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

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

LONG  energy budget up to >10 54 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 erg  short duration (< 5 s)  clean circum-burst environment  old stellar population SHORT Standard scenarios for GRB origin

 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) Open issues (several, despite obs. progress )

 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

 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 ?)  GRB (RAPTOR) contrary to GRB990123, the optical light curve follows the high energy light curve: evidence for same origin (internal shocks ?)  Prompt optical emission

~ -3 ~ -0.7 ~ ~ -2 10^2 – 10^3 s10^4 – 10^5 s 10^5 – 10^6 s ( 1 min ≤ t ≤ hours )  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  ) ?  flares: could be due to: refreshed shocks, IC from reverse shock, external density bumps, continued central engine activity, late internal shocks…

 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 (Ly  )  Circum-burst environment prompt afterglow

 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 ~10 50 – 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 ?

 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

 In the spectral lag – peak luminosity plane, GRB lies in the short GRBs region -> need for a new GRB classification scheme ? Gehrels et al., Nature, 2006  Short / long classification and physics  Swift GRB : 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 (and, less stringently, XRF )

 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 consistent with Ep,i-Eiso)  only outliers: GRB (very peculiar) and, possibly, INTEGRAL GRB > sub-class of sub-energetic GRBs ? Ep

 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

Recent highlights  In the last ~1 year three exceptional events:  The first GRB ( B ) 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, C, showing an isotropic-equivalent radiated energy of erg in 80s in the 1 keV – 10 GeV energy band)  The most distant GRB and source ever detected, , lying at a redshift of 8.1  All these discoveries involved Italian astronomers with primary roles

 the challenging “naked eye” GRB080319B: complexity and energetics

 GRB C: huge energetics and no spectral break/excess up to GeVs

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

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

 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 ,measured by EGRET-TASC shows a high energy component inconsistent with synchrotron shock model  Strong improvement expected form AGILE and, in particular, Fermi/GLAST  GRB VHE emission

 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,

 substantial improvement w/r CGRO/EGRET  Fermi/LAT performances and first results

 up to now 6 GRB detections in the GeV enegy range: C, C, B, , ,  most noticeable event: GRB C, the most energetic GRB ever (Eiso 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 shows a radiated energy similar to C but less photons above 100 MeV

 the huge radiated energy, the spectrum extending upto VHE without any excess or cut-off and time-delayed GeV photons of GRB C 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

 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 NARROW BAND BROAD BAND  GRB prompt emission spectrum and peak energy estimate Ep

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

 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

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 B, up to Mv 5.3, z =0.937), the most powerful source in the universe (GRB C, Eiso = erg), the most distant source ever detected (GRB 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

End of the talk

Backup slides

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

Basic observations  70s – 80s: GRBs = sudden and unpredictable bursts of hard X / soft gamma rays with huge flux  most of the flux detected from 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 – erg/cm 2

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

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

 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 > erg  short GRB lie at lower redshifts (<~1) and are less luminous (Eiso < ~10 52 erg) log(Eiso)= 1.0,  = 0.9

 BeppoSAX era prompt afterglow  Swift: transition from prompt to afterglow (Swift, >2005)  Swift era

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

 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

 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) GRB b Short Long  host galaxies (>1997, X-ray loc. + optical follow-up)

 GRB not only prototype event of GRB/SN connection but closest GRB (z = ) and sub-energetic event (Eiso ~ erg, Ek,aft ~ erg), outlier to E p,i -E iso correlation  GRB (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 on-axis -> truly sub-en.?  sub-energetic GRBs

Gamma-Ray Bursts  most of the flux detected from 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 – erg/cm 2 short long

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 The GRB phenomenon: VHE

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

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) GRB b Short Long  … and host galaxies

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

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

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

BeppoSAX era prompt afterglow  >2005 prompt – afterglow Swift era

 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) Conclusions - I

 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 Conclusions - II

Tests and debates

 Nakar & Piran and Band & Preece 2005: a substantial fraction (50-90%) of BATSE GRBs without known redshift are potentially inconsistent with the Ep,i-Eiso correlation for any redshift value  they suggest that the correlation is an artifact of selection effects introduced by the steps leading to z estimates: we are measuring the redshift only of those GRBs which follow the correlation  they predicted that Swift will detect several GRBs with Ep,i and Eiso inconsistent with the Ep,i-Eiso correlation  Ghirlanda et al. (2005), Bosnjak et al. (2005), Pizzichini et al. (2005): most BATSE GRB with unknown redshift are consistent with the Ep,i- Eiso correlation  different conclusions mostly due to the accounting or not for the dispersion of the correlation  Debate based on BATSE GRBs without known redshift

 Swift / BAT sensitivity better than BATSE for Ep ~100 keV but better than BeppoSAX/GRBM and HETE-2/FREGATE -> more complete coverage of the Ep-Fluence plane Band, ApJ, (2003, 2006) CGRO/BATSE Swift/BAT  Swift GRBs and selection effects Ghirlanda et al., MNRAS, (2008)

 fast (~1 min) and accurate localization (few arcesc) of GRBs -> prompt optical follow-up with large telescopes -> substantial increase of redshift estimates and reduction of selection effects in the sample of GRBs with known redshift  fast slew -> observation of a part (or most, for very long GRBs) of prompt emission down to 0.2 keV with unprecedented sensitivity –> following complete spectra evolution, detection and modelization of low-energy absorption/emission features -> better estimate of Ep for soft GRBs  drawback: BAT “narrow” energy band allow to estimate Ep only for ~15-20% of GRBs (but for some of them Ep from HETE-2 and/or Konus GRB060124, Romano et al., A&A, 2006

 all long Swift GRBs with known z and published estimates or limits to Ep,i are consistent with the correlation  the parameters (index, normalization,dispersion) obatined with Swift GRBs only are fully consistent with what found before  Swift allows reduction of selection effects in the sample of GRB with known z -> the Ep,i-Eiso correlation is passing the more reliable test: observations ! Amati 2006, Amati et al. 2008

 very recent claim by Butler et al.: 50% of Swift GRB are inconsistent with the pre-Swift Ep,i-Eiso correlation  but Swift/BAT has a narrow energy band: keV, nealy unesuseful for Ep estimates, possible only when Ep is in (or close to the bounds of ) the passband (15-20%) and with low accuracy  comparison of Ep derived by them from BAT spectra using Bayesian method and those MEASURED by Konus/Wind show they are unreliable  as shown by the case of GRB , missing the soft part of GRB emission leads to overestimate of Ep

 Full testing and exploitation of spectral-energy correlations may be provided by EDGE in the > 2015 time frame  in 3 years of operations, EDGE will detect perform sensitive spectral analysis in keV for ~ GRB  redshift will be provided by optical follow-up and EDGE X-ray absorption spectroscopy (use of microcalorimeters, GRB afterglow as bkg source)  substantial improvement in number and accuracy of the estimates of E p, which are critical for ultimately testing the reliability of spectral-energy correlations and their use for the estimates of cosmological parameters

 extending to high energies (1-2 MeV):  due to the broad spectral curvature of GRBs, a WFM with a limited energy band (< keV) will allow the determination of E p for only a small fraction of events and with low accuracy (e.g., Swift/BAT ~15%)  an extension up to 1-2 MeV with a good average effective area (~600cm keV) in the FOV of the WFM is required  -> baseline solution: WFM + GRB Detector based on crystal scintillator  accuracy of a few % for GRB with keV fluence > erg cm -2 over a broad range of E p ( ) WFM: 2mm thick CZT detector, eff. Area~500 keV, FOV~1.4sr WFM + GRBD (5cm thick NaI, 850cm2 geom.area) Pow.law fit

 extending to low energies (1 keV)  GRB was a very long event (~3000 s) and without XRT mesurement ( keV) Ep,i would have been over-estimated and found to be inconsistent with the Ep,i-Eiso correlation  Ghisellini et al. (2006) found that a spectral evolution model based on GRB can be applied to GRB and GRB031203, showing that these two events may be also consistent with the Ep,i-Eiso correlation  missing the X-ray prompt emission -> overestimate of Ep

 In some cases, Ep estimates form different instruments are inconsistent with each other -> particular attention has to be paid to systematics in the estimate of Ep (limited energy band, data truncation effects, detectors sensitivities as a function of energies, etc.)  include X-ray flares (and the whole afterglow emission) in the estimate of Ep,i and Eiso

 e.g., , ,

 Swift has shown how fast accurate localization and follow-up is critical in the reduction of selection effects in the sample of GRB with known z  increasing the number of GRB with known z and accurate estimate of Ep is fundamental for calibration and verification of spectral energy correlations and their use for estimate of cosmological parameters (sim. by G. Ghirlanda)

 consider BATSE keV fluences and spectra for 350 bright GRBs  Ep,i = K x Eiso m, Ep,i = Ep x (1+z), Eiso=(Sx4  D L 2 )/(1+z)  Ep,i-Eiso correlation re-fitted by computing Eiso from 25*(1+z) to 2000*(1+z) gives K ~100, m ~0.54,  (logEp,i) ~ 0.2, K max,2  ~ 250 -> S min,n  = min[(1+z) 2.85 /(4  D L 2 )] x (Ep/K max,n  ) 1.85 (min. for z = 3.8) Adapted from Kaneko et al., MNRAS,   only a small fraction (and with substantial uncertainties in Ep) is below the 2  limit !

Open issues (several, despite recent huge observational advancements)  Prompt and afterglow emission mechanisms: still to be settled  early afterglow (steep decay, flat decay, flares): to be understood  GeV / TeV emission: need for new measurements to test models  X/gamma-ray polarization: need of measurements to test models  short vs. long events: emission mech. and GRB/SN connection  sub-energetic GRB and GRB rate: to be investigated -> SFR evolution  collimated emission: yes or no ?  spectrum-energy correlations and GRB cosmology: explanations and test … and more !

 jet angles derived from the achromatic break time, are of the order of few degrees  the collimation-corrected radiated energy spans the range ~5x10 40 – erg-> more clustered but still not standard  recent observations put in doubt jet interpretation of optical break

 confirmation of expectations based on the fact that short GRBs are harder and have a lower fluence  spectra of short GRBs consistent with those of long GRBs in the first 1-2 s  evidences that long GRBs are produced by the superposition of 2 different emissions ?  e.g., in short GRBs only first ~thermal part of the emission and lack or weakness (e.g. due to very high  for internal shocks or low density medium for external shock) of long part  long weak soft emission is indeed observed for some short GRBs Ghirlanda et al. (2004)

 no secure detection of polarization of prompt emission (some information from INTEGRAL and BATSE ?)  polarization of a few % for some optical / radio afterglows  radiation from synchrotron and IC is polarized, but a high degree of polarization can be detected only if magnetic field is uniform and perpendicular to line of sight  small degree of polarization detectable if magnetic field is random, emission is collimated (jet) and we are observing only a (particular) portion of the jet or its edge  Polarization