Broad and Shifted Iron-Group Emission Lines in Gamma-Ray Bursts as Tests of the Hypernova Scenario

In the hypernova/collapsar model of γ-ray bursts, it is natural that radiation is emitted by the inner engine for some time after the burst. This has been discussed as a possible source of the X-ray line emission observed in some afterglows. We show here that the natural geometry of a hypernova—a source of radiation at the bottom of a deep funnel—has very significant consequences for the shape and central energy of the observed emission lines; the lines acquire a very broad scattering wing on the low-energy side and a characteristic second peak 1 Compton wavelength away from the initial energy due to the once- and twice-scattered photons. We suggest that this explains the large width of the observed lines. Furthermore, the downscattering lowers the central line energy (by up to 1 keV in the source rest frame, before the lines become unrecognizable) so that the observed line energies can become consistent with originating from cobalt and nickel, as expected in few-day-old supernova ejecta.

[1]  E. Rol,et al.  From X-Ray Binaries to Gamma-Ray Bursts , 2003 .

[2]  S. Djorgovski,et al.  The Observed Offset Distribution of Gamma-Ray Bursts from Their Host Galaxies: A Robust Clue to the Nature of the Progenitors , 2000, astro-ph/0010176.

[3]  Maarten Schmidt,et al.  Luminosity Function of Gamma-Ray Bursts Derived without Benefit of Redshifts , 2001, astro-ph/0101163.

[4]  Mario Vietri,et al.  Illuminated, and Enlightened, by GRB 991216 , 2000, astro-ph/0011580.

[5]  Titus J. Galama,et al.  High Column Densities and Low Extinctions of Gamma-Ray Bursts: Evidence for Hypernovae and Dust Destruction , 2000, astro-ph/0009367.

[6]  James R. Wilson,et al.  Gamma-Ray Bursts via the Neutrino Emission from Heated Neutron Stars , 2000, astro-ph/0002312.

[7]  D. Frail,et al.  Observation of X-ray lines from a gamma-ray burst (GRB991216): evidence of moving ejecta from the progenitor. , 2000, Science.

[8]  L. A. Antonelli,et al.  Discovery of a transient absorption edge in the X-ray spectrum of GRB 990705. , 2000, Science.

[9]  L. A. Antonelli,et al.  Discovery of a Redshifted Iron K Line in the X-Ray Afterglow of GRB 000214 , 2000, astro-ph/0010221.

[10]  J. van Paradijs,et al.  UvA-DARE ( Digital Academic Repository ) Gamma-Ray Burst afterglows , 2000 .

[11]  M. Boettcher,et al.  Line Emission from Gamma-Ray Burst Environments , 1999, astro-ph/9912030.

[12]  S. Djorgovski,et al.  The unusual afterglow of the γ-ray burst of 26 March 1998 as evidence for a supernova connection , 1999, Nature.

[13]  Chris L. Fryer,et al.  Black Hole-Neutron Star Mergers as Central Engines of Gamma-Ray Bursts , 1999, The Astrophysical journal.

[14]  E. Rol,et al.  Evidence for a Supernova in Reanalyzed Optical and Near-Infrared Images of GRB 970228 , 1999, astro-ph/9907264.

[15]  D. Reichart.  GRB 970228 Revisited: Evidence for a Supernova in the Light Curve and Late Spectral Energy Distribution of the Afterglow , 1999, astro-ph/9906079.

[16]  T. Bulik,et al.  Distribution of compact object mergers around galaxies , 1999, astro-ph/9903407.

[17]  M. Feroci,et al.  The X-Ray Afterglow of the Gamma-Ray Burst of 1997 May 8:Spectral Variability and Possible Evidence of an Iron Line , 1999, astro-ph/9902013.

[18]  T. Piran.  Gamma-ray bursts and the fireball model , 1998, astro-ph/9810256.

[19]  C. Kouveliotou,et al.  The Intensity Distribution of Faint Gamma-Ray Bursts Detected with BATSE , 1998, astro-ph/9809300.

[20]  California Institute of Technology,et al.  Gamma-Ray Bursts and the Cosmic Star Formation Rate , 1998, astro-ph/9807117.

[21]  B. Paczyński.  Are Gamma-Ray Bursts in Star-Forming Regions? , 1997, astro-ph/9710086.

[22]  J. Bloom,et al.  Gamma-ray bursts from stellar remnants - Probing the universe at high redshift , 1997, astro-ph/9708183.

[23]  J. Bloom,et al.  The Spatial Distribution of Coalescing Neutron Star Binaries , 1998, astro-ph/9805222.

[24]  D. Frail,et al.  The radio afterglow from the γ-ray burst of 8 May 1997 , 1997, Nature.

[25]  T. Totani.  Cosmological Gamma-Ray Bursts and Evolution of Galaxies , 1997, astro-ph/9707051.

[26]  S. Djorgovski,et al.  Spectral constraints on the redshift of the optical counterpart to the γ-ray burst of 8 May 1997 , 1997, Nature.

[27]  L. A. Antonelli,et al.  Discovery of an X-ray afterglow associated with the γ-ray burst of 28 February 1997 , 1997, Nature.

[28]  C. Kouveliotou,et al.  Transient optical emission from the error box of the γ-ray burst of 28 February 1997 , 1997, Nature.

[29]  M. Rees,et al.  Shocked by GRB 970228: the afterglow of a cosmological fireball , 1997, astro-ph/9704153.

[30]  M. Rees,et al.  Optical and Long-Wavelength Afterglow from Gamma-Ray Bursts , 1996, astro-ph/9606043.

[31]  S. Woosley,et al.  The Evolution and Explosion of Massive Stars. II. Explosive Hydrodynamics and Nucleosynthesis , 1995 .

[32]  C. Kouveliotou,et al.  Identification of two classes of gamma-ray bursts , 1993 .

[33]  S. Woosley.  Gamma-ray bursts from stellar mass accretion disks around black holes , 1993 .

[34]  M. Hernanz,et al.  Gamma-ray bursts as collimated jets from neutron star/black hole mergers , 1993, Nature.

[35]  M. Rees,et al.  Relativistic fireballs: energy conversion and time-scales , 1992 .

[36]  M. Livio,et al.  Nucleosynthesis, neutrino bursts and γ-rays from coalescing neutron stars , 1989, Nature.

[37]  R. McCray,et al.  Comptonization of X-rays by low-temperature electrons. [photon wavelength redistribution in cosmic sources , 1978 .