Speaker
Description
Formation rate (FR) history of extragalactic sources provide crucial information on how the universe emerged from the dark ages to its current stage. Best observed FR is that of stars (SFR) that rises from high redshifts to redshift of ~3 and declines to today. Active galactic nuclei (AGNs) show a similar trend but with different parameters. Transient sources like gamma-ray bursts (GRBs), and to some degree fast radio bursts (FRBs), by virtue of their large redshifts are good candidates for determining this history and its relation to SFR.
Methods: Achievement of this task requires determination of
the "Luminosity Function" (LF) and the cosmological
evolution of luminosity and FRs. This is often done by a
forward fitting of the data to some assumed parametric
forms for the LF and evolution, with many parameters, and
often ignoring luminosity evolution. However, recent works
use the non-parametric methods developed by Lynden-Bell and
Efron and Petrosian (EP), which determines the luminosity
evolution directly from "truncated" data. In this talk I
will review past and more recent estimates of LFs and FR
evolution of GRBs (and if time permits FRBs), using these
nonparametric methods.
Data: A critical issue here is to have samples with
measured "fluxes" and redshifts (z) with well defined
truncation determined by observational selection effects,
which we call "complete samples". The main source for GRBs
has been the Neil Gehrels Swift Observatory with some data
from Konus-Wind and Fermi-GBM. The main source for FRBs has
been the CHIME radio telescope. An important uncertainty is
the selection effects of securing redshifts. As I will
discuss use of Machine Learning (ML) to estimates redshifts
can help in this regard.
Theory: An important issue for the transient sources, which are often poorly localized is their progenitors. GRBs are divided into two groups; Long-soft and Short-hard. So- called collapsars are assumed to be the progenitors of GRBs, while SGRBs progenitors are believed to be neutron star (NS)-NS or NS-black hole mergers, which are also
sources of gravitational waves (GWs) and Kilonovae. FRBs
also come in two variety “repeaters” and “non-repeaters”
with magnetars as likely progenitors.
Results: GRBs consistently shown (1) presence of strong
luminosity evolution,a broken power-law LF and varied
formation rate evolution. Collapsars, being short lived
stars are expected to follow the SFR. We find that LGRBs
seem to follow the SFR at high-z’s but deviate
significantly from it, as a low low-z component that
increase with decreasing-z. SGRBs FR increases with
decreasing z’s, similar to the low redshift component of
LGRBs, often modeled by a delayed SFR expected from compact
object mergers. These finding raise doubt about the
standard separation by duration of GRBs and affects the
expected frequency of GWs and Kilonovae.
FRBs also show luminosity evolution and a delayed FR
supporting the magnetar origin.
This work is carried out with extensive collaboration with
Maria Dainotti and several graduate and undergraduate
students.