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
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)
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
Uniform/variable jet
PL-structured /universal jet
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)
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:
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
Spectral-energy Spectral-energy correlations: physics, correlations: physics,
peculiar GRB and peculiar GRB and possible outliers, tests, possible outliers, tests,
debatesdebates
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)
Origin of spectral-energy correlations
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)
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
Uniform/variable jet
PL-structured /universal jet
• 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
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
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)
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.)
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
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.)
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
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
Liang & Zhang,ApJ, 2005
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
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
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
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
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 (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)