Low-luminosity GRBs and Relativistic shock breakouts Ehud Nakar Tel Aviv University

1. Low-luminosity GRBs and Relativistic shock breakouts Ehud Nakar Tel Aviv University • Omer Bromberg • Re’em Sari • Tsvi Piran GRBs in the Era of Rapid Follow-up Liverpool, 2012

2. Low-luminosity GRBs • ~10-4 lower luminosity, <1048 erg/s • Much more frequent • Smooth light curve • Eg<< total energy • Not highly collimated • Mildly relativistic ejecta with energy ~ Eg • Delayed X-ray emission, with energy ~ Eg • Not “successful” jets Low luminosity Short Long • Low-luminosity GRB is NOT a regular GRB with low luminosity • Connection to long GRBs is mainly via the common association to broad-line Ic SNe Wanderman & Piran 2011

3. Outline • Propagation of a relativistic jet in a stellar envelope • Why low-luminosity GRBs are not generated by “successful” jets (as long GRBs) • Theory of relativistic shock breakout (gb>0.5) • Comparison of relativistic shock breakoutpredictions to low-luminosity GRB observations • A note on short GRB classification

4. Jet propagation in a stellar envelope Numerical Macfadyan, Woosley, Zhang ,Morsony, Lazzati, Mizuta, Aloy, Nagakura, Tominaga, Nagataki, Ioka ... Analytic Begelman, Matzner, Meszaros, Waxman, Lazzati, Bromberg, ...

5. Medium Jet

6. Head Medium Jet

7. Head Medium Jet Cocoon

8. Head Medium Jet Collimation Shock Cocoon

9. Morsony et al., 07

10. To observe a long GRB: the jet must break out • The head is slower than the jet material, and dissipates the jet energy. • In order to propagate the head needs to be pushed by the jet material. • The engine must work continuously until the jet breaks out.

11. To observe a GRB: the jet must break out • The head is slower than the jet material, and dissipates the jet energy. • In order to propagate the head needs to be pushed by the jet material. • The engine must work continuously until the jet breaks out – or it will fail. • Breakout time: Failed jet ʘ ʘ Bromberg, EN, Piran & Sari 11

12. Are low-luminosity GRBs produced by “successful” jets? (Bromberg, EN & Piran 2011)

13. tγ = te - tb After the jet breaks out energy flows (relatively) freely to large distances where the prompt GRB emission is emitted. tb tγ te

14. The engine is unaware that the jet breaks out tb tg Less likely te

15. Long GRBs # of bursts Low-luminosity 0.01 10 0.1 1 T90/tb Bromberg, EN & Piran 2012 Low-luminosity GRBs are most likely (2s) not produced by jets that successfully punches through their progenitor envelope

16. If not a successful jet then what is the g-ray source of low-luminosity GRBs? Even “failed” jets drive shocks that breakout of the stellar surface! “failed” jets are much more frequent than successful ones What are the observed signatures of the resulting shock breakouts?

17. Relativistic shock breakout (EN & Sari 2012)

18. Shock breakout Shock accelerates while its energy decreases Radiation dominated shock: t=c/v Shock width = distance to edge Shock breakout radiation-dominated shock

19. Three hydrodynamic stages • Shock breakout • Shock width = distance to edge • Planar expansion • Before breakout layer doubles its radius • Spherical expansion • After Breakout layer doubles its radius

20. Relativistic shock breakout Main physical properties: • Constant post shock rest frame temperature ~100-200 keV • Temperature dependent (pair) opacity • Significant post breakout acceleration Katz et. al., 10 Budnik et. al., 10 pairs TBB

21. The breakout emission - A flash of g-rays Colgate was correct after all (for wrong reasons) Three observables depend on two physical parameters: R and g Relativistic breakout relation for quasi-spherical breakout without wind

22. The breakout emission - A flash of g-rays Colgate was correct after all (for wrong reasons) Important note The photosphere is not in thermal equilibrium The blackbody radius (Rbb2 = L/4psT4) is meaningless

23. Emission following the shock breakout g-rays X-rays EN & Sari 12 Epshifts from g-rays to X-rays (Ex >Eg) ~

24. Are low-luminosity GRBs produced by relativistic shock breakouts? Colgate 1968, Kulkarni et al., 1998, Tan et al., 2001, Campana et al., 2006, Waxman et al., 2007, Wang et al., 2007, Katz et al., 2010

25. Predictions of relativistic shock breakouts from “failed” jets and a comparison to low luminosity GRBs: • Smooth light curve • Eg<< total energy • Relativistic ejecta with energy ~ Eg • Delayed X-ray emission, with energy ~ Eg • (e.g., X-ray echo of GRB 031203) Relativistic breakout relation ?

26. Low luminosity GRBs Relativistic breakout relation

27. Shock breakout from long GRBs A short, hard and faint pulse at the beginning of the burst

28. g-ray flares from relativistic shock breakouts are expected in a range of other explosions. For example, White dwarf explosions (Type Ia and .IaSNe and AIC): Extremely energetic and compact supernovae (e.g., SN 2002ap):

29. A note on short GRB classification BATSE T90 (50 - 300 keV) Swift T90 (15 - 350 keV) The threshold duration for Swift sample must be shorter than for BATSE sample !!! We show that it is 0.6-0.7 s (Bromberg, EN, Piran & Sari 12)

30. Summary • Relativistic breakouts produce g-ray flares with characteristic properties: • Ebo – Tbo – tbo relation (if quasi-spherical without wind) • smooth • a small fraction of total explosion energy • gto X-ray evolution • generate a relativistic outflow with E~Ebo • Low-luminosity GRBs, which are fundamentally different than long GRBs, show all these characteristics • Failed jets is the most natural mechanism (explains also the high low luminosity GRB rate) • Swift GRBs with 1s < T < 2s are most likely (>50%) collapsars

31. Thanks

32. Comparison with simulations L 51 Zhang et al., 04 Bromberg, EN, Piran Sari 11

33. Which explosions are expected to have relativistic breakouts? EN & Sari 11