A HIGH SIGNAL-TO-NOISE RATIO COMPOSITE SPECTRUM OF GAMMA-RAY BURST AFTERGLOWS

Journal Article · Tue Feb 01 00:00:00 EST 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/727/2/73· OSTI ID:21567563

Christensen, L ^[1]; Fynbo, J P. U. ^[2]; Prochaska, J X ^[3]; Thoene, C C; De Ugarte Postigo, A ^[4]; Jakobsson, P ^[5]

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1. Excellence Cluster Universe, Technische Universitaet Muenchen, Bolzmanstrasse 2, 85748 Garching (Germany)
2. Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Oe (Denmark)
3. Department of Astronomy and Astrophysics, UCO/Lick Observatory, University of California, 1156 High Street, Santa Cruz, CA 95064 (United States)
4. INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807, Merate (Italy)
5. Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, IS-107 Reykjavik (Iceland)

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 700-6600 A in the rest frame and has a mean signal-to-noise ratio of 150 per 1 A pixel and reaches a maximum of {approx}300 in the range 2500-3500 A. Equivalent widths are measured from metal absorption lines from the Ly{alpha} line to {approx}5200 A, and associated metal and hydrogen lines are identified between the Lyman break and Ly{alpha} 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 hr 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.

• OSTI ID: 21567563
• Journal Information: Astrophysical Journal, Vol. 727, Issue 2; Other Information: DOI: 10.1088/0004-637X/727/2/73; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABSORPTION
ABSORPTION SPECTRA
AFTERGLOW
COSMIC GAMMA BURSTS
HYDROGEN
LYMAN LINES
METALS
RED SHIFT
SIGNAL-TO-NOISE RATIO
COSMIC RADIATION
DIMENSIONLESS NUMBERS
ELEMENTS
IONIZING RADIATIONS
NONMETALS
PRIMARY COSMIC RADIATION
RADIATIONS
SORPTION
SPECTRA