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Understanding the Diversity of Gamma-Ray Bursts: Analyzing Optical and X-Ray Afterglows

This presentation by Gabriele Ghisellini at INAF-Osservatorio Astronomico di Brera explores the apparent discrepancies between optical and X-ray afterglows in gamma-ray bursts (GRBs). Utilizing a two-component model, the analysis distinguishes between standard afterglow behavior and a phenomenological component responding to late prompts. The findings address the prolonged activity of the central engine, potentially explaining the emergence of jet breaks and the mechanism behind extended afterglows. This research enhances our understanding of GRB light curves and the underlying astrophysical processes at play.

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Understanding the Diversity of Gamma-Ray Bursts: Analyzing Optical and X-Ray Afterglows

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  1. Unifying afterglow diversity Gabriele Ghisellini INAF-Osservatorio Astronomico di Brera - Italy with the help of G.Ghirlanda and M. Nardini

  2. X-ray and optical often behave differently optical TA X-ray Is this “real” afterglow? i.e. external shock? ~same spectrum What is TA ?

  3. 2 Component model • Component 1: Standard afterglow. Standard=forward shocks in the ISM (e.g. Panaitescu & Kumar 2000) • Component 2: Completely phenomenological. Used for the “flat-steep” part of the light curve. Constant SED (minimum # of parameters). • Both components emit optical and X-rays

  4. X-ray optical

  5. Standard afterglow theory (Panaitescu & Kumar 2000) Optical Afterglow X-ray Afterglow

  6. Component 2: completely phenomenological at this stage Component 2 Component 2

  7. aflat asteep

  8. Sum Sum

  9. X-ray optical

  10. X-ray Afterglow Optical Afterglow

  11. Component 2 Component 2

  12. aflat asteep

  13. Sum Sum

  14. Good fits for all 33 GRBs in the sample • What is “component 2”?

  15. asteep ~5/3 Decay index of steep part of comp 2

  16. ~70 Swift GRBs with z X-rays

  17. t-5/3

  18. t-5/4

  19. Comp 2 Ghisellini+ 2009

  20. t-5/3 Lazzati+ 2008 From averaging the luminosity of flares in different GRBs

  21. t-5/3 M from fallback MacFadyen+ 2001

  22. M from fallback Zhang Woosley Heger 2008 Fallback lasts for quite a long time 103 104 105 106 Time [s]

  23. Fallback Prolonged activity of the central engine • Can we explain TA?

  24. Early (normal) prompt: G>>1/qj Strong, erratic Smooth Late prompt: G>1/qj After the early prompt, the central engine decreases energy and decreases G monotonically Late prompt: G=1/qj Late prompt: G<1/qj ”real” after-glow Ghisellini et al. 2007 R~1013 cm

  25. Early (normal) prompt: G>>1/qj TA Late prompt: G>1/qj Tjet Late prompt: G=1/qj Late prompt: G<1/qj ”real” after-glow Ghisellini et al. 2007 R~1013 cm

  26. Conclusions • 2 components can explain the diversity • Component 2 can be associated with the prolonged activity of the entral engine • Fallback is long  late prompt “forever” • Flares as accretion of denser chunks of fallback material • TA can be explained • Next…

  27. NEXT (Marco Nardini): • Can we also explain the presence/absence of jet breaks?

  28. L ~ GMout ~ t-a3 t-a2 Flux G ~ t-Da/2 t-a3 TA Mout ~ t-(a3+a2)/2 Time

  29. L ~ GMout ~ t-a3 Very cheap L~ Maccr ? t-a2 Flux G ~ t-Da/2 t-a3 TA Mout ~ t-(a3+a2)/2 Time Long lasting engine with two-phase accretion? 1: Very dense torus, large B-field, erratic. 2: Less dense material, smaller B-field, smoother accretion – Fallback?

  30. Tests • SEDs vs light curves X-rays optical Log nFn Log n

  31. Tests • SEDs vs light curves • Some GRBs with no plateau: achromatic breaks?

  32. Tests • SEDs vs light curves • Some GRBs with no plateau: achromatic breaks? • Late ‘jump’ in X-ray light-curves revealing real X-ray afterglow TA Jump=end of late prompt X-ray Flux Time

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