1. GRB Prompt radiation mechanisms X-ray LC  progenitor star properties Outline † New Scenarios & Developments for Long GRBs Prompt Emission Models New developments led by: GRBs 080319B and 080916C GRBs 080319B and 080916C Alexandria, Egypt, April 2, 2009 † Work done with Ramesh Narayan & Rodolfo Burniol-Duran

2. Understanding GRB central engine central engine relativistic outflow Emission region The point to make: 1. 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…) 2. Understanding of emission region offer a way to getting a handle on the inner engine and deciding jet composition etc. and determining which of the proposed models for the central engine is operating in GRBs.

3. Mechanisms for prompt  -ray emission Internal/external shocks, magnetic reconnection etc. Conversion of jet energy to thermal energy Radiation mechanism Synchrotron, SSC, IC of external photon field, thermal radiation, jitter radiation Piran et al. ; Rees & Meszaros (1994); Thompson; Lyubarsky; Blandford, Spruit… Papathanassiou & Meszaros, 1996; Sari & Piran, 1997; Ghisellini et al. 2000; Thompson (1994); Lazzati et al. (2000); Medvedev (2000); Meszaros & Rees (1992-2007)….

4. Internal-External Fireball Paradigm Internal Shocks  -rays 10 13 -10 15 cm External Shock Afterglow 10 16 -10 18 cm Piran et al. 1993; Rees & Meszaros 1994; Paczynski & Xu 1994 Relativistic Outflow Inner Engine 10 6 cm The only point to make is that gamma-rays in internal shock And afterglow in external shock. Delete this slide?

5. My strategy:  -rays  jet  central engine From  -ray observations identify the radiation mechanism and determine the source properties such as N e, ,  e,  e … Use this information to determine the jet energy dissipation mechanism & (hopefully) jet composition & central engine properties. — In the remaining time we will apply this to two bright bursts: GRBs 080319B & 080916C.

6. GRB 080319B (naked eye burst) Burst detected by Swift and Konus satellites ; duration 50s. Z = 0.937 (Vreeswijk et al. 2008; Cucchiara & Fox 2008). 20keV — 7 MeV fluence = 5.7x10 -4 erg cm -2 ; isotropic equivalent energy = 1.3x10 54 erg (Golenetskii et al. 2008). Spectrum peaked at 650 keV and flux at the peak = 3 mJy. Time averaged  -ray spectrum: f  0.18  0.01 < 650 keV (Racusin et al. 2008) -2.87  0.44 > 650 keV (Racusin et al. 2008) -2.87  0.44 > 650 keV The low energy spectrum was 0.50  0.04 for t<8s. Optical peaked at 5.4 mag or 20 Jy (Karpov et al. 2008). Just say that -- bright burst at z~1, peaked at 650keV and the spectral properties … And of course bright optical at 5.4 mag

7. Synchrotron solution is ruled out as f  +0.18 Synchrotron peak at 650 kev  B  i 2  = 7x10 14 Electron cooling: t cool = ~ (2x10 -9 s)  i3 3  3 «  t ~ 0.1s Compton Y ~  e  c  i   c /  i ~ 10 -9 [  i  /(t GRB  e )] 1/2 6  m e c(1+z) —————  T B 2  i   f  -1/2 and NOT 0.18 as observed. 1. Jitter does not work for this burst (Y » 10 3 ). 2. Small pitch angle radiation (  <  i -1 ) can give f  +ve (Lloyd & Petrosian, 2000); but shock accelerations don’t produce small  distribution. This is basically Ghisellini et al. (2000) argument; Sari & Piran 97 Note: angle between e - momentum and B

8. Synchrotron-self-Compton solution Synchrotron  optical and IC   -rays 2. a < i /25 (because spectrum between 20 & 650 keV was 0.18 not +1 ) 3. The peak of IC at 650 keV and the flux is 3mJy Observational constraints: 1. mean optical flux 10 Jy; Synchrotron peak ( i ) unknown We use all these observational constraints to search for self-consistent solutions (we will make so assumption regarding the dissipation mechanism for jet energy).

9. f ic ≈ (8x10 -3 mJy)  -3 f op,1 ic5.8  t 0.9 R 16 0.64 A straightforward (although tedious) calculation finds: Where  i / a &  t  R/2c  2 No solution if one insists on taking  t ≈ 0.5s (the observed variability time), except if the source distance R » 10 18 cm (much larger than the deceleration radius!).  The IC flux is too small by a factor ~ 10 2 Solution is possible if we take  t ≈ 50s (burst duration), and R ≈ 2x10 17 cm; how to produce variability though? (Clearly NOT internal shocks) 0.25 2

10. In general, if  -rays are produced via the SSC process then the jet energy dissipation mechanism is highly unlikely to be internal shocks. The reason is that for SSC solutions E e  R 3 and E B  R -4  all internal shocks must take place within a narrow range of R (factor ~2) and that seems unlikely. range of R (factor ~2) and that seems unlikely. Kumar & Narayan, 2009 An aside:

11. IC solution when optical and  -rays are decoupled? 1. Optical flux is > a few Jy if  -rays are produced via SSC.  optical shell  -ray shell 2. Optical photons IC scattered in a different shell? Causality ensures that separation between shells  R/2  2 Moreover, it can be shown that electron LF in the two shells should be the same (to within a factor ~2). This suggests physically related shells.

12. Line of Sight 1/  R tt  Relativistic Turbulence Model (Narayan & Kumar, 2009; Lajar, Nakar and Piran, 2009) Variability time = R(1+z) ———— (2c  2 )  t 2 Synchrotron  optical (less variable) IC off of blobs   -rays (more variable) (Lyutikov & Blandford, 2003)

13. The correlation between  -rays and optical lightcurves suggests that they were produced in the same source. However, optical flux is 20 Jy which is 10 4 times larger than  -ray flux extrapolated to the optical band. Larger variability of  -ray LC Correlation between optical &  -rays is far from perfect. Optical depth of the source (  ) ~  -ray flux/optical flux ~ 10 -3 Racusin et al. (2008)

14. GRB 080319B: relativistic turbulence model Kumar & Narayan (2009) The ratio of IC & synchrotron energies is ~10, i.e. Y~ 10. Optical band lies above the synchrotron peak where f  -2.9 and so the total energy in synchrotron photons is larger than optical emission by a factor ~ 10. The total energy in 2 nd IC component is of order the 1 st IC scattering. The 2 nd IC lies in the K-N regime since  i /  >1, and the peak photon energy in e - rest frame is ~ 2 MeV. Focus on Y being order 10 and energy in 2nd IC of order 1st IC

15. Steep decline (~t -5 ) consistent with LAE (not RS) RS emission

16. GRB 080319B jet: a schematic sketch Poynting outflow Thompson 1994 & 06; Meszaros & Rees 97’; Lyutikov & Blandford(2003); Spruit et al. 2001 Swept up stellar gas

17. Some problems with the relativistic turbulence model 1. Not clear how to get asymmetric pulses in GRB lightcurve. 2. Correlation between pulse width and the gap preceding it (reported by Nakar and Piran). These problems and a few others (less serious) are discussed in Lazar, Nakar and Piran (2009). (Narayan and Piran might have a solution)

18. GRB 080916C (2 nd Fermi LAT burst) Burst detected by Fermi GBM & LAT; duration 55s. Z = 4.3 (Greiner et al. 2009). 10keV — 10 GeV fluence = 2.4x10 -4 erg cm -2 ; isotropic equivalent energy = 8.3x10 54 erg (Abdo et al. 2009). Spectrum peaked at 500 keV and flux at the peak = 2 mJy. Time averaged  -ray spectrum: f  0.02  0.02 < 500 keV (Abdo et al. 2008) -1.2  0.03 > 500 keV (Abdo et al. 2008) -1.2  0.03 > 500 keV The low energy spectrum was 0.42  0.04 for t<4s. Highest energy photon detected was 13GeV  > 10 3. The flux above 100 MeV was zero during the first 5s. LAT flux declined as t -1.2  0.2 for >100MeV and 5s 100MeV and 5s<t<1400s To say: 1) exceptionally bright burst at z=4.3 seen by GBM and LAT. 2) Highest energy photon detected was 13 GeV and 3) spectral properties very similar to the naked eye burst. 4) (optional) >100MeV photons lagged MeV Photons by about 5s.

19. Possible solutions 1. SSC would work only if t  ≈R/(2c  2 ), i.e. relativistic turbulence, and bright prompt optical flux >10Jy (5-mag). For this solution For this solution:  ~  I ~ 10 3 and the 2 nd IC peaks at ~ 2 TeV; (fluence in TeV photons is a factor ~5 larger than MeV photons) 2. Synchrotron process? Rapid cooling is a serious problem (same as naked eye burst) A clever way out of this problem was suggested by Nakar (2008) who pointed out that dn e /d  e   e -1 when cooling is dominated by IC scattering in K-N regime. The steady state solution of nene + = Q tt — (ene)(ene)  e ——— is n e   e   e -1 -1 -1  f  0 in this regime

20. Wang, Li, Dai & Meszaros (2009) invoke this mechanism for 080916C. Synchrotron solution continued… However, this requires t cool « t cool ic syn, and that leads to problems: 1. R < 3x10 13 cm and in that case Thomson optical depth is sufficiently large so that the IC flux at 2GeV exceeds the observed value by a factor ~ 3; also Y~10. 2.Moreover, at this small R, rapid production of e  modifies the low energy spectrum to f  -1/2 ! modifies the low energy spectrum to f  -1/2 ! Therefore, this very attractive solution can be ruled out for GRB 080916C.

21. It can be shown that there are only two synchrotron solutions that are self-consistent: 1.  >10 4,  i >10 4, R>10 15 cm & E B ~ 5x10 54 erg 2. Electrons are continuously accelerated so that the distribution below  i is actively maintained as   e -1. distribution below  i is actively maintained as   e -1. The bottom line is that the Fermi burst is the 2 nd burst for which internal shocks are ruled out, and a magnetic outflow is implicated (the 1 st burst was 080319B). Synchrotron solution continued… (cooling unimportant)

22. 2. For these bursts a relativistic magnetic outflow 2. For these bursts a relativistic magnetic outflow is indicated &  -ray source distance (from the is indicated &  -ray source distance (from the center of explosion) is found to be » 10 15 cm. center of explosion) is found to be » 10 15 cm. Summary 1. The 6-decades of frequency coverage for GRBs 080319B & 080916C has severely constrained the established paradigm for GRB jet energy dissipation mechanism and the emission process. 1. The 6-decades of frequency coverage for GRBs 080319B & 080916C has severely constrained the established paradigm for GRB jet energy dissipation mechanism and the emission process. Note: This is an animation slide. You need to click on the mouse several times to display all the texts. 3. Prompt Optical data together with Fermi GBM and 3. Prompt Optical data together with Fermi GBM and LAT provide unprecedented 1—10 10 eV coverage, LAT provide unprecedented 1—10 10 eV coverage, and that should finally solve the mystery of  -ray and that should finally solve the mystery of  -ray emission process in GRBs and the jet composition. emission process in GRBs and the jet composition.

25. Synchrotron peak: i = B ´  e 2  /1.8x10 8 ( i is in eV) Synchrotron peak flux: f p = 59 B ´  N e,55 mJy N e,55 = N e /10 55 N e,55 = N e /10 55 Self-absorption frequency: a = (18 eV) f p4 5/8 (  t/3s) -1/4 i -1/8 Y -1/8 R  15 -1 a = (18 eV) f p4 5/8 (  t/3s) -1/4 i -1/8 Y -1/8 R  15 -1 (The deceleration radius, R d, is about 10 17 cm) Note: We are finding this result -- R d / R  <10 -- for many bursts) R  15 = R  /10 15 cm (z=0.94) GRB 080319B (continued) Analysis of optical data Optical radiation, if from a thermal source, requires:  ≈1.2x10 7 (  t) 1 -1 T 5 -1/2 ; (  t) 1 =  t/10s; T 5 =T/10 5 K This suggests a nonthermal origin (likely synchrotron) f p4 = f p /10 Jy f p4 = f p /10 Jy

26. How is the jet energy dissipated? Jet is launched at ~10 7 cm and the Jet is launched at ~10 7 cm and the energy is dissipated at R>10 11 cm. energy is dissipated at R>10 11 cm. How are  -rays produced? The current paradigm is the so called internal shock model for energy dissipation and synchrotron + IC for radiation. Jet energy  radiation

27. So the optical band is below or close to the self-absorption frequency The observed peak optical flux of 20 Jy requires: R  > [1.4x10 16 cm] (  t/3s) -1/4 i -1/8 Y -1/8 Substituting this into  t ≈ R  /2c  2   ~ 375 (  t/3s) -5/8 i -1/16 Y -1/16 It can be shown that  e ≈ 120 (  t/3s) -1/8 i 3/16 Y 3/16 The radiation was produced at a large distance!

28. time flux steep fall off prompt GRB emission rapid decline X-ray plateau r ≈ 9  10 9 cm r ~ 1.5  10 11 cm f Ω ~  2  f Ω  2 3  10 10 cm   r -2.5 f    k Progenitor Star Properties Kumar, Narayan & Johnson (Science, July 2008)

29. The early steep decline occurs when accretion drops below ~ 10 -2 drops below ~ 10 -2.  M s -1 1) The neutrino cooled disk is replaced with a hot torus (ADAF) torus (ADAF) 2) The outer part of the stellar core has rapidly declining density profile  steep decline of dM/dt. declining density profile  steep decline of dM/dt. Accretion rate during a burst ~ 10 52 erg/s * 100/c 2 ~ 0.1.  M s -1 1% efficiency for jet The x-ray plateau occurs when the envelope of the star with  r -2.5 is accreted. Observed flux  accretion rate

31. Constraints  Flux  spectral index below the peak of spectrum  frequency at peak of spectrum  burst/pulse duration We also use the flux/upperlimits in early x-ray & optical when available. 5 unknowns and 3 constraints gives 2-D solution surface.

32. Factor ~ 10 3 drop in flux! The central engine comes back to life and is The central engine comes back to life and is active for hours and days! active for hours and days!

33. Fall-back time: t fb ~ 2(r 3 /GM r ) 1/2 = 2/  k Or r 10 ~ 1.5 t 2 2/3 M BH,1 1/3 ; r 10  r/10 10 cm, t 2  t/100s Specific angular momentum: j(r) =  (r) r 2  f   k r 2 Accretion time: t acc ~ 2/   k (r d ); r d : disk radius t acc ~  -1 f  3 t fb t acc ~  -1 f  3 t fb Progenitor Structure: Basic procedure

34. (Problem when you have good data!) Panaitescu et. Al, 2007 O’Brien et al. 2006 New puzzles posed by Swift data Flares lasting for hours - short and long GRBs Chromatic plateau In x-ray LCs Do we have the FS AG right? RS emission? Jet breaks? Do we have the FS AG right?

36. 3. No firm evidence for r -2 density structure (except perhaps in 1 or 2 cases). And very low density found in several cases is puzzling. 1. The nature of the central engine is not understood. Unsolved Problems 2. Is the energy from the explosion carried outward by magnetic field, e ±, or baryonic material? 4. Collisionless shocks, particle acceleration, magnetic field generation etc. poorly understood.

37. AGILE (an Italian mission) 30 Mev – 30 Gev & 10 – 40 kev is expected to launch in 2005 is expected to launch in 2005. ICECUBE, ANTARES will explore Neutrino emission from GRBs: 10 Gev – 10 5 Tev Neutrino emission from GRBs: 10 Gev – 10 5 Tev. AMANDA: at the south pole has an effective area of 10^4 m^2, and its sensitivity is about 3-orders of magnitude below the expected neutrino flux from GRBs (neutrino energy between 100 Tev And 10^7 Tev). Kilometer-cube size detectors are needed to detect neutrinos from GRBs. ICECUBE: is a one-cube-kilometer neutrino observatory being built in the clear deep ice on the South Pole. ANTARES: detectors in sea water, Mediterrarian sea, the expected size is several-cubic-kilometer. NESTER: is also a sea based detector (seems like it has been canceled) GLAST: anticipated launch is 2007. It will cover Energy range of 10 Kev to 300 Gev. GALST/LAT Will have a field of view of 2.5 stradians, and 50 times The sensitivity of CGRO/EGRET at 100 Mev and better At higher energies; the limiting flux is 10^{-9} photons/cm^2/s. It should be able to locate sources to better than 5 arc-min. INTEGRAL: International Gamma-ray Astrophysics Laboratory an ESA mission was launched on October 17, 2002 using a Soviet proton launcher in a 72 hr orbit. It has instruments covering 15 kev to 10 Mev; the mission cost 330M Euros. It will have a sensitivity of 10^{-6} photons/cm^2/s in 10-100 kev range for an integration time of 10^6 s, & 10^{-7} photons at 10 Mev. The imaging instrument has a resolution of 12 arc-minutes. AGILE: is an Italian mission which will have an energy range of 30Mev to 50Gev & 10-40 kev with 2 steradian field of view. AGILE is a small mission (weighing 80 kg!) which will be placed in an equatorial orbit at about 550 km height. The X-ray camera (10-40 kev) should have an angular resolution Of about 5 arc-min, and the gamma-ray telescope 35 arc-min. Sensitivity at 100Mev is expected to be 6x10^{-9} photons/cm^2/s for 10^6s integration time, and 4x10^{-11} at 1Gev, and the timing accuracy is 25-micro-sec. The spectral resolution in gamma-rays is expected to be about 1 i.e. dE/E~1. Future Missions EGRET energy range was 30Mev to 10Gev. GLAST, due for launch in 2007, will cover 10 Kev – 300 Gev, and detect > 200 GRBs yr -1. Gravitational waves from GRBs?