Properties of X- Ray Rich Gamma Ray Bursts and X-Ray Flashes

We study the spectrum of the prompt emission and the X-ray and optical afterglow fluxes of 54 X-Ray Rich Gamma Ray Burst (XRRs) and X-Ray Flashes (XRFs), observed by BeppoSAX  and HETE-2 . A comparison is then performed with classical Gamma Ray Bursts (GRBs). The goal of this paper is to investigate the nature of XRRs/XRFs, as high redshift GRBs or off-axis GRBs, analyzing both their prompt and afterglow properties. We find that the XRR/XRF spectral indexes of the Band function are similar to those of classical GRBs, whereas the peak energy is lower by a factor of 4. We study the optical and X-Ray afterglow properties of the XRRs/XRFs; in particular we analyze the XRR 011030 afterglow. We find that the X-ray and optical flux distributions and the lightcurves of the XRRs/XRFs sample are consistent with those of classical GRBs; in particular, they show evidence of a break and no rising temporal slope. We compare these results with the afterglow predictions of the high redshift scenario, where XRFs are GRBs at higher redshift and of the off-axis scenario, where the observed differences are due to viewing angle effects. In this last framework, we consider jets with a homogeneous, a -2 power-law shaped and a Gaussian luminosity angular distribution. We find that the high redshift scenario can explain some events but not the total sample of XRRs/XRFs. The off-axis model may be consistent with our findings when a homogeneous jet is considered. However, given the uncertainties on the selection effects in our sample, a Gaussian jet viewed at small angles from the Gaussian core and a power-law shaped cannot be ruled out.

[1]  M. Feroci,et al.  Prompt and Afterglow Emission from the X-Ray-Rich GRB 981226 Observed with BeppoSAX , 2000, astro-ph/0002527.

[2]  Bing Zhang,et al.  GAMMA-RAY BURST BEAMING: A UNIVERSAL CONFIGURATION WITH A STANDARD ENERGY RESERVOIR? , 2002 .

[3]  M. Feroci,et al.  The BeppoSAX catalog of GRB X-ray afterglow observations , 2005, astro-ph/0507708.

[4]  J. Fynbo,et al.  The red optical afterglow of GRB 030725 , 2005, astro-ph/0505243.

[5]  E. Rol,et al.  Very High Column Density and Small Reddening toward GRB 020124 at z = 3.20 , 2003, astro-ph/0307331.

[6]  Arnon Dar,et al.  On the origin of X-ray Flashes , 2004 .

[7]  G. Ghirlanda,et al.  The Collimation-corrected Gamma-Ray Burst Energies Correlate with the Peak Energy of Their νFν Spectrum , 2004, astro-ph/0405602.

[8]  W. Priedhorsky,et al.  Fast X-Ray Transients and Their Connection to Gamma-Ray Bursts , 2003, astro-ph/0304347.

[9]  Observatoire Midi-Pyrenees,et al.  On the nature of X-ray Flashes , 2004 .

[10]  J. Rhoads.  The Dynamics and Light Curves of Beamed Gamma-Ray Burst Afterglows , 1999, astro-ph/9903399.

[11]  Enrico Ramirez-Ruiz,et al.  Afterglow Observations Shed New Light on the Nature of X-Ray Flashes , 2005, astro-ph/0502300.

[12]  Merate,et al.  Late evolution of the X-ray afterglow of GRB 030329 , 2004, astro-ph/0402644.

[13]  Ryo Yamazaki,et al.  Peak energy-isotropic energy relation in the off-axis gamma-ray burst model , 2004 .

[14]  N. R. Tanvir,et al.  Discovery of the Near-IR Afterglow and of the Host of GRB 030528 , 2004 .

[15]  E. Rol,et al.  On the Afterglow of the X-Ray Flash of 2003 July 23: Photometric Evidence for an Off-Axis Gamma-Ray Burst with an Associated Supernova?* , 2004 .

[16]  Fiona Harrison,et al.  A Search for Discrete X-Ray Spectral Features in a Sample of Bright γ-Ray Burst Afterglows , 2004, astro-ph/0406210.

[17]  Enrico Ramirez-Ruiz,et al.  Beam models for gamma-ray bursts sources: outflow structure, kinematics and emission mechanisms , 2002 .

[18]  D. Eichler,et al.  An Interpretation of the hνpeak-Eiso Correlation for Gamma-Ray Bursts , 2004, astro-ph/0405014.

[19]  Vilspa,et al.  The X-ray afterglow of GRB 030329 , 2003, astro-ph/0305564.

[20]  Ryo Yamazaki,et al.  Cosmological X-Ray Flashes in the Off-Axis Jet Model , 2003 .

[21]  M. Feroci,et al.  A Comparative Study of the X-Ray Afterglow Properties of Optically Bright and Dark Gamma-Ray Bursts , 2002, astro-ph/0212298.

[22]  Usra,et al.  X-ray flashes and X-ray rich Gamma Ray Bursts , 2001, astro-ph/0111246.

[23]  Gregory Y. Prigozhin,et al.  Global Characteristics of X-Ray Flashes and X-Ray-Rich Gamma-Ray Bursts Observed by HETE-2 , 2005 .

[24]  C. Smith.  Additional Pleistocene Fishes from Kansas and Oklahoma , 1958 .

[25]  F. Frontera.  Gamma Ray Bursts in the Afterglow Era , 2003 .

[26]  M. Feroci,et al.  Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts , 2002, astro-ph/0205230.

[27]  M. Feroci,et al.  Prompt and afterglow X-ray emission from the X-Ray Flash of 2002 April 27 , 2004, astro-ph/0407166.

[28]  Ryo Yamazaki,et al.  X-Ray Flashes from Off-Axis Gamma-Ray Bursts , 2002 .

[29]  Tsvi Piran.  The Energy of Gamma-Ray Bursts , 2001 .

[30]  E. Rol,et al.  The host of GRB 030323 at z=3.372: A very high column density DLA system with a low metallicity , 2004, astro-ph/0403080.

[31]  Gregory Y. Prigozhin,et al.  GRB 010921: Localization and Observations by the High Energy Transient Explorer Satellite , 2002 .

[32]  T. Q. Donaghy,et al.  A Unified Jet Model of X-Ray Flashes, X-Ray-rich Gamma-Ray Bursts, and Gamma-Ray Bursts. I. Power-Law-shaped Universal and Top-Hat-shaped Variable Opening Angle Jet Models , 2003, astro-ph/0312634.

[33]  M. Feroci,et al.  Probing the environment in gamma-ray bursts : The case of an X-ray precursor, afterglow late onset, and wind versus constant density profile in GRB 011121 and GRB 011211 , 2004 .

[34]  D. Eichler,et al.  An Interpretation of the hν peakE iso Correlation for GRB , 2004 .

[35]  D. Palmer,et al.  BATSE observations of gamma-ray burst spectra. I: Spectral diversity , 1993 .

[36]  A. S. Fruchter,et al.  The fading optical counterpart of GRB 970228, 6 months and 1 year later , 1998 .

[37]  G. Zamorani,et al.  The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey , 1988 .

[38]  B. Gendre,et al.  X-ray continuum properties of GRB afterglows observed by XMM-Newton and Chandra , 2006 .

[39]  J. P. U. Fynbo,et al.  The line-of-sight towards GRB 030429 at z =2.66: Probing the matter at stellar, galactic and intergalactic scales , 2004, astro-ph/0407439.

[40]  L. A. Antonelli,et al.  GRB 990704: The most X-ray rich BeppoSAX gamma-ray burst , 2001, astro-ph/0108414.

[41]  J. G. Jernigan,et al.  Spectral analysis of 35 GRBs/XRFs observed with HETE-2/FREGATE , 2002 .

[42]  Elena M. Rossi,et al.  Afterglow Lightcurves, Viewing Angle and the Jet Structure of Gamma‐Ray Bursts , 2003 .

[43]  D. J. Helfand,et al.  The Einstein Observatory Detection of Faint X-Ray Flashes , 1996 .

[44]  D. Lazzati,et al.  Afterglow light curves, viewing angle and the jet structure of γ-ray bursts , 2002 .

[45]  J. G. Jernigan,et al.  Global characteristics of X-ray flashes and X-ray rich GRBs observed by HETE-2 , 2004, astro-ph/0409128.

[46]  A. Rau,et al.  The host of GRB/XRF 030528 – an actively star forming galaxy at z = 0.782 , 2005, astro-ph/0508394.

[47]  Heather Ting Ma,et al.  Rebrightening of XRF 030723: Further evidence for a two-component jet in a gamma-ray burst , 2003, astro-ph/0309360.

[48]  Ryo Yamazaki,et al.  Ep-Eiso Correlation in a Multiple Subjet Model of Gamma-Ray Bursts , 2005 .

[49]  J. Chiang,et al.  Fireball Loading and the Blast-Wave Model of Gamma-Ray Bursts , 1998, astro-ph/9804174.

[50]  Xinyu Dai,et al.  Quasi-universal Gaussian Jets: A Unified Picture for Gamma-Ray Bursts and X-Ray Flashes , 2004 .

[51]  S. G. Djorgovski,et al.  The First Two Host Galaxies of X-Ray Flashes: XRF 011030 and XRF 020427 , 2003, astro-ph/0303514.