GRB – Gamma Ray Bursts Gamma Ray Bursts are flashes of gamma rays caused apparently by energetic explosions. They are the brightest known source in the universe. They are followed by a long “afterglow” of the entire spectra. They emits a lot of radiation. But we don’t get all of it…
X-Ray absorption from host The missing radiation from the GRBs is assumed to go either by an obstacle near the source, or by junks of matter (IGM – Inter Galactic Medium) in the way. Absorption from near host obstacle is depended on distance (because energy dependency)
IGM – Inter Galactic Medium The material found scattered in the space between galaxies is called the IGM. The Inter galactic space (IGS) is consisted mostly of vacuum, and have estimated average density of 1 atom per m 3. The IGM is filaments of matter in IGS, and have estimated average density of 100 atoms per m 3. Most of the IGM is ionized.
Optical depth During the journey to the satellite some of the radiation from the GRBs is absorbed in the IGM. We define τ as the optical depth. We have good measurements of τ.
Optical depth measurement
Optical depth (2) In each segment of the way dl what governs the absorption rate is: −The density in this segment n(l) −The cross section for absorption for the materials (Z) in the segment for the specific energy (E(l)).
Optical depth factors - density The density is given by the hydrogen density
Radiative processes We are looking for radiation absorption. Photo excitation – Absorption lines: −The absorption lines have very strong cross section, but have very narrow bandwidth of wavelengths to happen, and there for are neglected Photo ionization: −Happens for energetic enough photons, and has a very wide bandwidth of wavelengths to happen.
Optical depth factors – cross section The cross section in the area of interest is governed by metals. The cross section is approximated as: 0.5 keV
Optical depth factors – cross section The cross section in the area of interest is governed by metals. The cross section is approximated as:
Optical depth factors – metallicity evolution Metals evolve as the universe ages, so we expect to see less metals as we move to higher redshift. A general metallicity evolution is given by:
Optical depth (3) - results Combining all the factors we get:
Data Vs. model (1) – z>4.4
Data Vs. model (2) – z<4.3
Conclusions Low redshift optical depth is dominated by absorption at host galaxy. The IGM can explain the optical depth saturation for high z GRBs (z>2). The redshift of saturation depends on the metallicity of the IGM. If the model is correct then we found the missing baryons
Discussion Does the high metallicity assumption holds? Non-Homogeneous IGM Behavior of Quasars Future missions – high spectral resolution for defining absorber (spectral lines)
Bibliography Behar E., Dado S., Dar A., Laor A., 2011, Can the Soft X-Ray Opacity Toward High- redshift Sources Probe the Missing Baryons? ApJ, 734, 26.