GRB redshifts from prompt emission SEDs

• Recently the very first detection of γ-ray resonant absorptions along the line of sight toward γ-ray bright quasars (QSOs), like 3C273, 3C279, PKS0528+0134 and BL Lacertae were reported. These detections resulted from the analysis of absorption troughs in SEDs derived on the base of mostly EGRET data that were collected during monitoring campaigns of the Virgo and galactic anticenter regions by the Compton Gamma Ray Observatory (CGRO), as well as during ToO observations of QSOs flares. Among three resonant absorption mechanisms that affect the γ-ray spectrum of point-like sources we point out the Δ-isobar resonance that has a very stable peak-energy of the absorption cross-section for all elements (nuclei), and for individual nucleons. By fitting the SED of γ-ray bright point-sources with a high-energy component, including long duration gamma-ray bursts (GRBs), two absorbers can be detected on the sight lines towards such a γ-ray bright point-source. One of those absorbers is at the GRB rest frame, and, therefore, the value of the GRB redshift can be found by fitting an appropriate trough in the SED. A distinct component of GRBs due to the synchrotron self-Compton (SSC) process is expected to be responsible for the significant GRB emission at GeV and TeV energies. Derishev, Aharonian and Kocharovsky suggested that this GeV-TeV emission is observable over a much broader range of angles relative to the direction of the GRB jet. Such GeV-TeV emission will have much harder spectrum and a much higher cut-off frequency compared to the values derived from the Doppler boosting considerations. These factors boost also our expectations for using GRB prompt emission to derive the redshift of host galaxy. Below we discuss a potential application of the gamma-ray absorption method (GRAM) for derivation of the redshift distribution of the long GRBs having a high-energy component, like GRB910503, GRB930131, GRB930506, GRB941017, and GRB980923. Successful application of this method will dramatically broaden the scope of cosmological studies with the use of GRBs.

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NATURVETENSKAP  -- Fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences (hsv//eng)

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Gamma-ray bursts

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