gamma-ray bursts (grb) as cosmological probes

Gamma-Ray Bursts (GRB) as Gamma-Ray Bursts (GRB) as cosmological probescosmological probes

Lorenzo AmatiLorenzo Amati

INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, BolognaBologna 43rd Rencontres de Moriond

La Thuile (Val d'Aosta, Italy) March 15 - 22, 2008

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; estimated true rate ~2 / day bimodal duration distribution fluences (= av.flux * duration) typically of ~10-7 – 10-4 erg/cm2

shortlong

isotropic distribution of GRBs directions paucity of weak events with respect to homogeneous distribution in euclidean space hints to cosmological origin of GRBs -> would imply huge luminosity but a “local” origin could not be excluded until 1997 !

in 1997 discovery of afterglow emission by BeppoSAX

first X, optical, radio counterparts

GRB970228, Van Paradijs et al., Nature, 1997

GRB 970508, Metzger et al., Nature, 1997

optical spectroscopy of afterglow and/or host galaxy –> first measurements of GRB redshift

redshifts higher than 0.1 -> GRB are cosmological

their isotropic equivalent radiated energy is huge (up to 1054 erg in a few tens of s !)

all GRBs with measured redshift (~100, including a few short GRB) lie at cosmological distances (z = 0.033 – 6.4) (except for the peculiar GRB980425, z=0.0085)

isotropic luminosities and radiated energy are huge and span several orders of magnitude: GRB are not standard candles (unfortunately)

Mean: Eiso = 1053 erg

Are GRB standard candles ?Are GRB standard candles ?

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

GRB have huge luminosity, a redshift distribution extending far beyond SN Ia

high energy emission -> no extinction problems

but need to investigate their

properties to find ways to standardize them (if possible)

GRB F spectra typically show a peak at photon energy Ep

for GRB with known redshift it is possible to estimate the cosmological rest frame peak energy Ep,i and the radiated energy assuming isotropic emission, Eiso

““Standardizing” GRB with spectrum-Standardizing” GRB with spectrum-energy correlationsenergy correlations

log(Ep,i )= 2.52 ,

= 0.43log(Eiso)= 1.0 ,

= 0.9

Amati et al. (2002) analyzed a sample of BeppoSAX events with known redshift found evidence of a strong correlation between Ep,i and Eiso , highly significant ( = 0.949, chance prob. 0.005%)

Further analysis with updated samples confirmed the correlation and extended it to X-Ray Flashes (XRF)

the scatter of the data around the best fit power-law can be fitted with a Gaussian with s(logEp,i) ~ 0.2 (~0.17 extra-Poissonian)

Amati et al., 2008

redshift estimates available only for a small fraction of GRB occurred in the last 10 years based on optical spectroscopy

pseudo-redshift estimates for the large amount of GRB without measured redshift -> GRB luminosity function, star formation rate evolution up to z > 6, etc.

use of the Ep,i – Eiso correlation for pseudo-redshift: most simple method is to study the track in the Ep,i - Eiso plane ad a function of z

not precise z estimates and possible degeneracy for z > 1.4

anyway useful for low –z GRB and in general when combined with optical

a first step: using Ep,i – Eiso correlation for z estimates

the Ep,i-Eiso correlation becomes tighter when adding a third observable: jet opening angle (jet -> E = [1-cos(jet)]*Eiso (Ghirlanda et al. 2004) or “high signal time” T0.45 (Firmani et al. 2006)

the logarithmic dispersion of these correlations is very low: they can be used to standardize GRB ?

jet angle inferred from break time in optical afterglow decay, while Ep,i-Eiso-T0.45 correlation based on prompt emission properties only

a step forward: standardizing GRB with 3-parameters spectral energy correlations

Methods (e.g., Ghirlanda et al, Firmani et al., Dai et al., Zhang et al.): general purpouse: estimate c.l. contours in 2-param surface (e.g. M-, M-w0)

general method: construct a chi-square statistics for a given correlation as a function of a couple cosmological parameters

general assumption: the correlation exists

method 1 – minimum correlation scatter: for each couple of cosm.parameters compute Ep,i and Eiso (or E), fit the points with a pl and compute the chi-square -> derive c.l. contours based on chi-square surface

method 2 – luminosity distance: for each couple of param. fit the correlation, derive Eiso from from Ep for each GRB through the best fit law, derive DL for each GRB from Eiso/fluence, construct chi-square based on difference from these DL and those derived form the z with the assumed couple of cosm. param. -> derive c.l. contours based on chi-square surface

method 3: more sophisticated bayesian method assuming that the correlation exists AND IS UNIQUE (which is reasonable given that it is expected that depends from the physics of GRB emission)

Ghirlanda et al., 2004,2007

GRB Hubble diagram with Ep-E using lum. dist. method:

Ghirlanda et al., Firmani et al.

Contour plots in m – plane with Ep-E correlation(left) and Ep-Lp-T0.45 correlation (right) with the bayesian method

Ghirlanda, Ghisellini et al. 2005, 2006,2007

What can be obtained with 150 GRB with known z and Ep and complementarity with other probes (SN Ia, CMB)

complementary to SN Ia: extension to much higher z even when considering the future sample of SNAP (z < 1.7), cross check of results with different probes

Combining spectrum-energy correlations with other (less tight) Luminosity correlations in GRB (Schaefer 2007)

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)

e.g., Ep,i -2 L1/2 t-1 for syncrotron emission from a power-law distribution of electrons generated in an internal shock (Zhang & Meszaros 2002, Ryde 2005); for Comptonized thermal emission

geometry of the jet (if assuming collimated emission) and viewing angle effects also may play a relevant role

lack of solid physical explanation

Drawbacks and circularity problemsDrawbacks and circularity problems

Lack of calibration differently to SN Ia, there are no low-redshift GRB (only 1 at z < 0.1) -> correlations cannot be calibrated in a “cosmology independent” way

would need calibration with a good number of events at z < 0.01 or within a small range of redshift -> neeed to substantial increase the number of GRB with estimates of redshift and Ep

Bayesian methods have been proposed to “cure” the circularity problem (e.g., Firmani et al., 2006), resulting in slightly reduced contours w/r to simple (and circularity free) scatter method (using Lp,iso-Ep,i-T0.45 corr.)

Very recently (Kodama et al., 2008; Liang et al., 2008) calibrated GRB spectrum—energy correlation at z < 1.7 by using the cosmology independent luminosity distance – redshift relation derived for SN Ia (RIess et al, 2007)

Obtained significant constraints on both M and but with this method GRB are no more an indipendent cosmological probe

“crisis” of 3-parameters spectrum-energy correlations Recent debate on Swift outliers to the Ep-E correlation (including both GRB with no break and a few GRB with achromatic break)

Recent evidence that the dispersion of the Lp-Ep-T0.45 correlation is significantly higher than thought before

Campana et al. 2007 Rossi et al. 2008

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, in order to avoid circularity problem

need solid physical interpretation

identification and understanding of possible sub-classes of GRB not following correlations

ConclusionsConclusions

GRB can be used as cosmological beacons for study of the IGM up to z > 6

Because of the association with the death of massive stars GRB allow the study the evolution of massive star formation and the evolution of their host galaxy ISM back to the early epochs of the Universe (z > 6)

GRB as cosmological beaconsGRB as cosmological beacons

EDGE Team

END OF THE TALK

BACK UP SLIDES


e.g., Ep,i -2 L1/2 t-1 for syncrotron emission from a power-law distribution of electrons generated in an internal shock (Zhang & Meszaros 2002, Ryde 2005)

e.g., Ep,i Tpk 2 L-1/4 or

under different assumptions and to be combined with and in scenarios in whch for comptonized thermal emission from the photosphere dominates (e.g. Rees & Meszaros 2005)

physics of prompt emission (e.g. Zhang & Meszaros 2002)

jet geometry and structure

XRF-GRB unification models

viewing angle effects

identification and nature of different classes of GRBs (as I will discuss next)

Uniform/variable jet

PL-structured /universal jet




Need of understending the physics behind spectral energy correlations

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 ?

Ep,i-E correlation need to clarify and understand the problem lack of chromatic breaks and possible outliers

GRB 061007, Mundell et al. Schady et al.

GRB 050416a

need to reduce systematics on Ep with broad energy band spectroscopy

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.)

need detectors extending to high energies (1-2 MeV)

due to the broad spectral curvature of GRBs, a WFM with a limited energy band (<150-200 keV) will allow the determination of Ep 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 (~600cm2@600 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 15-150 keV fluence > 10-6 erg cm-2 over a broad range of Ep (20-1500)

GRB with 15-150 keV fluence 10-6 erg cm-2 and Ep=300 keV. Left: WFM only, Right: WFM + GRBD

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

and to low energies (1 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 correlation

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 with the Ep,i-Eiso correlation

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)

need to increase number and reduce selection effects of redshift estimates

Next experiments: what is needed ?

to test the correlations, to better constrain their slope, normalization and dispersion, to understand the physics behind them and study the behaviour of different sub-classes of GRB, and to calibrate them for cosmological use, need to:

increase the number of z estimates, reduce selection effects and optimize coverage of the fluence-Ep plane in the sample of GRBs with known redshift

more accurate estimates of Ep,i by means of sensitive spectroscopy of GRB prompt emission from a few keV (or even below) and up to at least ~1 MeV

Swift is doing greatly the first job but 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)

Ep estimates for some Swift GRBs from HETE-2 (2-400 keV) and Konus (from 15 keV to several MeV) for simultaneously detected events

presently, Ep,i values are mainly provided by HETE-2 (but only up to 400 keV -> mostly CPL fits) or Konus/Wind (> 15 keV, fits with Band function); in some cases, useful spectral information also from BAT, Suzaku and INTEGRAL

2008(-2011 ?): GLAST (AGILE) + Swift:

accurate Ep and z estimate (plus full X-ray coverage and possibly study of GeV emission) for simultaneously detected events

however, even by assuming that Swift will follow-up ALL GLAST GRB, no more than 30 GRB with Ep and z in 3 years

need of new GRB detectors capable to extend down to a few keV (or even below) and up to at least ~1 MeV: e.g., a combination of a sensitive low energy detector (CZT, silicon microstrips or SDC diodes) and moderate area scintillator high energy detector (NaI, CsI, BGO)

In the 2011-2015 time frame a significant step forward expected from SVOM:

spectral study of prompt emission in 1-5000 keV -> accurate estimates of Ep and reduction of systematics (through optimal continuum shape determination and measurement of the spectral evolution down to X-rays)

fast and accurate localization of optical counterpart and prompt dissemination to optical telescopes -> increase in number of z estimates and reduction of selection effects in the sample of GRB with known z

optimized for detection of XRFs, short GRB, sub-energetic GRB

substantial increase of the number of GRB with known z and Ep -> test of correlations and calibration for their cosmological use

unique mission with these capabilities in the 2010-2015 (at least) time frame

SVOM will allow a step forward in the test, understanding and use (GRB physics, cosmology) of spectral – energy correlations

Full calibration and exploitation of spectral-energy correlation for cosmology may be provided by EDGE in the > 2015 time frame

in 3 years of operations, EDGE will detect perform sensitive spectral analysis in 8-2500 keV for ~150-180 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 Ep , which are critical for the estimates of cosmological parameters

THE ENDTHE END

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 naturally explained collimated emission

energy budget up to 1051 - 1052 erg short duration GRBs (< 5 s) clean circum-burst environment GRBs in the outer regions of the host galaxy

SHORT

before EDGE main contributions expected from GLAST+Swift and SVOM

EDGE will ~triplicate the GRB in the sample and substantially increase the accuracy in cosmological parameters

EDGE will allow cosm. param. estimates based on an homogeneous sample the very high number of GRB known z and Ep that will be reached

with EDGE will allow calibration of the correlations (i.e. find Ep for a sufficent number of GRB at same z within 10% and thus solve circularity problem)

EDGE GRB, SNAP SN Ia, other probes (e.g. clusters): complementarity and sinergy (different z ranges), cross-checks, reduction of systematics

EDGE will also allow a robust test of the correlations themselves (selection effects, outliers, etc.)

What is needed ? to test the correlations, to better constrain their slope, normalization and dispersion, to understand the physics behind them and study the behaviour of different sub-classes of GRB, and to calibrate them for cosmological use, need to:





Spectral-energy Spectral-energy correlations: physics, correlations: physics,

peculiar GRB and peculiar GRB and possible outliers, tests, possible outliers, tests,

debatesdebates


Origin of spectral-energy correlations


e.g., Ep,i -2 L1/2 t-1 for syncrotron emission from a power-law distribution of electrons generated in an internal shock (Zhang & Meszaros 2002, Ryde 2005)

e.g., Ep,i Tpk 2 L-1/4 or

under different assumptions and to be combined with and in scenarios in whch for comptonized thermal emission from the photosphere dominates (e.g. Rees & Meszaros 2005)





identification and nature of different classes of GRBs (as I will discuss next)





• 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)

• GRB031203 : the most similar case to GRB980425/SN1998bw: very close (z = 0.105), SN2003lw, sub-energetic

• the most common explanations for the (apparent ?) sub-energetic nature of GRB980425 and GRB031203 and their violation of the Ep-Eiso and Ep-E correlations assume that they are NORMAL events seen very off-axis (e.g. Yamazaki et al. 2003, Ramirez-Ruiz et al. 2005

are sub-energetic GRBs really outliers ?

but, contrary to GRB980425 and (possibly) GRB031203, GRB060218 is consistent with the Ep,i-Eiso correlation -> evidence that it is a truly sub-energetic GRB

also XRF 020903 is very weak and soft (sub-energetic GRB prompt emission) and is ocnsistent with the Ep-Eiso correlation

Amati et al., 2007

GRB 060218, a very close (z = 0.033, second only to GRB9809425), with a prominent association with SN2006aj, and very low Eiso (6 x 1049 erg) and Ek,aft -> very similar to GRB980425 and GRB031203

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 correlation

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 with the Ep,i-Eiso correlation

claims that a high fraction of BATSE events (without z) are inconsistent with the correlation (e.g. Nakar & Piran 2004, Band & Preece 2005

Ghirlanda et al. (2005), Bosnjak et al. (2005), Pizzichini et al. (2005): BATSE GRB with unknown redshift are consistent with the correlation

different conclusions but general consensus: Swift will allow reduction of selection effects both in GRB detection and in z estimates and will allow us to stringently test the correlation

Testing the Ep,i – Eiso correlation

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

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 !

adapted from Amati, NCimC, 2006

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: 15-150 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 060218, missing the soft part of GRB emission leads to overestimate of Ep

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 ?

The Ep,i-E correlation: the problem lack of breaks or of chromatic breaks and possible outliers

GRB 061007, Mundell et al. Schady et al.

GRB 050416a

Recent debate on Swift outliers to the Ep-Eg correlation (including both GRB with no break and a few GRB with achromatic break)

again, different conclusions based on light curve modeling and considering early or late break

additional reasons to prefer the Ep,i-Lp-T0.45 correlaiton for standardizing GRB (but needs to be confirmed with more GRBs)

Campana et al. 2007 Ghirlanda et al. 2007

to test the correlation, to better constrain its slope, normalization and dispersion (test of GRB/XRF prompt emission physics and geometry), to identify and understand sub-energetic GRBs and short GRBs, to calibrate spectral-energy correlation for cosmological use, need to





Observations

presently, Ep,i vaues are mainly provided by HETE-2 (but only up to 400 keV -> mostly CPL fits) or Konus/Wind (> 15 keV, fits with Band function); in some cases, useful spectral information also from BAT, Suzaku and INTEGRAL

some future and possible GRB experiments: AGILE and GLAST (2006, 2007) : study of GRBs form 10-20 keV up to tens or hundreds of GeV

ECLAIRs (2009) : spectral study of prompt emission in 4-300 keV and optical observation of prompt emission

Spectrum-RG/e-Rosita/Lobster (>2010 ?) : spectral study of GRB in the ~0.3-500 keV enegy band

ESTREMO/WXRT: Wide Field Monitor from a few keV to 500-700 keV need of new GRB detectors capable to extend down to a few keV (or even below) and up to at least ~1 MeV: e.g., a combination of a sensitive low energy detector (CZT, silicon microstrips or SDC diodes) and moderate area scintillator high energy detector (NaI, CsI, BGO)


the fixed 1-10000 keV energy band on which is computed Eiso could not be optimal -> energy band centerd on Ep,i or larger fixed energy band (e.g. 0.01 – 100000)

Analysis

study of the systematics on the estimates of Ep,i and Eiso

identify a golden sample of GRBs to allow accurate calibration

include X-ray flares (and the whole afterglow emission) in the estimate of Ep,i and Eiso

test theoretical models by computing Ep,i and Eiso in a specific way

cosmology with the Ep,i-Eiso correlation (only 2 parameters, high number of events) ?

“canonical” early afterglow light curve observed by Swift

~ -3

~ -0.7

~ - 1.3

~ -210^2 – 10^3 s 10^4 – 10^5 s

10^5 – 10^6 s

( 1 min ≤ t ≤ hours )

GRBs 061007 and 070110: peculiar afterglow decay

Swift GRB 061007: a long GRB with unusal monotonic power-law decay

no break up to at least 106 s -> very narrow (0.8°) jet angle or very bright event (E~1054 erg)

GRB 070110: very flat early X-ray decay followed by abrupt drop This behaviour challenges external shock scenarios “Late bump” likely originated by internal shocks due to long

lasting central engine emission

Both GRBs are consistent with Ep-Eiso correlation The Ep-Eiso correlation holds independently on early afterglow behaviour and the

presence of “jet” break Including the “bump” emission in the Eiso of GRB 070110 should make it fit even

better the correlation: a test for its internal shock origin

061007070110

Swift GRBs and Ep,i-Eiso correlation: summary Swift long GRBs are consistent with the Ep,i-Eiso correlation -> the

correlation is not significantly affected by selection effects

Short GRBs are all inconsistent with the correlation -> different emission mechanism or lack/weakness of the second longer part of the emission (the soft tail of GRB050724 is consistent with the correlation)

The local, sub-energetic, SN associated GRB 060218 is consistent with the correlation: evidence that it was not seen off-axis -> really sub-energetic -> GRB rate as high as 2000/Gpc3/year; GRB radiated energy vary much more than M of associated SN

The long GRB not associated with a bright GRB/SN, 060614, is consistent with the correlation -> properties of long GRBs do not depend on the properties of the associated SN GRBs with peculiar afterglow decay (061007, 070110) are consistent with the correlation; inclusion of early afterglow emission in Eiso may reduce the dispersion


the fixed 1-10000 keV energy band on which is computed Eiso could not be optimal -> energy band centerd on Ep,i or larger fixed energy band (e.g. 0.01 – 100000)

Analysis

study of the systematics on the estimates of Ep,i and Eiso

identify a golden sample of GRBs to allow accurate calibration

include X-ray flares (and the whole afterglow emission) in the estimate of Ep,i and Eiso

test theoretical models by computing Ep,i and Eiso in a specific way

cosmology with the Ep,i-Eiso correlation (only 2 parameters, high number of events) ?

A note on selection effects and systematics

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 predict that Swift will detect several GRBs with Ep,i and Eiso inconsistent with the Ep,i-Eiso correlation

BUT…

up to now, all long Swift GRBs with known z show Ep,i (values or upper/lower limits) and Eiso values consistent 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

Ghirlanda et al., ApJ, 2005

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 !

moreover (1): estimates of Ep,i of several BATSE GRBs may be biased by the lack detection of the X-ray emission, which can contribute significantly to the time-integrated spectrum -> overestimate of Ep,i moreover (2): for dim GRBs we may observe only the brightest and hardest portion of the event -> overestimate of Ep,i

BeppoSAX GRB960720 Swift GRB060124

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

the Ep,i-Eiso correlation is passing the more reliable test: observations !

adapted from Amati, NCimC, 2006

• GRB 980425, a normal GRB detected and localized by BeppoSAX WFC and NFI, but in temporal/spatial coincidence with a type Ic SN at z = 0.008 (chance prob. 0.0001)

GRB/SN connection and sub-energetic GRBs

Pian et al.,ApJ, 2000

• further evidences of a GRB/SN connection: bumps in optical afterglow light curves and optical spectra resembling that of GRB980425

• most prominent case: GRB030329

GRB 030329, Hjorth et al., Nature, 2003

GRB 030329, Stanek et al., ApJ, 2003

The Ep,i – E and Ep,i – Eiso – tb correlations

by substituting Eiso with the collimation corrected energy E the correlation still holds, with a lower dispersion and a steeper slope of ~0.7 (Ghirlanda et al. 2004, Dai et al. 2004)

Nava et al.. , A&A, 2005Dai et al., ApJ, 2004

very recently, Liang et al. (2005) performed a multi-variable correlation analysis between various observables of prompt and afterglow

they found a tight correlation between Epi, Eiso and tb

with respect to Ep,i – E correlation it has the advantage of being model independent

differently from the Ep,i-Eiso correlation, the Ep,i-E and Ep,i-Eiso-tb correlations can be used for only a fraction of events (a firm estimate of tb is needed)

use of the Ep,i-E and Ep,i-Eiso-tb correlations for the estimate of cosmological parameters, in a way similar to SN Ia

Ghirlanda et al.,ApJ, 2004

Ghisellini et al., NCIM, 2005


Liang & Zhang,ApJ, 2005


Ghisellini et al. 2005

use of 3-parameters spectral-energy correlations (Ep,i-E, Ep,i-Eiso-tb, Ep,i-Liso-T0.45 for cosmology


Ghisellini et al. 2005

Ghirlanda et al.Ghirlanda et al. 2006 A&A 2006 A&A

Ghirlanda et al. 2006 JOP Review, GRB Special Ghirlanda et al. 2006 JOP Review, GRB Special IIssssuuee

the physics behind the Ep,i-Eiso correlation is still to be understood





identification and nature of different classes of GRBs (short, sub-energetic)

Need to settle the physics behind this correlation in order to achieve a complete understanding of the GRB phenomenon and to use spectral-energy correlations to standardize GRBs for the estimate cosmological parameters

Physics

• “canonical” early afterglow light curve

~ -3

~ -0.7

~ - 1.3

~ -210^2 – 10^3 s 10^4 – 10^5 s

10^5 – 10^6 s

( 1 min ≤ t ≤ hours )

Adding pieces to the puzzle

the correlation is used regularly as a input or test ouptut for GRB/XRF syntesis models and simulations

pseudo redshift estimators

standardization of GRBs for estimate of cosmological parameters (albeit the addition of 1 more observable: tb, T0.45 )

Nava et al. (2006), Ghirlanda et al. (2004)

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

the “extra-statistical scatter” of the data was quantified by performing a fit whith a method (D’Agostini 2005) which accounts for sample variance

the “intrinsic” dispersion results to be int(logEp,i) = 0.14 (-0.02,+0.03)

with this method, the power-law index and normalization turn out to be ~0.5 and ~100, respectively (the commonly assumed values !)

Amati (2006)

the Ep-E shows slightly different slope and dispersion based on the assumptions on the circum-burst environment density profile (ISM or wind)

in the wind case the slope is more close to 1 (-> linear correlation) and the dispersion slightly lower

Nava et al.. , A&A, 2005: ISM (left) and WIND (right)

the correlation holds also when substituting Eiso with Liso or Lpeak,iso, with similar slope and scatter

Lamb et al., 2004, Schaefer 2007

Yonetoku et al., 2004, Ghirlanda et al., 2005

From Amati, MNRAS, 2006

increasing the sample of GRBs with known z and firm estimate of Ep,i increases the significance of the correlation

the correlation is characterized by a substantial dispersion, as indicated by the high chi-square values of the fits with a power-law

due to the scatter of the data around the best fit power-law, different sub-samples give different values of the power-law index (~0.4 – 0.6)

gamma-ray bursts (grb) as cosmological probes

Documents

eisostandardizing grb

short grb

low z grb

grb standard candles

redshift distribution

small fraction of grb

known redshift

gammaray bursts grb