Low-luminosity GRBs and Relativistic Shock Breakouts

1. Low-luminosity GRBs and Relativistic shock breakouts Ehud Nakar Tel Aviv University • Omer Bromberg • Tsvi Piran • Re’em Sari 2nd EUL Workshop on Gamma-Ray Bursts Moscow, 2013

2. Outline • Observational properties of Low-luminosity GRBs • Why low-luminosity GRBs are unlikely to be generated by “successful” jets (as long GRBs) • Theory of relativistic shock breakout (gb>0.5) • Comparison of relativistic shock breakout predictions to low-luminosity GRB observations • Shock breakout in regular long GRBs

3. Low-luminosity GRBs • There are 4 low-luminosity GRBs observed to date with a confirmed associated SNe and known redshifts. • Two with regular duration (~20 s) and two are very long (~2000 s) • All are nearby, ~40-400 Mpc. • All are associated with a very rare supernova type: Broad-line IcSNe • Nearby long GRBs are also associated with similar unique type of SNe • Low-luminosity GRB high energy emission is very different than that of long GRBs • The strong connection between the two types is based on the mutual association with Broad-line IcSNe

4. Properties of low-luminosity GRBs • Low luminosity 1046-1048 erg/s (~10-4 than long GRBs) and low energy 1048 - 1050 erg Swift GRBs

5. Properties of low-luminosity GRBs • Low luminosity 1046-1048 erg/s (~10-4 than long GRBs) and low energy 1048 - 1050 erg • High volumetric rate (x1000 that of long GRBs). • Not an extrapolation of long GRB rate to low luminosities Low luminosity Short Long Wanderman & Piran 2011

6. Properties of low-luminosity GRBs • Low luminosity 1046-1048 erg/s (~10-4 than long GRBs) and low energy 1048 - 1050 erg • High volumetric rate (x100 that of long GRBs). • Not an extrapolation of long GRB rate to low luminosities • Smooth light curves (very rare among long GRBs)

7. Properties of low-luminosity GRBs • Low luminosity 1046-1048 erg/s (~10-4 than long GRBs) and low energy 1048 - 1050 erg • High volumetric rate (x100 that of long GRBs). • Not an extrapolation of long GRB rate to low luminosities • Smooth light curves (very rare among long GRBs) • Eg<< total kinetic energy in the explosion (~1052 erg) • The gamma-rays are not highly collimated • Mildly relativistic ejecta with energy ~ Eg • Delayed X-ray emission, with energy ~ Eg • Low-Luminosity GRBs are very different than long GRBs. But, can they be produced in the same way?

8. Long GRBs are generated by relativistic jets that successfully “punch” through their progenitor envelopes Can low-luminosity GRBs be produced by “successful” jets? Zhang et al., 04

9. Before the jet punches through the star its energy is dissipated into its envelope After the jet breaks out energy flows (relatively) freely to large distances where the prompt GRB emission is emitted.

10. tγ = te- tb Time for jet to break out Engine Work time GRB duration tb tγ te

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

12. Long GRBs # of bursts Low-luminosity 0.01 10 0.1 1 T90/tb Bromberg, EN & Piran2011 Low-luminosity GRBs are most likely (2s) not produced by jets that successfully punches through their progenitor envelope

13. 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 (Bromberg et al 12) What are the observed signatures of the resulting shock breakouts?

14. Relativistic shock breakout (EN & Sari 2012)

15. Shock accelerates in steep density gradient Energy release radiation-dominated shock Shock breakout “first light” Continuous diffusion

16. Shock breakout A self-similar radiation dominated shock is accelerating through the envelope, gr-0.23(Johnson & Mckee 1971, Tan et al 2001, Pan & Sari 2006) log E log g

17. Shock breakout A self-similar radiation dominated shock is accelerating through the envelope, gr-0.23(Johnson & Mckee 1971, Tan et al 2001, Pan & Sari 2006) log g Shock width = distance to edge

18. 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

19. Colgate (1968): • SNe shocks before breakout: • very high Lorentz factor • radiation dominated at • thermal equilibrium Burst of g-rays (in some SNe and other explosions)

20. The temperature behind the shock • Constant (independent of gsh ) post shock rest frame temperature ~100-200 keV pairs Katz et. al., 10 Budnik et. al., 10 TBB

21. The Observed temperature • Following breakout the expanding gas accelerates up to • The gas is loaded with pairs, trapping the radiation • The trapped radiation can be released only when pairs annihilate at T`≈50 keV

22. Observed energy The breakout energy is released from a region with Thomson optical depth ~ 1 (without pairs) Observed duration Light travel time dominates the breakout duration

23. The Observed signature of a relativistic breakout Three observables depend on two physical parameters Relativistic breakout relation

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

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

26. Other Predictions of relativistic shock breakouts: • Smooth light curve • Eg<< total energy • Relativistic ejecta with energy ~ Eg • Delayed X-ray emission, with energy ~ Eg • If the breakout is due to failed jets than rate >> than long GRBs Relativistic breakout relation ?

27. Low luminosity GRBs Relativistic breakout relation

28. A Wolf-Rayet with a radius of a red supergiant? • Only a mass of 10-4Mʘ is needed at this radius to produce the observed shock breakout • Recent early time SNe light curves indicates on a compact massive mantle and a low mass extended envelope

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

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

31. Thanks

32. 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):