1. Gamma-ray Bursts in the Fermi Era Pawan Kumar June 13, 2012 Fermi data & developments of last 3 years Outline † High redshift bursts Review of main properties Problems with the current paradigm and possible solutions.

2. Gamma-ray Bursts were discovered (accidentally  ) by Vela satellites in 1967. For about 20 years the distance to GRBs was completely uncertain. History Picture showing Vela launch using a Tital II-c rocket; the nuclear test ban treaty was signed in 1963. Insert at the top shows Vela at Strategic Air & Space Museum, Nebraska. (GRBs)  Colgate (1968) anticipated GRBs -- associated with breakout of relativistic shocks from the surfaces of SNe.

3. Compton Gamma-ray Observatory (launched in 1991) established that the explosions are coming from random directions (isotropic) & have non-Euclidean space distribution. And therefore are at very large distances 

4. ISOTROPY Burst duration in Galactic coordinate. Note isotropy. We would see concentration along the Galactic plane if these bursts originated in our galaxy. Compton-GRO was launched in April 1991, and decommissioned on 4 June 2000.

5. A Italian/Dutch satellite – Beppo/SAX – launched in 96 localized long-bursts to 5-arcmin (a factor ~20 improvement) Which led to the discovery of optical afterglow, and redshift. Thus, it was discovered that energy (isotropic) E iso ~ 10 53 erg.

6. Stanek et al., Chornock et al. Eracleous et al., Hjorth et al., Kawabata et al. SN 1998bw: local, energetic, core-collapsed Type Ic GRB 030329: z=0.17 (afterglow-subtracted) ‏ Emission lines of CII, OII and OIII Long-GRB – collapse of a massive star (Woosley and Paczynski) X-ray flash 020903 also seems to show a spectrum like 98bw at 25 days after the burst (Soderberg astro-ph/0502553). GRB 030329 or SN2003dh

7. Wainwright, Berger & Penprase, 2007 Long GRBs: Exclusively in star-forming galaxies ⇒ Progenitors are massive stars HST/WFPC2 & ACS images of long-GRB host galaxies; each panel is 5-arcsec wide Host galaxies are typically 0.1 L * & ~ 0.1 Z sun

8. Short GRBs: Host Galaxies Berger et al. 2007 Large circle: Swift/XRT position Small circle: optical position (if available) Gemini & Magellan images are 20 ” on the side

9.  ≈ 5 0 Solid line: Spherical outflow in a uniform ISM; E 52 /n 0 =1 Dashed line: jet model with t j =10 days & E 52 /n 0 =20. Explosion speed (Taylor et al., 2004: GRB 030329) v  =5c v  =3c  ≈7 Angular Size (  as) Days After Burst

10. Panaitescu & Kumar (2001) These explosions are highly beamed (break in lightcurves); The energy release is determined by theoretical modeling of multi-wavelength afterglow data, and is found to be on average ~10 51 erg. (some bursts have >10 52 erg & others <10 50 erg) Afterglow theory: synchrotron radiation in external shock More energy comes out in these explosions in a few seconds than the Sun will produce in its 10 billion year lifetime!

11. The launch of Swift satellite – 11/20/04 – was another major milestone in the study of GRBs INTEGRAL satellite – Oct 17, 2002 launch – has discovered many GRBs and contributed much to our knowledge of these bursts.

12. Prompt  -ray generation mechanism O’Brien et al., 2006 Factor ~ 10 4 drop in flux! Another major discovery of Swift was that the x-ray flux declines rapidly at the end of γ-ray burst; this behavior was anticipated by Kumar & Panaitescu (2000) – 5 years before the discovery. If the rapid turn-off is due to a fast decline of accretion rate onto the newly formed black-hole, then we can “invert” the observed x-ray lightcurve and determine progenitor star structure.

13. 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 (2008) (Sophisticated simulation work of Lindner, Milosavljevic et al., 2010)Milosavljevic

14. Some interesting GRBs detected by Swift Naked Eye burst at z=0.94 (redshift measured by HET) movie made by Pi of the Sky, a Polish group that monitors transient events 2.5 million times more luminous (optical) than the most luminous supernova ever recorded T = 5.5s, fluence=3.1x10 -7 erg cm -2 (E p =49 keV) (similar to bursts at low z) Cucchiara et al. 2011 GRB 090429B: z=9.4, E iso =3.5x10 52 erg all 4 high-Z bursts have rest frame T 90 1 MeV in burst rest frame and bursts are known to be narrower and highly variable in high energy bands.

15. How far away can we see Bursts? Price et al. (2005) claim that 8 out of 9 Swift bursts (at z>1) could be detected at z=6.3 and 3 of these could be detected at z~20. So Swift can detect bursts like this to z ~ 15, when the universe was 270 million years old. Swift/BAT sensitivity is 1.2x10 -8 erg cm -2 s -1  Burstzt γ (s) flux ( erg cm -2 s -1 )E iso (erg) 050904 6.32253x10 -8 ~10 54 090423 8.210.26x10 -8 1.2x10 53 090429B 9.4 5.5 5x10 -8 3.5x10 52 SVOM: a Chinese-French mission (2017?) – more sensitive than Swift for GRBs with E peak < 20 keV ( 4--250 keV band); 2 IR telescopes (0.4 to 0.95 μm – located in Mexico & China) to look for z~8 bursts.  JANUS: funded for phase A study, but not selected for launch; x-ray (1-20 keV) and IR telescope (0.7—1.7 μm) can determine GRB redsfit to z=12. BAT sensitivity,1 5-150 kev, is 0.25 photons cm -2 s -1 or 1.2x10 -8 erg cm -2 s -1 for f  -1/2 )‏ 080913 6.7 87x10 -8 ~10 53

16. GRBs to probe the young Universe   The most distant quasar is at z=6.4 & galaxy at z~10. So we should be able to see bursts at much larger z, if they are there, which will help us explore properties of the first stars and objects that formed. Amati relation (2002): E_p \propto E_iso^0.52\pm 0.06 Ghirlanda relation (2004): E_p \propto E_jet_ga mma^0.6 9\pm0.04   Swift has seen two bursts at z=8.2 & 9.4 which are among the most distant objects we have seen. These bursts were NOT exceptionally bright – in fact their luminosity, spectra and lightcurves are similar to lower redshift GRBs. GRB 090423 (z=8.2)

17. Pre-Swift bursts Swift bursts comoving volume (dV/dz) Star formation rate (Porciani & Madau 01) Gehrels, Ramirez-Ruiz & Fox (2009)

18. Our understanding of GRBs has improved dramatically in last ~10 years. However, there are a number of fundamental questions that remain unanswered. The foremost amongst these are: 1. Whether a BH or a NS is produced in these explosions? 2. Composition of relativistic jets in GRBs:Baryons? e ± ? or B? We can answer these questions if we could understand how γ- rays are generated in GRBs, and use that to read the signatures of different central engine models and jet composition Woosley, 1993 Paczynski, 1998 Usov 1992, Thompson 1994 Wheeler et al. 2000 Thompson et al. 2004 magnetarblackhole

19. 6/11/2008 Fermi 8 KeV to 300 GeV One of the goals for Fermi is to understand γ-ray burst prompt radiation mechanism by observing high energy photons from GRBs. However, there were surprises in store for us: Fermi discovered that  Fermi discovered that  How are γ-rays generated?

20. 1. >10 2 MeV photons lag <10MeV photons (2-5s) 2. >100 MeV radiation lasts for ~10 3 s whereas emission below 10 MeV lasts for ~30s or less! Abdo et al. 2009

21. They suggested that high energy photons (>100 MeV) are produced in the External-shock via synchrotron Within a few months of these discoveries (by Fermi) Kumar & Barniol Duran (2009) proposed a model – now widely accepted – which will be discussed in the next few slides. Gehrels, Piro & Leonard: Scientific American, Dec 2002

22. Abdo et al. 2009 (GRB 080916C) Long lived lightcurve for >10 2 MeV (Abdo et al. 2009) f ν α ν -1.2 t -1.2 α = 1.5β – 0.5 (FS) α = 1.5β – 0.5 (FS)

23. >10 2 MeV data  expected ES flux in the X-ray and optical band (GRB 080916C) We can then compare it with the available X-ray and optical data. Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009 Long lived lightcurve for >10 2 MeV (Abdo et al. 2009) Kumar & Barniol Duran (2009)

24. 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 And that compares well with the available Fermi data. X-ray Optical > 100MeV 50 - 300keV Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009 Kumar & Barniol Duran (2009)

25. Using only >100MeV Fermi data Parameter search at t = 50 sec. How are Magnetic fields Generated in Shocks? 30 μG 5 μG Using late time x-ray, optical & radio data Parameter search at t = 0.5 day. 30 μG 5 μG Similar results found for two other Fermi/LAT bursts: 080916c & 090510. ε B is consistent with shock Recent work has provided a surprising answer: ε B is consistent with shock compressed magnetic field of CSM of ~ 10 μG compressed magnetic field of CSM of ~ 10 μG (Kumar & Barniol Duran 2009) [Thompson et al. (2009) find weak magnetic field amplification in SNe shocks] (A long standing open question) GRB 090902B

26. Melandri et al. (2008), Antonelli et al. (2006), Panaitescu & Vestrand (2011), Schulze et al. (2011), Stratta et al. (2009), Covino et al. (2010), Perley et al. (2008), Perley et al. (2009), Uehara et al. (2010), Guidorzi et al. (2011), Perley et al. (2011), Greiner et al. (2009),Yuan et al. (2010), Melandri et al. (2010) This result suggests a weak magnetic dynamo in relativistic shocks Santana et al. 2012

27. central engine relativistic outflow 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. Jet energy dissipation and γ-ray generation External shock radiation central engine  jet   -rays A good fraction of >10 2 MeV photons appear to be generated in external shock; (photo-pion & other hadronic processes might also contribute for ~30s or so) How about photons in the intermediate energy band, i.e. ~10keV – 0.1 GeV? Emission in this band lasts for <10 2 s, however it carries a good fraction of the total energy release in GRBs. And it offers the best link to the GRB central engine.

28. Radiation mechanisms Synchrotron, IC or SSC in internal shocks, RS or FS, or hadronic collision or photo-pion process... 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 … 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 (~10 2 man-years of work has gone into it) Radiation mechanism

29. GRB: current paradigm (internal shock model) Gehrels et al. (2002); Scientific American The next few slides will show that this paradigm is NOT working… We don’t know what replaces though…

30. It can be shown that shock-based mechanism for sub- MeV radiation from GRBs has severe problems Also, the natural expectation is to have two breaks in the spectrum – at ν c & ν i – which is never seen. ✫ ✫  e ± cool rapidly

31. 1. Thermal radiation + IC Thompson (1994 & 06); Liang et al. 1997; Ghisellini & Celloti 1999; Meszaros & Rees (2001); Beloborodov (2009) Daigne & Mochkovitch (2002); Pe’er et al. (2006)… (for prompt  -rays) Problem: we don’t see a thermal component in GRB prompt emission – Ryde (2004, 05) claims to find evidence for thermal spectrum, but Ghirlanda et al. 2007 do not. There are three possible solutions 2. Continuous acceleration of electrons If electrons can be continuously accelerated (as they lose energy to radiation) then some of the problems mentioned earlier disappear (Kumar & McMahon, 2008). However, is it NOT possible to accelerate electrons continuously in shocks. So this solution requires a magnetic jet and reconnection. 3. Relativistic turbulence (probably requires magnetic jet) Lyutikov & Blandford 03’; Narayan & Kumar 09’; Lazar, Nakar & Piran 09’

32. Line of Sight 1/  RsRs tt  Relativistic Turbulence Model Relativistic Turbulence Model Variability time = R s (1+z)‏ ———— (2c  2 )  t 2 Synchrotron emission in quiescent part of shell  less variable optical IC scattering of synchrotron off of blobs γ-rays (more variable)‏ Lyutikov & Blandford 03’; Narayan & Kumar 09’; Lazar, Nakar & Piran 09’ Consistent solutions for  -rays are found in this case & R s ~R d as suggested by observations. One possibility:

33. Black-hole vs. Magnetar & jet composition One of the best ways to determine jet composition is by looking for optical/IR radiation from RS–heated jet (and ~TeV ν e ν μ ). Improved sensitivity (~10) in optical/IR is needed on a timescale of less than 10 2 s from GRB trigger; recent ICECUBE result for neutrinos is interesting, but not yet too constraining of jet composition. ✫ ✫ ✫ Swift found that the x-ray flux at the end of GRBs declines very rapidly –– t -3 or faster. The expected decline of dipole luminosity for a magnetar is t -2 Some GRBs have E > 10 52 erg – more than expected of a magnetar. Recent work of Metzger et al. (2011) offers interesting suggestions regarding magnetars, but I see some problems... Although pulsar breaking index n (dΩ/dt α Ω n ) is found to be between 1.4 and 2.9 for 6 pulsars which implies a faster decay.

34. Summary We have learned many things about GRBs in the last 10 years: Produced in core collapse (long-GRB) & binary mergers (short-GRB) Highly relativistic jet (Γ ≥ 10 2 ), beamed (θ j ~ 5 0 ), E j ~10 51 erg They do occur at high redshifts (current record Z=9.4) High energy photons (>100 MeV) are produced in external shock Generation of magnetic fields in relativistic shocks is clarified But we don’t yet have answers to several basic questions: Are blackholes produced in these explosions (or a NS)? What is the GRB-jet made of? How are gamma-rays of ~MeV energy produced? ✫ ✫