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Register a new userProbing Intergalactic Neutral Hydrogen by the Lyman Alpha Red Damping Wing of Gamma-Ray Burst 130606A Afterglow Spectrum at z = 5.913
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The unprecedentedly bright optical afterglow of GRB 130606A located by Swift at a redshift close to the reionization era (z = 5.913) provides a new opportunity to probe the ionization status of intergalactic medium (IGM). Here we present an analysis of the red Ly alpha damping wing of the afterglow spectrum taken by Subaru/FOCAS during 10.4-13.2 hr after the burst. We find that the minimal model including only the baseline power-law and HI absorption in the host galaxy does not give a good fit, leaving residuals showing concave curvature in 8400-8900 A with an amplitude of about 0.6% of the flux. Such a curvature in the short wavelength range cannot be explained either by extinction at the host with standard extinction curves, intrinsic curvature of afterglow spectra, or by the known systematic uncertainties in the observed spectrum. The red damping wing by intervening HI gas outside the host can reduce the residual by about 3 sigma statistical significance. We find that a damped Ly alpha system is not favored as the origin of this intervening HI absorption, from the observed Ly beta and metal absorption features. Therefore absorption by diffuse IGM remains as a plausible explanation. A fit by a simple uniform IGM model requires HI neutral fraction of f_HI ~ 0.1-0.5 depending on the distance to the GRB host, implying high f_HI IGM associated with the observed dark Gunn-Peterson (GP) troughs. This gives a new evidence that the reionization is not yet complete at z ~ 6.
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The unprecedentedly bright afterglow of Swift GRB 130606A at z = 5.91 gave us a unique opportunity to probe the reionization era by high precision analyses of the redward damping wing of Ly alpha absorption, but the reported constraints on the neutral hydrogen fraction (f_HI) in intergalactic medium (IGM) derived from spectra taken by different telescopes are in contradiction. Here we examine the origin of this discrepancy by analyzing the spectrum taken by VLT with our own analysis code previously used to fit the Subaru spectrum. Though the VLT team reported no evidence for IGM HI using the VLT spectrum, we confirmed our previous result of preferring non-zero IGM HI (the best-fit f_HI ~ 0.06, when IGM HI extends to the GRB redshift). The fit residuals of the VLT spectrum by the model without IGM HI show the same systematic trend as the Subaru spectrum. We consider that the likely origin of the discrepancy between the two teams is the difference of the wavelength ranges adopted in the fittings; our wavelength range is wider than that of the VLT team, and also we avoided the shortest wavelength range of deep Ly alpha absorption (lambda_obs < 8426 A), because this region is dominated by HI in the host galaxy and the systematic uncertainty about host HI velocity distribution is large. We also study the sensitivity of these results to the adopted Ly alpha cross section formulae, ranging from the classical Lorentzian function to the most recent one taking into account fully quantum mechanical scattering. It is found that the preference for non-zero IGM HI is robust against the choice of the cross section formulae, but it is quantitatively not negligible and hence one should be careful in future analyses.
High-redshift gamma-ray bursts have several advantages for the study of the distant universe, providing unique information about the structure and properties of the galaxies in which they exploded. Spectroscopic identification with large ground-based telescopes has improved our knowledge of the class of such distant events. We present the multi-wavelength analysis of the high-$z$ Swift gamma-ray burst GRB140515A ($z = 6.327$). The best estimate of the neutral hydrogen fraction of the intergalactic medium (IGM) towards the burst is $x_{HI} leq 0.002$. The spectral absorption lines detected for this event are the weakest lines ever observed in gamma-ray burst afterglows, suggesting that GRB140515A exploded in a very low density environment. Its circum-burst medium is characterised by an average extinction (A$_{rm V} sim 0.1$) that seems to be typical of $z ge 6$ events. The observed multi-band light curves are explained either with a very flat injected spectrum ($p = 1.7$) or with a multi-component emission ($p = 2.1$). In the second case a long-lasting central engine activity is needed in order to explain the late time X-ray emission. The possible origin of GRB140515A from a Pop III (or from a Pop II stars with local environment enriched by Pop III) massive star is unlikely.
We demonstrate that gamma-ray burst afterglow spectra and light curves can be calculated for arbitrary explosion and radiation parameters by scaling the peak flux and the critical frequencies connecting different spectral regimes. Only one baseline calculation needs to be done for each jet opening angle and observer angle. These calculations are done numerically using high-resolution relativistic hydrodynamical afterglow blast wave simulations which include the two-dimensional dynamical features of expanding and decelerating afterglow blast waves. Any light curve can then be generated by applying scaling relations to the baseline calculations. As a result, it is now possible to fully fit for the shape of the jet break, e.g. at early time X-ray and optical frequencies. In addition, late-time radio calorimetry can be improved since the general shape of the transition into the Sedov-Taylor regime is now known for arbitrary explosion parameters so the exact moment when the Sedov-Taylor asymptote is reached in the light curve is no longer relevant. When calculating the baselines, we find that the synchrotron critical frequency and the cooling break frequency are strongly affected by the jet break. The synchrotron break temporal slope quickly drops to the steep late time Sedov-Taylor slope, while the cooling break first steepens then rises to meet the level of its shallow late time asymptote.
We report the detection of the most luminous high-redshift Lyman Alpha Emitting galaxy (LAE) yet seen, with log L(Ly alpha) = 43.9 ergs/s. The galaxy -- COSMOS Lyman alpha 1, or COLA1 -- was detected in a search for ultra-luminous LAEs with Hyper Suprime-Cam on the Subaru telescope. It was confirmed to lie at z = 6.593 based on a Lyman alpha line detection obtained from followup spectroscopy with the DEIMOS spectrograph on Keck2. COLA1 is the first very high-redshift LAE to show a multi-component Lyman alpha line profile with a blue wing, which suggests that it could lie in a highly ionized region of the intergalactic medium and could have significant infall. If this interpretation is correct, then ultra-luminous LAEs like COLA1 offer a unique opportunity to determine the properties of the HII regions around these galaxies which will help in understanding the ionization of the z ~ 7 intergalactic medium.
Starting as highly relativistic collimated jets, gamma-ray burst outflows gradually decelerate and become non-relativistic spherical blast waves. Although detailed analytical solutions describing the afterglow emission received by an on-axis observer during both the early and late phases of the outflow evolution exist, a calculation of the received flux during the intermediate phase and for an off-axis observer requires either a more simplified analytical model or direct numerical simulations of the outflow dynamics. In this paper we present light curves for off-axis observers covering the long-term evolution of the blast wave calculated from a high resolution two-dimensional relativistic hydrodynamics simulation using a synchrotron radiation model. We compare our results to earlier analytical work and calculate the consequence of the observer angle with respect to the jet axis both for the detection of orphan afterglows and for jet break fits to the observational data. We find that observable jet breaks can be delayed for up to several weeks for off-axis observers, potentially leading to overestimation of the beaming corrected total energy. When using our off-axis light curves to create synthetic Swift X-ray data, we find that jet breaks are likely to remain hidden in the data. We also confirm earlier results in the literature finding that only a very small number of local Type Ibc supernovae can harbor an orphan afterglow.
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