Why GRB Pulse Rises are Symptomatic of Relaxation Rather than of Energy Injection

1. Why GRB Pulse Rises are Symptomatic of Relaxation Rather than of Energy Injection 1 College of Charleston 2 University of Alabama in Huntsville A GRB pulse is defined by its intensity change. However, pulse spectral evolution evolves consistently before and after the pulse peak. This suggests that the intensity rise and the pulse peak are merely steps in the spectral evolutionary process. Jon Hakkila1, Rob Preece2

2. Black - summed 4-channel emission Green - high energy channel emission Red - low energy channel emission Observable Pulse Properties - obtained using semi-automated 4-parameter pulse model (Norris et al. 2005 ApJ, 627, 324; Hakkila et al. 2008 ApJ 677, L81) • Pulse peak flux (p256) - peak flux of summed multichannel data (black) measured on 256 ms timescale. • Pulse duration - time span when flux is e-3 of pulse peak flux. • Pulse peak lag- time span between channel 3 peak (100-300 keV; green) and channel 1 peak (25-50 keV; red). • Fluence - time-integrated flux. • Hardness - ratio of channel 3 fluence to channel 1 fluence. • Asymmetry - pulse shape measure; 0 is symmetric and 1 is asymmetric.

3. 300 keV - 1 MeV 100 keV - 300 keV 25 keV - 1 MeV 50 keV - 100 keV 3 2 1 25 keV - 50 keV Pulse-fitting example: GRB 950325a (BATSE 3480)

4. GRB pulses have correlated properties (1390 pulses in 646 BATSE GRBs).

5. Long GRB Pulse Correlations Short GRB Pulse Correlations Indisputable GRB pulse property correlations: short duration pulses have shorter lags, are brighter, are harder, and are more time symmetric than long duration pulses.

6. Peak luminosity (L) vs. duration (w) and lag (l0) for BATSE GRBs: Pulse relations replace bulk prompt emission relations (Hakkila et al. 2008, ApJ 677, L81). Correlated pulse properties are measured in the observer’s frame, indicating that cosmological effects and the inverse-square law are of only secondary importance. Essentially the GRB pulse luminosity function!!!

7. X-ray flares share many -ray pulse characteristics, with decreasing flare energetics and increasing pulse durations after the trigger -> less energy present as flares develop (Margutti et al. 2010, MNRAS, 406, 2149). Pulse lag and pulse duration vs. pulse luminosity relations are found in HETE-2 (Arimoto et al. 2010, PASJ, 62, 487) and Swift (Chincarini et al. (MNRAS, 2010, 406, 2113). Supportive Observations: HETE-2 and Swift

8. Since duration and lag are luminosity indicators, all other correlated pulse properties are also luminosity indicators, in the observer’s frame!!!

9. Correlated GRB Pulse Properties are Progenitor-Independent Short Swift GRBs (Norris, Gehrels, and Scargle 2011, ApJ submitted) continue the peak flux vs. duration relation to ms timescales. How can peak flux correlations be reconciled with differing luminosities of Short and Long GRBs? • Short and Long burst observable pulse properties appear to indicate a single distribution. • Short and Long GRB pulse peak luminosities do not differ systematically when the number of pulses per burst is taken into account (data from Ghirlanda et al. 2009, A&A 496, 585).

10. Peng et al. (2009, ApJ 698, 417) Correlated properties indicate a common pulse evolution. • Pulses start near-simultaneously at all energies (Hakkila and Nemiroff 2009, ApJ 705, 372). • Pulse Epeak values decay from the pulse onset (e.g. Peng et al. 2009, ApJ 698, 417).

11. Intensity Tracking pulse: BATSE trigger 1886 Evidence favors a single GRB pulse type: hard-to-soft. • 14 standard hard-to-soft (H2S) and 10 Intensity tracking (IT) pulses common to BATSE pulse database, discounting 3 ambiguously-classified pulses (data from Peng et al. 2009, ApJ 698, 417; Peng et al. 2010 NA, 332, 92; Lu et al. 2010 ApJ, 1146, 1154). • Most IT pulses (and some H2S pulses) are composed of two or more overlapping pulses. IT pulses typically come from complex, multi-pulsed GRBs. Unambiguous pulse sample reduced to 9 H2S pulses 2 IT pulses; 7 H2S pulses have clean fits, while no IT pulses do. • One IT pulse and 2 ambiguous pulses have similar long duration and time-symmetric light curves. Analyses of pulses with these characteristics (e.g. BATSE 1886) suggest they are composed of overlapping short-duration pulses.

12. A Conundrum • How does the detector sample observable pulse properties so that they always correlate with peak intensity (the maximum photon rate detected from the decaying pulse spectrum), since the peak intensity is strongly affected by instrumental spectral response, initial pulse spectrum, and GRB redshift? • Occam’s Razor: Rather than trying to understand how the detector samples pulse observables so that they are always correlated, we recognize that a pulse is more simply characterized by its hard-to-soft spectral evolution. All pulse properties, including peak intensity, are linked because they are part of the same evolution. • Since pulse intensity increases while the energy decays, peak intensity and pulse rise are symptoms of H2S evolution, rather than the climax. • Energy is injected at the beginning of the pulse, and the pulse we observe is simply the energy decay resulting from this initial injection.

13. Summary • GRB pulses have progenitor-independent correlated properties. • Correlated pulse properties are strong in the observer’s frame, indicating that cosmological effects and the inverse-square law are only of secondary importance. • Pulses start near-simultaneously in all energy channels. • Pulses represent hard-to-soft spectral evolution (lag, Epeak decay). • There appears to be only one pulse type. • Pulse peak intensity is altered by the convolution of instrumental spectral response, initial spectral characteristics, and GRB redshift. Yet, peak intensity correlates with all observable pulse properties. • A GRB pulse has been historically defined by its intensity change. • However, pulse spectral evolution evolves consistently before and after the pulse peak. This suggests that the intensity rise and the pulse peak are merely steps in the spectral evolutionary process. • Energy has been injected at the beginning of the pulse, and the pulse we observe is simply the energy decay resulting from this initial injection. This appears to be true for all GRB pulses, regardless of progenitor.