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Gamma-Ray Bursts in the Fermi/GLAST era

Gamma-Ray Bursts in the Fermi/GLAST era. Lorenzo Amati INAF – IASF Bologna (Italy). Outline Standard scenarios Main open issues Recent highlights The Fermi/GLAST contribution. Standard scenarios for GRB emission. prompt. afterglow.

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Gamma-Ray Bursts in the Fermi/GLAST era

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  1. Gamma-Ray Bursts in the Fermi/GLAST era Lorenzo Amati INAF – IASF Bologna (Italy)

  2. Outline • Standard scenarios • Main open issues • Recent highlights • The Fermi/GLAST contribution

  3. Standard scenarios for GRB emission prompt afterglow • ms time variability + huge energy + detection of GeV photons -> plasma occurring ultra-relativistic (G > 100) expansion (fireball or firejet) • non thermal spectra -> shocks synchrotron emission (SSM) • fireball internal shocks -> prompt emission • fireball external shock with ISM -> afterglow emission

  4. Standard scenarios for GRB origin LONG SHORT • energy budget up to >1054 erg • long duration GRBs • metal rich (Fe, Ni, Co) circum-burst environment • GRBs occur in star forming regions • GRBs are associated with SNe • likely collimated emission • energy budget up to 1051 - 1052 erg • short duration (< 5 s) • clean circum-burst environment • old stellar population

  5. Open issues (several, despite obs. progress) • GRB prompt emission physics • physics of prompt emission still not settled, various scenarios: SSM internal shocks, IC-dominated internal shocks, external shocks, photospheric emission dominated models, kinetic energy dominated fireball , poynting flux dominated fireball)

  6. most time averaged spectra of GRBs are well fit by synchrotron shock models • at early times, some spectra inconsistent with optically thin synchrotron: possible contribution of IC component and/or thermal emission from the fireball photosphere • thermal models challenged by X-ray spectra

  7. Prompt optical emission • GRB990123: first detection (by ROTSE) of prompt optical emission • optical light curve does not follow high energy light curve: evidence for a different origin (reverse shock ?) • GRB041219 (RAPTOR) contrary to GRB990123, the optical light curve follows the high energy light curve: evidence for same origin (internal shocks ?)

  8. Early X-ray afterglow • new features seen by Swift in X-ray early afterglow light curves (initial very steep decay, early breaks, flares) mostly unpredicted and unexplained • initial steep decay: continuation of prompt emission, high latitude emission, IC up-scatter of the reverse shock sinchrotron emission ? • flat decay: probably “refreshed shocks” (due either to long duration ejection or short ejection but with wide range of G) ? • flares: could be due to: refreshed shocks, IC from reverse shock, external density bumps, continued central engine activity, late internal shocks… ~ -3 ( 1 min ≤ t ≤ hours ) ~ -0.7 10^5 – 10^6 s ~ - 1.3 ~ -2 10^2 – 10^3 s 10^4 – 10^5 s

  9. Circum-burst environment afterglow • evidence of overdense and metal enriched circum-burst environment from absorption and emission features • emission lines in afterglow spectrum detected by BeppoSAX but not by Swift • Swift detects intrinsic NH for many GRB afterglows, often inconsistent with NH from optical (Lya) prompt afterglow

  10. Collimated or isotropic ? The problem of missing breaks • jet angles derived from the achromatic break time, are of the order of few degrees; the collimation-corrected radiated energy spans the range ~1050 – 1052 erg • lack of jet breaks in several Swift X-ray afterglow light curves, in some cases, evidence of achromatic break • challenging evidences for Jet interpretation of break in afterglow light curves or due to present inadequate sampling of optical light curves w/r to X-ray ones and to lack of satisfactory modeling of jets ?

  11. GRB/SN connection (see review by P. Mazzali) • are all long GRB produced by a type Ibc SN progenitor ? • which fraction of type Ibc SN produces a GRB, and what are their peculiarities ? • are the properties (e.g., energetics) of the GRB linked to those of the SN ? • long GRBs with no (or very faint) associated SNe

  12. Short / long classification and physics • Swift GRB 060614: a long GRB with a very high lower limit to the magnitude of an associated SN -> association with a GRB/SN is excluded • high lower limit to SN also for GRB 060505 (and, less stringently, XRF 040701) • In the spectral lag – peak luminosity plane, GRB060614 lies in the short GRBs region -> need for a new GRB classification scheme ? Gehrels et al., Nature, 2006

  13. Spectrum-energy correlations: GRB physics, short/long, sub-energetic • Strong correlation between Ep,i and Eiso for long GRBs: test for prompt emission models (physics, geometry, GRB/XRF unification models) • short GRB do not follow the correlation: clues to difference in emission mechanism (soft tail of short GRB 050724 consistent with Ep,i-Eiso) • only outliers: GRB 980425 (very peculiar) and, possibly, INTEGRAL GRB 031203 -> sub-class of sub-energeticGRBs ? Ep

  14. GRB cosmology ? • GRB have huge luminosities and a redshift distribution extending far beyond SN Ia and even beyond that of AGNs • high energy emission -> no extinction problems • potentially powerful cosmological sources • estimate of cosmological parameters through spectrum-energy correlations (see talk by M. Della Valle) • use of GRBs as tracers of star formation up to the dark ages of the universe • use of GRBs as cosmological beacons for the study of the ISM and the IGM (e.g., WHIM) evolution up to very high z

  15. Recent highlights • In the last ~1 year three exceptional events: • The first GRB (080319B) with a prompt optical counterpart, and the most distant object ever (z = 0.937) visible by naked eye (Mv up to 5.3) • The brightest GRB, and most powerful source in the universe, 080916C, showing an isotropic-equivalent radiated energy of 1055 erg in 80s in the 1 keV – 10 GeV energy band) • The most distant GRB and source ever detected, 090423, lying at a redshift of 8.1 • All these discoveries involved Italian astronomers with primary roles

  16. the challenging “naked eye” GRB080319B: complexity and energetics

  17. GRB 080916C: huge energetics and no spectral break/excess up to GeVs

  18. GRB 090423 - the most distance source in the universe (z = 8.1 !), discovered by Telescopio Nazionale “Galileo” during year 2009 celebrating Galileo ! 190 GRB

  19. The Fermi/GLAST contribution • key features of Fermi for the study of GRBs • Detection, arcmin localization and study of GRBs in the GeV energy range through the LAT instrument, with dramatic improvement w/r CGRO/EGRET • Detection, rough localization (a few degrees) and accurate determination of the shape of the spectral continuum of the prompt emission of GRBs from 8 keV up to 30 MeV through the GBM instrument

  20. GRB VHE emission • CGRO/EGRET detected VHE (from 30 MeV up to 18 GeV) photons for a few GRBs • VHE emission can last up to thousends of s after GRB onset • average spectrum of 4 events well described by a simple power-law with index ~2 , consistent with extension of low energy spectra • GRB 941017,measured by EGRET-TASC shows a high energy component inconsistent with synchrotron shock model • Strong improvement expected form AGILE and, in particular, Fermi/GLAST

  21. VHE emission of GRB predicted and explained in several scenarios / emission mechanisms: synchrotron self-compton in internal shocks, IC in external shocks, proton synchrotron emission in external shocks,

  22. Fermi/LAT performances and first results • substantial improvement w/r CGRO/EGRET

  23. up to now 6 GRB detections in the GeV enegy range: 080825C, 080916C, 081024B, 081215, 090323, 090328 • most noticeable event: GRB 080916C, the most energetic GRB ever (Eiso 1055 erg in 1 keV – 10 GeV cosm. rest-frame) and with spectrum extending up to tens of GeV (cosm. rest-frame) without any excess or cut-off • recent GRB 090323 shows a radiated energy similar to 080916C but less photons above 100 MeV

  24. the huge radiated energy, the spectrum extending upto VHE without any excess or cut-off and time-delayed GeV photons of GRB 080916C are challenging evidences for GRB prompt emission models • huge bulk Lorentz factor (> several hundreds), non thermal synchrotron emission favoured but lack of SSC poses problems, IC in residual collisions invoked to explain delayed GeV emission

  25. GRB prompt emission spectrum and peak energy estimate • the accurate measurement of the spectral parameters of GRB prompt emission is fundamental to test the emission models and, in particular, to test and use Ep-Eiso correlation • Swift cannot provide a high number of firm Ep estimates, due to BAT ‘narrow’ energy band (sensitive spectral analysis only from 15 up to ~200 keV) -> a broad energy band is needed • in last years, Ep estimates for some Swift GRBs from Konus, RHESSI and SUZAKU Ep NARROW BAND BROAD BAND

  26. Fermi/GBM performances and first results • very broad energy band: substantial improved spectral capabilities w/r BATSE and previous experiments -> accurate measurement of Ep for hundreds of GRBs

  27. Up to now: about 150 GBM GRBs: Ep estimates for 85%, 19 detected and localized by Swift (13%), 6 detected and localized by LAT, 9 with Ep and z estimates (6%) • 2008 pre-Fermi : 61 Swift detections, 5 BAT Ep (8%), 15 BAT+KON+SUZ Ep estimates (25%), 20 redshift (33%), 11 Ep + z (16%) • Fermi/GBM is increasing of about 60% the number of GRBs wth Ep and z (-> Ep-Eiso correlation) and providing Ep with high accuracy for medium/bright GRBs • GBM sample is growing fastly, we expect interesting results from statistical analysis

  28. Conclusions • Despite the huge observational progress of last 10 years, several open issues still affect our knowledge of the GRB phenomenon(e.g., prompt emission physics, understandung of the early X-ray afterglow phenomenology, collimation and jet structure, spectrum-energy correlation and GRB cosmology, GRB/SN connection, sub-classes of GRBs, short/long dicotomy) • We are living in a “golden era” for this field, with several space instruments performing greatly and complementing each other and excellent observational programs with the best on-ground facilities • This is confirmed by the exceptional results of the last year: the most distant source visible by naked eye (GRB 080319B, up to Mv 5.3, z =0.937), the most powerful source in the universe (GRB 080916C, Eiso = 1055 erg), the most distant source ever detected (GRB 090423 at z = 8.1) • Fermi/GLAST is already providing important results (detection, localization and study of GeV emission with unprecedented sensitivity, spectroscopy of the prompt emission with unprecedented broad band and accuracy) and is expected to provide more in the next years

  29. End of the talk

  30. Backup slides

  31. Outline • Observational picture and standard scenarios • Main open issues • Recent highlights • The Fermi/GLAST contribution to GRB science

  32. Basic observations • 70s – 80s: GRBs = sudden and unpredictable bursts of hard X / soft gamma rays with huge flux • most of the flux detected from 10-20 keV up to 1-2 MeV • measured rate (by an all-sky experiment on a LEO satellite): ~0.8 / day; estimated true rate ~2 / day • complex and unclassifiable light curves • fluences (= av.flux * duration) typically of ~10-7 – 10-4 erg/cm2

  33. The contribution of CGRO/BATSE in the 90s • bimodal distribution of durations • characterization of GRB non thermal spectra • isotropic distribution of directions short long

  34. BeppoSAX era: afterglow emission, optical /IR/radio counterparts, host-gal.(late ’90s, XMM, Chandra, opt/IR/radio telesc.):

  35. Distance and luminosity • from optical spectroscopy (OT or HG) -> redshift estimates • all GRBs with measured redshift (~100) lie at cosmological distances(except for the peculiar GRB980425, z=0.0085) • isotropic equivalent radiated energies can be as high as > 1054 erg • short GRB lie at lower redshifts (<~1) and are less luminous (Eiso < ~1052 erg) log(Eiso)= 1.0 , s = 0.9

  36. prompt afterglow • Swift: transition from prompt to afterglow (Swift, >2005) • Swift era • BeppoSAX era

  37. GRB 980425, a normal GRB detected and localized by WFC and NFI, but in temporal/spatial coincidence with a type Ib/c SN at z = 0.008 (chance prob. 0.0001) • further evidences of a GRB/SN connection: bumps in optical afterglow light curves and optical spectra resembling that of GRB980425 (e.g., GRB 030329) • GRB/SN connection

  38. X-Ray Flashes (late ’90s, main contribution by BeppoSAX and HETE-2) • GRBs with only X-ray emission (BeppoSAX, HETE-2) • distribution of spectral peak energies has a low energy tails

  39. host galaxies (>1997, X-ray loc. + optical follow-up) • host galaxies long GRBs: blue, usually regular and high star forming, GRB located in star forming regions • host galaxies of short GRBs (very recent): elliptical, irregular galaxies, away from star forming region (but still unclear) Long Short GRB 050509b

  40. sub-energetic GRBs • GRB980425 not only prototype event of GRB/SN connection but closest GRB (z = 0.0085) and sub-energetic event (Eiso ~ 1048 erg, Ek,aft ~ 1050 erg), outlier to Ep,i-Eiso correlation • GRB031203 (by INTEGRAL): the most similar case to GRB980425/SN1998bw: very close (z = 0.105), SN2003lw, sub-energetic • normal GRBs seen off-axis or truly sub-energetic ? GRB 060218 on-axis -> truly sub-en.?

  41. Gamma-Ray Bursts • most of the flux detected from 10-20 keV up to 1-2 MeV • measured rate (by an all-sky experiment on a LEO satellite): ~0.8 / day • bimodal duration distribution • fluences (= av.flux * duration) typically of ~10-7 – 10-4 erg/cm2 short long

  42. The GRB phenomenon: VHE • CGRO/EGRET detected VHE (from 30 MeV up to 18 GeV) photons for a few GRBs • VHE emission can last up to thousends of s after GRB onset • average spectrum of 4 events well described by a simple power-law with index ~2 , consistent with extension of low energy spectra

  43. afterglow emission (> 1997): X, optical, radio counterparts

  44. … and host galaxies • host galaxies long GRBs: blue, usually regular and high star forming, GRB located in star forming regions • host galaxies of short GRBs (very recent): elliptical, irregular galaxies, away from star forming region (but still unclear) Long Short GRB 050509b

  45. in 1997 discovery of afterglow emission by BeppoSAX • first X, optical, radio counterparts, redshifts, energetics GRB 970508, Metzger et al., Nature, 1997

  46. GRB 980425, a normal GRB detected and localized by WFC and NFI, but in temporal/spatial coincidence with a type Ic SN at z = 0.008 (chance prob. 0.0001) • further evidences of a GRB/SN connection: bumps in optical afterglow light curves and optical spectra resembling that of GRB980425 • 1998: GRB/SN connection

  47. circum-burst environment • evidence of overdense and metal enriched circum-burst environment from absorption and emission features found in a few cases

  48. prompt afterglow • >2005 prompt – afterglow • Swift era • BeppoSAX era

  49. Conclusions - I • The Ep,i-Eiso correlation is the most firm GRB correlation followed by all normal GRB and XRF • Swift results and recent analysis show that it is not an artifact of selection effects • The existence, slope and extrinsic scatter of the correlation allow to test models for GRB prompt emission physics • The study of the locatios of GRB in the Ep,i-Eiso plane help in indentifying and understanding sub-classes of GRB (short, sub-energetic, GRB-SN connection)

  50. Conclusions - II • Given their huge luminosities and redshift distribution extending up to at least 6.3, GRB are potentially a very powerful tool for cosmology and complementary to other probes (CMB, SN Ia, clusters, etc.) • The use of spectrum-energy correlations to this purpouse is promising, but: • need to substantial increase of the # of GRB with known z and Ep (which will be realistically allowed by next GRB experiments: Swift+GLAST/GBM, SVOM,…) • need calibration with a good number of events at z < 0.01 or within a small range of redshift • need solid physical interpretation • identification and understanding of possible sub-classes of GRB not following correlations

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