Gamma-ray echo (pair echo)
from TeV sourcesand extragalactic magnetic fields
Keitaro Takahashi
Nagoya University, Japan
Dec./15/2010 @APC
with K. Ichiki, S. Inoue, M. Mori
K. Murase, S. Nagataki, T. Nakamura, B. Zhang

generation and observation of B
magnetogenesis
・reionization
・structure formation
・first stars
・primordial fluctuations
・phase transition・inflation
observation
・Faraday rotation
・CMB anisotropy

・B as a remnant
・probe early universe
through B・preserved in voids
・tiny in general(10 -25 ～10-15 G)

sensitivity ～ 10 -9 G

Secondary gamma-rays (pair echo) from blazars
and GRBs can be used to measure tiny B.

pair echo

CMBEBL

γ-ray(TeV)e±creation

γ-ray(MeV-GeV)
blazarGRB

IC

γ-ray(GeV)

observertime delay (angular spreading)spectral modification (TeV→GeV)
extended halo

pair echo

Plaga (1995), Dai & Lu (2002), Wang et al. (2004)
Razzaque et al. (2004), Ando (2004), KT+ (2008, 2009, 2010)

CMB

EBL

γ-ray(TeV)e±creation

γ-ray(MeV)
blazarGRB

B fields

ICγ-ray(GeV)

observerenhanced by Bcharacteristic scalesEBL Eγ 
EIR = 0.1eV 1TeV −1CMB Eγ 
Eecho = 0.6GeV 1TeV Eγ = 1TeV

 nIR −3  0.1cm λγγ = 20Mpc −1 Eγ λIC = 0.4Mpc  1TeV −1−2

 Edelay   B   − 20 ∆t B = 0.5 day  1GeV   10 G 22

typical situation
GRB・blazar～ 10Mpc
measure B ofe ± region only

3 approaches

Neronov & Vovk, 2010

1. spectral modification
2. extended halo
3. lightcurve (time delay)

KT et al.,this talkAndo & Kusenko, 2010

comparison
spectralmodification

extendedhalotime delay

targetblazar(quiescent)blazar(quiescent)blazar flare
GRBsensitivity

10advantage

known target

known target

disadvantage assumption
on past TeVemission*PSF subtlety*assumption
on past TeVemission-20-14- 10-16

G 10-14- 10G 10-20-16- 10G

no assumption
on past TeVemission*transient*confusion
with othercomponents

GRB and TeV blazarGRB- TeV emission is theoretically natural
but has not been observed due to the sensitivity
and mobility of the current TeV telescopes.
- prompt emission and afterglow
TeV blazar- TeV emission is already observed.
- lightcurve is complicated (flare and quiescent emission)
Let’s start with GRBs.

GRB①: pair echo spectrum

Pair echo
appears after
promptemission
disappears.
High-energy
echo becomes
dim earlier.
Large B=> small flux
but long lastingz = 0.3

KT+, 08, 09

GRB②: pair echo lightcurve
lightcurve (@1GeV)
stronger B => dimmer and longer-lasting pair echoFermi

GRB③: pair echo lightcurve
lightcurve (@100GeV)
stronger B => dimmer and longer-lasting pair echo

Fermi

GRB④: dependence on cutoff and EBL
 Eγ 
Eecho = 0.6GeV 1TeV 2z = 0.3

detectability of pair echo

preferable condition:near, weak B, large cutoff energy, afterglow

Fermi

for CTA,z ～ several
For very weak B,
pair echo has noinformation on B,
even if it isobserved.

afterglow
afterglow 1053-54 erg

Pair echo tends to behidden by afterglow
in early phase. Butit would appear after jet break.

Fermi

pair echo from high-z GRB

KT+, 10high-z GRB (z > 10)・purely primordial B・target of pair annihilation is only CMB

Challenging
even for CTA.

GRB and TeV blazar
GRB is a simple example to consider.
But we should be lucky to observe TeV emission of GRB.
We have to wait...
We already know TeV emission of blazars.
This is more promising observationally.
Let’s move on to TeV blazar.

Mkn421

Nearest, hardest, best-observed TeV blazar

TeV flux (Crab)

1010.116 years
0.01

49000

51000
53000
day (MJD)

55000

Mkn421

TeV flux (Crab)

10Nearest, hardest, best-observed TeV blazar

log normal distribution
mean: 0.73 Crabstandard deviation: 0.99 => factor 2.67

10.1
16 years
0.01

49000

51000
53000
day (MJD)

55000

Mkn421

Nearest, hardest, best-observed TeV blazar

TeV flux (Crab)

1010.10.01

To calculate pair echo at some time,
we must integrate all contributions
from the past TeV emission.
49000

51000
53000
day (MJD)

55000

How distant past is important?
log[flux * time (arbitrary unit)]

0-21

B = 10-20.5G, 10-20G, 10-19.5

G, 10-19G, 10G

89-1-2-3@1GeV-434567log[time to ☆ (sec)]

10Dermer et al., 2010①

No observation by
Fermi in 0.1-100GeVrange.
If the blazar has emitted
TeV gamma-rays formore than 100 years
at the level of 2009,
B = 10-18 G isconstrained.
Because 1ES 0229+200
has been observed forseveral days in 2005,
2006 and 2009,
this is a strong
assumption.

Dermer et al., 2010②
For B = 10 -18 G,
1-year emission is
sufficient to constrain.
This would be a morereasonable assumption.

4 years =>Relatively robust,
but still based on
an assumption on
the past TeV emission.

mock Mkn421 data①
What is the lightcurve of pair echo from blazars?
Demonstration with a mock TeV lightcurve of Mkn421

observational data

extrapolated flux

mock Mkn421 data②
赤：primary (TeV)
青：pair echo (GeV, B = 10 -20 G)
桃：pair echo (GeV, B = 10 -19.5 G)

mock Mkn421 data 赤：primary (TeV)
青：pair echo (GeV, B = 10 -20 G)
桃：pair echo (GeV, B = 10 -19.5 G)
Rapid response
for weaker B.

Simultaneous observation of GeV-TeV is crucial.

Mkn501 flare①
Abdo et al., 2010
multiwavelength observation for 480 days
flare for (more than) 3 days

flare

quiescentMkn501 flare②

KT+, in preparation
flare (unabsorbed, absorbed)

quiescent(unabsorbed,absorbed)

pair echo
from flare
1, 10, 100 days
after flare

pair echo
from quiescent
TeV emission

Mkn501 flare③

lightcurve of 1-10GeV

Fermi data (7-days average)= primary, flare echo, quiescent echo

quiescent pair echoflare pair echo
B = 10-19, 10 -19.5 , 10-20 G

Mkn501 flare④

lightcurve of 1-10GeV

Fermi data (7-days average)= primary, flare echo, quiescent echo

Time bin should be shorter. (Now in progress.)=> Larger flux error.
Constraint on B?
If obtained, it is a robust constraint
because it is based on the observed TeV emission.

summary
・Pair echo (halo) can probe the weakest B.
・3 ways to observe pair echo.
‐spectral modification‐extended halo‐time delay
・Time delay has an advantage that it does notneed to assume TeV activity in unobserved past.
・Robust constraints could be obtained bysimultaneous observations of blazars
in GeV-TeV range.

commentKT+, 06

magnetogenesis from primordial fluctuations- KT+, 05, 06, 07, 08- Pitrou+, 10
Are we consistent?
B spectrum @ recombination(KT+, 06)

B spectrum @ z = 0(Pitrou+, 10)