The repeating FRB 121102 (the "repeater") shows repetitive bursting activities and was localized in a host galaxy at z = 0.193. On the other hand, despite dozens of hours of telescope time spent on follow-up observations, no other FRBs have been observed to repeat. Yet, it has been speculated that the repeater is the prototype of FRBs, and that other FRBs should show similar repeating patterns. Using the published data, we compare the repeater with other FRBs in the observed time interval (∆t) -flux ratio (S i /S i+1 ) plane. We find that whereas other FRBs occupy the upper (large S i /S i+1 ) and right (large ∆t) regions of the plane due to the non-detections of other bursts, some of the repeater bursts fall into the lower-left region of the plot (short interval and small flux ratio) excluded by the non-detection data of other FRBs. The trend also exists even if one only selects those bursts detectable by the Parkes radio telescope. If other FRBs were similar to the repeater, our simulations suggest that the probability that none of them have been detected to repeat with the current searches would be ∼ (10 −4 − 10 −3 ). We suggest that the repeater is not representative of the entire FRB population, and that there is strong evidence of more than one population of FRBs.
Gamma ray bursts (GRBs) are classified into long and short categories based on their durations. Broad band studies suggest that these two categories of objects roughly correspond to two different classes of progenitor systems, i.e. compact star mergers (Type I) vs. massive star core collapse (Type II). However, the duration criterion sometimes leads to mis-identification of the progenitor systems. We perform a comprehensive multi-wavelength comparative study between duration-defined long GRBs and short GRBs as well as the so-called "consensus" long GRBs and short GRBs (which are believed to be more closely related to the two types of progenitor systems). The parameters we study include two parts: the prompt emission properties including duration (T 90 ), spectral peak energy (E p ), low energy photon index (α), isotropic γ-ray energy (E γ,iso ), isotropic peak luminosity (L p,iso ), and the amplitude parameters (f and f eff ); and the host galaxy properties including stellar mass (M * ), star formation rate (SFR), metallicity ([X/H]), half light radius (R 50 ), angular and physical (R off ) offset of the afterglow from the center of the host galaxy, the normalized offset (r off = R off /R 50 ), and the brightness fraction F light . For most parameters, we find interesting overlapping properties between the two populations in both 1D and 2D distribution plots. The three best parameters for the classification purpose are T 90 , f eff , and F light . However, no single parameter alone is good enough to place a particular burst into the right physical category, suggesting a need of multiple criteria for physical classification.
Recent arcsecond localizations of fast radio bursts and identifications of their host galaxies confirmed their extragalactic origin. While FRB 121102 resides in the bright region of a dwarf star-forming galaxy, other FRBs reside in more massive galaxies and are related to older stellar populations. We compare the host galaxy properties of nine FRBs with those of several types of stellar transients: from young to old populations, long-duration gamma-ray bursts (LGRBs), superluminous supernovae (SLSNe), SNe Ibc, SNe II, SNe Ia, and short-duration gamma-ray bursts (SGRBs). We find that the stellar mass and star formation rate of the FRB host galaxies, taken as a whole sample, prefer a medium to old population, and are against a young population, similar to LGRBs and SLSNe by a null probability of 0.02. Individually, the host of FRB 121102 is consistent with that of young population objects; the environment of FRB 180924 is similar to that of SGRBs; and the environment of FRB 190523 is similar to those of SNe Ia. These results are consistent with the magnetar engine model for FRBs, if magnetars produced from extreme explosions (GRBs/SLSNe) and those from regular channels (e.g., those producing Galactic magnetars) can both produce FRBs.
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