1. 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?

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

3. 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

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

5. 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

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

7. 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 G)

8. 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

9. The case for low-energy particles
Sironi et al. (2013)
(2013ApJ S) 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

10. The case for low-energy particles
Sironi et al. (2013)
(2013ApJ S) 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

11. 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

12. 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 W)
Note large populations at GeV energies!
See Ellison+ (2013), Warren+ (2015) for more info on nonlinear Fermi process in rel shocks
(2013ApJ E) (2015MNRAS W)
Point out equal normalizations at high-energy end of electron tails

13. The consequences of low-energy particles
Warren et al. (2017)
(2017ApJ W)
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

14. 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

15. 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

16. Conclusions
Sironi et al. (2013)
(2013ApJ S)
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

17. 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