LOW-RESOLUTION SPECTROSCOPY OF GAMMA-RAY BURST OPTICAL AFTERGLOWS: BIASES IN THE SWIFT SAMPLE AND CHARACTERIZATION OF THE ABSORBERS

By Johan Peter Uldall Fynbo, P. Jakobsson, J. Xavier Prochaska, D. Malesani, C. Ledoux, A. D. U. Postigo, M. Nardini, P. Vreeswijk, K. Wiersema, J. Hjorth and 32 more. 2009

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We present a sample of 77 optical afterglows (OAs) of Swift detected gamma-ray bursts (GRBs) for which spectroscopic follow-up observations have been secured. Our first objective is to measure the redshifts of the bursts. For the majority (90%) of the afterglows, the redshifts have been determined from the spectra. We provide line lists and equivalent widths (EWs) for all detected lines redward of Lyα covered by the spectra. In addition to the GRB absorption systems, these lists include line strengths for a total of 33 intervening absorption systems. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray-selected statistical sample of Swift bursts with optimal conditions for ground-based follow-up from the period 2005 March to 2008 September; 146 bursts fulfill our sample criteria. We derive the redshift distribution for the statistical (X-ray selected) sample and conclude that less than 18% of Swift bursts can be at z > 7. We compare the high-energy properties (e.g., γ-ray (15–350 keV) fluence and duration, X-ray flux, and excess absorption) for three subsamples of bursts in the statistical sample: (1) bursts with redshifts measured from OA spectroscopy; (2) bursts with detected optical and/or near-IR afterglow, but no afterglow-based redshift; and (3) bursts with no detection of the OA. The bursts in group (1) have slightly higher γ-ray fluences and higher X-ray fluxes and significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fractions of dark bursts, defined as bursts with an optical to X-ray slope βOX < 0.5, is 14% in group (1), 38% in group (2), and >39% in group (3). For the full sample, the dark burst fraction is constrained to be in the range 25%–42%. From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight lines. This should be taken into account when determining, e.g., the redshift or metallicity distribution of GRBs and when using GRBs as a probe of star formation. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular the damped Lyα absorbers (DLAs). On average GRB absorbers are characterized by significantly stronger EWs for H i as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. However, the distribution of line strengths is very broad and several GRB absorbers have lines with EWs well within the range spanned by QSO-DLAs. Based on the 33 z > 2 bursts in the sample, we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e35f51167909c0fbda8c49142bf30297e258e294

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A POPULATION OF MASSIVE, LUMINOUS GALAXIES HOSTING HEAVILY DUST-OBSCURED GAMMA-RAY BURSTS: IMPLICATIONS FOR THE USE OF GRBs AS TRACERS OF COSMIC STAR FORMATION (2013, background): “GRB 090417B GRB 090417B was a moderately bright but very long burst; its prominent X-ray afterglow was immediately recognized to be coincident with an SDSS galaxy (Fynbo et al. 2009a) whose redshift was measured early to be 0.345 (Berger & Fox 2009).”
Swift J1112.2−8238: a candidate relativistic tidal disruption flare (2015, background): “If the emission mechanisms were similar to GRBs this would identify the counterpart of Swift J1112-8238 as a dark burst, and would imply significant extinction (Fynbo et al. 2009; Perley et al. 2013), the correction for which would make the afterglow the brightest seen.”
THE HOST GALAXIES OF GAMMA-RAY BURSTS. II. A MASS–METALLICITY RELATION FOR LONG-DURATION GAMMA-RAY BURST HOST GALAXIES (2010, background): “As a result, these surveys are limited in their ability to sample “dark” LGRBs; Fynbo et al. (2009) estimate an overall dark burst fraction of 25% - 42% based on their survey of 77 Swift LGRBs.”
THE OPTICALLY UNBIASED GAMMA-RAY BURST HOST (TOUGH) SURVEY. I. SURVEY DESIGN AND CATALOGS (2012, background): “…2011c), 12 http://www.swift.ac.uk/xrt_curves/ Tanvir et al. (2008), Thöne et al. (2008, 2010), Jaunsen et al. (2008), Xu et al. (2009), Elı́asdóttir et al. (2009), Ferrero et al. (2009), Fynbo et al. (2009, 2008a), Malesani et al. (2009, 2010), Svensson et al. (2012), and Schulze et al. (2012).”
A RADIO-SELECTED SAMPLE OF GAMMA-RAY BURST AFTERGLOWS (2011, background): “However, Fynbo et al. (2009) have measured the average redshift of Swift sample to be 〈z〉 = 2.2, based on a sample of 146 Swift bursts, which is closer to the mean redshift of our radio-selected sample.”
THE AFTERGLOW OF GRB 130427A FROM 1 TO 1016 GHz (2013, background): “…of long-duration gamma-ray bursts (GRBs) detected by orbiting satellites are extremely energetic events originating from the distant universe: the mean redshift among Swift GRBs is z ≈ 2.0, and ∼ 80% of Swift events originate from z > 1 (e.g., Jakobsson et al. 2006, 2012; Fynbo et al. 2009).”
Evolution of the AGN UV luminosity function from redshift 7.5 (2018, background): “Statistical constraints from H I column density measurements in gamma-ray burst (GRB) afterglow spectra suggest an even lower escape fraction of 0.5% (Chen et al. 2007; Fynbo et al. 2009; Tanvir et al. 2018).”
Spectroscopy of superluminous supernova host galaxies. A preference of hydrogen-poor events for extreme emission line galaxies (2014, background): “In addition, there is evidence supporting that H-poor SLSN environments are more extreme and have a harder ionization field than those of GRBs.”
STELLAR POPULATIONS OF Lyα EMITTERS AT z ∼ 6–7: CONSTRAINTS ON THE ESCAPE FRACTION OF IONIZING PHOTONS FROM GALAXY BUILDING BLOCKS (2010, methodology): “…of star-forming galaxies (e.g., Steidel et al. 2001; Shapley et al. 2006; Iwata et al. 2009) and the distribution of neutral hydrogen column densities in the afterglow spectra of long duration GRBs (e.g., Chen et al. 2007; Fynbo et al. 2009), although the results do not agree well with each other.”
The luminosity function and the rate of Swift's Gamma Ray Bursts (2009, background): “Recently, Fynbo et al. (2009) obtained emission lines redshift measurements for GRBs host galaxies that were not measured before.”
THE HOST GALAXIES OF SWIFT DARK GAMMA-RAY BURSTS: OBSERVATIONAL CONSTRAINTS ON HIGHLY OBSCURED AND VERY HIGH REDSHIFT GRBs (2009, background): “An alternate hypothesis for the galaxy detected in our imaging associates it with the z = 0.8104 Mg II absorber in Table 54 of Fynbo et al. (2009), rather than the true host.”
A complete sample of bright Swift long gamma-ray bursts: testing the spectral-energy correlations (2011, background): “Finally, the redshift of this source isz = 0.346 (Fynbo et al. 2009).”
THE COSMIC RATE, LUMINOSITY FUNCTION, AND INTRINSIC CORRELATIONS OF LONG GAMMA-RAY BURSTS (2009, background): “This is good in the sense that our rate estimates will be largely independent of those based primarily on afterglow or host galaxy studies (e.g., Perley et al. 2009a; Fynbo et al. 2009).”
The metallicity of the long GRB hosts and the fundamental metallicity relation of low-mass galaxies (2010, background): “It is still not known if this dust is spread across the host galaxy, which thus is likely to be metal-rich (Fynbo et al. 2009), or is directly associated to the GRB itself (Perley et al. 2009), maybe without a clear dependece on metallicity.”