217 research outputs found
Heterogeneity in Short Gamma-ray Bursts
We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective
Bayesian Block procedure to extract temporal descriptors of the bursts' initial
pulse complexes (IPCs). The sample comprises 12 and 41 bursts with and without
extended emission (EE) components, respectively. IPCs of non-EE bursts are
dominated by single pulse structures, while EE bursts tend to have two or more
pulse structures. The medians of characteristic timescales - durations, pulse
structure widths, and peak intervals - for EE bursts are factors of ~ 2-3
longer than for non-EE bursts. A trend previously reported by Hakkila and
colleagues unifying long and short bursts - the anti-correlation of pulse
intensity and width - continues in the two short burst groups, with non-EE
bursts extending to more intense, narrower pulses. In addition we find that
preceding and succeeding pulse intensities are anti-correlated with pulse
interval. We also examine the short burst X-ray afterglows as observed by the
Swift/XRT. The median flux of the initial XRT detections for EE bursts (~ 6 x
10^-10 erg cm^-2 s^-1) is ~> 20 x brighter than for non-EE bursts, and the
median X-ray afterglow duration for EE bursts (~ 60,000 s) is ~ 30 x longer
than for non-EE bursts.
The tendency for EE bursts toward longer prompt-emission timescales and
higher initial X-ray afterglow fluxes implies larger energy injections powering
the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest
that a significant fraction explode into more dense environments than non-EE
bursts, or that the sometimes-dominant EE component efficiently powers the
afterglow. Combined, these results favor different progenitors for EE and
non-EE short bursts.Comment: 30 pages, 11 figures, 3 tables; accepted to The Astrophysical Journa
Correlations Between Lag, Luminosity, and Duration in Gamma-ray Burst Pulses
We derive a new peak lag vs. peak luminosity relation in gamma-ray burst
(GRB) pulses. We demonstrate conclusively that GRB spectral lags are pulse
rather than burst properties and show how the lag vs. luminosity relation
determined from CCF measurements of burst properties is essentially just a
rough measure of this newly derived relation for individual pulses. We further
show that most GRB pulses have correlated properties: short-lag pulses have
shorter durations, are more luminous, and are harder within a burst than
long-lag pulses. We also uncover a new pulse duration vs. pulse peak luminosity
relation, and indicate that long-lag pulses often precede short-lag pulses.
Although most pulse behaviors are supportive of internal shocks (including
long-lag pulses), we identify some pulse shapes that could result from external
shocks.Comment: 14 pages, 4 figures, 1 table; accepted for publication in
Astrophysical Journal Letter
Classification of Swift's gamma-ray bursts
Two classes of gamma-ray bursts have been identified in the BATSE catalogs
characterized by durations shorter and longer than about 2 seconds. There are,
however, some indications for the existence of a third class. Swift satellite
detectors have different spectral sensitivity than pre-Swift ones for gamma-ray
bursts. Therefore, it is worth to reanalyze the durations and their
distribution. We analyze, the maximum likelihood estimation, the bursts
duration distribution, published in The First BAT Catalog, whether it contains
two, three or more groups. The three log-normal fit is significantly (99.54%
probability) better than the two for the duration distribution. Monte-Carlo
simulations also confirm this probability (99.2%). Similarly, in previous
results we found that the fourth component is not needed. The relative
frequencies of the distribution of the groups are 7% short 35% intermediate and
58% long. Although the relative frequencies of the groups are different than in
the BATSE GRB sample, the difference in the instrument spectral sensitivities
can explain this bias on a natural way. This means theoretical models may be
needed to explain three different type of gamma-ray bursts.Comment: Accepted in AA, added bibliograph
Gamma-Ray Burst Pulse Correlations as Redshift Indicators
Correlations among pulse properties in the prompt emission of long GRBs can
potentially be used as cosmological distance indicators to estimate redshifts
of GRBs to which these pulses belong. We demonstrate application of this
technique to a sample of GRBs for which redshifts are not known. We also study
the scatter of predicted redshifts of pulses found within individual bursts. We
explore the characteristics of this scatter in hopes of identifying systematic
corrections and/or pulse subsets that can be used to increase the technique's
reliability.Comment: 3 pages and 6 figures, to appear in the proceedings of the Sixth
Huntsville Gamma-Ray Burst Symposium, edited by C.A. Meegan, N. Gehrels, and
C. Kouvelioto
Detailed Classification of Swift's Gamma-Ray Bursts
Earlier classification analyses found three types of gamma-ray bursts (short,
long and intermediate in duration) in the BATSE sample. Recent works have shown
that these three groups are also present in the RHESSI and the BeppoSAX
databases. The duration distribution analysis of the bursts observed by the
Swift satellite also favors the three-component model. In this paper, we extend
the analysis of the Swift data with spectral information. We show, using the
spectral hardness and the duration simultaneously, that the maximum likelihood
method favors the three-component against the two-component model. The
likelihood also shows that a fourth component is not needed.Comment: Accepted for publication in The Astrophysical Journa
How Sample Completeness Affects Gamma-Ray Burst Classification
Unsupervised pattern recognition algorithms support the existence of three
gamma-ray burst classes; Class I (long, large fluence bursts of intermediate
spectral hardness), Class II (short, small fluence, hard bursts), and Class III
(soft bursts of intermediate durations and fluences). The algorithms
surprisingly assign larger membership to Class III than to either of the other
two classes. A known systematic bias has been previously used to explain the
existence of Class III in terms of Class I; this bias allows the fluences and
durations of some bursts to be underestimated (Hakkila et al., ApJ 538, 165,
2000). We show that this bias primarily affects only the longest bursts and
cannot explain the bulk of the Class III properties. We resolve the question of
Class III existence by demonstrating how samples obtained using standard
trigger mechanisms fail to preserve the duration characteristics of small peak
flux bursts. Sample incompleteness is thus primarily responsible for the
existence of Class III. In order to avoid this incompleteness, we show how a
new dual timescale peak flux can be defined in terms of peak flux and fluence.
The dual timescale peak flux preserves the duration distribution of faint
bursts and correlates better with spectral hardness (and presumably redshift)
than either peak flux or fluence. The techniques presented here are generic and
have applicability to the studies of other transient events. The results also
indicate that pattern recognition algorithms are sensitive to sample
completeness; this can influence the study of large astronomical databases such
as those found in a Virtual Observatory.Comment: 29 pages, 6 figures, 3 tables, Accepted for publication in The
Astrophysical Journa
The Spectral Lag of GRB060505: A Likely Member of the Long Duration Class
Two long gamma-ray bursts, GRB 060505 and GRB 060614, occurred in nearby
galaxies at redshifts of 0.089 and 0.125 respectively. Due to their proximity
and durations, deep follow-up campaigns to search for supernovae (SNe) were
initiated. However none were found in either case, to limits more than two
orders of magnitude fainter than the prototypical GRB-associated SN, 1998bw. It
was suggested that the bursts, in spite of their durations (4 and 102 s),
belonged to the population of short GRBs which has been shown to be unrelated
to SNe. In the case of GRB 060614 this argument was based on a number of
indicators, including the negligible spectral lag, which is consistent with
that of short bursts. GRB 060505 has a shorter duration, but no spectral lag
was measured. We present the spectral lag measurements of GRB 060505 using
Suzakus Wide Area Monitor and the Swift Burst Alert Telescope. We find that the
lag is 0.36+/- 0.05 s, inconsistent with the lags of short bursts and
consistent with the properties of long bursts and SN-GRBs. These results
support the association of GRB 060505 with other low-luminosity GRBs also found
in star-forming galaxies and indicates that at least some massive stars may die
without bright SNe.Comment: Accepted by ApJL, 5 pages, 3 Figure
A distinct peak-flux distribution of the third class of gamma-ray bursts: A possible signature of X-ray flashes?
Gamma-ray bursts are the most luminous events in the Universe. Going beyond
the short-long classification scheme we work in the context of three burst
populations with the third group of intermediate duration and softest spectrum.
We are looking for physical properties which discriminate the intermediate
duration bursts from the other two classes. We use maximum likelihood fits to
establish group memberships in the duration-hardness plane. To confirm these
results we also use k-means and hierarchical clustering. We use Monte-Carlo
simulations to test the significance of the existence of the intermediate group
and we find it with 99.8% probability. The intermediate duration population has
a significantly lower peak-flux (with 99.94% significance). Also, long bursts
with measured redshift have higher peak-fluxes (with 98.6% significance) than
long bursts without measured redshifts. As the third group is the softest, we
argue that we have {related} them with X-ray flashes among the gamma-ray
bursts. We give a new, probabilistic definition for this class of events.Comment: accepted for publication in Ap
Time-Resolved Spectroscopy of the 3 Brightest and Hardest Short Gamma-Ray Bursts Observed with the FGST Gamma-Ray Burst Monitor
From July 2008 to October 2009, the Gamma-ray Burst Monitor (GBM) on board
the Fermi Gamma-ray Space Telescope (FGST) has detected 320 Gamma-Ray Bursts
(GRBs). About 20% of these events are classified as short based on their T90
duration below 2 s. We present here for the first time time-resolved
spectroscopy at timescales as short as 2 ms for the three brightest short GRBs
observed with GBM. The time-integrated spectra of the events deviate from the
Band function, indicating the existence of an additional spectral component,
which can be fit by a power-law with index ~-1.5. The time-integrated Epeak
values exceed 2 MeV for two of the bursts, and are well above the values
observed in the brightest long GRBs. Their Epeak values and their low-energy
power-law indices ({\alpha}) confirm that short GRBs are harder than long ones.
We find that short GRBs are very similar to long ones, but with light curves
contracted in time and with harder spectra stretched towards higher energies.
In our time-resolved spectroscopy analysis, we find that the Epeak values range
from a few tens of keV up to more than 6 MeV. In general, the hardness
evolutions during the bursts follows their flux/intensity variations, similar
to long bursts. However, we do not always see the Epeak leading the light-curve
rises, and we confirm the zero/short average light-curve spectral lag below 1
MeV, already established for short GRBs. We also find that the time-resolved
low-energy power-law indices of the Band function mostly violate the limits
imposed by the synchrotron models for both slow and fast electron cooling and
may require additional emission processes to explain the data. Finally, we
interpreted these observations in the context of the current existing models
and emission mechanisms for the prompt emission of GRBs.Comment: 14 pages, 10 figures, 9 tables, Accepted for publication in the
Astrophysical Journal September, 23 2010 (Submitted May, 16 2010)
Corrections: 1 reference updated, figure 10 captio
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