Jerry T. Bonnell

CONNECTION BETWEEN ENERGY-DEPENDENT LAGS AND PEAK LUMINOSITY IN GAMMA-RAY BURSTS

We suggest a connection between the pulse paradigm at gamma-ray energies and the recently demonstrated luminosity distribution in gamma-ray bursts: The spectral evolution timescale of pulse structures is anticorrelated with peak luminosity and with quantities that might be expected to reflect the bulk relativistic Lorentz factor, such as spectral hardness ratio. We establish this relationship in two important burst samples using the cross-correlation lags between low (25-50 keV) and high (100-300 keV and >300 keV) energy bands. For a set of seven bursts (six with redshifts) observed by CGRO/BATSE and BeppoSAX that also have optical or radio counterparts, the γ/X peak flux ratios and peak luminosities are anticorrelated with spectral lag. For the 174 brightest BATSE bursts with durations longer than 2 s and significant emission above 300 keV, a similar anticorrelation is evident between gamma-ray hardness ratio or peak flux and spectral lag. For the six bursts with redshifts, the connection between peak luminosity and spectral lag is well fitted by a power law, L53 ≈ 1.3 × (τ/0.01 s)-1.15. While GRB 980425 (if associated with SN 1998bw) would appear to extend this trend qualitatively, with a lag of ~4-5 s and luminosity of ~1.3 × 1047 ergs s-1 , it falls below the power-law relationship by a factor of several hundred. As noted previously by Band, most lags are concentrated on the short end of the lag distribution, near 100 ms, suggesting that the gamma-ray burst luminosity distribution is peaked on its high end.

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

Long-Lag, Wide-Pulse Gamma-Ray Bursts

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 (tens of seconds), have systematically lower peaks in νF(ν), and have 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 ~5 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 νF(ν) spectral peaks, and relatively steep upper power-law spectral indices indicate that Swift will detect many such bursts.

Gamma-ray burst peak duration as a function of energy

Gamma-ray burst time histories often consist of many peaks. These peaks tend to be narrower at higher energy. If gamma-ray bursts are cosmological, the energy dependence of gamma-ray burst timescales must be understood in order to correct the timescale dependence due to the expansion of the universe. By using the average autocorrelation function and the average pulse width, we show that the narrowing with energy follows, quite well, a power law. The power-law index is ~-0.4. This is the first quantitative relationship between temporal and spectral structure in gamma-ray bursts. It is unclear what physics causes this relationship. The average autocorrelation has a universal shape such that one energy range scales linearly with time into all other energy ranges. This shape is approximately the sum of two exponentials.

Temporal and spectral characteristics of terrestrial gamma flashes

We have analyzed the Burst and Transient Source Experiment (BATSE) high-resolution timing data for 13 terrestrial gamma flashes (TGFs) to better characterize this newly identified phenomenon, which may be related to atmospheric lightning. We find that the minimum timescale for TGF variability is ∼25–250 μs, with 50 μs near typical. In general, TGFs are spectrally much harder than cosmic gamma ray bursts (GRBs). We additionally find that as with GRBs, individual pulses within a TGF tend to peak earlier at higher energies. This time-asymmetry rules out models such as sweeping beams. We also find that different pulses can have different spectra, with spectra typically softening as a pulse progresses. Event-averaged spectra for the TGFs were examined and found to be better fit in the 25–500 keV range by a power law than by a blackbody model. However, in general, even a power law is not a perfect fit. We find correlation between minimum TGF timescale and the power law spectral index, with rapidly varying TGFs appearing softer. From empirical comparisons of timescales and structures we speculate that if TGFs are somehow related to known high-atmospheric lightning events, then they are more probably related to red sprites than to blue jets or transionospheric pulse pairs.

Prospects for GRB Science with the Fermi Large Area Telescope

The Large Area Telescope (LAT) instrument on the Fermi mission will reveal the rich spectral and temporal gamma-ray burst (GRB) phenomena in the >100 MeV band. The synergy with Fermis Gamma-ray Burst Monitor detectors will link these observations to those in the well explored 10-1000 keV range; the addition of the >100 MeV band observations will resolve theoretical uncertainties about burst emission in both the prompt and afterglow phases. Trigger algorithms will be applied to the LAT data both onboard the spacecraft and on the ground. The sensitivity of these triggers will differ because of the available computing resources onboard and on the ground. Here we present the LATs burst detection methodologies and the instruments GRB capabilities.

Correlations between Lag, Luminosity, and Duration in Gamma-Ray Burst Pulses

We derive a new peak lag versus 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 versus 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 versus 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.

Duration distributions of bright and dim BATSE gamma-ray bursts

We have measured the T(sub 90) and T(sub 50) durations of bright and dim gamma-ray bursts detected by the Compton Gamma-Ray Observatorys (CGRO) Burst and Transient Source Experiment (BASTE). The T(sub 90) T(sub 50) duration is defined as the interval over which 5% (25%) to 95% (75%) of the burst counts accumulate. Out of 775 bursts observed by BATSE 159 bursts were analyzed; bursts with durations shorter than 1.5 s were excluded. A Kolmogorov-Smirnov test yields a probability of 6 x 10(exp -5) that the T(sub 50) durations of the dim and bright samples are drawn from the same parent population. We find that the centroid and extent of the duration distribution for the dim sample are scaled by approximately a factor of 2 relative to those of the bright sample. The measured time-dilation factor is not sensitive to choice of energy band. These results are quantitatively consistent with previous tests for time dilation in a smaller sample of BATSE bursts. The sources of dimmer bursts, if cosmological, would lie at redshifts of order 2.

Gamma-Ray Burst Intensity Distributions

We use the lag-luminosity relation to calculate self-consistently the redshifts, apparent peak bolometric luminosities LB, and isotropic energies Eiso for a large sample of BATSE gamma-ray bursts. We consider two different forms of the lag-luminosity relation; for both forms the median redshift for our burst database is 1.6. We model the resulting Eiso sample with power-law and Gaussian probability distributions without redshift evolution, both of which are reasonable models. The power-law model has an index of αE = 1.76 ± 0.05 (95% confidence), where p(Eiso) ∝ E. The simple universal jet profile model suggested but did not require αE = 2, and subsequent physically reasonable refinements to this model permit greater diversity in αE, as well as deviations from a power law; therefore, our observed Eiso probability distribution does not disprove the universal jet model.

Gravitationally Lensed Gamma-Ray Bursts as Probes of Dark Compact Objects

If dark matter in the form of compact objects comprises a large fraction of the mass of the universe, then gravitational lensing effects on gamma-ray bursts are expected. We utilize BATSE and Ulysses data to search for lenses of different mass ranges, which cause lensing in the milli, pico, and femto regimes. Null results are used to set weak limits on the cosmological abundance of compact objects in mass ranges from 10-16 to 10-9 M☉. A stronger limit is found for a much-discussed Ω=0.15 universe dominated by black holes of mass ~106.5 M☉, which is ruled out at the ~90% confidence level.