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Recent Advances in our Understanding of GRB emission mechanism

Recent Advances in our Understanding of GRB emission mechanism. Pawan Kumar. Outline †. •. Constraints on radiation mechanisms ♪. •. •. High energy emission from GRBs and our understanding of Fermi data. •. ♪ My goal is to generate a good discussion of this topic. Moscow, October 9, 2013.

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Recent Advances in our Understanding of GRB emission mechanism

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  1. Recent Advances in our Understanding of GRB emission mechanism Pawan Kumar Outline† • Constraints on radiation mechanisms♪ • • High energy emission from GRBs and our understanding of Fermi data. • ♪My goal is to generate a good discussion of this topic Moscow, October 9, 2013

  2. • • Understanding the radiation mechanism for ~10keV – 10 MeV band is one of the most challenging problems in GRBs. Emission in this band lasts for <102s, however it carries a good fraction of the total energy release in GRBs. And it offers the best link to the GRB central engine. Jet energy dissipation and γ-ray generation relativistic outflow External shock radiation central engine central engine jet-rays A good fraction of >102MeV photons appear to be generated in external shock; (photo-pion & other hadronic processes might also contribute for ~30s or so) Make point 1 ONLY: 2. Central engine is completely hidden from our view so the progress that is being made is via numerical simulation of core collapse and other very interesting works (woosley et al. Quataert et al…)‏ 1. The relativistic jet energy produced in these explosions is dissipated at some distance from the central engine and then a fraction of that energy is radiated away as gamma-rays. CONSIDERING OUR LACK OF UNDERSTANDING OF GRB JET COMPOSITION IT IS BEST TO TREAT JET DISSIPATION AND GAMMA-RAY PRODUCTION SEPARATELY. DO NOT seond more than 30d on this slide.

  3. • Conversion of jet energy to thermal energy Internal/external shocks, magnetic reconnection etc. Piran et al. ; Rees & Meszaros; Dermer; Thompson; Lyubarsky; Blandford, Lyutikov; Spruit… • Radiation mechanism (sub-MeV photons) 1. Synchrotron 2. SSC (or IC of external photons) 3. photospheric mechanism… Papathanassiou & Meszaros, 1996; Sari, Narayan & Piran, 1996; Liang et al. 1996; Ghisellini et al. 2000; Thompson (1994); Lazzati et al. (2000); Medvedev (2000); Meszaros & Rees 1992-2007; Totani 1998;Paczynski & Xu 1994; Zhang & Meszaros 2000; Meszaros & Rees 1994; Pilla & Loeb 1996; Dermer et al. 2000; Wang et al. 2001 & 06; Zhang & Meszaros 2001; Sari & Esin 01’; Granot & Guetta 2003; Piran et al. 2004; Fan et al. 2005 & 08; Beloborodov 2005; Fan & Piran 2006; Galli & Guetta 2008; Pe’er et al. 06; Granot et al. 08;Bošnjak, Daigne & Dubus 09; Katz 1994; Derishev et al. 1999; Bahcall & Meszaros 2000; Dermer & Atoyan 2004; Razzaque & Meszaros 2006; Fan & Piran 2008; Gupta & Zhang 2008; Granot et al. 08; Daigne, Bošnjak & Dubus 2011 …

  4. Energy dissipation: internal shocks (current paradigm) (Prof. Bosnjak will talk about this model in detail) Gehrels et al. (2002); Scientific American

  5. Distance (Rs) of γ-ray source from the center of explosion 2. Prompt bright optical flash from GRBs: Rs 1016cm(GRB 080319B – Zou, Piran & Sari 2009) t-5 (Too steep to be RS) Kumar & Panaitescu, 09 Prompt -ray emission from GRB 080319B also suggests Rs 1016cm; Kumar & Narayan; Racusin et al. 2008 (RS) > > > ~ ~ ~ GRB 080319B: x-ray & optical LCs (This would help determine if radiation mechanism is photospheric or not) Steep decline of flux at end of GRB prompt phase suggests: Rs ≈ 2c2δt ~ 1016cm (Lyutikov; Lazzati & Begelman; Kumar et al.) (Rs can be smaller if the steep decline is due to central engine activity) Shen & Zhang (2009) provide a limit on Rs from prompt optical for a number of GRBs.

  6. 3. Detection of high energy -ray photons by Fermi/LAT (GRB 080916C…)   103 Rs = 2cΓ2δt 1016cm > > ~ ~ However, Zou, Fan & Piran (2011), Hascoet et al. (2012) suggest Γ~300 This implies Rs~1015 cm, and that is still much larger than photospheric radius (~1012 cm) – this is for MeV photon emission and δt ~ 0.1 s. So the photospheric radiation is not the correct mechanism for MeV γ-rays at least for some GRBs GRB 080916C, a very bright Fermi burst, had a very stringent upper limit on thermal component (Zhang & Pe’er, 2009). Incidentally, Γ>103 would rule out baryonic and leptopic thermal fireball model for GRBs since Γmax ~ 850 L521/4 R0,7-1/4; where R0 the jet launching radius. •

  7. 6mec(1+z) ————— TB2i  tcool = ~ (7x10−7 s) i332 « t ~ 0.1s MeV γ-ray radiation mechanism 1. Synchrotron • Synchrotron peak at ~102 kev  Bi2 ~ 2x1013 • Electron cooling  f −1/2(or α = −1.5)which holds for only a small fraction of GRBs This is basically Ghisellini et al. (2000) argument; Sari & Piran 1997 Note: 1. Synchrotron solutions with α = −2/3 is possible provided that Rs>1016cm, and Γ> 300 –– Kumar & McMahon (2008), Beniamini & Piran (2013) –– but in this case the variability time can’t be smaller than a few sec. 2. IC cooling in KN regime (Nakar, Ando & Sari, 2009; Bosnjak et al.; Barniol Duran et al.) helps but not enough.. 3. Continuous acceleration of electrons can fix the low energy spectral index problem.

  8. 2. Synchrotron-self-Compton solutions • INTEGRAL: black BATSE: violet Fermi/GBM: red The spectral peak (Ep) for SSC: α γi4 so one would expect a broad distribution for Ep but that is not what GRB observations find Bosnjak et al. (2013) • It can be shown that for SSC solutions Ee α R3and EB α R−4 • emission must be produced within a narrow range of R (factor ~2) and that seems unlikely -- especially for the IS model. • There is another problem with the SSC solution: A lack of an excess in the Fermi/LAT band (100 MeV to 100 GeV), and absence of a bright optical flash severely constrains the SSC model (e.g. Piran, Sari and Zou, 2009). No reason that jet energy should dissipate at the minimum of Ee+EB

  9. Thompson (1994 & 06); Liang et al. 1997; Ghisellini & Celloti 1999; Meszaros & Rees (2001); Daigne & Mochkovitch (2002); Pe’er et al. (2006), Beloborodov (2009)… 3. Thermal radiation + IC (for prompt -rays) Observational constraints • Photospheric radius ~ 1012cm 3−3 Lj53; so the IC of thermal radiation is expected to take place at a much smaller radius than Rs ~ 1016cm we are finding. • Low energy spectrum should be fν ν—ν2 which is rarely seen. However, recent work of Burgess et al. (arXiv:1304.4628) claims to see a thermal component for 5 out of 8 Fermi GRBs they analyzed. Theoretical constraints Vrum et al. (2013) & Asano & Meszaros (2013) provide general constraints on photospheric models for MeV emission (Vrum’s talk on Monday) They find that a large fraction of jet energy should be dissipated at a radius of 1010–1011 cm –– optical depth ~10 –– and jet LF at this radius should be order a few 10s, i.e. the dissipation should take place at a high but not too large optical depth, i.e. some fine tuning needed.

  10. Since GRB spectra are largely non-thermal, there are many different proposals as to how to modify the photospheric radiation so that the emergent spectrum is non-thermal. Let us consider one particular photosphere model – n-p collision • Consider a baryonic jet consisting of n & p+. Neutrons accelerate with the fireball expansion as long as they collide frequently with protons. • Eventually at some radius (Rnp) n & p+ decouple & hereafter nare no longer accelerated whereas p+ Lorentz factor could continue to increase with R as long as Γ(Rnp) < η. • The resulting differential velocity between n & p+ result in theircollision and conversion of a fraction of jet KE to thermal energy below the photosphere. ★ ★ ★ ★

  11. n–p decoupling radius is given by – or For n – p to develop differential velocity: Rnp < Rs = R0 η Thus, GRB jets consisting of n& p& terminal Lorentz factor > 400 will undergo n– pcollisions below the Thomson photosphere & convert a fraction of jet kinetic energy to radiation & e± thermal energy (Beloborodov 2010; Vurm et al. 2011 & Meszaros & Rees 2011)

  12. n– pdifferential motion can also arise in internal shocks ★ Beloborodov, 2010

  13. Radius where internal collisions occur: Rcol = c Γ2δt ★ And the radius where the probability of n-p collisions drop below 0.5 is: Rnp α Γ-3 ★ • Rcol/Rnp α Γ5 For an efficient conversion of outflow kinetic energy to thermal energy via n–p collisions these radii should be approximately equal, and that requires: ★ 50 < Γ < 102 Which does not appear to be consistent with GRB data.

  14. Origin of high energy photons (>100 MeV) Prompt phase: high energy photons during this phase might have a separate origin than photons that come afterwards if rapid fluctuations and correlation with MeV lightcurve is established. Observers need to quantify the statistical significance of this! • Hadronic processes: proton synchrotron, photo-meson … Bottcher and Dermer, 1998; Totani, 1998; Aharonian, 2000; Mucke et al., 2003; Reimer et al., 2004; Gupta and Zhang, 2007b; Asano et al., 2009; Fan and Piran, 2008; Razzaque et al. 2010; Asano and Meszaros, 2012; Crumley and Kumar, 2013…. Inefficient process – typically requires several order more energy than we see in the MeV band (unless Γ were to be small, of order a few hundred, which few people believe is the case for Fermi/LAT bursts), e.g. Razzaque et al. 2010, Crumley & Kumar 2013. • Internal shock and SSC: e.g. Bosnjak et al. 2009, Daigne et al. 2011

  15. Afterglow: external shock synchrotron, IC in forward or reverse shock of prompt radiation or afterglow photons; IC of CMB photons by e± in IGM; pair enrichment of external medium and IC… Dermer et al., 2000; Zhang and Meszaros, 2001; Wang et al. 2001; Granot and Guetta, 2003; Gupta and Zhang, 2007b; Fan and Piran, 2008; Zou et al., 2009; Meszaros and Rees 1994; Beloborodov 2005; Fan et al., 200; Dai and Lu 2002; Dai et al. 2002; Wang et al. 2004; Murase et al. 2009; Beloborodov 2013….

  16. GRB 130427A (Perley et al. arXiv:1307.4401) MeV duration (T90) = 138s, LAT duration (TGeV) > 4.3x103s; TGeV/T90 > 31 Highest energy photon (95 GeV) detected 242s after T0; z=0.34; Eγ,iso= 7.8x1053erg

  17. GRB 110731A (Ackermann et al. 2013)

  18. Kumar & Barniol Duran (2009) and Ghisellini, Ghirlanda & Nava (2010) showed that high energy γ-ray radiation from GRBs, after the prompt phase, are produced in the external-forward shock via the synchrotron process. The reasoning for this will be described in the next several slides. Gehrels, Piro & Leonard: Scientific American, Dec 2002

  19. Flux above νc is independent of density and almost independent of εB • Consider GRB circumstellar medium density profile: • Blast wave dynamics follows from energy conservation: • Observer frame elapsed time: • Comoving magnetic field in shocked fluid: • Synchrotron characteristic frequency: • Observed flux at νm: . . . Synchrotron cooling frequency: • Observed flux at ν:

  20. The flux from the external shock above the cooling frequency is given by: 0.2 mJy E55(p+2)/4 εep-1 εB(p-2)/4(1+z)(p+2)/4 _______________________________________ fν = dL282(t/10s)(3p-2)/4 ν8p/2 (1+Y) Y << 1 due to Klein-Nishina effect for electrons radiating 102MeV photons. Note that the flux does not depend on the external medium density or stratification, and has a very weak dependence on εB. •

  21. The expected decline of the >100 MeV lightcurve according to the external shock model is t-(3p-2)/4. For p=2.2 the expected decline is t-1.1 which is in agreement with Fermi/LAT observations. Temporal decay index in Fermi/LAT band; Ackermann et al. 2013

  22. Table of expected and observed 100 MeV flux Time (observer frame in s) Expected flux♪ from ES in nJy Observed flux (nJy) z Eγ,54 _____________________________________________________________ 8.8 0.11 3.6 0.6 0.78 150 100 50 100 600 080916C 090510 090902B 110731A 130427A 4.3 0.9 1.8 2.83 0.34 50 9 300 8 48 67 14 220 ~5 ~40 ♪We have taken energy in blast wave = 3Eγ, εe=0.2, p=2.4, εB=10-5

  23. According to the external shock model the LAT flux should be proportional to E(p+2)/4 εep-1 or ~ (Eεe) (E is proportional to Eγ,iso and PIC simulations suggest εe~0.1-0.2) t-(3p-2)/4 ≈ t-1.1 (independent of n, ε ) Nava et al. 2013 -- arXiv:1308.5442 B

  24. Long lived lightcurve for >102MeV (Abdo et al. 2009) (GRB 080916C) fν α ν-1.2 t-1.2 α = 1.5β – 0.5 (FS) Abdo et al. 2009

  25. Long lived lightcurve for >102MeV (Abdo et al. 2009) >102MeV data  expected ES flux in the X-ray and optical band (GRB 080916C) Kumar & Barniol Duran (2009) Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009 We can then compare it with the available X-ray and optical data.

  26. Or we can go in the reverse direction… Assuming that the late (>1day) X-ray and optical flux are from ES, calculate the expected flux at 100 MeV at early times > 100MeV Optical 50 - 300keV Kumar & Barniol Duran (2009) X-ray Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009 And that compares well with the available Fermi data.

  27. A Brief Summary The expected flux between 100 MeV and ~10 GeV due to synchrotron emission in external shock is within a factor 2 of the observed flux (as long as electrons are accelerated as per Fermi mechanism). ★ The predicted flux is independent of ISM density and εB. And hence the flux predictions are robust. An alternate mechanism to explain the >100 MeV flux observed by Fermi/LAT would have to make a more compelling case than the external shock model. ★ Let us look at one recent proposal…

  28. According to the recent proposal of Beloborodov et al. (2013) – IC scattering of MeV photons by e± produced in the external medium – when R(1+z)/2cΓ2 (observer frame time for arrival of IC photons) exceeds a few time T90 the GeV flux should decline sharply. In other words this model suggests TLAT < 3 T90,MeV ~ T90,MeV (s) TLAT (Power-law decline part) in s TLAT/T90,MeV ___________________________________________________________ 080916C 090510 090902B 110731A 130427A >400 120 700 550 >4300 >7 360 23 75 >30 60 0.3 30 7.3 138

  29. • There is little evidence for high density CBM required for this model to work (A* ~ 0.5). Moreover, the high density is likely to over produce 100 keV flux at tobs< T90,MeV • The large optical flux according to this model (~1 Jy) could have escaped detection. However, its IC scattering off of e± produces ~10 keV photons with flux ~ τ± fopt ~ a few mJy that is harder to hide. Deceleration radius (for wind medium) Rd = 2x1015 E55 A*-1 Γ0,3-2 cm

  30. What about 10 GeV – 95 GeV photons detected from GRB 130427A? Could these be produced by the synchrotron process? Highest energy photon (95 GeV) was detected 242s after the trigger (z=0.34, Eγ,iso= 7.8x1053erg) when Γ~ 102. ★ q γe2 ΓB Highest possible energy for synchrotron photons is when ★ 2π mec electrons lose half their energy in one Larmor time (Because electrons gain energy by a factor ~2 in shock acceleration in ~ a few Larmor time) me γe c Synchrotron loss rate σT B2 γe2c Larmor time = ★ = qB 6π Synchrotro loss rate Larmor time x < meγe c2 9mec3 Γ  = 50 Γ MeV νmax = < < 10GeV ~ 16π q2 >10GeV photons might be due to IC in external shock, however, perhaps the above limit could be violated by inhomogeneous B.

  31. Summary The mechanism for generation of photons of energy between ~10 keV and 10 MeV remains elusive. ✫ High energy photons (>100 MeV), after the prompt phase, are produced by the simplest possible mechanism one could imagine, i.e. synchrotron in external shock. However, it is unclear how >10 GeV photons are produced. ✫

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