Thermal electrons in GRB afterglows, or Don Warren RIKEN – ABBL 6 Jun 2017 With: Don Ellison (NCSU) Maxim Barkov (RIKEN/Purdue) Shigehiro Nagataki (RIKEN), and more?

Preview slide! Radio/optical/X-ray for afterglow models with/ without thermal electrons

Preview slide! Radio/optical/X-ray for afterglow models with/ without thermal electrons For same optical/X-ray, models with only cosmic rays radio faint

Outline Background The case for low-energy particles The consequences of low-energy particles Conclusion

Background Afterglow is long-lived (hours, days, months) multiwavelength relic of GRB External shock wave γ-rays Central engine X-rays Optical Radio Prompt emission Afterglow

Background Perley+ (2014) (2014ApJ...781...37P) Observations of GRB afterglows span orders of magnitude in time, energy 0.007 d 130 d 109 1025 Hz

Background Many different models to explain broadband spectra and light curves However, current afterglow studies assume extremely simple model for electron population downstream from shock Gao+ (2013) (2013NewAR..57..141G)

Background Many different models to explain broadband spectra and light curves However, current afterglow studies assume extremely simple model for electron population downstream from shock Early time Late time (Assume electrons form power law with index constant in time) But, with Fermi accel, Have “non-nonthermal” particles: crossed shock but didn’t enter accel process Spectral index varies with Lorentz factor (will not be constant in time) N(E) N(E) Emin Emax Emin Emax Energy Energy

The case for low-energy particles Sironi et al. (2013) (2013ApJ...771...54S) εB Know this from particle-in-cell (PIC) simulations of relativistic low-magnetization shocks Critical results: Plasma instabilities UpS from shock transfer energy from ions to electrons Electrons, ions both cross shock at E ~ γ0mpc2 Only small fraction (few %) enter Fermi accel process & become cosmic rays σ = 0 σ = 10-5 σ = 10-4 σ = 10-3 Make sure you point out/explain magnetization and spectra

The case for low-energy particles Sironi et al. (2013) (2013ApJ...771...54S) εB Know this from particle-in-cell (PIC) simulations of relativistic low-magnetization shocks Critical results: Plasma instabilities UpS from shock transfer energy from ions to electrons Electrons, ions both cross shock at E ~ γ0mpc2 Only small fraction (few %) enter Fermi accel process & become cosmic rays σ = 0 “Low-energy”: few to few tens of GeV σ = 10-5 σ = 10-4 σ = 10-3

The consequences of low-energy particles Use PIC results to guide Monte Carlo simulation of Fermi process in GRB afterglow Why MC? PIC sims ~109 cm across, forward shock >1013 cm. Too large space/time domain for computation MC approach balances versatility with simplicity— computable on desktop

The consequences of low-energy particles Model Fermi process at select shock ages/speeds, then compute photon production Consider 3 cases: CR-only: only particles that entered Fermi process produce photons TP (test particle): all particles produce photons, but injection inefficient NL (nonlinear): as TP, but efficient injection into Fermi process alters shock structure & resultant CR spectra Warren et al. (2017) (2017ApJ...835..248W) Note large populations at GeV energies! See Ellison+ (2013), Warren+ (2015) for more info on nonlinear Fermi process in rel shocks (2013ApJ...776...46E) (2015MNRAS.452..431W) Point out equal normalizations at high-energy end of electron tails

The consequences of low-energy particles Warren et al. (2017) (2017ApJ...835..248W) Model Fermi process at select shock ages/speeds, then compute photon production Photon processes: Synchrotron Inverse Compton CMB SSC ISRF p-p pion production Absorption SSA (at radio) EBL (at GeV+) Large early deviation if thermal particles included Later agreement, since photons produced by (equal) nonthermal tails

The consequences of low-energy particles PRELIMINARY Model Fermi process at select shock ages/speeds, then compute photon production Photon processes: Synchrotron Inverse Compton CMB SSC ISRF p-p pion production Absorption SSA (at radio) EBL (at GeV+) High B-fields, energy transfer mean νm well above radio for a while Thermal pop makes huge difference in radio intensity

The consequences of low-energy particles Model Fermi process at select shock ages/speeds, then compute photon production Photon processes: Synchrotron Inverse Compton CMB SSC ISRF p-p pion production Absorption SSA (at radio) EBL (at GeV+) PRELIMINARY R/O/X afterglows from Chandra & Frail (2012) CR-only model radio-faint despite consistency in optical, X-ray Thermal particles necessary to match broadband info? Redshift dependence At tobs = 11 h Explain the figure

Conclusions Sironi et al. (2013) (2013ApJ...771...54S) Relativistic shocks must produce “non-nonthermal” electrons at GeV energies Photons from thermal electrons create huge departure from CR-only model For reasonable GRB parameters, CR-only model too radio-faint, despite consistent optical/X-ray intensity Only requirement: some particles injected into Fermi process (i.e. high Lorentz factors, low UpS B fields)  robust

Conclusions Relativistic shocks must produce “non-nonthermal” electrons at GeV energies Photons from thermal electrons create huge departure from CR-only model For reasonable GRB parameters, CR-only model too radio-faint, despite consistent optical/X-ray intensity Only requirement: some particles injected into Fermi process (i.e. high Lorentz factors, low UpS B fields)  robust