The distribution of equivalent widths in long GRB afterglow spectra

Abstract

Context. The extreme brightness of gamma-ray burst (GRB) afterglows and their simple spectral shape make them ideal beacons to study the interstellar medium of their host galaxies through absorption line spectroscopy at almost any redshift. Aims. We describe the distribution of rest-frame equivalent widths (EWs) of the most prominent absorption features in GRB afterglow spectra, providing the means to compare individual spectra to the sample and identify its peculiarities. Methods. Using 69 low-resolution GRB afterglow spectra, we conduct a study of the rest-frame EWs distribution of features with an average rest-frame EW larger than 0.5 Å. To compare an individual GRB with the sample, we develop EW diagrams as a graphical tool, and we give a catalogue with diagrams for the 69 spectra. We introduce a line strength parameter (LSP) that allows us to quantify the strength of the absorption features in a GRB spectrum as compared to the sample by a single number. Using the distributions of EWs of single-species features, we derive the distribution of their column densities by a curve of growth (CoG) fit. Results. We find correlations between the LSP and the extinction of the GRB, the UV brightness of the host galaxies and the neutral hydrogen column density. However, we see no significant evolution of the LSP with the redshift. There is a weak correlation between the ionisation of the absorbers and the energy of the GRB, indicating that, either the GRB event is responsible for part of the ionisation, or that galaxies with high-ionisation media produce more energetic GRBs. Spectral features in GRB spectra are, on average, 2.5 times stronger than those seen in QSO intervening damped Lyman-α (DLA) systems and slightly more ionised. In particular we find a larger excess in the EW of C ivλλ1549 relative to QSO DLAs, which could be related to an excess of Wolf-Rayet stars in the environments of GRBs. From the CoG fitting we obtain an average number of components in the absorption features of GRBs of 6.00 +1.00 −1.25. The most extreme ionisation ratios in our sample are found for GRBs with low neutral hydrogen column density, which could be related to ionisation by the GRB emission.

Spectroscopy of the short-hard GRB 130603B

“…To study the absorption properties, we measure the equivalent widths of each of the features in the different spectra, as displayed in Table 3. These absorption features can be compared to the sample of absorption lines in the line of sight of long GRB spectra (Fynbo et al 2009;de Ugarte Postigo et al 2012). A way of doing this is by using the line strength parameter (LSP; de Ugarte Postigo et al 2012).…”

“…The former class has been studied in detail for over 15 years, with optical spectroscopy of their afterglows available for over 200 of them (e.g. Fynbo et al 2009;de Ugarte Postigo et al 2012;Cucchiara et al 2013b). Their broad-band emission is powered by synchrotron radiation, they explode in star forming galaxies, and are accompanied by supernova emission (detected in almost all cases close enough to be studied in detail; Hjorth & Bloom 2012), which establishes that they are produced in the collapse of massive stars.…”

Molecular hydrogen in the damped Lyman αsystem towards GRB 120815A atz= 2.36

“…In addition significant dustdepletion factors (e.g., Savaglio & Fall 2004;Thöne et al 2013;Hartoog et al 2013), large amounts of reddening (e.g., Greiner et al 2011;Zafar et al 2012) and strong metal lines (e.g., Savaglio et al 2003;Christensen et al 2011;de Ugarte Postigo et al 2012) are regularly detected towards GRBs.…”

What can we learn from GRBs?

“…When computed in the source cosmological rest-frame, the plateau and the late power-law decay exhibit new features (see figure 3 (c)): 1) the overlapping of the afterglow phases, which have typical slopes of −1.7 α −1.3 and show a characteristic common power-law behavior [46]; the nested property, which shows that the duration (the luminosity) of the plateau phase is inversely (directly) proportional to the energy of the GRB emission, i.e., the more energetic the source, the smaller (higher) the duration (the luminosity) of the plateau [47]. The use of the overlapping of the afterglows as a distance indicator has been explored by inferring the redshifts of GRB 101023 [34], and has been applied to predict the occurrence of the SN associated to GRB 130427A before its discovery [80], later confirmed by the observations [81][82][83][84]. In the IGC scenario, the FPA originates from the SN ejecta [9,79].…”