A High Signal-to-Noise Ratio Composite Spectrum of Gamma-Ray Burst Afterglows

Abstract

We present a composite spectrum of 60 long duration gamma-ray burst (GRB) afterglows with redshifts in the range 0.35 < z < 6.7 observed with low resolution optical spectra. The composite spectrum covers the wavelength range 700-6600Å in the rest frame and has a mean signal-to-noise ratio of 150 per 1Å pixel and reaches a maximum of ∼300 in the range 2500-3500Å. Equivalent widths are measured from metal absorption lines from the Lyα line to ∼5200Å, and associated metal and hydrogen lines are identified between the Lyman break and Lyα line. The average transmission within the Lyman forest is consistent with that found along quasar lines of sight. We find a temporal variation in fine-structure lines when dividing the sample into bursts observed within 2 hours from their trigger and those observed later. Other lines in the predominantly neutral gas show variations too, but this is most likely a random effect caused by weighting of individual strong absorption lines and which mimics a temporal variation. Bursts characterized with high or low prompt GRB energy release produce afterglows with similar absorption line strengths, and likewise for bursts with bright or faint optical afterglows. Bursts defined as dark from their optical to X-ray spectral index have stronger absorption lines relative to the optically bright bursts. The composite spectrum has strong Ca ii and Mg ii absorption lines as commonly found in dusty galaxies, however, we find no evidence for dust or a significant molecular content based on the non-detection of diffuse interstellar bands. Compared to starburst galaxy spectra, the GRB composite has much stronger fine-structure lines, while metal absorption lines are weaker.

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

“…The dashed lines show synthetic H 2 models corresponding to the 1σ errors on the measured molecular content. In the top panel, we also mark prominent metal absorption lines previously detected in GRB-DLAs (Christensen et al 2011) and those of the intervening systems. In the lower panels, J0, J1, and so forth denote transitions from the J = 0, J = 1 rotational level of the vibrational ground state of the Lyman-Werner bands of H 2 .…”

“…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.…”

Unveiling the population of orphanγ-ray bursts

“…A final identification is given by optical spectroscopy. The spectral continuum and the absorption lines present in the optical-near-IR (NIR) spectra of the afterglow are very different from those typically found in SNe or blazars (Fynbo et al 2009;Christensen et al 2011). Nonetheless, spectra with a sufficient signal-to-noise ratio are needed.…”

An X-Shooter composite of bright 1 <z< 2 quasars from UV to infrared

“…Examples of such composite spectra include template spectra of various classes of galaxies (Mannucci et al 2001;Shapley et al 2003;Dobos et al 2012), quasi-stellar object (QSO; Cristiani & Vio 1990;Boyle 1990;Francis et al 1991;Zheng et al 1997;Brotherton et al 2001;Vanden Berk et al 2001;Telfer et al 2002;Richards et al 2006a;Glikman et al 2006;Lusso et al 2015) and gamma-ray burst (GRB) afterglows (Christensen et al 2011). When investigating dust in QSOs or in galaxies along QSO sightlines, template spectra can be used to determine the amount of extinction by artificially reddening the template to match an observed spectrum (e.g., Glikman et al 2007;Urrutia et al 2009;Wang et al 2012;Fynbo et al 2013;Krogager et al 2015).…”