Gamma-Ray Bursts: Understanding Prompt Emission, Afterglow, and the Central Engine

1. What we know, What we believe we know (but we are not sure) and What we would like to know about gamma-ray bursts Robert Mochkovitch (IAP) - LIA meeting, Kunming Nov.1-5, 2018 relativistic jet internal dissipation: prompt emission Gamma-ray bursts: the global picture photosphere central engine RS FS afterglow emission jet break

2. The prompt emission What we know: • There are short and long bursts • Light curves are very diverse • Spectra are simple: broken power-laws • Mostly at cosmological distance

3. Energetics •  isotropic energy release from redshift and measured fluence • But GRBs are beamed: achromatic jet et break in afterglow when 1/G > qj Eiso = 1051 – 1054erg + a few sub-energetic, nearby events True energy: erg • Transparency condition • Pair creation: g+g e++e- mostly prevented if G > 100

4. What we believe we know (but we are not sure): • The “early steep decay” as “high latitude emission” • Sub-dominant thermal (blackbody?) component in some spectra tb DR dt Rdiss~ 6 1014G22tb(s) cm Photospheric emission ? . Rphot~ 6 1012 E52/G23 cm

5. What we would like to know: • What is the main dissipation process in the relativistic jet? • and where does it take place? • (i) at Rdiss~ cG2 tb~ 1015 cm : Shocks? Reconnection? Some mix of both  ICMART • (ii) at the photosphere Rdiss ~ RphotcG2 tb : sub-photospheric dissipation + comptonization •  (i) supported by the “early steep decay” but there are alternatives • shocks less efficient (or even suppressed) if the ejecta is highly magnetized (s1) • What is the dominant radiative process ? • In case (i) synchrotron (+ IC at very high energy) but there are problems: • to be efficient synchrotron should be in the so-called “fast cooling regime” •  low energy spectral index : a = -3/2 • A majority of GRBs show harder spectra • Include IC • B decreasing in the emission region • Extend spectral range for the analysis ? •  SVOM

6. In case (ii) recipe: start from black body add tail at high energy via IC add contributions at low energy (possibly synchrotron) Which radiative process is to be preferred? Clues from prompt optical emission? But data is sparse and diverse: optical data correlated to g’s … or not (GRB 990123) extreme case of the “naked eye burst” SVOM (GWAC) contribution expected !!! E2N(E) IC syn Planck → broken PL

7. The afterglow • What we know: • -Results from the deceleration of the ejecta by the surrounding medium • - Relativistic analog of the Sedov solution + synchrotron emission •  light curve and spectrum made of power-law segments • good fits of the data after a few hours • Slopes depending on the ordering: nobs , nsyn, ncool

8. What we believe we know (but we are not sure): Playing the exercise of fitting the multi-l afterglow to obtain the burst parameters • GRB 090423 @ z=8.3 • But degeneracies • Uniform medium preferred when wind would be expected

9. What we would like to know: • The origin of plateaus and flares during the early afterglow revealed by Swift • Late injection of energy by the central engine? A lot ! continuous/impulsive • Radiative process initially inefficient ? • Reverse shock contribution  “tomography of the ejecta”

10. The central engine • What we know: • Short time scale variability  compact object • Long GRBs  type Ic supernovae “collapsars” • Short GRBs found also in elliptical galaxies  compact object mergers: NS+NS or BH+NS • [confirmed with GRB 170817A in NGC 4993] • In both cases central compact object surrounded by a thick accretion torus •  source of energy: gravitational (accretion) or rotational

11. What we believe we know (but we are not sure): • Extraction of energy • nnannihilation (low efficiency) or magnetic breaking (favored) • mainly along the system axis • Acceleration • (i) thermally (fireball) or (ii) magnetically driven outflow (Blandford-Znajek in BH case) • respective contribution may vary from burst but (ii) likely dominates in most cases • What we would like to know: • In case (ii) residual magnetization at “infinity”: s • s < 1 for shocks to be possible • What is the central object? Kerr black hole or “magnetar” •  early afterglow plateaus powered by Poynting flux from a magnetar?? field lines

12. GRB 170817A • What we know • Many firsts: • First direct observation of the coalescence of two NS • First “kilonova” observed in great details • First GRB seen off-axis (qv=20-25°): sub-luminous by 4-5 orders of magnitude • What we believe we know (but we are not sure): • Cocoon structure with a successful central jet jet superluminal motion detected 75 days 230 days bapp= 4.1 Lazzati et al., 2017

13. What we would like to know • How is the GRB produced ? Probably not central GRB seen off-axis • Shock break-out from the cocoon ? (Nakar & Piran, 2018) • Seems we were lucky with the first one: GW+GRB+KN+AG full display ! • What can we expect from future events ? • Simulation: DH,grav =100 Mpc ; source detected in GW and radio (3 GHz) •  Distributions in distance and viewing angle • Observed distributions will constrain input parameters: • f(E), f(n), f(eB) … ~15-20% at D  40 Mpc black dots: all sources detected in GW red dots: sources detected in radio are concentrated at small viewing angles and large distances GRB 170817A

14. Conclusions • A glorious future in the GW + SVOM era ! • Explore the diversity intrinsic/ viewing angle of short GRBs • Tens to hundreds NS+NS GW event/yr in the coming LIGO/Virgo observing runs ! • but detection of simultaneous GRB more difficult as DH,gravincreases • When will we capture the first BH + NS event ? • But still much to do with “normal” GRBs at cosmological distance • - dissipative and radiative process during the prompt emission? [SVOM large l range +z ] • - origin of plateau and flares during the early afterglow?