Bounds on the Cosmic TeV
Gamma-ray BackgroundYoshiyuki Inoue(ISAS/JAXA, JAXA International Top Young Fellow)
Collaborators: Kunihito Ioka, Yasuyuki T. Tanaka

The extreme Universe viewed in very-high-energy gamma-rays 2015, Kashiwa, 2016-01-14

Cosmic Gamma-ray Background Spectrum at >0.1 GeVCGB Spectrum• Updated LAT measurement of IGRB spectrum

Ackerman+’15• Fermihas resolved
30%200of MeV
the –CGBat ~1GeV
andmore– Extendedenergy range:100 GeVx 100
MeV– 820
GeV athigherenergies.
• Significant
high-energy cutoff feature in IGRB spectrum– Consistent with simple source populations attenuated by EBL• Blazars, star-forming galaxies and radio galaxies can explain the intensity
Componentsof the Cosmic GeV Gamma-ray Backgroundand the spectrum of the EGB

Preliminary
Ajello, YI +’15

Asgals.
usual: (YI’11)
it does
include the systematic
uncertainty
on the EGBmake!
• Blazars (Ajello+’15), Radio, &notStar-forminggalaxies(Ackermann+’12)
up almost

100% of CGB from 0.1-1000 GeV.
• Next frontiers will be• Anisotropy (e.g. Ando & Komatsu ’06, Ackermann+’11, Camero+’13, Shirasaki+’14)• Cosmic MeV Gamma-ray Background (e.g. YI+’08, Ajello+’09, YI+’13)• Cosmic TeV Gamma-ray Background (This talk)Γ = 2.14 (thin). The shaded rectangle indicates the IceCube
data [4]. The atmospheric muon neutrino background [21]and the diﬀuse IGB data by Fermi /LAT [14] are depicted.

isCosmic
TeV
Gamma-rayBackground(4)
10-3 !"#(, ! -./0 12 3 34 5

E2 dN/dE [MeV2/cm2/s/sr/MeV])#) #! #!

ntly asr limitto Γ ≤−1eV)] .
easuredsr−1#'!"-410(5)#$Total CGB(Fermi)
6/427)"!"
IceCube (i per flavour)
6/427 )"!)!"#$ %&!"#&-5(10!"

FermiCGB-610.

)*+,-.+ /012#%!"#$10-7!"#!"
103" 104!"!

!"%'

IceCube TeVPeV νs105 )+( 108 '10*6107!"
!"
!"!"
!"
Photon Energy [MeV], -./05

109 &!"

10%10!"

Murase+’13• (6)Above 1 TeV, there is no gamma-ray data, though it is important
FIG. 2: The same as Fig. 1, but for Γ = 2.0 (thick) andis comfor neutrino
studies.
Γ = 2.18(thin) with the star-formation history [23].

ub-TeVthe • Iceextragalacticpp
scenarioforIceCube
eventsisconstrainedby
theis because ξz in Eqs. (2) and (4) is similar and cancels
os mustCGB (Murase+’13;Bechtol+’15). Eq. (5). This conclusion largely holds
out in obtaining
ergalac-GeV-TeV Gamma-ray Connection: Cascade
TeV
Sourceγ-raye+γ-rayeEBLObserverCMB• TeV gamma-ray photons are absorbed by EBL• electron-positron pairs are created• pairs scatter CMB via inverse-Compton process• 1 TeV (primary) -> ~1 GeV (secondary)• Note: plasma instability may suppress the cascade(Broderick+’12, but see also Sironi & Giannios ‘14)

Upper Bound on the Cosmic Gamma-ray BackgroundCascadeIntrinsic

UL

Absorbed

YI & Ioka ’12• Cascade component from the TeV background can notexceed the Fermi data (Coppi & Aharonian ’97, YI & Ioka ’12, Murase+’12,Ackermann+’14).

Galaxy Counts: Lower Bound on theCosmic Optical/Infrared Background
100iIi [nW/m2/sr]

10Stars

1

0.1
0.1

1Dust

10100h[µm]Madau & Pozzetti ’00 (HST)
Elbaz et al. ’02 (ISO)
Papovich et al. ’04 (Spitzer)
Fazio et al. ’04 (Spitzer)
Xu et al. ’05 (GALEX)
Dole et al. ’06 (Spitzer)
Frayer et al. ’06 (Spitzer)
Gardner et al. ’00 (HST)
Berta et al. ’11 (Hershel/PEP)
Wright & Reese ’00 (DIRBE)
Wright ’04 (DIRBE)
Levenson et al. ’07 (DIRBE)
Levenson & Wright ’08 (DIRBE)
Bernstein ’07 (HST)
Matsuoka et al. ’11 (Pioneer)
Matsumoto et al. ’11 (IRTS)
Matsuura et al. ’11 (AKARI)
CambrØsy et al. ’01 (DIRBE)
Dwek & Arendt ’98 (DIRBE)
Gorijian et al ’00 (DIRBE)
Finkbeiner et al ’00 (DIRBE)
Hauser et al ’98 (DIRBE)
Lagache et al ’00 (DIRBE)
Edelstein et al ’00 (Voyger)
Brown et al ’00 (HST/STIS)
Albert et al ’08 (MAGIC)(X, zc)=(1.0, 0.0)
(X, zc)=(50.0, 10.0)(X, zc)=(100.0, 10.0)

1000

10000

YI+’13

Known TeV sources
10-910-10z = 0.031
Mrk 421z = 0.034
Mrk 501z = 0.044
1ES2344+514

z = 0.045
Mrk 180 (high)z = 0.048
1ES1959+650

z = 0.049
AP Librae

10-11
10-12

10-13
10-9
E2dN/dE [TeV/cm2/s]

10-10• Select 35 known TeV

sources at |b|>10 deg
from the default
TeVcat catalog.
• low-state data only

10-11
10-12
10-13
10-9
10-10

• 30 are blazars, 3 arez = 0.069
PKS 0548-322

z = 0.069
BL Lacz = 0.071
PKS 2005-489

10-11
10-12
10-13
10-4 10-3 10-2 10-1 100 10110-4 10-3 10-2 10-1 100 10110-4 10-3 10-2 10-1 100 101
Energy [TeV]
Energy [TeV]
Energy [TeV]YI & Tanaka ‘15

radio galaxies, 2 arestarbursts• 3FGL SED data for theGeV data.

Source Count Distribution
N(>E2dN/dE) at |b|>10 deg

1000.3 TeV
1 TeV
3 TeV
10 TeV

10

1

10-14

10-13

10-12
10-11
10-10
E2dN/dE [TeV/cm2/s] YI & Tanaka ’15• diﬀerent from a uniform distribution.
• More uniform and wide sky coverage is required.

E2 dN/dE [GeV2/cm2/s/sr/GeV]

Lower Bound on the Cosmic Gamma-ray Background10-610-7

10-8Total CGB (Fermi)
Resolved CGB (Fermi)
2FHL Source Fluxes
TeV Source Fluxes
Mrk 421
Mrk 501

10-910-10
10-11
10-12 010101231010
Photon Energy [GeV]

10410

5

YI & Tanaka ’15• TeV source counts give lower limit on to the cosmic gamma-ray background.
• Fermi has resolved more portion of the TeV sky than IACTs do.
• CTA & HAWC surveys will be important (YI, Totani, & Mori 10; Dubus, YI,+’13)

How large fraction of the VHEsky resolved by Fermi?
1Fraction against total CGB

0.8

0.6

0.4

0.2

0102

Unresolved CGB (Bechtol at HEM2014)
Resolved CGB (Bechtol at HEM2014)103

104
Photon Energy [MeV]105106

The Fermi-LATcollaboration ‘15
FIG. 4: Cumulative source count distribution N (> S) withthe uncertainty bands as in Fig. 3 together with the theoretical predictions from Ref. [12] (blue dashed line), [4] (reddashed line) and [13] (green band). The vertical dotted brownline shows the 5 mCrab flux reachable by CTA in 240 hrs ofexposure [14].

• Fermi has resolved 50-80% of the VHE sky (0.1-1 TeV).

• CTA survey (at >50 GeV) will not drastically change thesource counts.

Our best-fit model for the flux distribution dN/dS is
therefore, for S & 10 12 ph cm 2 s 1 , a broken power-

Why Fermi has resolved thesky more even at ~1 TeV?
S. Funk, J.A. Hinton / Astroparticle Physics 43 (2013) 348–355

10-9Crab Nebula
Synchrotron

10-10

LAT - 10 yrs (inner Galaxy)
Inverse Compton

10-11

H.E.S.S. - 100 hrs

10-12-13

LAT - 10 yrs (extragalactic)

CTA - 100 hrs

10CTA - 1000 hrs

-14

10102

103

104

105106

Photon Energy (MeV)107

differences between instruments in both quantities. Below
100 GeV
Fermi-LAT outperforms all ground-based instruments-10 the Mrk421
10
in both angular and energy resolution. This is due to inherent fluctuations in thosez = particles
0.031 above the Cherenkov threshold high in theatmosphere for showers initiated by low energy primaries. So even
if the differential sensitivity of the Fermi-LAT and CTA is the same ata given energy, the Fermi-LAT will be able to do a better measure-11
ment
10 of a source. While HAWC’s performance in these quantities
is rather modest, its main goal is to detect new sources and study
variability and find transients. HAWC is not shown in Fig. 1 as differential sensitivity curves has not been provided by the HAWC collabMAGIC (low-state)oration and indeed, it is
not the relevant quantity for the3FGL
aforementionedgoals. In theenergy range at which this study is
2FHL-1210
focused, HAWC
is notcompetitivewith
the Fermi-LATand CTA
10-4
10-3
10-2
10-1
100101except perhaps for the detection of very short timescale transients
Energy [TeV]such as GRBs.

E2dN/dE [TeV/cm2/s]

Differential Flux E 2 dN/dE (erg cm-2 s-1)

10-8349

108Fig. 1. ‘‘Differential’’ sensitivity (integral sensitivity in small energy bins) for aminimum significance of 5r in each bin, minimum 10 events per bin and 4 bins per
decade in energy. For Fermi-LAT, the curve labeled ‘‘inner Galaxy’’ corresponds tothe background estimated at a position of l ¼ 10" ; b ¼ 0" , while the curve labeled‘‘extragalactic’’ is calculated using the isotropic extragalactic diffuse emission only.
For the ground-based instruments a 5% systematic error on the backgroundestimate has been assumed. All curves have been derived using the sensitivity
model described in Section 2. For the Fermi-LAT, the pass6v3 instrument response
function curves have been used. As comparison, the synchrotron and Inverse
Compton measurements for the brightest persistent TeV source, the Crab Nebula
are shown as dashed grey curves.

• 14 sources at the highest energy (585-2000 GeV) bin in the 2FHL samples, while 30 sources at>585 GeV in our sample.

The sensitivity model
Sky coverage is not the2. cause.

• The dominant object Mrk 421 is variable.

The sensitivity of gamma-ray detectors is determined by threebasic characteristics: the effective collection area, residual background rate and angular resolution, all of which are typically astrong function of gamma-ray energy. For Fermi-LAT the relevant
curves are taken from [29] for instrument response function
pass6_v3, and for CTA from [30]. It should be noted that the usageof the enhanced pass7 response-functions for the Fermi-LAT will
not substantially change the presented results. The difference in
effective area above 1 GeV is $ 10%. We also note that the CTA per-• The CGB is the time-averaged spectrum. e.g. Fermi accumulated data 80 months for 2FHL.

the sensitivity information provided is insufficient to make a detailed
comparison of the performance in the overlapping region•
which motivates this study.
As can be seen from the figure, the Fermi-LAT is photon starved

We need long-term monitoring of TeV sources.

HAWC & current IACTs in the CTA era.

Lower Bound on the Cosmic Gamma-ray BackgroundE2 dN/dE [GeV2/cm2/s/sr/GeV]

10-6

Total CGB (Fermi)
Resolved CGB (Fermi)
2FHL Source Fluxes
TeV Source Fluxes

10-7
10-8
10-9
10-10
10-11
10-12 010

101

102
103
Photon Energy [GeV]

104

105

YI & Tanaka ’15• The bounds above 100 GeV is approximated as:
3 ⇥ 108✓

E100 GeV◆ 0.6exp✓E
2 ⇥ 103 GeV◆[GeV/cm2 /s/sr]• Exponential cutoﬀ may not be due to the EBL attenuation.

The gamma-ray horizon energy for Mrk 421 & 501 is ~7 TeV.

Bounds on the Cosmic TeV Gamma-ray BackgroundE2 dN/dE [GeV2/cm2/s/sr/GeV]

10-610-710-810-910-1010

Total CGB (Fermi)
Upper LimitIceCube(InoueTeV andSource(i perIokaflavour)
Counts2012)
Upper LimitIceCube(Inoue and(i perIokaflavour)
2012)
IceCube (i per flavour)

-11

10-12
100

101

102

103
104
105
106
107
YI & Tanaka ‘15
Photon Energy [GeV]• Current limit at 0.1-10 TeV is-8

• 3x10(E/0.1 TeV)-0.82-7exp(-E/2 TeV) < E dN/dE < 1x10 (E/0.1 TeV)

-0.5

2[GeV/cm /s/sr]Requirement for CTA tomeasure the CGB spectrum
10-2Total CGB (Fermi)
Resolved CGB (Fermi)
TeV Source Fluxes
Upper Limit (II12)
IceCube (i per flavour)
AMS (e++e-)

10-4
10-6
10-7

2210-5

2E dN/dE [GeV /cm /s/sr/GeV]

10-3

10-8
10-9
10-10
10-11
10-12 010101

102

103
104
105
Photon Energy [GeV]106107• Need to remove electron background events which is 104-5

times higher than the CGB events.

Summary• Cosmic TeV gamma-ray background is not well investigatedyet.
• Current GeV gamma-ray background gives upper limits on theTeV gamma-ray background through the cascade argument• Ensemble of low-state TeV blazar flux gives lower limit on to

the cosmic gamma-ray background.
• Current limit on the TeV background is• 3x10-8 (E/0.1 TeV)-0.8 exp(-E/2 TeV) < E2dN/dE < 1x10-7 (E/0.1TeV)-0.5 [GeV/cm2/s/sr].