Searching for differences in Swift's intermediate GRBs

Abstract

Gamma-ray bursts are usually classified through their high-energy emission into short-duration and long-duration bursts, which presumably reflect two different types of progenitors. However, it has been shown on statistical grounds that a third, intermediate population is needed in this classification scheme, although an extensive study of the properties of this class has so far not been done. The large amount of follow-up studies generated during the Swift era allows us to have a suficient sample to attempt a study of this third population through the properties of their prompt emission and their afterglows. Our study is focused on a sample of GRBs observed by Swift during its first four years of operation. The sample contains those bursts with measured redshift since this allows us to derive intrinsic properties. Intermediate bursts are less energetic and have dimmer afterglows than long GRBs, especially when considering the X-ray light curves, which are on average one order of magnitude fainter than long bursts. There is a less significant trend in the redshift distribution that places intermediate bursts closer than long bursts. Except for this, intermediate bursts show similar properties to long bursts. In particular, they follow the Epeak vs. Eiso correlation and have, on average, positive spectral lags with a distribution similar to that of long bursts. Like long GRBs, they normally have an associated supernova, although some intermediate bursts have shown no supernova component. This study shows that intermediate bursts are different from short bursts and, in spite of sharing many properties with long bursts, there are some differences between them as well. We suggest that the physical difference between intermediate and long bursts could be that for the first the ejecta are thin shells while for the latter they are thick shells.

Accepted for publication in Astronomy and Astrophysics. 16 pages, 17 figures, 5 tables

References in corpus (46)

• An online repository of Swift/XRT light curves of GRBs
• Long gamma-ray bursts and core-collapse supernovae have different environments
• An optical supernova associated with the X-ray flash XRF 060218
• No supernovae associated with two long-duration gamma ray bursts
• An enigmatic long-lasting gamma-ray burst not accompanied by a bright supernova
• The Galaxy Population Hosting Gamma-Ray Bursts
• UV star-formation rates of GRB host galaxies
• Making a Short Gamma-Ray Burst from a Long one: Implications for the Nature of GRB 060614
• Swift Identification of Dark Gamma-Ray Bursts
• The Host Galaxies of Swift Dark Gamma-Ray Bursts: Observational Constraints on Highly Obscured and Very High-Redshift GRBs
• Observations of the Naked-Eye GRB 080319B: Implications of Nature's Brightest Explosion
• A Comparison of the Afterglows of Short- and Long-Duration Gamma-Ray Bursts
• The Host Galaxies of Short-Duration Gamma-Ray Bursts: Luminosities, Metallicities, and Star Formation Rates
• The GRB 060218/SN 2006aj event in the context of other Gamma-Ray Burst Supernovae
• Spatially resolved properties of the GRB 060505 host: implications for the nature of the progenitor
• The Connection between Gamma-Ray Bursts and Extremely Metal-Poor Stars: Black Hole-forming Supernovae with Relativistic Jets
• Correlations of Prompt and Afterglow Emission in Swift Long and Short Gamma Ray Bursts
• Deceleration of arbitrarily magnetized GRB ejecta: the complete evolution
• Very different X-ray to optical column density ratios in gamma-ray burst afterglows: ionisation in GRB environments
• A unifying view of Gamma Ray Burst Afterglows
• Hypernova Signatures in the Late Rebrightening of GRB 050525A
• Discovery of the afterglow and host galaxy of the low redshift short GRB 080905A
• Numerical Study on GRB-Jet Formation in Collapsars
• The optical afterglows and host galaxies of three short/hard gamma-ray bursts
• A new type of long gamma--ray burst
• Gamma-Ray Burst afterglows as probes of environment and blastwave physics I: absorption by host galaxy gas and dust
• Further Study of the Gamma-Ray Bursts Duration Distribution
• A Putative Early-Type Host Galaxy for GRB 060502B: Implications for the Progenitors of Short-Duration Hard-Spectrum Bursts
• On the Spectral Lags of the Short Gamma-Ray Bursts
• GRB Energetics in the Swift Era
• GRB 071003: Broadband Follow-up Observations of a Very Bright Gamma-Ray Burst in a Galactic Halo
• The prompt, high resolution spectroscopic view of the "naked-eye" GRB080319B
• Explosive Nucleosynthesis from GRB and Hypernova Progenitors: Direct Collapse versus Fallback
• The complex light-curve of the afterglow of GRB071010A
• The Spectral Lag of GRB060505: A Likely Member of the Long Duration Class
• Testing the Randomness in the Sky-Distribution of Gamma-Ray Bursts
• The nature of the X-Ray Flash of August 24 2005
• The prompt optical/near-infrared flare of GRB 050904: the most luminous transient ever detected
• Classification of BeppoSAX's Gamma-Ray Bursts
• On the Early Time X-ray Spectra of Swift Afterglows I: Evidence for Anomalous Soft X-ray Emission
• GRB 060605: multi-wavelength analysis of the first GRB observed using integral field spectroscopy
• Swift-UVOT captures the earliest ultraviolet spectrum of a Gamma Ray Burst
• The bright optical flash from GRB 060117
• Light Curve Calculations of Supernovae from Fallback Gamma-Ray Bursts
• GRB 050826: A Subluminous Event at z = 0.296 Finds its Place in the Luminosity Distribution of Gamma-Ray Burst Afterglows
• Constraints on an optical afterglow and on supernova light following the short burst GRB 050813