CTA Extragalactic Survey Discovery
Potential and the Impact of Axion-Like
Particles and Secondary Gamma Rays
Andrea De Franco (Oxford -> EBG MedAustron GmbH)
Yoshiyuki Inoue (ISAS/JAXA), Miguel A. Sánchez-Conde (OKC),
Garret Cotter (Oxford)

CTA-Japan Meeting @ Kashiwa, 2016-12-15

Current View of VHE Gamma-Ray Sky©TeVCat• ~200 very high energy (VHE; >50 GeV) blazars by Fermi/LAT (seethe 2FHL catalog)• ~70 VHE blazars by imaging atmospheric Cherenkov telescopes(IACTs; see TeVCat)VHE Extragalactic Sky Survey by CTA
G. Dubus et al. / Astroparticle Physics 43 (2013) 317–330• Future survey will enable

us to study• High energy phenomena

around SMBHs• Cosmological evolutionof blazars• We have investigated the

expected source counts(YI, Totani, & Mori ’10; Dubus, YI+’13; YI,
Kalashev, Kusenko+’14 ).

Dubus, YI, +’13
cumulative source counts as a function of the integral gamma-ray flux of VHE blazars. The fivecorrespondto different photon
energies, as indicated inforpanelsCTA
SpecialIssue

en curves correspond to a blank field all-sky survey, blue curves for a follow-up of Fermi blazars (assuming a Fermi/LAT sensitivity limit ofs cm!2 s!1 above 100 MeV). The horizontal thin solid line is the total expected number of blazars above the Fermi/LAT sensitivity. Solid curves include EBLA New Blazar Luminosity Functione Astrophysical Journal Letters, 800:L27 (7pp), 2015 February 20

Redshift Distribution

Ajello et al.

Source Count Distribution

Ajello, MASC, YI+’15Figure 1. Observed redshift (upper left), photonAjello,
MASC,
index (upper
right),YI+’15
0.1–100 GeV so

distributions of Fermi-LAT blazars. For the upper panels, the continuous solid line is the PLE
et al. 2010c), while for the lower ones, it represents the predictions of the LF models. The 68%
and SED model for the 10–500 GeV source counts. Error bars compatible with zero are 1σ

the EBL:• Utilizing the latest 403 Fermi/LAT
blazar samples, a new1 E ¬Ha  E ¬Hb ¯­­ ° MASC,
(Ajello,· eU (E, z ) YI,+’15)
 K ¡¡  ­­­­° E b ­®
,
(11)
dE
¡¢  E b ­®
°±(ph cm2s1GeV1).

blazar luminosity function is constructeddNH.

We rely on the EBL model of Finke et al. (2010) and use Ha = 1.7

dN /d
blaza
index
2012
corre
simu
rando
descr

a factor 4 (which is reasonable for illustrative purposes and is con[25]. Each of these possibilities produces a substantially better fit tosistent with observations [30,32,31]), while the secondary compothe data than a primary gamma ray spectrum obtained in the abnent remains constant. Although the flaring spectral data revealed
sence of cosmic rays. This is remarkable because the shape of thelittle difference between the two scenarios, a comparison of thehigh-energy spectrum is quite robust and independent of any modhigh and low states shows some marked differences. Firstly, theel parameters. This successful fit to the data provides additional
spectral shape of the higher redshift source’s low state is significredence to the idea that distant gamma-ray sources are dominatedcantly softer than the lower redshift source. Secondly, the ratio
by a hard, secondary spectrum. However, the spectral shape and fit
HINTS OF THE EXISTENCE OF AXIONLIKE PARTICLES . . .
PHYSICAL REVIEW D 79, 123511 (2009)
to experimental data alone cannot distinguish between the differ1 10 7
100 of EBL and different redshifts. One can, however, use
ent models
5 10 8flaring and variability data to infer further information on the tran1000
10-1
sition fromprimary to secondary spectrum and to gain information
about boththe EBL and the redshift of the source.
10-2
1 10 8
While the shape of the spectrum is robust, its normalization de100 5 10 9pends on10thesource luminosity in protons Lp , which is not known.
-3
IGMF
In what follows,
we obtain the best fit for isotropic equivalentsource powerin protons Lp;iso ¼ ð1 $ 10Þ & 1049 erg/s. The true910-4
10 1 10intrinsic luminosity can be smaller by several orders of magnitude,
5 10 10'10-5 on the jet opening angle h. In particular, for h ¼ 3 , thedependingrequired intrinsic luminosity is comparable to the Eddington lumi1-6810
nosity of a black hole with a mass M ( ð1 $ 8Þ & 10 M) . Of course,
1 10 10
0.1
1
100
1000
0.1
1.0
10.0 10100.0
1000.00.1
1.0 energy is10.0
1000.0 par10 4
only a fractionof the jet
transferred100.0
to high-energy
E (GeV)E (GeV)ticles. For a non-spherical accretion with a jet, the Eddington lumiE GeV
1.00
Essey & Kusenko ‘14
nosity is not necessarily a limitingSánchez-Conde+’09
factor, and there is growing
Fig. 1. Predicted spectrum of PKS 1424 + 240 (solid black line), assuming z ¼ 0:6,
evidence of super-Eddington luminosities in relativistic outflows
the lowest redshift consistent with the lower bound [23], for EBL model of Ref. [24].
in GRBs and in very powerful blazars [33]. The required luminosity
The blue dot-dashed line is primary spectrum (spectral index c ¼ 1:65); red dashedis consistent with general principles of acceleration, as long asline is secondary gamma rays for the mean IGMF of B ¼ 10$15 G with a correlation
length of 1 Mpc; black solid line is the combined spectrum of primary andmost of 0.10
the accretion energy is converted efficiently to the kinetic
secondarygamma rays. Also shown are the
VERITAS and Fermi data points. The(e.g.
Deaccretion
Angelis+07;
Sánchez-Conde+09;
Horns+12)
energy of the jet, rather than to thermal radiationof thepurple line represents the total spectrum with primary signal suppressed by aflow [8].
factor of 4 (low state). (For interpretation of the references to colour in this figure
Some assumptions must be made about the strengths of intercaption, the reader is referred to the web version of this article.)galactic magnetic fields (IGMFs) along the line of sight. If IGMFs
1
0.01
are greaterthan ( 3 & 10$14 G, they can cause sufficient deflectionsof protons to diminish a contribution of secondary gamma rays. For
1 10 7$17very small
below 10’10,G,Essey+’10,
the low-energyEssey+’11,
part of(e.g.magnetic
Essey fields,& Kusenko
Murase+’12,Takami+’13)
5 10 80.1 (which1.0
1000.0may exceed
10000.0
0.1
1.0
10.0
100.0
1000.010000.0
the spectrum
is not as 10.0
robust as 100.0
the TeV part)E (GeV)E (GeV)the observations of Fermi [12]. Therefore, we choose to fit the data

Axion-Like Particles & Secondary γ-rays
E2dN dE GeV cm 2 s 1

3C 279 (z=0.536)
Eﬀect of ALPs
B
= 0.1 nG

Axion boost factor

Intensity

PKS 1424+240 (z=0.6)
Secondary Gamma Rays

attenuation eﬀect

Axion boost factor

Intensity• Axion-Like Particles (ALPs) modify the gamma-ray.

• Secondary gamma rays from cosmic rays along line of sights 1.

N (>F) [deg-2]> 30 GeV> 300 GeVN (>F) [deg-2]

Cumulative Source Count Distribution> 2 TeV> 10 TeV

E x F [erg cm-2 s-1]

E x F [erg cm-2 s-1] De Franco+’17

Assuming EBL attenuation only and 50 hr obs. per FoV>30 GeV>300 GeV>2 TeV>10 TeV5x10-2 deg-2
2x10-2 deg-2
5x10-3 deg-2
3x10-4 deg-2
Figure 2: Cumulative source count distribution in di↵erent energy bands for the di↵erent scenarios discussed in the text: intrinsic spectrum(black solid line); EBL absorption (blue solid); EBL+ALPs (red lines); EBL + secondary gamma rays (magenta lines). In the last two
scenarios, dashed and solid curves refer to two di↵erent values of the IGMF strength, see legend. In the energy bands in which information
are available, we also show the integral flux sensitivity of CTA-South (vertical, orange dashed-dotted) and North (5 , 50hr observation per
FoV) (vertical, red dashed-dotted); Hybrid (5 , 1 year) (vertical, blue dashed-dotted); LHAASO (5 , 1 year) (vertical, purple dashed-double
dotted); HAWC (5 , 1 year) (vertical, green dashed-triple dotted).

200 hrs Blind Surveys
CTA-South/North – 0.5 hr

CTA-South/North– 0.5
hr>30 GeV >300
GeVNo EBLNoEBL
EBL
10ALP (B=10
G)
EBL
11
ALP
(B=10
ALP (B=10 10 G)
G)
11 15 G)
Secondary(B=10
ALP (B=10
G)
17
Secondary(B=10
Secondary (B=10 15 G)
G)

>30
GeV
56/22
35/13
56/22
19/7
35/13
24/9
19/7
47/18
24/9
76/32
47/18

>300
GeV
156/6832/15
156/68
25/12
32/15
30/14
25/12
36/16
30/14
45/20
36/16

Secondary (B=10 17 G)

76/32

45/20

>2 TeV>10 TeV>2
96⇤TeV/36⇤⇤ /4
10
96 /36
⇤⇤
8
10 /4
/4
⇤
10
/4
8⇤ /4
⇤
12
10⇤ /5/4⇤
14
12⇤ /6/5>10
20⇤TeV/8⇤20-//8-/-/-/-/⇤
1-//11⇤⇤ //-14⇤ /6 De Franco+’171⇤ /-FoV of 5°, 7°, 9° at >30 GeV, >300 GeV, >2 TeV

ed source• count for a blind survey of 200 hours with observations of 0.5 hr with CTA-South and CTA-N
for the models considered in this paper. See text for further details. Note: ‘-’ mark denotes no expected
ed sourcetocountfor ofa blindof 200hours
with observationsof 0.5 hr with CTA-South and CTA-N
espondsa survey
half ofsurvey
the sky,which
is completed
in ⇡130 hours
for the models considered in this paper. See text for further details. Note: ‘-’ mark denotes no expected• a survey of half of the sky, which is completed in ⇡130 hours
esponds to

FoV of 7° at > 2 TeV for CTA-North (no SSTs)• ALPs reduce # of blazars at >30 GeV.
1010

• 2x more blazars with the secondary scenario.

Cumulative Redshift Distribution> 300 GeV

> 2 TeV> 10 TeV

N (>z) [deg-2]

N (>z) [deg-2]> 30 GeV

A 200 hr survey
for CTA-South0.5 hr / FoVzz

Figure 4: Same as Figure 3, but assuming 0.5 hr exposure observations.

• z~2 blazars will be detected.

• Secondary gamma rays will enable us to detect z~0.4

objects even at >2 TeV(see also YI, Kalashev, & Kusenko ‘14).
HAWC / LHAASO / Hybrid – 5 years

De Franco+’17

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

100

0.3 TeV
1 TeV
3 TeV
10 TeV

10

1

10

-14

10

-13

-12

-11

-10

10
10
10
E2dN/dE [TeV/cm2/s] YI & Tanaka ’16• With current IACTs’ data
• diﬀerent from a uniform distribution.
• More uniform and wide sky coverage is required.

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
Mrk 421
Mrk 501

10-7
10-8
10-9
10-10
10-11
10-12 0
10

101

102
103
Photon Energy [GeV]

104

105

YI & Tanaka ’16• 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 in order to check this.

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

10-6
10

-7

10-8
10

-9

10-10
10

Total CGB (Fermi)
Upper LimitIceCube(Inoue
TeV and
Source(ν perIokaflavour)
Counts2012)
Upper LimitIceCube(Inoue and(ν perIokaflavour)
2012)
IceCube (ν per flavour)

-11

10-12
100

101

102

103
104
105
Photon Energy [GeV]• 3x10sr](E/0.1 TeV)

-0.8

107

YI & Tanaka ’16• Current limit at 0.1-10 TeV is
-8

106

2

-7

exp(-E/2 TeV) < E dN/dE < 1x10 (E/0.1 TeV)

-0.5

2

[GeV/cm /s/

Redshift Distribution

2

dI/d(log z) [TeV/cm /s/sr]

10-10

0.3-100 TeV

10-11
10-12
10-13
10-14
10-10

2

dI/d(log z) [TeV/cm /s/sr]

0.5-100 TeV

1-100 TeV

3-100 TeV

10-11

Flim = 0 w/ EBL
Flim = 10-12
Flim = 10-11

10-12
10-13
10-14
0.01

0.1
Redshift z

1

0.01

0.1
Redshift z• Redshift distribution of gamma-ray fluxes• at high energies, samples are not enough.

1

Summary• We revisit prospect for future CTA extragalactic survey taking

into account 1): the latest blazar luminosity function, 2): axionlike particles, and 3): secondary gamma rays.
• the source density will be 5x10-2 deg-2 at >30 GeV with 50 hr

obs/FoV.
• 7-76 blazars will be detected at >30 GeV with two (North &South) 200 hr blind surveys (-> less than a year).
• ALPs would reduce # of blazars at >30 GeV.
• Secondary gamma rays will allow us to see out to z~1 at >2 TeV.

ch is independentof theiroscillate
mass. They are predicted bywhere z is the redshift of the source, is a photon index
mma
rays would
model
in
[14].
We set zminintrinsic= 0.001,
zmax = 4,lumi-m
ral extensions of the Standard Model and can consti(see eq. 3), L is thesource gamma–ray
52
1
rce
field
anddensityin (see, e.g.,
all ormagnetic
part of the dark
matter
[48]
=
3.5,
L
=
10
erg
s
as in [14].
Lminosityintheenergyband
0.1
100GeV.
is the blazar
max
max
a recent review).
couplein the
GLF and V is the comoving volume [58]. For the GLF, we
IGMF).
ALPsPhotons
would
notwith ALPs
intrinsic
sourceluminosity corresponding to theence of external magnetic fields, therefore would such
use the luminosity dependent density evolution (LDDE)
resultinga VHE
less gamma
opaquedof particles inexist,
rays
oscillateat Earth.
130would
tected
WeWeconstrain
Lmin
great
model in [14].
set zmin = 0.001,
zmaxto= 4,bemin= 1,52
forth to ALP
in thesourceobmagnetic field42and in1
erg s 1 as in [14]. Lmin is themax = 3.5, Lmax = 10
sk anda result,
CTA
would
following[14].
To
relateLtoaminintergalactic magnetic field (IGMF). ALPs10
would erg
not s intrinsic
source luminosity corresponding to the flux deistantdue toresulting
this in a flux
ractwith sources
the EBL, possibly
less opaquelimit, we
makeuse Weof constrain
an average
blazarSEDttectedat Earth.
Lmin tobe greater
than
verse
VHE gamma
rays. As athus
result, CTA would ob1042 erg s 1 following [14]. To relate Lmin to a certain
be tovariable
objects,presented
in
[14]:
e
spectralhardening[49]
ofdistant
sourcesduetothisflux limit, we make use of an average blazar SED
template
The Astrophysical Journal Letters, 800:L27 (7pp), 2015 February 20
Ajelloet al.
ectralhardeninginindict. Yet, blazars are known to be variable objects, thus
presented
in [14]:• Blazar
SED
template
ing statistically-significant
g.
Instead, in this spectral
paper,hardening in indi"✓ " ◆# 1
ual sources may be challenging. Instead, in this paper,
✓
◆
#
✓ 1.7
◆1.7 ✓ ◆2.62.61
estudyexpected
to
see
a
hint
whether CTA could be expected to seedNa hintE E
EdN
E⌧ (E,z) ⌧ (E
(E,
,
z)
/
+
e
, (2)
(E,
,
z)
/
+
e
LPs
in
the
sourcecount
distribution,
which
would
be
dE
E
E
ribution, which would be
b
b
dE
E
E
b
b
dified, especially at the highest energies probed by thestenergies probed by theeriment.
where the dependency from measured derives from:High and ultra-high-energy cosmic ray (UHECR) source•dependency
Breakenergy
E
b
where
the
from
measuredfr
log Eb (GeV) = 9.25 4.11 derives (3)leration has not yet been uniquely identified, and AGN
smic
ray (UHECR)
source range [50] (butd contributeto it up to the multi-EeV
For the opacity coefficient ⌧ due to absorption of gammaalso[51]).
Protons escaped
the AGN interact withlog Eonb (GeV)=we9.25
4.11
uely
identified,
andfrom
AGN
rayphotons
the
EBL,
adopt
the
model of [59] asintergalactic photon fields via photo-pion production
multi-EeV[50] (but pair production our reference one. We verified that there is no significant! p⇡ 0 / n⇡ + )range
and electron/positron
in results computed
other EBL modFor thedisagreement
opacity coefficient⌧ duewithto absorptiono!
). Both channels
leadwithto cascades of particles
mpethee+AGN
interactels [60–62]. Below, we explore the e↵ect of two di↵erent
secondary gamma rays, which would be detected along
photons
onvs. measured
theEBL,
we source
adopt
the
model
Figure2. Left panel: simulated
break energy E (for 135
Equationray
(11)
with
= 2.6)
power-law
photonindex
for acountset ofsimulatedblazars. Theof
H = 1.7, H scenarios
Ajello+’15
aphoto-pion
production
on thecumulative
distribution.
line
of
sight
of
the
blazar[36,
37,
52–57],
providedthatdashed line represents the best ﬁt described in the text. Right panel: photon index (α, at 300 MeV) and curvature β (black data points) of the best-ﬁtting logParabolareferenceverifiedthere
no(184)sig
IGMF
is not pairstrongenoughspectra
to significantly
bendthe2.6). TheWe
gray data
points show thethat
parameters
for all theisblazars
modelsto simulated
double power-law(e.g., Equationour
(11)
with
H = 1.7 and H =one.
positronproduction
2.1. Axion-like particles
whosecurvature
is signiﬁcantly
3LAC
catalog
ectoryof theprimary detectedcosmicin the
rays
[52,
54].(Ackermann
Protonset al. 2014c).
disagreement
in
results
computed
with
other
EB
ad
to
cascadesofparticles
The spectral distortions induced by photon/ALP mixh energies around ⇠1 EeV, i.e. below the Greisen–
els
Below,
weEBL-absorbed
exploretheofaretwo
epin–Kuzmin
be absorbed
bying
on top ofEGB
thesourcee↵ect
spectrum
comBecausethe
LF cuto↵,displayswould
steepnotpower
laws [60–62].
at highunresolved
intensity.
Furthermore,
it is difﬁcult
to d
habove.
wouldbe(GZK)
detectedalong
minSpectral Model in Ajello+’15
125

130

135

b

edshift and luminosity, the only limit that matters is L

a

b

a

b

140

,

accommodate a blazar population that produces a larger

s-2] E2 dNγ/dE (>F) [GeV cm-2 s-2]

Spectral Modeling
z=0.3; Γ=1.7

E [GeV]

z=3; Γ=1.7

E [GeV]De Franco+’17

Figure 1: Averaged blazar SED with di↵erent photon index
and redshift z as observed at Earth for four di↵erent scenarios: intrinsic
spectrum (black solid line); EBL absorption (blue solid); EBL+ALPs (red lines); EBL + secondary gamma rays (magenta lines). In the lasttwo scenarios, dashed and solid curves refer to two di↵erent values of the IGMF strength, see legend. Normalization is set by the primarygamma–ray flux at 0.1 GeV, and it is equal to 1 GeV cm 2 s 1 .

• We use an average blazar SED template in Ajello, MASC, YI+’15 andthe YI+’13 EBL attenuation model.

150

-11
-10 nor on thepresent the case for which this e↵ect is evident: an IGMF
the bolometric proton luminosityof the source,
•
ALPs,
use
the
modelinby
MASC+’09
with spectrum
B = 10 [36, and
10 G.
strengthForof B=
10 10 we
G and
10 11
G, uniform
1 Mpc
proton injection52]. However
the gamma–
domains but whose orientation randomly varies from one
ray flux is proportional to the source bolometric luminosdomain to another. It is important to emphasize that theity, Lp,bol . In Ref. [74], the authors estimated Lp,bol to be• For secondary gamma rays, we use themodel by Kalashev+’12 with
ALP scenario presented in [44] for the intergalactic caseof the same order, or slightly higher, than L ,bol for BL-15
only represents
case overB = 10an-17averageand 10G a large number of Lac objects, while being one or two orders of magnitude
realizations of the IGMF configuration (strength and origreater than L ,bol for FSRQs. The argument assumes a

How large fraction of the VHE sky resolved
by Fermi?
1

Fraction against total CGB

0.8

0.6

0.4

0.2

0
102

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

104
Photon Energy [MeV]

105

106

The Fermi-LAT
collaboration ‘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 of
exposure [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 the sky more evenat ~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

10

CTA - 1000 hrs

-14

10

102

103

104

105

106

Photon Energy (MeV)107

differences between instruments in both quantities. Below
100 GeVthe Fermi-LAT outperforms all ground-based instruments
Mrk 42110-10
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 measurementof a source. While HAWC’s performance in these quantities
10-11
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 the
3FGL
aforementioned
goals. In theenergy range at which this study is
2FHL
-12
10
focused,
HAWC is notcompetitivewith
the Fermi-LAT
and CTA
10-4
10-3
10-2
10-1
100
101
except 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)

10-8

349

108

Fig. 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.