Status and perspectives of EBL limits fromFermi/LAT and IACTs
Daniel Mazin
Institute for Cosmic Ray Research, University of Tokyo
Max Planck Institute for Physics, Munichmazin@icrr.u-tokyo.ac.jp

CTA Meeting
The extreme universe viewed in very-high-energy gamma rays 20151

Outline
Extragalactic Background
Light (EBL)
Indirect methods tomeasure EBL
Instruments to measure
HE/VHE gamma rays
Limits by Fermi-LAT
Limits by Imaging
Cherenkov Telescopes
Future perspective: CTA
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 2016

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Stars and Dustin Galaxiesintrinsic
dN/dEAGN

EnergyHE/VHE ϒ-Rays
UV/O/IR
PhotonsdN/dE

measured

Pair production:
γTeV γEBL ➞ e+ e2 2ee+Energy

2E EEBL ⇡ 4(me c ) ⇡ 1 MeV
EEBL ⇠ eV ! E ⇠ TeV
Nikishov (1962), Jelley (1966), Gould & Schreder (1966)

Stars and Dust
in Galaxies

intrinsic
dN/dEAGN

EnergyHE/VHE ϒ-Rays
UV/O/IR
Photons

Constrain the EBL density- Measured spectrum + assumptions about theUnique information- Strong foregrounds hamper direct measurements- Redshift resolved- True integrated measurement
Stecker, de Jager 1992, Aharonian et al 2006, Mazin & Raue 2007 ...

dN/dEintrinsic spectrum- Many sources at different redshifts to disentangle
eEBL and intr. spectrume+measuredEnergyStars and Dustin Galaxies

AGN

HE/VHE ϒ-Rays
UV/O/IR
Photons

Investigate EBL sources- Star & dust in galaxies- Population III stars- Exotic contributions
Study star formation rate density- Structure formation history

Santos et al. 2002, Fernadez & Komatsu 2006, Raue, Kneiske, Mazin
2009, Gilmore 2011, Raue & Meyer 2012, ...

ee+The EBL energy density: data
DM et al., APh, 43 (2013), 241

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EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 2016

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The EBL energy density: models
Dominguez et al., MNRAS 410, 2556 (2010)

Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 20167

Imprint of the EBL on spectra of HE/VHE
gamma ray spectra of distant sources
Cross section of pair production:
peaks at ~4*Ethreshold
Delta function approximation is
not precise

Attenuation of Gamma-ray flux is
calculated by integrating over number
density of EBL, angles between
photons, and distance to the source.
The attenuation factor is sensitive tothe EBL density

Dominguez et al., MNRAS 410, 2556 (2010)
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 20168

Indirect methods to measure theEBL: Sources
Typical extragalactic source at HE/VHE gamma rays is ablazar
blazarAGN

Factories of violent, broad band (up to highenergy) non-thermal radiation!
Blazars: relativistic plasma jet, highly variable!
TeV blazars (above 100 GeV): ~50 out of 61
are HBL (High-peaked BL Lacertae)
Daniel Mazin

Typical energy spectrum has a 2-bump structure!
First bump is synchrotron emission from electrons!
Second bump probably Inverse Compton (IC)from the same population of photons (SSC) orIC from external photon fields (EC)EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 20169

Indirect methods to measure theEBL: Assumptions
There is no pile-up at highest gamma rays energies
measured
PKS 2155-304

B

Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 2016

A

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Indirect methods to measure theEBL: AssumptionsThere is no hardening of the energy spectrum between HE
regime (typically measured by Fermi-LAT)and VHE regime measured by the imaging Cherenkov telescopes

DM et al, Aph 43, 241-251 (2013)
intrinsic (not affected by EBL)
Daniel Mazin

absorbed (affected by EBL

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201611

Indirect methods to measure theEBL: Assumptions
The intrinsic energy spectrum is smoothno wiggles with correct EBL density

wiggles with in-correct EBL density
DM et al, Aph 43, 241-251 (2013)
Daniel MazinEBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201612

Indirect methods to measure the
EBL: Instruments
High energy gamma rays are measured by Fermi-LAT (also Agile)

100 MeV < E < 300 GeV
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201613

Indirect methods to measure the
EBL: Instruments
Very High energy gamma rays are measured by Imaging Atmospheric Cherenkov Telescopes (IACTs):
H.E.S.S., MAGIC and VERITAS (as well as Water Cherenkov Detectors (HAWC) which, however, have amuch worse energy resolution)

50 GeV < E < 100 TeV

VERITASMAGIC+30

HESS-30

Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201614

Limits from Fermi-LAT
Idea: F(E)observed = F(E)intrinsic ∙ exp(-b ∙ τmodel)
with b as a free parameter
F(E)intrinsic is extrapolated from measured spectrum in unabsorbed
energies to higher energies from the same data.
F(E)observed is measured spectrum.
τmodel is gamma-ray optical depth calculated from a given theoretical
EBL model
Expected result:b=0: there is no EBL
b=1: the EBL model is correct
Fermi-LAT, Science, 338, 1190 (2012)
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201615

Limits from Fermi-LAT
Average of the residuals with respect !
to the best fit models for all z<0.2 bin.

Assumption: BL Lac spectra follow
a parabolic shape
Residuals for best fit b are flat

Results: EBL is detected and consistent with state-of-the-art EBL models
Fermi-LAT, Science, 338, 1190 (2012)
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201616

Limits from Fermi-LAT
EBL is detected and consistent with state-of-the-art EBL models

Fermi-LAT, Science, 338, 1190 (2012)
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201617

Limits from Fermi-LAT
2FHL: Second Fermi Hard Source Catalog, 50GeV < E < 2 TeV

Fermi-LAT, ApJ submitted, arXiv:1508.04449
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201618

Limits from Fermi-LAT
2FHL: Second Fermi Hard Source Catalog, 50GeV < E < 2 TeV
Source classes:

distribution for 158 BL Lacs that arecommon for 1FHL, 2FHL and 3FGL

Fermi-LAT, ApJ submitted, arXiv:1508.04449
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EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201619

Limits from Fermi-LAT
Using 2FHL: No unexpected features found
Intrinsic spectral index: no evolution with redshift

Dominguez+Ajello, ApJ, 813 (2015) L34
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EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201620

Limits from IACTs
Reminder: effect of EBL:

H.E.S.S. from several sources
H.E.S.S., A&A 550, 11 (2013)

Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 2016

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MAGIC from single sources
MAGIC, submitted (2015)

Test Statistics (TS)Limits from IACTs

EBL model normalization
preliminary

preliminary

Using data from 1ES1011+496 (z=0.211)
Daniel Mazin

EBL normalization = 1.07(+0.09-0.13)stat(+0.11-0.07)syst

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201622

Limits from IACTs
Combined results from many sources and data from
diﬀerent experiments makes results more robust
Biteau&Williams, arXiv:1502.04166

Meyer, DM,++ A&A 542 A59 (2012)

EBL detection

Stringent upper limits

Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201623

Cosmology: Hubble parameterOnce EBL is known, the imprint of the EBL on VHE spectra ofAGNs can be used to independently estimate their distanceDominguez++, ApJ 770, 77 (2013)

Biteau&Williams, arXiv:1502.04166

H0=( 71.8+4.6-5.6 (stat) +7.2-13.8 (syst)) km/s/Mpc

H0= (88 +/-8 (stat) +/- 13 (syst)) km/s/Mpc

Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201624

10 fold sensitivity of current instruments
10 fold energy range
improved angular resolution
two sites (North / South)
operated as observatory

The future in
VHE gamma ray
astronomy:!

World-wide cooperation
32 countries
150 institutes>1400 scientists

Topic of this conferenceFuture prospects: CTA
CTA will have two observatories, North and South,
cover 20 GeV to >300 TeV, survey capabilities
sensitivity vs energy

sensitivity vs time at low energies

Plots from Funk&Hinton, AP 43, 348-355 (2013)
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201626

CTA: Key Science Projects
Three of 10 KSPs will deal (at least partially) with EBL and IGMF:AGN survey:
obtain high quality spectra of ~50 bright blazars at diﬀerent z
extensive Target of Opportunity program
GRBs: chance to obtain high quality signal from z>1 (actually there is noupper limit on z)
Extragalactic survey: find new hard spectra GeV-TeV sources
6040

Fermi/LATCTA exposure
for AGN survey

200-20-40

Detectabl

AGNs detgalactic latitude (deg)80hours+90°+180°-180°

144 extragalactic sources
486 galactic sources

AGNs in CTA ExG

Sources CTA Gal

Sources in Fermi/-60

AGNs detected by-80-90°

galactic longitude (deg)Time distribution for extragalactic KSPs
Daniel Mazin

CTA Galactic and extragalactic surveys

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201627

Future prospects: CTA
CTA can measure the evolution of the EBL,
which can help constraining the star and galaxy evolution
scenarios
sensitivity of CTA to resolve EBL evolution

Star formation rate is notwell determined at z>1

EBL scaling factor

HESS limits
Fermi/LAT limits

2CTA prediction

1.8
1.6
1.4
1.2
10.8
0.6
0.4

preliminary

0.2
00

0.2

0.4

0.6

0.8

DM, in preparation
Daniel Mazin

11.2

1.4

1.6

1.8
2
redshift z

Horiuchi & Beacom, ApJ 723,329-341 (2010)

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201628

Conclusion
EBL can be constrained and detected indirectly with Fermi/LATand Imaging Cherenkov Telescopes
Derived limits are consistent with the EBL from resolved
sourceswhich means that the current limits and measurements suggestthat at z=0 there is no significant contribution to the EBL byunresolved sources
Some pile-ups at high energies in the spectra of blazars are
intriguing but not significant yet (could be a sign of axion-likeparticles)
CTA will be an eﬀective machine to resolve EBL evolution to z=2
Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 2016

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BACKUP30

Limits from IACTs: axions
Contradicting results:
J. Biteau & D. Williams (2015) vs. D. Horns & M. Meyer (2012)

Daniel Mazin

EBL limits @ CTA Meeting, ICRR, Kashiwa, Jan 14, 201631