K. S. Dhuga

SPECTRAL LAGS AND THE LAG-LUMINOSITY RELATION: AN INVESTIGATION WITH SWIFT BAT GAMMA-RAY BURSTS

Spectral lag, the time difference between the arrival of high-energy and low-energy photons, is a common feature in gamma-ray bursts (GRBs). Norris et al. reported a correlation between the spectral lag and the isotropic peak luminosity of GRBs based on a limited sample. More recently, a number of authors have provided further support for this correlation using arbitrary energy bands of various instruments. In this paper, we report on a systematic extraction of spectral lags based on the largest Swift sample to date of 31 GRBs with measured redshifts. We extracted the spectral lags for all combinations of the standard Swift hard X-ray energy bands: 15-25 keV, 25-50 keV, 50-100 keV, and 100-200 keV and plotted the time dilation corrected lag as a function of isotropic peak luminosity. The mean value of the correlation coefficient for various channel combinations is -0.68 with a chance probability of {approx}0.7 x 10{sup -3}. In addition, the mean value of the power-law index is 1.4 +- 0.3. Hence, our study lends support to the existence of a lag-luminosity correlation, albeit with large scatter.

Minimum variability time-scales of long and short GRBs

ABSTRACT We have investigated the variability of a sample of long and short Fermi/GBM Gammaray bursts (GRBs) using a fast wavelet technique to determine the smallest timescales. The results indicate different variability time scales for long and short burstsin the source frame and that variabilities on the order of a few milliseconds are notuncommon. The data also indicate an intriguing relation between the variability scaleand the burst duration.Key words: Gamma-ray bursts 1 INTRODUCTIONThe prompt emission from Gamma-ray Bursts (GRBs)shows very complicated time profiles that hitherto elude asatisfactory explanation. Fenimore & Ramirez-Ruiz (2000)reported a correlation between variability of GRBs and thepeak isotropic luminosity. The existence of the variability-luminosity correlation suggests that the prompt emissionlight curve is embedded with temporal information relatedto the microphysics of GRBs. Several models have been pro-posed to explain the observed temporal variability of GRBlightcurves. Leading models such as the internal shock model(reference) and the photospheric model (reference) link therapid variability directly to the activity of the central en-gine. Others invoke relativistic outflow mechanisms to sug-gest that local turbulence amplified through Lorentz boost-ing leads to causally disconnected regions which in turnact as independent centers for the observed prompt emis-sion. In more recent models, both Morsony et al. (2010)and Zhang & Yan (2011) argue that the temporal variabil-ity may show two different scales depending on the physicalmechanisms generating the prompt emission.In order to further our understanding of the promptemission phase of GRBs and to explicitly test some of thekey ingredients in the various models it is clearly importantto extract the variability for both short and long gamma-ray bursts in a robust and unbiased manner. It is also clear

Probing Curvature Effects in the Fermi GRB 110920

Curvature effects in gamma-ray bursts (GRBs) have long been a source of considerable interest. In a collimated relativistic GRB jet, photons that are off-axis relative to the observer arrive at later times than on-axis photons and are also expected to be spectrally softer. In this work, we invoke a relatively simple kinematic two-shell collision model for a uniform jet profile and compare its predictions to GRB prompt-emission data for observations that have been attributed to curvature effects such as the peak-flux-peak-frequency relation, i.e., the relation between the νF{sub ν} flux and the spectral peak, E{sub pk} in the decay phase of a GRB pulse, and spectral lags. In addition, we explore the behavior of pulse widths with energy. We present the case of the single-pulse Fermi GRB 110920 as a test for the predictions of the model against observations.

The minimum variability time‐scale and its relation to pulse profiles of Fermi GRBs

We present a direct link between the minimum variability time-scales extracted through a wavelet decomposition and the rise times of the shortest pulses extracted via fits of 34 Fermi Gamma-ray Burst Monitor (GBM) Gamma-Ray Burst (GRB) light curves comprised of 379 pulses. Pulses used in this study were fitted with lognormal functions, whereas the wavelet technique used employs a multiresolution analysis that does not rely on identifying distinct pulses. By applying a corrective filter to published data fitted with pulses, we demonstrate agreement between these two independent techniques and offer a method for distinguishing signal from noise.

Pion double charge exchange above the Δ(1232) resonance

Abstract First measurements of exclusive cross sections for the ( π + , π − ) reaction at incident energies of 300–500 MeV are reported. For the analog-state transitions in 14 C and 18 O the 5° cross sections are found to remain nearly constant in this energy interval. This behavior is in disagreement both with predictions based on six-quark cluster models and with simple expectations based on recent single-charge-exchange measurements.

GAMMA-RAY BURSTS: TEMPORAL SCALES AND THE BULK LORENTZ FACTOR

For a sample of Swift and Fermi GRBs, we show that the minimum variability timescale and the spectral lag of the prompt emission is related to the bulk Lorentz factor in a complex manner: For small

The Hurst exponent of Fermi gamma-ray bursts

\Gamma

A new frequency–luminosity relation for long gamma-ray bursts?

s, the variability timescale exhibits a shallow (plateau) region. For large

Charge-symmetry violation in pion scattering from three-body nuclei

\Gamma

A NEW CORRELATION BETWEEN GRB X-RAY FLARES AND THE PROMPT EMISSION

s, the variability timescale declines steeply as a function of