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Gamma-ray Bursts in the Fermi Era

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

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Gamma-ray Bursts in the Fermi Era

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

  2. 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. History Gamma-ray Bursts (GRBs) were discovered (accidentally) by Vela satellites in 1967. For about 20 years the distance to GRBs was completely uncertain. 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) Eiso ~ 1053 erg.

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

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

  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. Explosion speed (Taylor et al., 2004: GRB 030329) ≈7 v=3c v=5c Angular Size (as) Days After Burst Solid line: Spherical outflow in a uniform ISM; E52/n0 =1  ≈ 50 Dashed line: jet model with tj =10 days & E52/n0 =20.

  10. 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 ~1051 erg. (some bursts have >1052 erg & others <1050 erg) Afterglow theory: synchrotron radiation in external shock Panaitescu & Kumar (2001) More energy comes out in these explosions in a few seconds than the Sun will produce in its 10 billion year lifetime!

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

  12. Prompt -ray generation mechanism 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. O’Brien et al., 2006 Factor ~ 104 drop in flux! • 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. Progenitor Star Properties -2.5   r r ≈ 9  109 cm flux 2fΩ2 3  1010 cm prompt GRB emission r ~ 1.5  1011 cm fΩ~ rapid decline X-ray plateau steep fall off time Kumar, Narayan & Johnson (2008) (Sophisticated simulation work of Lindner, Milosavljevic et al., 2010) f k

  14. Some interesting GRBs detected by Swift Naked Eye burst at z=0.94 (redshift measured by HET) GRB 090429B: z=9.4, Eiso=3.5x1052erg T = 5.5s, fluence=3.1x10-7 erg cm-2 (Ep=49 keV) all 4 high-Z bursts have rest frame T90 < 1s this might be due to the fact that Swift 150keV band is > 1 MeV in burst rest frame and bursts are known to be narrower and highly variable in high energy bands. 2.5 million times more luminous (optical) than the most luminous supernova ever recorded (similar to bursts at low z) Cucchiara et al. 2011 movie made by Pi of the Sky, a Polish group that monitors transient events

  15. How far away can we see Bursts? Burst z tγ(s) flux(erg cm-2 s-1) Eiso(erg) BAT sensitivity,15-150 kev, is 0.25 photons cm-2 s-1 or 1.2x10-8 erg cm-2 s-1 for f-1/2)‏ 050904 6.3 225 3x10-8 ~1054 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. 080913 6.7 8 7x10-8 ~1053 090423 8.2 10.2 6x10-8 1.2x1053 090429B 9.4 5.5 5x10-8 3.5x1052 Swift/BAT sensitivity is 1.2x10-8erg cm-2s-1 So Swift can detect bursts like this to z ~ 15, when the universe was 270 million years old. SVOM: a Chinese-French mission (2017?) – more sensitive than Swift for GRBs with Epeak< 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. 

  16. GRBs to probe the young Universe GRB 090423 (z=8.2) 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.  Ghirlanda relation (2004): E_p \propto E_jet_gamma^0.69\pm0.04  Amati relation (2002): E_p \propto E_iso^0.52\pm0.06 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. The most distant quasar is at z=6.4 & galaxy at z~10.

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

  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? blackhole magnetar Usov 1992, Thompson 1994 Wheeler et al. 2000 Thompson et al. 2004 Woosley, 1993 Paczynski, 1998 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

  19. Fermi 8 KeV to 300 GeV 6/11/2008 How are γ-rays generated? 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 

  20. 1. >102MeV photons lag <10MeV photons (2-5s) Abdo et al. 2009 2. >100 MeV radiation lasts for ~103s whereas emission below 10 MeV lasts for ~30s or less!

  21. 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. They suggested that high energy photons (>100 MeV) are produced in the External-shock via synchrotron Gehrels, Piro & Leonard: Scientific American, Dec 2002

  22. 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

  23. 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.

  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 > 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.

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

  26. Santana et al. 2012 This result suggests a weak magnetic dynamo in relativistic shocks 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)

  27. • 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) • How about photons in the intermediate energy band, i.e. ~10keV – 0.1 GeV? 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 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.

  28. Radiation mechanisms (~102 man-years of work has gone into it) • Radiation mechanism Synchrotron, IC or SSC in internal shocks, RS or FS, or hadronic collision or photo-pion process... 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 …

  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 e± cool rapidly  Synchrotron: νc < νilow energy spectrum ν-1/2 (this is rarely seen) or Rγ >1017cm (deceleration radius) SSC: It produces very bright prompt optical which is seen in just a few cases. It produces a 2nd IC peak between 100 MeV and 100 GeV which Fermi has NEVER seen. ✫ ✫ Also, the natural expectation is to have two breaks in the spectrum – at νc & νi – which is never seen.

  31. There are three possible solutions Thompson (1994 & 06); Liang et al. 1997; Ghisellini & Celloti 1999; Meszaros & Rees (2001); Beloborodov (2009) Daigne & Mochkovitch (2002); Pe’er et al. (2006)… 1. Thermal radiation + IC (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. 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 accelerateelectrons 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. Rs(1+z)‏ ———— (2c2)t2 Rs t 1/  Line of Sight Relativistic Turbulence Model Lyutikov & Blandford 03’; Narayan & Kumar 09’; Lazar, Nakar & Piran 09’ Variability time = Consistent solutions for -rays are found in this case & Rs~Rd as suggested by observations. One possibility: Synchrotron emission in quiescent part of shell  less variable optical IC scattering of synchrotron off of blobs γ-rays (more variable)‏

  33. Black-hole vs. Magnetar & 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 Although pulsar breaking index n (dΩ/dt α Ωn) is found to be between 1.4 and 2.9 for 6 pulsars which implies a faster decay. Some GRBs have E > 1052 erg – more than expected of a magnetar. ✫ Recent work of Metzger et al. (2011) offers interesting suggestions regarding magnetars, but I see some problems... One of the best ways to determine jet composition is by looking for optical/IR radiation from RS–heated jet (and ~TeV νe νμ). Improved sensitivity (~10) in optical/IR is needed on a timescale of less than 102s from GRB trigger; recent ICECUBE result for neutrinos is interesting, but not yet too constraining of jet composition. ✫

  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 ≥ 102), beamed (θj ~ 50), Ej~1051 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?

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