On the Hardness‐Intensity Correlation in Gamma‐Ray Burst Pulses

Abstract

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 the Burst And Transient Source Experiment (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. A number of the other cases can still be explained with the power law model if various limitations of the observations are taken into account.The distribution of the power law indices γ, 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 p of a random coincidence is shown to be very small (< 2 × 10 −5 ). In most cases, the indices are equal to within the uncertainties. These results demand a physical model to be able to reproduce multiple pulses with similar characteristics for an individual burst, but with a large diversity for pulses from an ensemble of bursts. This is particularly relevant when comparing the external versus the internal models.In our analysis, we also use a new method for studying the hardness-intensity correlation, in which the intensity is represented by the peak value of the EF E spectrum, where E is the energy and F E is the energy flux spectrum. We compare it to the traditional method in which the intensity over a finite energy range is used instead, which may be an incorrect measure of the bolometric intensity. 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 and examples of its application are given.

Gamma-Ray Burst Prompt Correlations

“…The measured scatter of the peak -tot relation is 0.17 ± 0.08. This result was reported for the first time by Golenetskii et al [178] and later confirmed by Borgonovo and Ryde [179]; Ghirlanda et al [180]; Guiriec et al [181]; Ghirlanda et al [165].…”

The spectral catalogue of INTEGRAL gamma-ray bursts

“…2. There may be biases in the results when the analysis is performed on a sample of the brightest events, since there is a tendency for bright GRBs (having higher photon fluxes) to have higher spectral peak energies than faint GRBs (Borgonovo & Ryde 2001;Mallozzi et al 1995). For example, Kaneko et al (2006) burst selection criteria required a peak photon flux on the energy range 50−300 keV greater than 10 photons s −1 cm −2 or a total energy fluence in 20−2000 keV energy range larger than 2.0 × 10 −5 erg cm −2 .…”

Spectral-luminosity relation within individualFermigamma rays bursts

“…Borgonovo & Ryde 2001;Ryde & Petrosian 2002) that the off-latitude emission that follows an abrupt switch-off of the fireball introduces a spectral-energy dependence, since the observer sees progressively less beamed (and less blue-shifted) emission. However, this could explain only the decaying phase of the pulse.…”