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 >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

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 ?)

GRB041219 (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

~ - 1.3

~ -210^2 – 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

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 ~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 ?

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, GRB060614 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 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)

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-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 (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

the challenging “naked eye” GRB080319B: complexity and energetics

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

GRB 090423 - 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 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

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: 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

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

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 BANDBROAD 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 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

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 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

The contribution of CGRO/BATSE in the 90s bimodal distribution of durations

characterization of GRB non thermal spectra

isotropic distribution of directionsshort

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 > 1054 erg

short GRB lie at lower redshifts (<~1) and are less luminous (Eiso < ~1052 erg)

log(Eiso)= 1.0 ,

= 0.9

BeppoSAX era

prompt

afterglow

Swift: transition from prompt to afterglow (Swift, >2005)

Swift era

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

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 050509b

Short

Long

host galaxies (>1997, X-ray loc. + optical follow-up)

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.?

sub-energetic GRBs

Gamma-Ray BurstsGamma-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

shortlong

• 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 counterpartsafterglow 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 050509b

ShortLong

… and host galaxies

in 1997 discovery of afterglow emission by BeppoSAXin 1997 discovery of afterglow emission by BeppoSAX

first X, optical, radio counterparts, redshifts, energeticsfirst X, optical, radio counterparts, redshifts, energetics

GRB 970508, Metzger et al., Nature, 1997

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

• 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 - IConclusions - 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 - IIConclusions - II

Tests and debatesTests 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 redshiftDebate based on BATSE GRBs without known redshift

Swift / BAT sensitivity better than BATSE for Ep < ~100 keV, slightly worse 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 effectsSwift GRBs and selection effects

Ghirlanda et al., MNRAS, (2008)

fast (~1 min) and accurate localization (few fast (~1 min) and accurate localization (few arcesc) of GRBs -> prompt optical follow-up with arcesc) of GRBs -> prompt optical follow-up with large telescopes -> large telescopes -> substantial increase of substantial increase of redshift estimates and reduction of selection redshift estimates and reduction of selection effects in the sample of GRBs with known effects in the sample of GRBs with known redshiftredshift

fast slew -> observation of a part (or most, fast slew -> observation of a part (or most, for very long GRBs) of prompt emission down to for very long GRBs) of prompt emission down to 0.2 keV with unprecedented sensitivity –> 0.2 keV with unprecedented sensitivity –> following complete spectra evolution, detection following complete spectra evolution, detection and modelization of low-energy and modelization of low-energy absorption/emission featuresabsorption/emission features -> better estimate -> better estimate of Ep for soft GRBsof Ep for soft GRBs

drawback: BAT “narrow” energy band allow drawback: BAT “narrow” energy band allow to estimate Ep only for ~15-20% of GRBs (but to estimate Ep only for ~15-20% of GRBs (but for some of them Ep from HETE-2 and/or Konusfor 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 correlationall 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 beforethe 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 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: observationspassing the more reliable test: observations ! !

Amati 2006, Amati et al. 2008

very recent claim by Butler et al.: 50% of Swift very recent claim by Butler et al.: 50% of Swift GRB are inconsistent with the pre-Swift Ep,i-Eiso GRB are inconsistent with the pre-Swift Ep,i-Eiso correlationcorrelation

but Swift/BAT has a narrow energy band: 15-but Swift/BAT has a narrow energy band: 15-150 keV, nealy unesuseful for Ep estimates, 150 keV, nealy unesuseful for Ep estimates, possible only when Ep is in (or close to the possible only when Ep is in (or close to the bounds of ) the passband (15-20%) and with low bounds of ) the passband (15-20%) and with low accuracyaccuracy

comparison of Ep derived by them from BAT comparison of Ep derived by them from BAT spectra using Bayesian method and those spectra using Bayesian method and those MEASURED by Konus/Wind show they are MEASURED by Konus/Wind show they are unreliableunreliable

as shown by the case of GRB 060218, missing as shown by the case of GRB 060218, missing the soft part of GRB emission leads to the soft part of GRB emission leads to overestimate of Epoverestimate of Ep

Full testing and exploitation of spectral-energy correlations may be provided by Full testing and exploitation of spectral-energy correlations may be provided by EDGE in the > 2015 time frameEDGE in the > 2015 time frame

in 3 years of operations, EDGE will detect perform sensitive spectral analysis in 8-2500 keV in 3 years of operations, EDGE will detect perform sensitive spectral analysis in 8-2500 keV for ~150-180 GRBfor ~150-180 GRB

redshift will be provided by optical follow-up and EDGE X-ray absorption spectroscopy (use redshift will be provided by optical follow-up and EDGE X-ray absorption spectroscopy (use of microcalorimeters, GRB afterglow as bkg source)of microcalorimeters, GRB afterglow as bkg source)

substantial improvement in number and accuracy of the estimates of Esubstantial improvement in number and accuracy of the estimates of Epp , which are critical , which are critical for ultimately testing the reliability of spectral-energy correlations and their use for the estimates for ultimately testing the reliability of spectral-energy correlations and their use for the estimates of cosmological parametersof cosmological parameters

extending to high energies (1-2 MeV):extending to high energies (1-2 MeV):

due to the broad spectral curvature of GRBs, a WFM with a limited energy band (<150-200 due to the broad spectral curvature of GRBs, a WFM with a limited energy band (<150-200 keV) will allow the determination of EkeV) will allow the determination of Epp for only a small fraction of events and with low accuracy for only a small fraction of events and with low accuracy (e.g., Swift/BAT ~15%)(e.g., Swift/BAT ~15%)

an extension up to 1-2 MeV with a good average effective area (~600cman extension up to 1-2 MeV with a good average effective area (~600cm22@600 keV) in the @600 keV) in the FOV of the WFM is requiredFOV of the WFM is required

-> baseline solution: WFM + GRB Detector based on crystal scintillator-> baseline solution: WFM + GRB Detector based on crystal scintillator

accuracy of a few % for GRB with 15-150 keV fluence > 10accuracy of a few % for GRB with 15-150 keV fluence > 10-6-6 erg cm erg cm-2-2 over a broad range of over a broad range of EEpp (20-1500) (20-1500)

WFM: 2mm thick CZT detector, eff. Area~500 cm2@100 keV, FOV~1.4sr

WFM + GRBD (5cm thick NaI, 850cm2 geom.area)

Pow.law fit Pow.law fit

extending to low energies (1 keV)extending to low energies (1 keV)

GRB060218 was a very long event (~3000 s) and without XRT mesurement (0.3-10 keV) GRB060218 was a very long event (~3000 s) and without XRT mesurement (0.3-10 keV) Ep,i would have been over-estimated and found to be inconsistent with the Ep,i-Eiso correlationEp,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 GRB060218 can be Ghisellini et al. (2006) found that a spectral evolution model based on GRB060218 can be applied to GRB980425 and GRB031203, showing that these two events may be also consistent applied to GRB980425 and GRB031203, showing that these two events may be also consistent with the Ep,i-Eiso correlationwith the Ep,i-Eiso correlation

missing the X-ray prompt emission -> overestimate of Epmissing 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., 060218, 061007, 070110

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 25-2000 keV fluences and spectra for 350 bright GRBs

Ep,i = K x Eisom , Ep,i = Ep x (1+z), Eiso=(Sx4DL2)/(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, Kmax,2 ~ 250

-> Smin,n = min[(1+z)2.85/(4DL2)] x (Ep/Kmax,n)1.85 (min. for z = 3.8)

Adapted from Kaneko et al., MNRAS, 2006

2

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 ~5x1040 – 1052 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