Fast Radio Burst and Non-thermal Afterglow
from Binary Neutron Star Mergers
戸谷友則 (TOTANI, Tomonori)
Dept. Astronomy, Univ. Tokyo

Outline•

Three recent papers by my students:•

repeating and non-repeating FRBs from binary neutron star mergers• Yamasaki, TT, & Kiuchi ’18, PASJ, 70, 39•

A new, more natural modeling of electron energy distribution for the nonthermal afterglow of GW 170817• Lin, TT, & Kiuchi ’18, arXiv:1810.02587•IceCube neutrinos from cosmic-rays in star-forming galaxies: a latest
calculation by cosmological galaxy formation model• Sudoh, TT, & Kawanaka ’18, PASJ, 70, 49

• repeating and non-repeating FRBs from binary neutronstar mergers•

Shotaro Yamasaki, TT, & Kiuchi ’18, PASJ, 70, 39

Fast Radio Bursts: A New Transient Population at
Cosmological Distances✦

intrinsic pulse width <~ 1 msec (observedwidth broadened by scattering)✦

event rate ~ 103-4 /sky /day✦large dispersion measure implies z ~ 1Thoronton

Whatʼs the origin of FRBs?
✦✦✦

FRB 121102 is a repeater!
✦most likely a young neutron star✦only one FRB detected by Arecibo (the faintest ﬂux)

✦dwarf, star-forming host galaxy identiﬁed at z = 0.19✦

strong persistent radio ﬂux detected (180 uJy, size < 0.7 pc)✦

only one case of conﬁrmed repeating FRB: a diﬀerent population from others?

some FRBs show low rotation measure (e.g., FRB 150807, Ravi+ʼ16)✦

highly magnetized environment like young supernova remnants or dense star
forming regions not favored✦clean environment such as neutron-star merger?

FRB 171020 does not have any persistent radio counterpart similar to FRB 121102(Mahony+ʼ18)

(non-repeating) FRBs from NS-NS mergers
TT 2013, PASJ, 65, L12✦

FRB rate vs. NS-NS merger rate✦

FRB rate 103-104 /day/sky at z~1 is roughly 103-104 /Gpc3/yr at z=0✦✦c.f. short GRBs ~1-10 /Gpc3/yr✦

high end of NS-NS merger rate estimate before GW 170817✦

now NS-NS rate 1540+3200-1220 /Gpc3/yr (LVC ʼ17 PRL 119, 161101)

predicted radio ﬂux by dipole radiation is similar to FRBs, if✦

dipole with B ~ 1012 G and r ~ 10 km✦

rotation period ~ msec✦

radio conversion eﬃciency similar to pulsars (~10-4)

NS-NS merger ejecta vs. radio emission✦

10-3~10-2 M◉ ejecta expected from merger✦

no radio emission if they are absorbed by thick ejecta?

NS-NS merger simulation by K. Kiuchi

ejecta proﬁle in merger simulation✦

ejecta appears at r > 30 km only ~ 1 msec after the spin of merged star becomesmaximum✦

There is a time window (1-2 msec) to produce a FRB before hidden by ejecta✦

ejecta formation gives a possible explanation for no repeating bursts for many FRBs

Yamasaki, TT, & Kiuchi ’17

orbital rotation

spin of NSs

ejecta column densityrepeating FRB from NS-NS mergers
Yamasaki, TT, & Kiuchi ’17✦

a long-lived massive NS may be left after a fraction of NS-NS mergers, depending
on EOS✦

event rate of NS-NS mergers much (~100x) higher than SLSN rate (40 /Gpc3/yr)✦merger ejecta becomes transparent in 1-10 yrs to radio signals✦✦

c.f. ~10-100 yrs for supernova scenario

repeating burst rate of FRB 121102 broadly consistent with NS-NS merger rate if
the repeater life time is ~10 yrs✦

persistent radio emission from pulsar wind nebular interacting with merger ejecta✦

prediction:✦✦✦

ejecta much faster than supernova ̶> source size evolution may be seen for
FRB 121102 in the future
repeating FRBs also from elliptical/passive galaxies
a repeating FRB appears ~10 yrs following a fraction of NS-NS mergers detectedby GW

• A new, more natural modeling of electron energy

distribution for the non-thermal afterglow of GW 170817•Haoxiang Lin, TT, & Kiuchi ’18, arXiv:1810.02587

nonthermal afterglow of GW170817

Lin+’18••••

synchrotron emission from accelerated electrons in (mildly) relativistic shock, like GRB
afterglow
radially stratified spherical shell or off-axis and angularly extended jet
best-fit by previous studies is single-power-law from radio to X-rays in all timemeans all observed frequencies above νm (corresponding to the minimum electron energy)

synchrotron tail?
•observed data of GW170817 do not show clear evidence of the synchrotron tail(ν < νm)Sari+’98

previous models, and our work•

previous studies all assumed that all electrons in the shock are accelerated as nonthermal particles!
• following standard GRB afterglow modelings (e.g. Sari+’98)• energy fraction of accelerated electrons is controlled by the minimum energy ofthe electron energy distribution• simple, but obviously unphysical (c.f. supernova remnants)•This work:• add a new, but natural model parameter, so that nonthermal electron energy
fraction is variable• total number of nonthermal electrons: corresponds to the acceleration
efficiency• the minimum electron energy: corresponds to the electron-ion equipartition

MCMC fits•with two standard
geometrical models:• radially stratified
spherical outflow• off-axis, angularly
extended jetNew solution in synchrotron tail!

Main Results•a more natural electron energy distribution leads to the synchrotron tail in earlyradio bands!
• confirming only a small fraction is accelerated to nonthermal• close to electron-ion equipartition•

low-frequency early radio observations highly encouraged in future events• would give important information for:• particle acceleration efficiency• electron-ion equipartition•

jet energy ~ 1052 erg (isotropic-equivalent to the jet direction), about 10 times
larger than the conventional modeling• still consistent with the distribution of the short GRB energy distribution•

ambient matter density n ~ 10-3-10-2 cm-3, about 10 times larger than theconventional modeling• consistent with the hot gas density in typical giant elliptical galaxies

•

IceCube neutrinos from cosmic-rays in star-forming galaxies:
a latest calculation by cosmological galaxy formation model•

Takahiro Sudoh, TT, & Kawanaka ’18, PASJ, 70, 49

Main Points of This Work•

Neutrinos produced by cosmic-ray interaction in star-forming galaxies should havesome contribution to IceCube neutrinos, but its fraction is controversial•

We present a new model of gamma-ray and neutrino emission from a star-forming
galaxy, from the quantities of (1) stellar mass, (2) gas mass, (3) star formation rate,and (4) disk radius.

•This model nicely reproduces gamma-ray luminosities of nearby galaxies detected byFermi, from dwarfs to starbursts.
• → good calibration for the prediction of neutrino flux•

This model is combined with a semi-analytical galaxy formation model in LambdaCDM cosmology to predict neutrino background from star-forming galaxies•It is extremely difficult to explain the IceCube neutrinos by star-forming galaxies inthe standard picture of galaxy formation.

modeling gamma-ray and neutrino emission•CR production rate: ∝ SFR
CR energy spectrum: power-law with index Γ•target ISM gas density: from Mgas, Reff, and disk scale height H∝Reff•

velocity scale in the disk: virial equilibrium along the disk height• G (Mgas + Mstar) / Reff2 ~ σ2 / H•

CR diffusion and escape• energy-dependent diffusion coefficient:• RL < l0 → Kolomogorov turbulence• RL > l0 → small angle scattering• RL > (H l0)1/2 → free streaming• l0: coherent length of turbulence, assumed to be 10 pc from observations•magnetic field• equilibrium with the energy density injected by star formation within the dynamical
time scale H/σ•→ you can calculate luminosity and spectrum of gamma-ray and neutrinos via pion
production in ISM•

Calibration by gamma-rays from Nearby galaxies
OutputSFRInput•

6 nearby galaxies with good measurements of gamma-ray luminosity from CRinteractions•including various types (dwarfs to starbursts)•

good measurements of galaxy properties: star formation rate (SFR), gas mass(Mgas), stellar mass (Mstar), disk effective radius (Reff)•Can we make a physical model to predict gamma-ray luminosity from galaxy
properties for these galaxies?

Comparison with nearby galaxies••

our model reproduces gamma-ray luminosities fairly well!
better than the simple Lgamma ∝ SFR or Lgamma ∝ SFR x Mgas

cosmological galaxy formation model•

use a semi-analytic model ofhierarchical galaxy formation in theCDM framework• Nagashima & Yoshii ’04• gives necessary inputs (SFR,Mstar, Mgas, size)•reproduces local galaxy statistics(luminosity function, luminositysize relation, etc.)•tested against various high-z galaxy
data set (e.g. Ly-break galaxies)•major mergers produce starburst
galaxiesneutrino background•accounts for only 0.4% of IceCube
neutrinos with the plausible Γinj~2.3!

•15% even if we take an extreme
value of Γinj=2 for all galaxies

solid: alldashed: SBG