Gammaray Bursts in the Fermi Era Outline Pawan
Gamma-ray Bursts in the Fermi Era Outline† Pawan Kumar • 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
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.
Compton Gamma-ray Observatory (launched in 1991) established that the explosions are coming from random directions (isotropic) & have non-Euclidean space distribution. And therefore at very large distances
ISOTROPY
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.
Long-GRB – collapse of a massive star (Woosley and Paczynski) GRB 030329: z=0. 17 (afterglow-subtracted) SN 1998 bw: local, energetic, core-collapsed Type Ic Stanek et al. , Chornock et al. Eracleous et al. , Hjorth et al. , Kawabata et al.
Wainwright, Berger & Penprase, 2007 Long GRBs: Exclusively in star-forming galaxies ⇒ Progenitors are massive stars HST/WFPC 2 & ACS images of long-GRB host galaxies; each panel is 5 -arcsec wide Host galaxies are typically 0. 1 L* & ~ 0. 1 Zsun
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
Explosion speed Angular Size ( as) (Taylor et al. , 2004: GRB 030329) ≈7 v =3 c v =5 c ≈ 50 Days After Burst
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) Panaitescu & Kumar (2001) 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!
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.
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.
Progenitor Star Properties flux Kumar, Narayan & Johnson (2008) (Sophisticated simulation work of Lindner, Milosavljevic et al. , 2010) r -2. 5 prompt GRB emission r ≈ 9 109 cm � 2� � � 2 � fΩ rapid decline 10 3 10 11 r~ fΩ ~ X-ray plateau cm steep fall off time f k 5 1. 10 cm
Some interesting GRBs detected by Swift Naked Eye burst at z=0. 94 (redshift measured by HET) 2. 5 million times more luminous (optical) than the most luminous supernova ever recorded GRB 090429 B: z=9. 4, Eiso=3. 5 x 1052 erg T = 5. 5 s, fluence=3. 1 x 10 -7 erg cm-2 (Ep=49 ke. V) (similar to bursts at low z) Cucchiara et al. 2011 movie made by Pi of the Sky, a Polish group that monitors transient events
How far away can we see Bursts? Burst z tγ(s) flux (erg cm-2 s-1) Eiso(erg) 050904 080913 6. 7 225 8 3 x 10 -8 7 x 10 -8 ~1054 ~1053 090429 B 8. 2 10. 2 6 x 10 -8 5 x 10 -8 1. 2 x 1053 3. 5 x 1052 9. 4 5. 5 Swift/BAT sensitivity is 1. 2 x 10 -8 erg cm-2 s-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 ke. V ( 4 --250 ke. V 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 ke. V) and IR telescope (0. 7— 1. 7 μm) can determine GRB redsfit to z=12.
GRBs to probe the young Universe Swift has seen two bursts at z=8. 2 & 9. 4 which are among the most distant objects we have seen. GRB 090423 (z=8. 2) These bursts were NOT exceptionally bright – in fact their luminosity, spectra and lightcurves are similar to lower redshift GRBs. 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.
Gehrels, Ramirez-Ruiz & Fox (2009) comoving volume (d. V/dz) Swift bursts Star formation rate (Porciani & Madau 01) Pre-Swift bursts
• Our understanding of GRBs has improved dramatically in last ~10 years. • However, there a number of fundamental questions that remain unanswered. The foremost amongst these are: Thompson et al. 2004 magnetar Wheeler et al. 2000 Woosley, 1993 Paczynski, 1998 blackhole Usov 1992, Thompson 1994 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 γrays are generated in GRBs, and use that to read the signatures of different central engine models and jet composition
Fermi 6/11/2008 8 Ke. V to 300 Ge. V 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
Abdo et al. 2009 1. >102 Me. V photons lag <10 Me. V photons (2 -5 s) 2. >100 Me. V radiation lasts for ~103 s whereas emission below 10 Me. V lasts for ~30 s or less!
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 Me. V) are produced in the External-shock via synchrotron Gehrels, Piro & Leonard: Scientific American, Dec 2002
Long lived lightcurve for >102 Me. V (Abdo et al. 2009) (GRB 080916 C) α = fν α 1. ν -1. 5β 2 t -1. – 0. 2 5 (F S) Abdo et al. 2009
Long lived lightcurve for >102 Me. V (Abdo et al. 2009) Kumar & Barniol Duran (2009) >102 Me. V data expected ES flux in the X-ray and optical band (GRB 080916 C) 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.
Or we can go in the reverse direction… > 100 Me. V Optical 50 - 300 ke. V X-ray Kumar & Barniol Duran (2009) Assuming that the late (>1 day) X-ray and optical flux are from ES, calculate the expected flux at 100 Me. V at early times Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009 And that compares well with the available Fermi data.
How are Magnetic fields Generated in Shocks? (A long standing open question) ε Recent work has provided a surprising answer: B is consistent with shock compressed magnetic field of CSM of ~ 10 μG (Kumar & Barniol Duran 2009) Using late time x-ray, optical & radio data Parameter search at t = 0. 5 day. G GRB 090902 B μG 30 5μ Parameter search at t = 50 sec. Using only >100 Me. V Fermi data Similar results found for two other Fermi/LAT bursts: 080916 c & 090510. [Thompson et al. (2009) find weak magnetic field amplification in SNe shocks]
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)
• A good fraction of >102 Me. V photons appear to be generated in external shock; (photo-pion & other hadronic processes might also contribute for ~30 s or so) • How about photons in the intermediate energy band, i. e. ~10 ke. V – 0. 1 Ge. V? Emission in this band lasts for <102 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. Jet energy dissipation and γ-ray generation relativistic outflow central engine jet -rays External shock radiation
• 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 …
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…
• It can be shown that shock-based mechanism for sub. Me. V radiation from GRBs has severe problems e± cool rapidly ν c < νi ✫ Synchrotron: low energy spectrum ν -1/2 (this is rarely seen) or Rγ >1017 cm (deceleration radius) ✫ SSC: It produces very bright prompt optical which is seen in just a few cases. It produces a 2 nd IC peak between 100 Me. V and 100 Ge. V which Fermi has NEVER seen. Also, the natural expectation is to have two breaks in the spectrum – at νc & νi – which is never seen.
There are three possible solutions 1. Thermal radiation + IC (for prompt -rays) Thompson (1994 & 06); Liang et al. 1997; Ghisellini & Celloti 1999; Meszaros & Rees (2001); Beloborodov (2009) Daigne & Mochkovitch (2002); Pe’er et al. (2006)… 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 & Mc. Mahon, 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’
Relativistic Turbulence Model Lyutikov & Blandford 03’; Narayan & Kumar 09’; Lazar, Nakar & Piran 09’ Rs(1+z) Variability time = ———— (2 c 2) t 2 1/ Rs t Line of Sight 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)
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 ~Te. V νe νμ). Improved sensitivity (~10) in optical/IR is needed on a timescale of less than 102 s from GRB trigger; recent ICECUBE result for neutrinos is interesting, but not yet too constraining of jet composition.
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 Me. V) 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 ~Me. V energy produced?
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