On the Subclasses in Swift Long Gamma-Ray Bursts: A Clue to Different Central Engines

Abstract

Analyzing light curves of a complete sample of bright Swift long gamma-ray bursts (LGRBs) of which the peak photon fluxes constructed with the bin width of 1 second in the Swift 15-350 keV energy band exceed 2.6 photons cm${}^{-2}$s${}^{-1}$, we confirm that there do exist the third class in GRBs in addition to short and long GRBs. Being different from previous works based on the duration, fluence, etc. our classification method is based on two properties both quantified with light curve shapes of the prompt emission: the Absolute Deviation from the Constant Luminosity of their cumulative light curve $ADCL$, and the ratio of the mean counts to the maximum counts $\bar{C}/C_{\max }$. These are independent of the distance and the jet opening angle. A cluster analysis via the Gaussian mixture model detects three subclasses: one consisting of LGRBs with small $ADCL$ and large $\bar{C}/C_{\max }$ values referred to as Type I, one with large $ADCL$ and large $\bar{C}/C_{\max }$ referred to as Type II, and one with intermediate $ADCL$ and small $\bar{C}/C_{\max }$, which is composed of contaminating short GRBs with the extended emission. This result is reinforced by different temporal and spectral indices of their X-ray afterglows. The difference is prominent in the temporal index of the steep decay phase in particular: the indices for Type I LGRBs distribute between $-6$ and $-3$ while those for Type II LGRBs between $-3$ and $-2$. From these properties, we propose a possible scenario with different central engines: an accreting black hole and a magnetar.

22 pages, 16 figures. Accepted for publication in PASJ

References in corpus (12)

• The Hubble Diagram to Redshift >6 from 69 Gamma-Ray Bursts
• A complete sample of bright Swift Long Gamma-Ray Bursts: Sample presentation, Luminosity Function and evolution
• A Cosmology-Independent Calibration of Gamma-Ray Burst Luminosity Relations and the Hubble Diagram
• Relativistic Jets and Long-Duration Gamma-ray Bursts from the Birth of Magnetars
• A new definition of the intermediate group of gamma-ray bursts
• A unifying view of Gamma Ray Burst Afterglows
• Gamma-Ray Bursts in $1.8<z<5.6$ Suggest that the Time Variation of the Dark Energy is Small
• Prior Emission Model for X-ray Plateau Phase of Gamma-Ray Burst Afterglows
• Classification of Swift's gamma-ray bursts
• Classification of BeppoSAX's Gamma-Ray Bursts
• Cosmological Constraints from calibrated Yonetoku and Amati relation implies Fundamental plane of Gamma-ray bursts
• Identifying Subclasses of Long Gamma-Ray Bursts with Cumulative Light Curve Morphology of Prompt Emissions

Cited by in corpus (9)

• A comprehensive statistical study on gamma-ray bursts
• Gaussian-mixture-model-based cluster analysis of gamma-ray bursts in the BATSE catalog
• Some statistical remarks on the Giant GRB Ring
• Classifying GRB170817A / GW170817 in a Fermi duration - hardness plane
• Distinction of groups of gamma-ray bursts in the BATSE catalog through fuzzy clustering
• Multivariate analysis of BATSE gamma-ray burst properties using skewed distributions
• Multidimensional analysis of Fermi GBM gamma-ray bursts
• GRB Optical and X-ray Plateau Properties Classifier Using Unsupervised Machine Learning
• Discovery of a Universal Correlation For Long and Short GRBs and Its Application on the Study of Luminosity Function and Formation Rate