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Exploring the Non-thermal Universe with Gamma Rays
Exploring the Non-thermal Universe with Gamma Rays
On the occasion
of the 60th birthday of Felix Aharonian
On the occasion of the 60th birthday of Felix Aharonian
Barcelona, November 6 - 9, 2012
Barcelona, November 6 - 9, 2012
EBL
Extragalactic
Background Light
Joel Primack & Alberto Domínguez
The Detection of the
The Detection of the
Cosmic
γ
-ray
Horizon
Cosmic γ-ray Horizon
The Detection of
o
Cosmic γ-ray Ho
Alberto Domíngue
(University of California, River
Extragalactic Background Light (EBL)
Joel Primack & Alberto Domínguez
Data from (non-) attenuation of gamma rays from blazars and
gamma ray bursts (GRBs) give upper limits on the EBL from the
UV to the mid-IR that are only a little above the lower limits from
observed galaxies. New data on attenuation of gamma rays
from blazers now lead to statistically significant measurements
of the cosmic gamma ray horizon (CGRH) as a function of
source redshift and gamma ray energy that are independent of
EBL models. These new measurements are consistent with
recent EBL calculations based both on multiwavelength
observations of thousands of galaxies and also on semianalytic models of the evolving galaxy population. Such
comparisons account for all the light, including that from
galaxies too faint to see. Catching a few high-redshift GRBs
with Fermi or low-threshold atmospheric Cherenkov telescope
(ACT) arrays could provide important new constraints on the
high-redshift star formation history of the universe.
Gamma Ray Attenuation due to γγ → e+e-
Illustration: Mazin & Raue
If we know the intrinsic spectrum, we can infer the
optical depth τ(E,z) from the observed spectrum. In
practice, we typically assume that dN/dE|int is not harder
than E-Γ with Γ = 1.5, since local sources have Γ ≥ 2.
More conservatively, we can assume that Γ ≥ 2/3.
Local EBL Observations
Γ ≥ 1.5
Γ ≥ 2/3
Evolution Calculated from Observations
Using AEGIS Multiwavelength Data
Alberto DomÍnguez, Joel Primack, et al. (MNRAS, 2011)
0.7 ☐°
http://aegis.ucolick.org/
χ SED Fitting
2
Le PHARE code for fitting the SWIRE templates in FUV, NUV, B, R, I, Ks, IRAC1, 2, 3, 4 and MIPS24
Quiescent
Starburst
AGN-type
Best SED Fits
Star-forming
Worst SED Fits
Domínguez+ 11
SED-Type Evolution
Local fractions, z<0.2:
Goto+ 03, morphologically classified from Sloan
converted to spectral classification using results
from Galaxy Zoo
Skibba+ 09 ~6% blue ellipticals
Schawinski+ 09 ~25% red spirals
Results:
Maximum uncertainty due to
photometry and fit errors
35% red-type galaxies
65% blue-type galaxies
High-redshift universe, z>1:
Two approaches:
1. Keep constant the fractions of our last redshift bin (Fiducial Model), or
2. Quickly increase starburst population from 16% at z = 0.9 to 60% at z ≥ 2
We find that the differences in the predicted EBL are small except at long
wavelengths, affecting attenuation only for E ≥ 5 TeV.
Domínguez+11
Local Luminosity Density
Domínguez+11
Local EBL
Observations
Domínguez+
11
vs. Domínguez+11
Propagating errors in SED fits
and redshift extrapolation
Γ ≥ 1.5
Γ ≥ 2/3
EBL Calculated by Forward Evolution using SAMs
When we first tried doing this (Primack & MacMinn 1996,
presented at Felix Aharonian’s first Heidelberg conference),
both the stellar initial mass function (IMF) and the values of
the cosmological parameters were quite uncertain. After
1998, the cosmological model was known to be ΛCDM
although it was still necessary to consider various
cosmological parameters in models. Now the parameters
are known rather precisely, and our latest semi-analytic
model (SAM) uses the current (WMAP5) cosmological
parameters. With improved simulations and better galaxy
data, we can now normalize SAMs better and determine the
key astrophysical processes to include in them.
Remaining uncertainties include whether the IMF is
different in different galaxies (possibly “bottom-heavy” in
massive galaxies), feedback from AGN, the nature of submm galaxies, and the star formation rate at high redshifts.
z=5.7 (t=1.0 Gyr)
Forward Evolution Present status of ΛCDM
“Double Dark” theory:
• cosmological parameters
are now well constrained
by observations
z=1.4 (t=4.7 Gyr)
Cluster Data
time
z=0 (t=13.6 Gyr)
Springel et al. 2005
Wechsler et al. 2002
• mass accretion history of
dark matter halos is~1012
represented by ‘merger
trees’ like the one at left
Galaxy Formation in ΛCDM
• gas is collisionally heated when perturbations ‘turn
around’ and collapse to form gravitationally bound
structures
• gas in halos cools via atomic line transitions
(depends on density, temperature, and metallicity)
• cooled gas collapses to form a rotationally
supported disk
• cold gas forms stars, with efficiency a function of
gas density (e.g. Schmidt-Kennicutt Law)
• massive stars and SNae reheat (and in small halos
expel) cold gas and some metals
• galaxy mergers trigger bursts of star formation;
‘major’ mergers transform disks into spheroids and
fuel AGN
• AGN feedback cuts off star formation
White & Frenk 91; Kauffmann+93; Cole+94;
Somerville & Primack 99; Cole+00; Somerville,
Primack, & Faber 01; Croton et al. 2006; Somerville
+08; Fanidakis+09; Guo+2011; Somerville, Gilmore,
Primack, & Domínguez 12 (discussed here)
Some Results from our Semi-Analytic Models
z=0 Luminosity Density
Modelling of the EBL and gamma-ray spectra
Evolving Luminosity Density
WMAP1
WMAP5
re 2. Left: the luminosity density of the local universe. The solid black line is the WMAP5 model, and the dotted line is the C!CDM mode
Gilmore,
Somerville,
Domínguez
(2012)
er of wavelengths are shown from GALEX (blue circles), SDSS (red stars;
Montero-Dorta
& PradaPrimack,
2009), 6dF&(light
blue squares;
Jones e
SS (green stars; Cole et al. 2001; Bell et al. 2003). In the mid- and far-IR, the orange squares are from IRAS (Soifer & Neugebauer 1991),
Some Results from our Semi-Analytic Models
Evolving Luminosity Functions
K-band
B-band
-1
-1
-1
-1
-2
-2
-2
-2
-3
-3
-3
-3
-4
-4
-4
-4
-5
-5
-5
-5
-6
-6
-6
18
20
22
24
18
20
22
24
-6
18
26
20
22
24
-1
-1
-1
-1
-2
-2
-2
-2
-3
-3
-3
-3
-4
-4
-4
-4
-5
-5
-5
-5
-6
-6
-6
18
20
22
24
26
18
20
22
24
26
18
20
22
24
18
20
22
24
-6
18
20
22
24
An advantage of the SAM approach is that it is
possible to compare predictions and observations
Gilmore, Somerville, Primack, & Domínguez (2012)
at all redshifts and in all spectral bands.
Some Results from our Semi-Analytic Models
3.6, 8, 24 and 24, 70, 160, &
Number Counts in
850 μm Bands
UV, b, v, i, and z Bands
Somerville, Gilmore, Primack, & Domínguez (2012)
Worst failure is at 850 μm
Modelling of the EBL and gamma-ray spectra
EBL from our Semi-Analytic Models
3195
Propagating D+11 errors in SED
fits and redshift extrapolation
WMAP1
Gilmore, Somerville, Primack,
& Domínguez (2012)
from all but the nearest extragalactic sources. The change in the functional form of
the EBL means that a simple z-dependent scaling model is inadequate.
Evolution of the EBL
Physical Coordinates
Co-moving Coordinates
FIGURE 5. The evolution of the EBL in our WMAP5 Fiducial model. This is plotted on the left panel
The evolution of the EBL in our WMAP5 Fiducial model. This is plotted on the left panel in
in standard units. The right panel shows the build-up of the present-day EBL by plotting the same
standard
units. The right panel shows the build-up of the present-day EBL by plotting the
quantities in comoving units. The redshifts from 0 to 2.5 are shown by the different line types in the
same quantities in comovingkey
units.
The
fromFig.
0 to52.5
are shown by the different
in the
leftredshifts
panel. (From
of [9].)
line types in the key in the left panel.
Gilmore, Somerville, Primack, & Domínguez (2012)
ments, and by the Large
Fermi gamma-ray space
AGILE (Tavani et al. 200
The Fermi LAT spends
and with its large area o
f
finding high-energy sour
685 high-energy sources
et al. 2010a). While the F
e
to ∼300 GeV, it has a mu
generation of ground-ba
se
ment is therefore most us
ef
threshold of these IACT
s,
EBL constraints availab
le
and gamma-ray bursts (G
paper by the Fermi collab
or
its on the EBL available
fr
the UV flux predicted in
presented here.
In this section and the f
effect of th
198Predicted
R. C. Gilmore
et
al.
Gamma Ray Attenuation
ments, and by
Increasing distance causes
Fermi
gammaabsorption features to
AGILE (Tavan
increase in magnitude
and
appear at lower energies.
The Fermi L
The plateau seen between
and
with
1 and 10 TeV
at low
z isits
a la
product of the
mid-IR high-en
finding
valley in the EBL spectrum.
685 high-energ
et al. 2010a). W
to ∼300 GeV,
generation of g
ment is therefo
threshold of th
EBL constrain
and gamma-ra
WMAP5 Fiducial
paper by the Fe
WMAP5 Fixed
its on the EBL
Domiínguez+11
the UV flux p
presented here
Gilmore, Somerville, Primack, & Domínguez (2012)
In this secti
Modelling of the EBL and gamma-ray spectra
3195
Figure 4. The predicted z = 0 EBL spectrum from our fiducial WMAP5 model (solid black) and WMAP5+fixed (dash–dotted violet) dust parameters, and
C!CDM (dotted black) models, compared with experimental constraints at a number of wavelengths. D11 is shown for comparison in dashed–dotted red with
the shaded area indicating the uncertainty region. Data: upward pointing arrows indicate lower bounds from number counts; other symbols are results from
direct detection experiments. Note that some points have been shifted slightly in wavelength for clarity. Lower limits: the blue–violet triangles are results from
HST and Space Telescope Imaging Spectrograph (STIS; Gardner et al. 2000), while the purple open triangles are from GALEX (Xu et al. 2005). The solid green
and red triangles are from the Hubble Deep Field (Madau & Pozzetti 2000) and Ultra Deep Field (Dolch & Ferguson, in preparation), respectively, combined
with ground-based data, and the solid purple triangle is from a measurement by the Large Binocular Camera (Grazian et al. 2009). In the near-IR J, H and K
bands, open violet points are the limits from Keenan et al. (2010). Open red triangles are from IRAC on Spitzer (Fazio et al. 2004), and the purple triangle at
15 µm is from ISOCAM (Hopwood et al. 2010) on ISO. The lower limits from MIPS at 24, 70 and 160 µm on Spitzer are provided by Béthermin et al. (2010)
(solid blue) and by Chary et al. (2004), Frayer et al. (2006) and Dole et al. (2006) (solid gold, open gold and open green, respectively). Lower limits from
Herschel number counts (Berta et al. 2010) are shown as solid red triangles. In the submillimetre, limits are presented from the BLAST experiment (green
points; Devlin et al. 2009). Direct detection: in the optical, orange hexagons are based on data from the Pioneer 10/11 Imaging Photopolarimeter (Matsuoka
et al. 2011), which are consistent with the older determination of Toller (1983). The blue star is a determination from Mattila et al. (2011), and the triangle
at 520 nm is an upper limit from the same. The points at 1.25, 2.2 and 3.5 µm are based upon DIRBE data with foreground subtraction: Wright (2001, dark
red squares), Cambrésy et al. (2001, orange crosses), Levenson & Wright (2008, red diamond), Gorjian et al. (2000, purple open hexes), Wright & Reese
(2000, green square) and Levenson et al. (2007, red asterisks). In the far-IR, direct detection measurements are shown from DIRBE (Schlegel, Finkbeiner &
Davis 1998; Wright 2004, solid red circles and blue stars) and FIRAS (Fixsen et al. 1998, purple bars). Blue–violet open squares are from IR background
measurements with the AKARI satellite (Matsuura et al. 2011).
Wavelength range
WMAP5 (fiducial)
WMAP5+fixed
C!CDM
D11
Optical–near-IR peak (0.1–8 µm)
Mid-IR (8–50 µm)
Far-IR peak (50–500 µm)
Total (0.1–500 µm)
29.01
4.89
21.01
54.91
24.34
5.16
22.94
52.44
26.15
5.86
24.08
56.09
24.47
5.24
39.48
69.19
"
C 2012 The Authors, MNRAS 422, 3189–3207
C 2012 RAS
Monthly Notices of the Royal Astronomical Society "
ersus
Table 1. The integrated flux of the local EBL in our models (WMAP5 with evolving and fixed
dust parameters, and the C!CDM model) and the model of D11. Units are nW m−2 sr−1 .
Gamma Ray Attenuation due to γγ → e+e-
Illustration: Mazin & Raue
If we know the intrinsic spectrum, we can infer the
optical depth τ(E,z) from the observed spectrum. In
practice, we typically assume that dN/dE|int is not harder
than E-Γ with Γ = 1.5, since local sources have Γ ≥ 2.
More conservatively, we can assume that Γ ≥ 2/3.
Reconstructed Blazar Spectral Indexes
With our SAM based
on current WMAP5
cosmological
parameters and
Spitzer (Rieke+09)
dust emission
templates, all high
redshift blazars have
spectral indexes
Γ≥1.5, as expected
from nearby sources.
Γ=1.5
(Of course, Felix can
make them much
harder!)
H 1426+428
1ES 0229+200
Gilmore, Somerville, Primack,
& Domínguez (2012)
mid-IR valley in the EBL spectrum.
With a 50 GeV
threshold, we
see to z≈1.5-3
with less than
1/e attenuation!
Cosmic Gamma-Ray Horizon
100 GeV
Threshold
50 GeV
Threshold
Gilmore, Somerville, Primack, & Domínguez (2012)
Mon. Not. R. Astron. Soc. 000, 1–?? (2011)
Printed 31 October 2012
(MN LATEX style file v2.2)
Approved by the Fermi Publication Board 31 Oct 2012
Detection of the cosmic -ray horizon from
multiwavelength observations of blazars
Submitted but
not on arXiv
A. Domı́nguez1? , J. D. Finke2 , F. Prada3,4,5 , J. R. Primack6 , F. S. Kitaura7 , B. Siana1 ,
8,9
10,11
12,13
S.
Ciprini
,
J.
Knödlseder
and
D.
Paneque
1
Dept. Physics & Astronomy, University of California, Riverside, CA 92521, USA
Naval Research Laboratory, Space Science Division, Code 7653, 4555 Overlook Ave. SW, Washington, DC, 20375 USA
3 Campus of International Excellence UAM+CSIC, Cantoblanco, E-28049 Madrid, Spain
4 Instituto de Fı́sica Teórica, (UAM/CSIC), Universidad Autónoma de Madrid, Cantoblanco, E-28049 Madrid, Spain
5 Instituto de Astrofı́sica de Andalucı́a (CSIC), Glorieta de la Astronomı́a, E-18080 Granada, Spain
6 Department of Physics, University of California, Santa Cruz, CA 95064, USA
7 Leibniz-Institut für Astrophysik (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
8 INAF, Observatory of Rome, Monte Porzio Catone, Roma, Italy
9 Agenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), Italy
10 Institut de Recherche en Astrophysique et Planétologie (CESR), F-31028 Toulouse Cedex 4, France
11 GAHEC, Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
12 Kavli Institute for Particle Astrophysics and Cosmology, SLAC, Stanford University, Stanford, CA 94305, USA
13 Max-Planck-Institut für Physik, D-80805 München, Germany
2 U.S.
Presented at the 4th Fermi Symposium in Monterey, CA
31 October 2012
The Detection of the
Cosmic γ-ray Horizon
ABSTRACT
The first statistically significant detection of the cosmic -ray horizon (CGRH) that
is independent of any extragalactic background light (EBL) model is presented. The
CGRH is a fundamental quantity in cosmology. It gives an estimate of the opacity
of the Universe to very-high energy (VHE) -ray photons due to photon-photon pair
production with the EBL. The only estimations of the CGRH to date are predictions
from EBL models and lower limits from -ray observations of cosmological blazars and
-ray bursts. Here, we present synchrotron/synchrotron self-Compton models (SSC) of
the spectral energy distributions of 15 blazars based on (almost) simultaneous observa(University
California,
Riverside)
tions from
radio up toofthe
highest energy
-rays taken with the Fermi satellite. These
Alberto Domínguez
SED multiwavelength fits
A one-zone synchrotron/SSC model is fit to the multiwavelength data excluding the
Cherenkov data, which are EBL attenuated. Then, this fit is extrapolated to the VHE regime
representing the intrinsic VHE spectrum. Technique similar to Mankuzhiyil et al. 2010.
y
r
a
n
i
m
li
e
r
P
PKS 2155-304
z = 0.116
Variability time
scale 104 s (fast)
y
r
a
n
i
im
l
e
Pr
1ES 1218+304
z = 0.182
Variability time
scale 105 s (slow)
Domínguez+12
Cosmic γ-ray Horizon: results
Propagating D+11 errors in SED
fits and redshift extrapolation
There are 4 out of 15 cases where our maximum likelihood methodology could not be applied since the prediction from the
synchrotron/SSC model was lower than the detected flux by the Cherenkov telescopes.
Two other cases where the statistical uncertainties were too high to set any constraint on E0.
Domínguez+12
Published Online in Science 4 Nov 2012
ABSTRACT The light emitted by stars and
Reports
accreting compact objects through the
history of the universe is encoded in the
intensity
process, a gamma-ray of
photonthe
of ener- extragalactic background
gy E and an EBL photon of energy
The Imprint of the Extragalactic
E
annihilate
and create anKnowledge
electronlight
(EBL).
of the EBL is
positron pair. This process occurs for
Background Light in the Gamma-Ray important
head-on collisions when (e.g.) E
to understand the nature of star
×E
m c ) , where m c is the
rest mass energy of the electron. This
Spectra of Blazars
formation
galaxy evolution, but direct
introduces an attenuation and
in the spectra
of gamma-ray sources above a critical
M. Ackermann, M. Ajello, * A. Allafort, P. Schady, L. Baldini, J. Ballet, G.
measurements
of the EBL are limited by
gamma-ray energy of E (z
Barbiellini, D. Bastieri,
R. Bellazzini, R. D. Blandford, E. D. Bloom, A. W.
170(1+z)
GeV (7, 8).
Borgland, E. Bottacini, A. Bouvier, J. Bregeon, M. Brigida,
P. Bruel,
The detection of
the gamma-ray
galactic
and
other foreground emissions.
R. Buehler, * S. Buson,
G. A. Caliandro, R. A. Cameron, P. A. Caraveo, E.
horizon (i.e., the point beyond which
Cavazzuti, C. Cecchi,
E. Charles, R. C. G. Chaves, A. Chekhtman, C. C.
the emission
of gamma-ray
sources is
Here,
we
report
absorption
in the
combined
spectra of a sample of
Cheung, J.
Chiang,
G. Chiaro,an
S. Ciprini,
R. Claus, J.feature
Cohen-Tanugi,seen
strongly attenuated) is one of the priJ. Conrad,
S. Cutini, F. D’Ammando,
F. de Palma,
C. D.
mary scientific drivers of the Fermi
gamma-ray
outA. to
a redshift
1.6.
This
feature
is caused
by attenuation
of
Dermer, S. W. Digel, blazars
E. do Couto e Silva,
Domínguez,
P. S. Drell, of
A. z ∼ Gamma-ray
Space Telescope
(9–11).
The
Imprint
of
the
EBL
Drlica-Wagner, C. Favuzzi,
S. J. Fegan, W. B. Focke, A. Franckowiak, Y.
Several attempts have been made in
Fukazawa,
S.
Funk,
P.
Fusco,
F.
Gargano,
D.
Gasparrini,
N.
Gehrels,
the past but none
detected the longgamma
rays
by the
EBL M.
atGiroletti,
optical
to ultraviolet
frequencies
and allowed us to measure
S. Germani,
N. Giglietto,
F. Giordano,
T. Glanzman, G.
sought EBL attenuation (12–14). So
in
Godfrey, I. A. Grenier, J. E. Grove, S. Guiriec, M. Gustafsson, D.
far, limits on the EBL density have
the
EBL
flux
density
in
this
frequency
band.
Hadasch, M. Hayashida,
E. Hays, M. S. Jackson,
T. Jogler, J.
been inferred from the absence of abEBL
EBL
2,3
2
9,10
2
2
2
2
31
32
2
19,20
2
12
2,35
38,39
2
18
13,14
31
33
34
33
33
11
2
2
14
13,14
6
16
25
13,14
15
13,14
17
2
2
13,14
crit
21
19,29,30
2
2
37
24,20
18
2
15
6
23
26,27,28
13,14
2
2
e
6
2
11
16
19,20
22
5
2
12
9,10
18
4
11
2 2
36,27
2
2
9
2
26,27,40
Kataoka, J. Knödlseder,
M. Kuss, J. Lande, S. Larsson,
L.
Latronico,41 F. Longo,7,8 F. Loparco,13,14 M. N. Lovellette,31 P. Lubrano,19,20 M. N.
Mazziotta,14 J. E. McEnery,33,42 J. Mehault,25 P. F. Michelson,2 T. Mizuno,43 C.
Monte,13,14 M. E. Monzani,2 A. Morselli,44 I. V. Moskalenko,2 S. Murgia,2 A.
Tramacere,45 E. Nuss,25 J. Greiner,4 M. Ohno,46 T. Ohsugi,43 N. Omodei,2 M.
Orienti,34 E. Orlando,2 J. F. Ormes,47 D. Paneque,48,2 J. S. Perkins,33,49,50,51 M.
Pesce-Rollins,11 F. Piron,25 G. Pivato,10 T. A. Porter,2, 2 S. Rainò,13,14 R. Rando,9,10
M. Razzano,11,12 S. Razzaque,21 A. Reimer,52,2* O. Reimer,52,2 L. C. Reyes,53 S.
Ritz,12 A. Rau,4 C. Romoli,10 M. Roth,54 M. Sánchez-Conde,2 D.A. Sanchez,55 J.
D. Scargle,56 C. Sgrò,11 E. J. Siskind,57 G. Spandre,11 P. Spinelli,13,14 ukasz
Stawarz,46,58 D. J. Suson,59 H. Takahashi,32 T. Tanaka,2 J. G. Thayer,2 D. J.
Thompson,33 L. Tibaldo,9,10 M. Tinivella,11 D. F. Torres,16,60 G. Tosti,19,20 E.
Troja,33,61 T. L. Usher,2 J. Vandenbroucke,2 V. Vasileiou,25 G. Vianello,2,62 V.
Vitale,44,63 A. P. Waite,2 B. L. Winer,64 K. S. Wood,31 M. Wood2
Downloaded from www.sciencemag.org on November 4, 2012
1
7,8
e
the Spectra of Blazars
sorption features in the spectra of individual blazars (13, 15), distant galaxies
with bright gamma-ray emission powered by matter accreting onto central,
massive black holes. While this feature
is indeed difficult to constrain for a
single source, we show that it is detected collectively in the gamma-ray
spectra of a sample of blazars as a cutoff that changes amplitude and energy
with redshift. We searched for an attenuation of the spectra of blazars in
the 1-500 GeV band using the first 46
months of observations of the Large
Area Telescope (LAT) on board the
Fermi satellite. At these energies
gamma rays are absorbed by EBL
photons in the optical to UV range.
Thanks to the large energy and redshift
coverage, Fermi-LAT measures the
intrinsic (i.e., unabsorbed) spectrum up
to 100 GeV for any blazar at z < 0.2,
and up to 15 GeV for any redshift.
The LAT has detected > 1000
blazars to date (16). We restricted our
search to a subset of 150 blazars of the
BL Lacertae (BL Lac) type that are
Presented at the 4th Fermi Symposium in Monterey, CA
The Imprint of the EBL
in
the Spectra of Blazars
*To whom correspondence should be addressed. E-mail: [email protected] (M.A.);
[email protected] (R.B.); [email protected] (A.R.)
Affiliations are listed at the end of the paper.
The light emitted by stars and accreting compact objects through the history of the
universe is encoded in the intensity of the extragalactic background light (EBL).
Knowledge of the EBL is important to understand the nature of star formation and
galaxy evolution, but direct measurements of the EBL are limited by galactic and
other foreground emissions. Here, we report an absorption feature seen in the
combined spectra of a sample of gamma-ray blazars out to a redshift of z 1.6. This
feature is caused by attenuation of gamma rays by the EBL at optical to ultraviolet
frequencies and allowed us to measure the EBL flux density in this frequency band.
Marco Ajello1,2,
Anita Reimer3, Rolf Buehler1
on behalf of the Fermi-LAT
collaboration
Analysis Procedure
We look for the collective deviation of the spectra of blazars from their intrinsic spectra
• 
We use 46months of P7V6 1-500 GeV data
• 
We define 3 redshift bins with 50 sources
each:
–  z= 0-0.2, 0.2-0.5, 0.5 -1.6
• 
All BL Lacs are modeled with a LogParabola
spectrum
• 
We perform a combined fit where:
–  The spectra of all sources are fit
independently
–  The spectra of all sources are modified
by a common e-b τ(E,z) term
• 
We evaluate 2 cases:
1.  Null hypothesis b=0 : there is no EBL
2.  Null hypothesis b=1 : the model
predictions are correct
preliminary
Fit to ‘unabsorbed’ data
Simulated SEDs
Simulated data
F(E) absorbed = F(E) int rinsic ⋅ e
−b⋅τ mod el
7
Measurement of Tau with Energy and Redshift
•  We use the composite likelihood in small
energy bins to measure the collective
deviation of the observed spectra from
the intrinsic ones
Best-fit EBL model!
•  The cut-off moves in z and energy as
expected for EBL absorption (for low
opacity models)
Ackermann+12
•  It is difficult to explain this attenuation
with an intrinsic property of BL Lacs
1.  BL Lacs required to evolve across the
z=0.2 barrier
2.  Attenuation change with energy and
redshift cannot be explained by an
intrinsic cut-off that changes from
source to source because of redshift
and blazar sequence effects
Best-fit intrinsic cut-off!
1
Composite Likelihood Results: 2
•  A significant steepening in the blazars’ spectra is detected
•  This is consistent with that expected by a ‘minimal’ EBL:
–  i.e. EBL at the level of galaxy counts
–  4 models rejected above 3sigma
•  All the non-rejected models yield a significance of detection of
5.6-5.9 σ
•  The level of EBL is 3-4 times lower than our previous UL (Abdo+10,
ApJ 723, 1082)
EBL Detection
Significance
Model Rejection
Significance
Ackermann+12
10
Cosmic Gamma-Ray Horizon
Dominguez+12
+
Ackermann+12
Cosmic -ray horizon [TeV]
Domı́nguez+ 11
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results. The Cherenkov Telescope Array (CTA; The CTA Consortium 2010) is another possible source of constraining events. The
CTA will have a lower threshold energy than current-generation
ground-based instruments and may be able to detect sources at
Mon. Not. R. Astron. Soc. 420, 800–809 (2012)
doi:10.1111/j.1365-2966.2011.20092.x
much higher redshift than currently achieved from the ground. Detections with either of these instruments could potentially shed new
light onfrom
star formation
thegamma-ray
re-ionizationbursts
era. and active galactic nuclei at
The Fermi satellite has detected GeV emission
a numberinof
ABSTRACT
420, 800–809 (2012)
Rudy C. Gilmore
Constraining the near-infrared background
light from Population III stars using highredshift gamma-ray sources
Constraining the near-infrared background
light
from Population
III sources place on the
the detections
of gamma-rays
from several of these
AC K N OW
L EEmission
D G M E Nfrom
T S these primordial stars, particularly
contribution
of Population
III stars togamma-ray
the extragalactic background
light.
stars
using
high-redshift
sources
could be derived from future detections of high-redshift sources
high6.redshift,
z ≳1.5.figure,
We examine
the constraints
that
Figure
As in the previous
but for a cut-off
redshift zr = 9.
redshifted Lyman α emission, can interact with gamma-raysRCG
to produce
electron–positron
pairsbyand
create an
optical depth to the
was supported
during this work
a SISSA
postdoctoral
with the Fermi LAT or future telescopes. In these plots, the axes
fellowship,
W. B.therefore
Atwood, constrain
J. Primack the
andproduction
A. Bouvier of this
propagation
ofand
gamma-ray
emission,
andenergy
the detection
of emission
at and
>10thanks
GeV can
!
of
a
refer
to
the
redshift
highest
observed
photon
E
γ
Rudy
C. Gilmore
for helpful discussions related to this project, and J. Colucci and the
background.
We source.
consider
initial
mass
functions
hypothetical
gamma-ray
Thetwo
source
is then
assumed
to havefor the early stars and use derived spectral energy distributions for each to put
SISSA, via Bonomea 265, 34136 Trieste, Italy
anonymous referee for reading the manuscript and providing useful
a normalization
at lower
energy
such that the
expected
number
of
upper limits
on the
star formation
rate
density
of massive
early
stars from redshifts 6 to 10. Our limits are complementary to those
comments. Some calculations in the paper were performed on the
in the
photon
and near-infrared
above Eγ is 1 [Nbackground
x (>Ehigh ) = 1]
setcounts
on a athigh
flux
by absence
ground-based
TeV-scale
observations
and show
that current data can limit star
SISSA
High-Performance
Computing
Cluster.
of any
background
field.
The
spectrum
of
the
source
is
set
here
to
Accepted 2011 October 27. Received 2011 October 27; in original form 2011 September 1 −1
formation
in theoflate
stages of
less than
Mpc−3 . Our results also show that the total background flux
−2.25,
near the mean
the sources
in re-ionization
Table 1, and thetop-EBL
is 0.5 M⊙ yr
ignored.
Given
these parameters,
the contours
on the plotsless
show
EFER
E N C E Sgalaxies at wavelengths below 1.5 μm.
from
Population
III stars must
be considerably
than thatRfrom
resolved
ABSTRACT
Abdo A. A. et al., 2009, Sci, 323, 1688
Abdo
A. A. et from
al., 2010a,
ApJ, 710,
810
The Fermi satellite has detected GeV
emission
a number
of gamma-ray
bursts and active
A. A.
al., 2010b,the
ApJ,constraints
723, 1082 that the detections of
galactic nuclei at high redshift, z Abdo
! 1.5.
Weet examine
Abel T., Bryan G. L., Norman M. L., 2000, ApJ, 540, 39
gamma-rays from several of these sources
place on the contribution of Population III stars to the
Aguirre A., Schaye J., Theuns T., 2002, ApJ, 576, 1
extragalactic background light. Emission
from these primordial stars, particularly redshifted
Aharonian F. et al., 2006, Nat, 440, 1018
Lyman α emission, can interact with
gamma-rays
to produce
pairs and create
Albert
J. et al., 2008,
Sci, 320,electron–positron
1752
an optical depth to the propagationAtwood
of gamma-ray
emission,
detection of emission at
W. B. et al.,
2009, ApJ,and
697,the
1071
Becker
R. H. et al.,
AJ, 122, 2850 We consider two initial
>10 GeV can therefore constrain the
production
of2001,
this background.
Giammanco
C., 2010,
Mem.
Soc. Astron. Ital.,
81, 460
mass functions for the early starsBeckman
and useJ.,derived
spectral
energy
distributions
for each
to
Bouché N., Lehnert M. D., Aguirre A., Péroux C., Bergeron J., 2007, MNput upper limits on the star formation rate density of massive early stars from redshifts 6 to
RAS, 378, 525
10. Our limits are complementaryBouwens
to thoseR.set
a high near-infrared
background
flux ApJ,
by 670,
J., on
Illingworth
G. D., Franx M.,
Ford H., 2007,
ground-based TeV-scale observations928
and show that current data can limit star formation in
−3
Mpc
. Rev.,
Our 46,
results
the late stages of re-ionization to Breit
less than
0.5 M!
yr−1
G., Wheeler
J. A.,
1934,
Phys.
1087 also show that
Bromm V.,III
Coppi
S., Larson
R. B., 2002, ApJ,
23 that from
the total background flux from Population
starsP. must
be considerably
less564,
than
Bromm1.5
V.,µm.
Loeb A., 2004, New Astron., 9, 353
resolved galaxies at wavelengths below
Brown R. L., Mathews W. G., 1970, ApJ, 160, 939
Cambrésy
L., Reach
W. T., Beichman
C. A., radiation.
Jarrett T. H., 2001, ApJ, 555,
Key words: gamma-ray burst: general
– stars:
Population
III – diffuse
563
Chabrier G., 2003, PASP, 115, 763
Cooray A., Yoshida N., 2004, MNRAS, 351, L71
Domı́nguez A.
et al.,is2011,
MNRAS,
410, 2556
cosmological
impact
therefore
the primary
way of understanding
Star Formation Rate Density
5σ
(Madau Plot)
3σ
2σ
Upper bounds on the redshift z = 6 - 9 Pop-III
SFRD in two possible scenarios with future
Fermi GRBs, in the Larson IMF case. The solid
lines show the limits from a GRB with the same
redshift and spectral characteristics of GRB
080916C (z = 4.35), but with a highest energy
observed photon of 30 GeV (160 GeV as
emitted) instead of 13.2 GeV, in combination
with the 5 most constraining z ≳ 2 sources
(Abdo+2010). The dotted lines show a case
with a GRB at z = 7 and a highest energy
observed photon at 15 GeV (120 GeV emitted).
Conclusions
New data on attenuation of gamma rays from blazers
● X-ray + Fermi + ACT SSC fits to 9 blazars (Dominguez+12)
● Fermi data on 150 blazars at z = 0 - 1.6 (Ackermann+12)
now lead to statistically significant measurements of the cosmic
gamma ray horizon and EBL as a function of source redshift
and gamma ray energy
These new measurements are consistent with recent EBL
calculations based both on multiwavelength observations of
thousands of galaxies and also on semi-analytic models of the
evolving galaxy population. Such comparisons account for all
the light, including that from galaxies too faint to see.
Catching a few high-redshift GRBs with Fermi or low-threshold
atmospheric Cherenkov telescope arrays could provide
important new constraints on the high-redshift star formation
history of the universe.
Happy Birthday Felix!