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GRB and X-Ray Flash perspective with ESTREMO

ESTREMO WXRT: Meeting on scientific requirements Bologna, 2006 May 4-5. GRB and X-Ray Flash perspective with ESTREMO. A. Galli, L. Piro & GRB Working Group. X-ray Flash and X-ray Rich Gamma Ray Burst.

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GRB and X-Ray Flash perspective with ESTREMO

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  1. ESTREMO WXRT: Meeting on scientific requirements Bologna, 2006 May 4-5 GRB and X-Ray Flash perspective with ESTREMO A. Galli, L. Piro & GRB Working Group

  2. X-ray Flash and X-ray Rich Gamma Ray Burst • Very soft class of GRB discovered by WFC of BeppoSAX (Heise et al. 2001). • According to Lamb&Graziani (2003) we classify the events according to their hardness ratio, Hh=S(2,30)/S(30,400): • Hh≤0.32  GRB • 0.32 ≤Hh≤ 1 XRR • Hh≥ 1  XRF • Spectral parametersαand β consistent with that of GRB • Lower average peak energy, about 36 keV for XRR/XRF and 162 keV for GRB in the BeppoSAX sample (D'Alessio, Piro & Rossi 2005) ESTREMO WXRT-Bologna, 2006, May 4-5

  3. Nature of XRR/XRF High redshift scenario XRF and GRB have the same intrinsic properties and their differencies are due only to distance effects. From the observed peak energies and assuming zGRB=1 we expect: - zXRF~8!!! -X-ray flux ratio at 11 hours from the burst ~ 12 -No optical afterglow because of the Lyman-α forest A large energy band and a rapid multi-wavelength follow-up are important ESTREMO WXRT-Bologna, 2006, May 4-5

  4. The off-axis scenario Differencies between GRB, XRR and XRF are due to differencies in the observer line of sight. Three different jet structures are proposed: 1. Off-axis Homogeneus (OH) model, the energy is uniformly distribuited within the jet aperture θj and drops sharply to zero outside 2. Universal Power-law (UP) jet, the energy is constant within a core angle θc and and falls as a power-law with an index -2 outside it 3. Quasi-Universal Gaussian (QUG) jet, the energy is nearly constant within a core angle θk and decreases exponentially outside it, exp-θ2/θk2 ESTREMO WXRT-Bologna, 2006, May 4-5

  5. Model predictions in the off-axis scenario I OH model D'Alessio, Piro & Rossi 2005 No jet break expected !! Jet break rarely observed in Swift light curves!!!! θ0=0, 1, 1.03, 4θj θj=6.1° 20 ks θ0=θj GRB(blu dotted line)θ0=1.03θj XRF (red dashed line)X-ray flux ratio ~ 1.1 ESTREMO WXRT-Bologna, 2006, May 4-5

  6. Model predictions in the off-axis scenario II UP jet D'Alessio,Piro & Rossi 2005 θ0=0, 3.1, 9.4, 36 θc θc=2° θ0,GRB=3.1θcblu dotted line, θ0,XRF=9.4θcred dashed line X-ray flux ratio~ 20 ESTREMO WXRT-Bologna, 2006, May 4-5

  7. Model predictions in the off-axis scenario III QUG jet D'Alessio,Piro & Rossi 2005 The afterglow light curve flattens for increasing value ofθ0. In the limit case it assume a profile similar to that predicted in the OH model ┤ θ0=0, 0.7, 2.2, 8.0 θk θk=5.7° θ0,GRB=0.7θkblu dotted line,  θ0,XRF=2.2θkred dashed line X-ray flux ratio ~11 ESTREMO WXRT-Bologna, 2006, May 4-5

  8. We have the following picture: - The UP and QUG jet models predict different X-ray fluxes for XRF and GRB. - The OH model predict smilar X-ray fluxes for XRF and GRB. Actually the data agree with this model (D'Alessio, Piro & Rossi 2005). - Selection effects play an important role in discriminating between different models. Our sample of XRR/XRF has not to be biased towards nearest events which afterglows appear bright as that of GRBs. In the QUG model the flux goes below the sensibility limit of WFC forθ0,XRF/θ0,GRB > 3.5 for z=1. Thus for z>1 we can miss events wihtθ0≥3.5θk ESTREMO WXRT-Bologna, 2006, May 4-5

  9. - An important test is the X-ray afterglow flux to promptγ-ray flux ratio, which estimates the kinetic energy ratio before and after the prompt if the emission is dominated by the line of sight part of the jet. In the UP and QUG jet models this is true for small angles, θk<θ0<2θk. If the prompt to afterglow radiation efficiency ratio is constant we expect a similar ratio for XRF and GRB. In the OH model the ratio strongly depends on the jet Lorentz factorΓand the viewing anlgeθ0. - XRF have a larger EISO distribution, thus a larger X-ray flux distribution is expected for XRF in comparison to GRB. We conclude that it is important to observe also dim/soft events to clarify the nature of XRR/XRF. A large energy band estending to low energy is required. ESTREMO WXRT-Bologna, 2006, May 4-5

  10. Beppo SAX GRB distribution The LogN-Log S is based on: -prompt X-ray fluence (2-10 keV) -last updated BeppoSAX catalog (http://grb.iasf-roma.inaf.it/catalog) - No field of view correction ~ 230 GRB/year - With the field of view correction ~ 1014 GRB/year This is a preliminary result and a more accurate investigation is required, but it is consistent with results from Batse (Fishman & Meegan 95). We can assume a normalization factor of 1000 GRB/year in all the sky. ESTREMO WXRT-Bologna, 2006, May 4-5

  11. XRR/XRF Hete II distribution - The Hete II sample (2000-2003) now offers the better statistic on XRF - Hete II sample is not affected by observational selection effects related to the different spectral slope of the three kinds of GRB (Sakamoto et al. 2005) Hete II: 41 XRR/XRF over 65 events, about the 63% of the sample. The numbers of the three kinds of GRB are roughly equal (Sakamoto et al. 2005). BeppoSAX: 15 XRR/XRF over 63 events, about the 24% of the total sample Swift: only 6 XRR/XRF over more than 100 GRB in about 1 year ESTREMO WXRT-Bologna, 2006, May 4-5

  12. Intensity and spectral Hete II XRR/XRF distribution We reduce our analysis to a sub-sample of Hete II XRR/XRF with both known γ-ray fluence and hardness ratio. Hh=S(2,30)/S(30,400) P XRR/XRF with H.R ≥Hh e S≥Sγ Sγ[erg cm-2] 60% (22 XRR/XRF over 37) 32% (12 XRR/XRF over 37) 0 0.5 1 5 10-7 24% (9 XRR/XRF over 37) 8% (3 XRR/XRF over 37) 0 0.5 1 5 10-6 11 % (4 XRR/XRF over 37) 0 0 0.5 1 5 10-5 ESTREMO WXRT-Bologna, 2006, May 4-5

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