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We present a database of spectral lags and internal luminosity function (ILF) measurements for gamma-ray bursts (GRBs) in the BATSE catalog. Measurements were made using 64 ms count rate data and are defined for various combinations of the four broadband BATSE energy channels. We discuss the processes used for measuring lags and ILF characteristics. We discuss the statistical and systematic uncertainties in measuring these attributes, as well as the role of temporal resolution in measuring lags and/or ILFs—these are particularly noticeable for GRBs belonging to the Short class. Correlative and clustering properties of the lag and ILF are examined, including the ability of these attributes to predict GRB time history morphologies. We conclude that the ILF and lag have great potential for studying GRB physics when used with other burst attributes.

A simple scheme delineates Long GRBs with similar time histories using the Internal Luminosity Function power-law index and the spectral lag. Several generalizations are made about time history morphologies: 1) GRBs with long spectral lags contain fewer pulses that are broader than those found in bursts with short spectral lags, 2) short-lag bursts with small ILF power-law indices have many narrow pulses and are highly variable, while long-lag bursts with small ILF power-law indices are characterized by broad, smooth pulses and have low variability. GRB time history morphologies primarily identify intrinsic rather than extrinsic characteristics based on correlations with gamma-ray luminosity, afterglow luminosity, and numbers of pulses. These characteristics result because internal relativistic effects due to bulk Lorentz factor are larger than cosmological effects, and because the numbers and shapes of pulses indicate different efficiencies and forms of GRB energy release. Single-pulsed GRBs are characterized either by large ILF power indices (indicating a range of jet opening angles and Lorentz factors, with a FRED pulse shape), or they have long lags (large jet opening angles with low Lorentz factors, with either a FRED pulse shape or an unpeaked, smooth pulse shape). They also have lower-luminosity afterglows than multi-pulsed GRBs. GRBs with simple time histories are often associated with Type Ibc supernovae. This suggests that some single-pulsed GRBs contain single, beamed blast waves that are similar to and have characteristics that overlap with those of many supernovae. Such a connection may not exist between multi-pulsed GRBs and supernovae., Comment: Accepted for publication in The Astrophysical Journal. 29 pages, 10 figures, 1 table

Gamma-Ray Burst (GRB) prompt emission contains information that can be used to infer structure of the relativistic outflow. Spectral lags, the Internal Luminosity Function (ILF), and Color-Color Diagrams are attributes that provide diagnositcs with which jet structure can be studied. These attributes help delineate properties of internal shocks originating in the large Lorentz factor, tightly-beamed central core of the jet and external shocks resulting when the jet slows down by decelerating into the external medium. The external shock signature has a smaller Lorentz factor and a larger beaming angle than the central jet core, yet both components are present in the prompt emission of some GRBs.

We have identified two quiescent GRBs (bursts having two or more widely-separated distinct emission episodes) in which the post-quiescent emission exhibits distinctly different characteristics than the pre-quiescent emission. In these two cases (BATSE GRBs 960530 and 980125), the second emission episode has a longer lag, a smoother morphology, and softer spectral evolution than the first episode. Although the pre-quiescent emission satisfies the standard internal shock paradigm, we demonstrate that the post-quiescent emission is more consistent with external shocks. We infer that some observed soft, faint, long-lag GRBs are external shocks in which the internal shock signature is not observed. We further note that the peak luminosity ratio between quiescent episodes is not in agreement with the ratio predicated by the lag vs. peak luminosity relationship. We briefly discuss these observations in terms of current collapsar jet models.

Isolated Swift gamma-ray burst (GRB) pulses, like their higher-energy BATSE counterparts, emit the bulk of their pulsed emission as a hard-to-soft component that can be fitted by the Norris et al. (2005) empirical pulse model. This signal is overlaid by a fainter, three-peaked signal that can be modeled by an empirical wave-like function (Hakkila and Preece, 2014): the two fits combine to reproduce GRB pulses with distinctive three-peaked shapes. The precursor peak appears on or before the pulse rise and is often the hardest component, the central peak is the brightest, and the decay peak converts exponentially decaying emission into a long, soft, power-law tail. Accounting for systematic instrumental differences, the general characteristics of the fitted pulses are remarkably similar. Isolated GRB pulses are dominated by hard-to-soft evolution; this is more pronounced for asymmetric pulses than for symmetric ones. Isolated GRB pulses can also exhibit intensity tracking behaviors that, when observed, are tied to the timing of the three peaks: pulses with the largest maximum hardnesses are hardest during the precursor, those with smaller maximum hardnesses are hardest during the central peak, and all pulses can re-harden during the central peak and/or during the decay peak. Since these behaviors are essentially seen in all isolated pulses, the distinction between "hard-to-soft" and "intensity-tracking" pulses really no longer applies. Additionally, the triple-peaked nature of isolated GRB pulses seems to indicate that energy is injected on three separate occasions during the pulse duration: theoretical pulse models need to account for this., Comment: accepted for publication in The Astrophysical Journal

The connection between Gamma-Ray Bursts (GRBs) and their afterglows is currently not well understood. Afterglow models of synchrotron emission generated by external shocks in the GRB fireball model predict emission detectable in the gamma-ray regime ($\gax 25$ keV). In this paper, we present a temporal and spectral analysis of a subset of BATSE GRBs with smooth extended emission tails to search for signatures of the ``early high-energy afterglow'', i.e., afterglow emission that initially begins in the gamma-ray phase and subsequently evolves into X-Ray, uv, optical, and radio emission as the blast wave is decelerated by the ambient medium. From a sample of 40 GRBs we find that the temporal decays are best described with a power-law $\sim t^{\beta}$, rather than an exponential, with a mean index $ \approx -2$. Spectral analysis shows that $\sim 20%$ of these events are consistent with fast-cooling synchrotron emission for an adiabatic blast wave; three of which are consistent with the blast wave evolution of a jet, with $F_{\nu} \sim t^{-p}$. This behavior suggests that, in some cases, the emission may originate from a narrow jet, possibly consisting of ``nuggets'' whose angular size are less than $1 / \Gamma$, where $\Gamma$ is the bulk Lorentz factor., Comment: 17 pages, 4 color PostScript figures, to appear in ApJ May 10, 2002 issue

We present evidence for burst emission from SGR 1900+14 with a power-law high energy spectrum extending beyond 500 keV. Unlike previous detections of high energy photons during bursts from SGRs, these emissions are not associated with high-luminosity burst intervals. Not only is the emission hard, but the spectra are better fit by Band's GRB function rather than by the traditional optically-thin thermal bremsstrahlung model. We find that the spectral evolution within these hard events obeys a hardness/intensity anti-correlation. Temporally, these events are distinct from typical SGR burst emissions in that they are longer (~ 1 s) and have relatively smooth profiles. Despite a difference in peak luminosity of > 1E+11 between these bursts from SGR 1900+14 and cosmological GRBs, there are striking temporal and spectral similarities between the two kinds of bursts, aside from spectral evolution. We outline an interpretation of these events in the context of the magnetar model., 11 pages (text and figures), submitted to ApJ Letters, corrected erroneous hardness ratios

The lunar scientific community is currently exploring and planning a new vision of scientific experimentation and exploration using the lunar surface as a platform for scientific investigations that include Earth observations, lunar science, Solar System studies, and the Universe that are uniquely enabled on the lunar surface. This lunar exploration science begins with robotic precursor missions, eventually followed by human missions to the lunar surface. The concept of a central lunar operations facility can be envisioned to support the challenge of coordinating lunar operations and science across a myriad of participating institutions. A Center for Lunar Exploration Operations (CLEO) could be implemented at a facility such as the Mission Control Center at NASA’s Johnson Space Center where significant infrastructure is readily available.

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. Kouveliotou

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., 14 pages, 4 figures, 1 table; accepted for publication in Astrophysical Journal Letters

Gamma‐ray burst (GRB) spectral lags are pulse rather than burst characteristics. Most GRB pulses have well‐defined properties: short‐lag pulses have shorter durations and are brighter than long‐lag pulses. These properties are present in the observer's frame, implying that pulse characteristics are intrinsic and are more pronounced than cosmological effects. We show how the GRB lag measured by the cross‐correlation function is primarily that of the brightest, shortest‐lag pulse, and is insensitive to the presence of longer‐lag pulses. The hypothesis that long‐lag pulses are related to external shocks is thus placed in doubt because (1) short‐lag pulses often precede long‐lag ones temporally, and (2) the range of pulse lags seems to span the range of GRB lags.

The scientific method encourages sharing data with other researchers to independently verify conclusions. Currently, technical barriers impede such public scrutiny. A strategy for offering scientific data for public analysis is described. With this strategy, effectively no requirements of software installation (other than a web browser) or data manipulation are imposed on other researchers to prepare for perusing the scientific data. A prototype showcasing this strategy is described.

Long‐lag GRBs have two different morphologies that can be delineated using the Internal Luminosity Function (ILF). Both burst types are typically dominated by a single broad pulse. Long‐lag GRBs with large ILF indices are long FRED (Fast Rise Exponential Decay) bursts having well‐defined peaks, while those with small ILF indices are long smooth bursts having no well‐defined short timescale pulse structure. Long smooth bursts typically have have the longest lags of any type of GRB time history. We examine the hypothesis that long‐lag GRBs are external shock signatures and contrast this with the hypothesis that they are merely broad internal shock pulses.

Currently, the best available probe of the early phase of gamma-ray burst (GRB) jet attributes is the prompt gamma-ray emission, in which several intrinsic and extrinsic variables determine GRB pulse evolution. Bright, usually complex bursts have many narrow pulses that are difficult to model due to overlap. However, the relatively simple, long spectral lag, wide-pulse bursts may have simpler physics and are easier to model. In this work we analyze the temporal and spectral behavior of wide pulses in 24 long-lag bursts, using a pulse model with two shape parameters -- width and asymmetry -- and the Band spectral model with three shape parameters. We find that pulses in long-lag bursts are distinguished both temporally and spectrally from those in bright bursts: the pulses in long spectral lag bursts are few in number, and ~ 100 times wider (10s of seconds), have systematically lower peaks in nu*F(nu), harder low-energy spectra and softer high-energy spectra. We find that these five pulse descriptors are essentially uncorrelated for our long-lag sample, suggesting that at least five parameters are needed to model burst temporal and spectral behavior. However, pulse width is strongly correlated with spectral lag; hence these two parameters may be viewed as mutual surrogates. We infer that accurate formulations for estimating GRB luminosity and total energy will depend on several gamma-ray attributes, at least for long-lag bursts. The prevalence of long-lag bursts near the BATSE trigger threshold, their predominantly low nF(n) spectral peaks, and relatively steep upper power-law spectral indices indicate that Swift will detect many such bursts., 57 pages, 26 figures, 6 tables. Accepted for publication in the Astrophysical Journal

The Internal Luminosity Function (ILF) is the distribution of luminosity within a gamma‐ray burst and is thus an intrinsic burst property. The ILF of most bursts can be described accurately using a power‐law index modified by a power‐law curvature parameter. A flat power‐law index indicates a large amount of high luminosity emission relative to low luminosity emission, while a steep power‐law index indicates a large amount of low luminosity emission relative to high luminosity emission. Bursts with steep power‐law indices tend to show large ILF curvatures, indicating that they are also deficient in lower‐luminosity emission. The ILF anticorrelates with duration and fluence, indicating that bursts with large ILF curvatures are longer and brighter (as measured by fluence) than bursts emitting a broad range of luminosities. The ILF might be an indicator of Long vs. Short GRB class.

We have identified two quiescent GRBs (bursts having two or more widely-separated distinct emission episodes) in which the post-quiescent emission exhibits distinctly different characteristics than the pre-quiescent emission. In these two cases (BATSE GRBs 960530 and 980125), the second emission episode has a longer lag, a smoother morphology, and softer spectral evolution than the first episode. Although the pre-quiescent emission satisfies the standard internal shock paradigm, we demonstrate that the post-quiescent emission is more consistent with external shocks. We infer that some observed soft, faint, long-lag GRBs are external shocks in which the internal shock signature is not observed. We further note that the peak luminosity ratio between quiescent episodes is not in agreement with the ratio predicated by the lag vs. peak luminosity relationship. We briefly discuss these observations in terms of current collapsar jet models., Comment: 19 pages, 4 figures, 2 tables, accepted for publication in The Astrophysical Journal

The College of Charleston (CofC) is one of three institutions that belong to a consortium led by the Division of Science and Mathematics at the University of the Virgin Islands (UVI) to maintain and operate a research grade telescope at Etelman Observatory on the island of St. Thomas (18deg, 21′ N, 65deg W at an elevation ∼ 1325 ft with ⩽ 1″ seeing). The location provides 80% sky coverage of the southern celestial hemisphere and, on average, ∼ 6 hours of clear sky per night, except during the peak of hurricane season. This makes the observatory an ideal facility for observing Gamma‐Ray Bursts (GRBs). The observatory will serve a variety of needs to the consortium members that include research, teaching, and public outreach. The primary research function of this facility will be to perform rapid, automated followup observations of Gamma‐Ray Bursts (GRBs) observed with NASA’s Swift spacecraft, to be launched in June 2004, via the GCN. The newly renovated observatory houses a new robotic 0.5 m Cassegrain tel...

The GRB ToolSHED, a Gamma‐Ray Burst SHell for Expeditions in Data‐Mining, is now online and available via a web browser to all in the scientific community. The ToolSHED is an online web utility that contains pre‐processed burst attributes of the BATSE catalog and a suite of induction‐based machine learning and statistical tools for classification and cluster analysis. Users create their own login account and study burst properties within user‐defined multi‐dimensional parameter spaces. Although new GRB attributes are periodically added to the database for user selection, the ToolSHED has a feature that allows users to upload their own burst attributes (e.g. spectral parameters, etc.) so that additional parameter spaces can be explored. A data visualization feature using GNUplot and web‐based IDL has also been implemented to provide interactive plotting of user‐selected session output. In an era in which GRB observations and attributes are becoming increasingly more complex, a utility such as the GRB ToolSHED may play an important role in deciphering GRB classes and understanding intrinsic burst properties.

We classify BATSE gamma‐ray bursts using unsupervised clustering algorithms in order to compare classification with statistical clustering techniques. BATSE bursts detected with homogeneous trigger criteria and measured with a limited attribute set (duration, hardness, and fluence) are classified using four unsupervised algorithms (the concept hierarchy classifier ESX, the EM algorithm, the Kmeans algorithm, and a kohonen neural network). The classifiers prefer three‐class solutions to two‐class and four‐class solutions. When forced to find two classes, the classifiers do not find the traditional long and short classes; many short soft events are placed in a class with the short hard bursts. When three classes are found, the classifiers clearly identify the short bursts, but place far more members in an intermediate duration soft class than have been found using statistical clustering techniques. It appears that the boundary between short faint and long bright bursts is more important to the classifiers than is the boundary between short hard and long soft bursts. We conclude that the boundary between short faint and long hard bursts is the result of data bias and poor attribute selection. We recommend that future gamma‐ray burst classification avoid using extrinsic parameters such as fluence, and should instead concentrate on intrinsic properties such as spectral, temporal, and (when available) luminosity characteristics. Future classification should also be wary of correlated attributes (such as fluence and duration), as these bias classification results.

The Internal Luminosity Function (ILF) is the differential distribution of luminosity measured within a gamma‐ray burst (GRB). Most GRBs are found to have pseudo power‐law ILFs; the properties of the ILF power‐law index α has been examined by Horack and Hakkila for a sample of 50 bright GRBs (ApJ 479, 371). We measure α values for 348 BATSE GRBs, then correlate these with other measured GRB properties.

The GRB ToolShed is an online suite of induction‐based machine learning and statistical tools designed for gamma‐ray burst classification and cluster analysis. The ToolSHED also includes a large preprocessed gamma‐ray burst database. We report on the current status of the ToolSHED.

We study the angular anisotropies of faint intermediate and extremely short BATSE bursts in CGRO spacecraft coordinates. Our goal is to determine whether or not biases in detector effeciency can account for the observed anisotropies. We conclude that the faint intermediate burst anisotropy is not statistically meaningful, and that some of the very shortest bursts might be transient events other than gamma‐ray bursts.

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., 29 pages, 6 figures, 3 tables, Accepted for publication in The Astrophysical Journal

We present evidence for burst emission from SGR 1900+14 with a power-law high-energy spectrum extending beyond 500 keV. Not only is the emission hard, but the spectra are better fit by Band’s GRB function rather than by the traditional optically-thin thermal bremsstrahlung model. We find that the spectral evolution within these hard events obeys a hardness/intensity anti-correlation. Temporally, these events are distinct from typical SGR burst emissions in that they are longer (∼1 s) and have relatively smooth profiles. Despite a difference in peak luminosity of ≳1011 between these bursts from SGR 1900+14 and cosmological GRBs, there are striking temporal and spectral similarities between the two kinds of bursts, aside from spectral evolution. We outline an interpretation of these events in the context of the magnetar model.

A possible relationship between the presence of a radio afterglow and gamma-ray burst spectral hardness is discussed. The correlation is marginally significant; the spectral hardness of the bursts with radio afterglows apparently results from a combination of the break energy Ebreak and the high-energy spectral index beta. If valid, this relationship would indicate that the afterglow does carry information pertaining to the GRB central engine., Comment: 5 pages, 3 figures, presented at the 5th Huntsville Gamma-Ray Burst Symposium

We present preliminary results of an investigation of the temporal variability of gamma-ray bursts using Fourier analysis. Power density spectra were generated in the frequency range 0 2. The limitations of applying the Fourier transform to nonstationary GRB time series will be discussed in the context of the distribution of power-law indices.

In this Letter, we present the first evidence in the BATSE data for a prompt high-energy (25-300 keV) afterglow component from a gamma-ray burst (GRB), GRB980923. The event consists of rapid variabilty lasting ~40 s followed by a smooth power law emission tail lasting ~400 s. An abrupt change in spectral shape is found when the tail becomes noticeable. Our analysis reveals that the spectral evolution in the tail of the burst mimics that of a cooling synchrotron spectrum, similar to the spectral evolution of the low-energy afterglows for GRBs. This evidence for a separate emission component is consistent with the internal-external shock scenario in the relativistic fireball picture. In particular, it illustrates that the external shocks can be generated during the gamma-ray emission phase, as in the case of GRB990123., 4 pages, 4 figures, accepted for publication in Astrophysical Journal Letters