1. GAMMA-RAY BURSTS&2. FAST RADIO BURSTS
WITH

TAM, Pak Hin(Sun Yat-sen University/ICRR)GAMMA-RAY BURST OBSERVATIONSWITH CTALESSONS LEARNT FROM FERMI/LAT
TAM, Pak Hin (Sun Yat-sen University/ICRR, U Tokyo)

Kashiwanoha, ICRR
15/12/2016

What are GRBs?
Intense bursts of gamma-rays
Duration: ~10ms - hundreds of seconds
happen at a random position on the sky
never repeat

Burst spectra: “Band” function
Most GRB spectra canbe fit by the Band (1993)
function

which is aphenomenologicalfunction without being
motivated by any theory.

GRB 970228, van Paradijs et al. (1997)

Afterglow
GRB 970508

synchrotron spectrum

Decays connected by PLs, from optical,X-rays to GeV, GRB 090510

XRT afterglow light curves

A “canonical” X-ray decay
light curve without flares

Strong late-time activities,
thought to be central engine

X-ray flares can be IC up-scattered to very high energy gamma-rays

The Fireball Model
gamma-rays

X-ray, optical, radio afterglow

Photosphere

Prompt emission(short-lived, seconds)

Afterglow emission (long-lasting,up to days to weeks)c.f. Piran (2004)

What does Fermi see?

LAT does not see GeV emission from most GRBs
(see, e.g., Ackermann et al., 2012)

GRB 080916C

H.E.S.S.
MAGIC
VERITAS
CTA

HXMT
HE

HAWC
LHASSOMWL/GeV photons during
GRB afterglow
GRB 090510

GRB 090926A
GBM

LAT

Abdo et al. (2010)

Swenson et al. (2010)

LAT (Fermi): 100 MeV-100 GeV

CTA: >10 GeV

Fermi vs ground-based tels
Fermi-LAT is able to, and did see photons above 10 GeV,
most notably a 27.4 GeV photon from GRB 080916C (atz=4.35, it is a 147 GeV one at origin), and a 95 GeV
photon from GRB 130427A
As time passes, Fermi-LAT will see more of them, butwill find it hard to obtain good photon number statistics
for spectral and light curve construction.
Ground-based gamma-ray detectors have much larger
eﬀective collection area.

Fermi-CTA sensitivity

GRB

Very bright GRB 130427A
GRB 130427A emits many high-energy gamma-rays during theprompt & afterglow period, T90~138s

95 GeV73 GeVa 95 GeV photon arrived at T0 + 243s,
corresponding to an intrinsic
photon energy 128 GeV at z=0.34

Fan, Tam et al. (2013)

Ackermann et al. (2014)

Spectral evolution
synchrotron

inverseComptonLiu et al. (2013)

Power law index doesn’t change!

Significance of broken power law
over power law

Tam et al. (2013)

>10 GeV afterglow
emission mechanism
Synchrotron emission (e.g., Kumar & Barniol 2009,Ghisellini et al. 2010)
but there exists a maximum synchrotron energy, it ishard to explain the >10 GeV photons

Fan, Tam, et al. (2013)
also see Ackermann et al. (2013)

Contemporaneous X-ray/GeV flares?
within 3-sigma

LAT detection: TS=32

Abdo et al. (2011)

GRB 100728A: LAT detection during X-ray flares

Power-law component during prompt phase
GRB 090902B~10 GeVGRB 090510

10 GeV

Abdo et al. (2009)

Ackermann, et al. 2010

~10 sec after trigger~1 sec after trigger

GRB 131231A: showing very hard
0.1-100 GeV spectrum since T0100 GeV
Liu, B. et al. (2014)
Probably the strongest case for IC emission, besides 130427A

Why bother the very highenergy photons of GRBs?
10-100 GeV is the last observing window of GRBs
The radiation mechanism at >GeV energies, both inprompt or afterglow phase, is still under debate
The energy band where extragalactic background light(EBL) attenuation starts to modify the intrinsic spectraof the sources (e.g., AGN, GRBs)
GRBs are more distant than AGNs, but a few nearby
GRBs will be good ..

Historical observations of GRBs
Over the last twenty years or so, ground-based telescopes have not detectedGRBs at significantly high confidenceSome early claims:
GRB 970417A by MILAGRITO （2.7σ），
GRB 991208 by Tibet-ASγ （1.88σ, z=0.706）steep fall-oﬀ of optical flash like
GRB 990123 was also seen
Not even MAGIC II/H.E.S.S. II/VERITAS/HAWC (yet)Some evidence for Tupi detected photo-muons associated with several GRBs
(Augusto et al. 2016)
High energy threshold (thus absorbed by the EBL) is a major reason, otherreasons include low sensitivity, time delay, etc. (see, e.g., Xue, Tam, et al., 2009).

HAWC is observing GRBs
With less than 1/3 of the array active, the HAWC
observatory obtained limits for GRB 130702A, which isat a close redshift of z = 0.145, and a limit for GRB
130427A
Simulated HAWC light curve of GRB 090510

CTA
simulation

Inoue et al. (2013)

FAST RADIO BURSTS & CTA
THE LOW & THE HIGH
TAM, Pak Hin (Sun Yat-sen University/ICRR)

Kashiwanoha, ICRR
15/12/2016

Fast Radio Bursts
Discovered in 2007
(Lorimer et al.)ms-duration
Large dispersion
measure (DM), about
10 times the Milky Way
contribution
Lorime+ (2007)

Fast Radio Bursts
Some or all excess DM may come from IGM Sources
outside the Galaxy. Are they cosmological?
Rate ~ 5,000 sky-1 day-1
Without EM signals in other wavelengths, our current
knowledge of FRBs are pretty much like that of GRBs in
the 1980s
Many models proposed: some more discussed ones are:magnetar flares, neutron star collapse, binary mergers.
Number of models > number of FRBs!

Recent developments (2016)
A possibly fading radio afterglow & host galaxy (Keane+2016) of FRB 150418; this might be an AGN-like
activity, though (Williams & Berger 2016)
the first repeating FRB 121102 (Spitler+ 2016)
No counterpart in any otherwavelengths (Scholz+ 2016)

FRB 131104/
SwJ0644.5-5111

Significance of association~3.2 sigma

DeLaunay+ (2016)

TeV observation of FRB150418

Observations started 14.5 hr after the FRB, lasting for1.1 hrNo detection

CTA prospects?
Prompt follow-up observations need coordination withradio facilities
Pointed observations of repeating FRBs?
Compared to GRBs, FRBs are more nearby, reducing theeﬀect of EBL.
Gamma-rays are less absorbed by FRB local environment
Crucial: whether FRBs emit gamma-rays (or anycounterpart at all) is an open question

SummaryGRBs: 10-100 GeV is probably the last observing
window which is still missing (apart from a few photons seen
by Fermi/LAT). It contains important clues on GRBs, EBL andrelated astrophysics.

Fast Radio Bursts: origin unknown, but they areenergetic and probably related to compact objects (seealso talk by T. Totani tomorrow)