GRB Spectral-Energy correlations: perspectives and issues G. Ghirlanda + G. Ghisellini, L. Nava, Z. Bosnjak, C. Firmani, I. Cabrera, F. Tavecchio & A. Celotti giancarlo.ghirlanda@brera.inaf.it GRB Spectral-Energy correlations: perspectives and issues Why? 1) tools for understanding GRB physics 2) tools to standardize GRB energetics  cosmology G. Ghirlanda – 2008 Nanjing Gamma Ray Burst Conference

true physics or selection effects? Epeak  Eiso0.57 Epeak  Eg1.05 Liso  Ep1.62 T45-0.49 2red=7.2 (60 dof) s=0.2 Q. Spectral-energy correlations: true physics or selection effects? 2red=0.89 (23 dof) s=0.08 2red=0.7 (16 dof) s=0.06 Amati et al. 2002 Ghirlanda et al. 2004 Firmani et al. 2005 Physical interpretation (e.g. Rees & Meszaros 2006, Thomson, Meszaros & Rees 2006) Study selection effects Liso  t - 1.05 Liso  V 3.0 2red=9.6 (34 dof) 2red=100 (46 dof) Liso0.57  Epeak 1.85 (I) GRBs with z (II) + GRBs without z (IV) Thermal component? Reichart et al. 2000 Ramirez-Ruiz & Fenimore 2000 Norris et al. 2000 Yonetoku et al. 2004 (III) Still not convinced ? (V) news

Epeak – Eiso 76 GRBs with z and Epeak 9 SAX (GRBM+WFC) 5 CGRO (Batse) Amati et al. 2002, 2006, Nava et al. 2006, Ghirlanda et al. 2008 … etc 76 GRBs with z and Epeak 35 before Nov. 2004 41 since Nov. 2004 9 SAX (GRBM+WFC) 5 CGRO (Batse) 15 Hete-II (Fr.+WXC) 20 Konus et al. 27(/41) Swift (BAT) Passaggio: With this considerably large sample of busrsts we can also study of there is any evolution of this correlation with redshift. Indded, this sample of 76 events contains very low redshift bursts (at z<0.1) as well as very distant one (the record holder at z=6.3).

Evolution with redshift? NO Slope of the 76 GRBs 1. No redshift evolution; 2. Except for very low z, redsfhits are distributed along the correlation

No segregation in z in the obs. frame REST FRAME GRBs with F>Flim Ep  [Emin, Emax] Instrumental selection effcts OBSERVER FRAME obs frame Ep-fluence correlation : (Lloyd, Petrosian & Mallozzi 2000; Lamb et al. 2005; Sakamoto et al. 2005) 1) No segregation in z means that the rest frame correlation cannot be due to the “stretching” of a blob of points due to the multiplication of strong functions of redshifts. 2) Any GRB instrument detected GRBs above some limiting flux and within a certain energy range No segregation in z in the obs. frame

Selection effects Obs Frame  Epeak - F low fluence – intemediate/high Epeak Selection effects High fluence – intemediate/low Epeak

a=1 b=2.3 Epeak Fbol Background Detector response Trigger threshold: which flux to trigger? (Band 2003, 2006) Spectral threshold: which fluence to measure Epeak? Assume GRB spectrum Background Detector response a=1 b=2.3 Epeak Fbol the error on Epeak(fit) < 100% in 97.7% of cases Fit with single powerlaw is excluded at 2σ

Trigger threshold: which flux to trigger? (Band 2003, 2006) Spectral threshold: which fluence to measure Epeak? Ghirlanda et al. 2008 BATSE could not detect this burst BATSE could detected this burst BATSE certainly detected this burst and measured Epeak

76 GRBs (updated to Oct. 2007) with z and spectrum CONCLUSIONS (I) 76 GRBs (updated to Oct. 2007) with z and spectrum 1) No evolution of the Epeak-Eiso correlation with redshift. 2) A correlation is found in the observer frame 3.1) no z segregation 3.2) Instrumental selection effect: a) trigger threshold  not biasing b) spectral analysis threshold  yes on Swift no on Batse/Sax

HOW is populated the Ep-Fluence plane? Q: are there intermediate/low fluence bursts (i.e. between those with z and the spectral analisys curves)??

HOW is populated the Ep-Fluence plane?  Add GRBs without redshift From the literature Passaggio: these are not complete samples!! Also the Batse bright bursts is not complete because it was cut in both fluence and peak flux. We need a complete sample of bursts. Sakamoto et al. 2005 Butler et al. 2007 (freq) Kaneko et al. 2005 GCNs (Golenetski et al. …) Nava et al. 2008 submitted

Extend the Bright Batse GRB sample (Kaneko et al. 2005) to lower fluences Build a complete spectral sample of BATSE bursts down to ~2x10-6 erg/cm2 Peak energy distribution Ep = 160 keV Bright BATSE Fainter BATSE

BATSE bursts

Outliers of the Epeak – Eiso correlation 6% of BATSE bursts are outliers Nava et al. 2008 submitted

Isotropic luminosity Isotropic energy The Ep-Liso “Yonetoku” correlation The Ep-Eiso “Amati” correlation Isotropic luminosity Isotropic energy

The Ep-Flux plane and the outliers of the Ep-Liso correlation Nava et al. 2008 submitted

Ep-Fluence or Ep-Peak flux show strong correlations CONCLUSIONS (II) Ep-Fluence or Ep-Peak flux show strong correlations Add bursts without redshifts (+ a complete BATSE sample) Strong Ep-Fluence correlation Strong Ep-Peak flux correlation The 20?? Ep-Eiso correlation will have a different slope & larger scatter but maybe not for the Ep-Liso correlation 6% of outliers of the Ep-Eiso 0.3% of outliers of the Ep-Liso

Still not convinced ? Are the spectral-energy correlations revealing a physical process or simply due to selection effects? Liang, Dai & Wu 2004 noted that an Ep-Liso correlation holds WITHIN single bursts!

Still not convinced ? Are the spectral-energy correlations revealing a physical process or simply due to selection effects? We have studied the time resolved spectra of BATSE GRBs with measured redshifts

Physical origin for this ! Ep-Liso is equivalent to Ep(t)-Liso(t) Ep-Liso correlation found with time integrated spectra holds also within a burst!! Ep-Liso “Yonetoku” Physical origin for this ! Bosnjak et al. 2008 (to be subm.)

Interpretation of the <Epeak>  Eg,iso0.5 (4) Interpretation – Thermal BB Interpretation of the <Epeak>  Eg,iso0.5 “Geometrical” models: Eichler & lenvinson 2005a,b; Toma et al. 2005 “Radiative” models: Rees & Meszaros 2005; Tompson 2006; Thompson, Meszaros & Rees 2006 If the spectrum of GRBs is dominated by a thermal balck body component then the luminosity is naturally LINKED to the peak energy. Evidence of Black Body in GRBs: Ghirlanda, Celotti, Ghisellini 2003  980326, 970111, 911118, 910807, 910927 [Spectrum is thermal black body in the inital phase (~2 sec), later a non-thermal component dominates.] Bosnjak, Celotti, Ghirlanda 2005  990413 Ryde 2005, 2006  Fit with Black body + Powerlaw

Black Body + powerlaw fits Thermal components in GRB spectra Black Body + powerlaw fits Band model fits

Thermal interpretation of the Amati relation Time integrated spectrum = sum of time resolved Time resolved spectra (BB+PL) 5 GRBs detected by BATSE and with WFC data the BB+PL fit to the BATSE data is inconsistent with the X-ray (WFC) data. A single Band model is the best fit. Ghirlanda et al. 2007

Last slide … more News: the Ep-Eg corrlation Pre swift-era bursts Swift era bursts (up to March 2007) Swift era bursts of the last year Jet Breaks from the Optical Ghirlanda et al. 2007