integrated
spectrum is equal to the exponential decay constant. We study analytically how
this general behavior is modified in more realistic situations and show that
diversity is then introduced in the properties of time-integrated GRB pulse
spectra. We argue that further diversity will occur in time-integrated
multi-pulse (complex) GRB spectra. The total energy received per cm^2 is
approximately the decay constant times the maximum peak energy of the pulse.
Our analytical results connect the properties of the time-integrated pulse
spectrum with those of the time-resolved spectra, and can thus be used when
studying observed GRB pulse spectra. We illustrate with the bright burst GRB
910807 and comment on GRB 910525 and GRB 921207.Comment: 7 pages, 6 postscript figures, accepted by the Astrophysical Journa
The conspicuous gamma-ray burst of 30 May 1996
The spectra of the majority of bursts exhibit a low-energy power law index,
alpha, that is either a constant or becomes softer with time. However, in the
burst of 30 May 1996 alpha becomes harder. Here we show that this behavior can
be explained by a hybrid model consisting of a thermal and a non-thermal
component. In this burst the power-law index of the non-thermal component
changes drastically from s ~ -1.5 to s ~ -0.67 at approximately 5 seconds after
the trigger, thereby revealing, at low energies, the thermal component with its
hard Rayleigh-Jeans tail. This leads to the large alpha-values that are found
if the Band function is fitted to the spectra. We suggest that the change in s
could be due to a transition from fast to slow cooling of the electrons
emitting in the BATSE range. This could be due to the fact that the magnetic
field strength becomes weaker.Comment: Submitted to Il Nuovo Cimento (4th Workshop Gamma-Ray Bursts in the
Afterglow Era, Rome, 18-22 October 2004
On the Time Evolution of Gamma-Ray Burst Pulses: A Self-Consistent Description
For the first time, the consequences of combining two well-established
empirical relations, describing different aspects of the spectral evolution of
observed gamma-ray burst (GRB) pulses, are explored. These empirical relations
are: i) the hardness-intensity correlation, and ii) the hardness-photon fluence
correlation. From these we find a self-consistent, quantitative, and compact
description for the temporal evolution of pulse decay phases within a GRB light
curve. In particular, we show that in the case of the two empirical relations
both being valid, the instantaneous photon flux (intensity) must behave as
1/(1+ t/\tau) where \tau is a time constant that can be expressed in terms of
the parameters of the two empirical relations. The time evolution is fully
defined by two initial constants, and two parameters. We study a complete
sample of 83 bright GRB pulses observed by the Compton Gamma-Ray Observatory
and identify a major subgroup of GRB pulses (~45 %), which satisfy the
spectral-temporal behavior described above. In particular, the decay phase
follows a reciprocal law in time. It is unclear what physics causes such a
decay phase.Comment: 4 pages, 1 figure, 2 tables, to appear in ApJ
Clustering of gamma-ray burst types in the Fermi-GBM catalogue: indications of photosphere and synchrotron emissions during the prompt phase
Many different physical processes have been suggested to explain the prompt
gamma-ray emission in gamma-ray bursts (GRBs). Although there are examples of
both bursts with photospheric and synchrotron emission origins, these distinct
spectral appearances have not been generalized to large samples of GRBs. Here,
we search for signatures of the different emission mechanisms in the full Fermi
Gamma-ray Space Telescope GBM catalogue. We use Gaussian Mixture Models to
cluster bursts according to their parameters from the Band function (,
, and ) as well as their fluence and . We find five
distinct clusters. We further argue that these clusters can be divided into
bursts of photospheric origin (2/3 of all bursts, divided into 3 clusters) and
bursts of synchrotron origin (1/3 of all bursts, divided into 2 clusters). For
instance, the cluster that contains predominantly short bursts is consistent of
photospheric emission origin. We discuss several reasons that can determine
which cluster a burst belongs to: jet dissipation pattern and/or the jet
content, or viewing angle.Comment: Accepted for publication in MNRA
On the Hardness-Intensity Correlation in Gamma-Ray Burst Pulses
We study the hardness-intensity correlation (HIC) in gamma-ray bursts (GRBs).
In particular, we analyze the decay phase of pulse structures in their light
curves. The study comprises a sample of 82 long pulses selected from 66 long
bursts observed by BATSE on the Compton Gamma-Ray Observatory. We find that at
least 57% of these pulses have HICs that can be well described by a power law.
The distribution of the power law indices, obtained by modeling the HIC of
pulses from different bursts, is broad with a mean of 1.9 and a standard
deviation of 0.7. We also compare indices among pulses from the same bursts and
find that their distribution is significantly narrower. The probability of a
random coincidence is shown to be very small. In most cases, the indices are
equal to within the uncertainties. This is particularly relevant when comparing
the external versus the internal shock models. In our analysis, we also use a
new method for studying the HIC, in which the intensity is represented by the
peak value of the E F_E spectrum. This new method gives stronger correlations
and is useful in the study of various aspects of the HIC. In particular, it
produces a better agreement between indices of different pulses within the same
burst. Also, we find that some pulses exhibit a "track jump" in their HICs, in
which the correlation jumps between two power laws with the same index. We
discuss the possibility that the "track jump" is caused by strongly overlapping
pulses. Based on our findings, the constancy of the index is proposed to be
used as a tool for pulse identification in overlapping pulses.Comment: 20 pages with 9 eps figures (emulateapj), ApJ accepte
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