Gamma-Ray Bursts Early Afterglows

1. Gamma-Ray Bursts Early Afterglows Bing Zhang Physics Department University of Nevada, Las Vegas Jun. 21, 2005, Torun, Poland

2. Collaborators • S. Kobayashi (PSU) • P. Meszaros (PSU) • Y. Z. Fan (UNLV & PMO) • D. M. Wei (PMO) • J. Dyks (UNLV & NCAC) • Swift collaborations (D. Burrows, P. Roming, G. Tagliaferri; A. Blustin; J. Nousek; N. Gehrels, et al)

3. Gamma-ray bursts: the most violent explosions in the universe!

4. A Generic GRB Fireball UV/opt/IR/radio gamma-ray gamma-ray X-ray UV/optical IR mm radio central photosphere internal external shocks engine (shocks) (reverse)(forward)

5. Why Is Early Afterglow Essential? • Diagnose the composition of the “fireball” ejecta (Late afterglow is the emission from the medium) • Diagnose the immediate environment of GRBs (ISM or wind? Density clumps? etc) • Diagnose the emission site of GRB emission (bridge between the prompt emission phase and afterglow phase) • Diagnose the central engine ativity of the fireball

6. Expectations in Optical band:fireball composition

7. GRB Composition • Baryonic component • Protons and electrons • Neutrons • Magnetic fields (Poynting flux) Answer: from Early Afterglows!

8. A Generic GRB Fireball UV/opt/IR/radio gamma-ray gamma-ray X-ray UV/optical IR mm radio central photosphere internal external shocks engine (shocks) (reverse)(forward)

9. Early optical afterglow lightcurves(Zhang, Kobayashi & Meszaros, 2003)

10. GRB 990123 Zhang, Kobayashi & Meszaros, 2003

11. GRB 021211(Fox et al. 2003) Zhang & Kobayashi 2004:

12. GRB 041219a Fan, Zhang & Wei, 2005

13. An analytic MHD shock Solution for GRB reverse shocks (Zhang & Kobayashi 2004) Two free parameters: Blandford-McKee (1976) Kennel-Coroniti (1984)

14. t-1 t1/2 Optical, forward shock emission

15. t-2 t? t1/2 t-1 Optical, forward + reverse shock emission σ ~ 0

16. t-2 t? t1/2 t-1 Optical, forward + reverse shock emission σ ~ 0.01

17. t-2 t? t1/2 t-1 Optical, forward + reverse shock emission σ ~ 1

18. t-2 t? t1/2 t-1 Optical, forward + reverse shock emission σ ~ 10

19. t1/2 t-1 Optical, forward + reverse shock emission σ ~ 100

20. Neutron-fed GRB afterglows • If GRB jets are dominated by the baryonic component, then the neutron component has an important impact on GRB afterglow lightcurves (Derishev et al. 1999; Beloborodov 2003) • Detailed dynamical evolution & lightcurve calculation (Fan, Zhang & Wei 2005 - astro-ph/0412105)

21. Neutron-fed fireball dynamics (wind)(Fan, Zhang & Wei, 2005)

22. Neutron-fed early afterglow (wind)(Fan, Zhang & Wei, 2005)

23. Neutron-fed fireball dynamics (ISM)(Fan, Zhang & Wei, 2005)

24. Neutron-fed early afterglow (ISM)(Fan, Zhang & Wei, 2005)

25. The GRB Explorer Mission: . A Multiwavelength Observatory for Rapid-Response Observations of Transient Targets Launched on Nov. 20, 2004 Prime institution: NASA/GSFC Leading university partner: Penn State University Country involved: USA, Italy, UK

26. GRB 041219a • Contemporaneous • IR flash & early • Afterglow – PAIRITEL • Blake et al. 2005 • Contemporaneous • optical flash tracking • GRB lightcurves – • RAPTOR, Vestrand • et al. 2005. Neutron-rich internal shocks & mildly magnetized reverse shock + forward shock model Fan, Zhang & Wei, 2005

27. GRB 050525a • Early reverse shock • Transition to forward shock • Re-brightening • Jet break • Blustin et al., 2005

28. UVOT Dark Bursts Lack of reverse shock Highly magnetized flow? Roming et al., 2005

29. Surprises in the X-ray bandemission site & central engine

30. GRB Emission Site • Internal • Internal shocks • Internal magnetic dissipation • External shock • “Innermost” model: dissipative photosphere Answer: from early X-ray bridge between prompt emission and afterglow!

31. A Generic GRB Fireball UV/opt/IR/radio gamma-ray gamma-ray X-ray UV/optical IR mm radio central photosphere internal external shocks engine (shocks) (reverse)(forward)

32. Surprise (1): Rapid initial decays • Steep-to-shallow transition • Common signature • Tagliaferri et al., 2005

33. Rapid initial decays • May or may not connect to the prompt emission • Tagliaferri et al., 2005

34. Interpretations • Tail of prompt GRB emission or the late central engine emission – curvature effect (Kumar & Panaitescu 2000; Dyks et al. 2005) • Important implication: GRBs and afterglows come from different locations! tail afterglow GRB

35. Surprise (2): Re-brightening bumps • GRB 050406 • by a factor of 6 • Burrows et al., 2005

36. Re-brightening bumps • GRB 050502B • by a factor of 1000! • Burrows et al., 2005

37. Interpretations • Reverse shock Inverse Compton flare (Kobayashi, Zhang, Meszaros & Burrows 2005) • Density bumps? • Refreshed shocks? • Two component jets? • Late internal shocks(Burrows et al. 2005; Zhang et al. 2005) !!

38. RS SSC bump • Spectrum • Kobayashi et al., 2005 RS - SSC

39. RS SSC bump • Lightcurve • Can interpret 050406, but difficult to interpret 050502b • Kobayashi et al., 2005

40. Late Internal Shocks • Rapid falling rules out density bump, refreshed shocks & two-component models • A factor of 1000 re-brightening is difficult for the SSC model • The central engine is active again hundreds of seconds later! • Implications for XRFs. • Burrows et al., 2005 • Zhang et al. 2005

41. Conclusions • Swift is revolutionizing our understanding of GRBs; • Early UV/optical/IR observations are consistent with the reverse shock + forward shock interpretation; • There might be diverse fireball composition among bursts; • Early X-ray observations reveal the bridge between prompt emission and afterglow; • Prompt emission is originated from a different site from afterglow; • GRB central engine is still alive after the GRB ceases.