A Possible Subclassification of Fast Radio Bursts

Abstract

Although fast radio bursts (FRBs) have been an active field in astronomy and cosmology, their origin is still unknown to date. One of the interesting topics is the classification of FRBs, which is closely related to the origin of FRBs. Different physical mechanisms are required by different classes of FRBs. In the literature, they usually could be classified into non-repeating and repeating FRBs. Well motivated by the observations, here we are interested in the possible subclassification of FRBs. By using the first CHIME/FRB catalog, we propose to subclassify non-repeating (type I) FRBs into type Ia and Ib FRBs. The distribution of type Ia FRBs is delayed with respect to the cosmic star formation history (SFH), and hence they are probably associated with old stellar populations, while the distribution of type Ib FRBs tracks SFH, and hence they are probably associated with young stellar populations. Accordingly, the physical criteria for this subclassification of type I FRBs have been clearly determined. We find that there are some tight empirical correlations for type Ia FRBs but not for type Ib FRBs, and vice versa. These make them different in physical properties. Similarly, we suggest that repeating (type II) FRBs could also be subclassified into type IIa and IIb FRBs. A universal subclassification scheme is given at the end. This subclassification of FRBs might help us to reveal quite different physical mechanisms behind them, and improve their applications in astronomy and cosmology.

Could Fast Radio Bursts Be Standard Candles?

“…Since the distance along the path in DM records the expansion history of the universe, it plays a key role in the FRB cosmology. The observed DM of FRB at redshift z can be separated into [8][9][10][11][12][13][14][15][16] DM obs = DM MW + DM halo + DM IGM + DM host /(1 + z) ,…”

“…Obviously, the main contribution to DM of FRB comes from IGM. The mean of DM IGM is given by [8][9][10][11][12][13][14][15][16] DM…”

“…Thus, DM IGM = DM IGM for any observed FRB, since the plasma density fluctuates along the line of sight [17][18][19][20] (see also e.g. [8][9][10][11][12][13][14][15][16]). In the literature, the deviation of DM IGM from DM IGM is usually characterized by the uncertainty σ IGM (z), which is a function of redshift z in principle.…”

“…So, one can only consider various empirical σ IGM (z) in the literature, which is usually large (∼ O(10 2 ) to O(10 3 ) pc cm −3 ) [17][18][19][20] (see also e.g. [8][9][10][11][12][13][14][15][16]). In fact, the large uncertainty σ IGM (z) in DM IGM is one of the main troubles in the FRB cosmology.…”

“…Note that the host galaxy of FRB and hence its contribution to DM are also poorly known. Therefore, DM host was extensively assumed to be constant in the literature [8][9][10][11][12][13][14][15][16]. Of course, this simplification prevents us from understanding the cosmic expansion history better.…”