Indications for a cascade component in {\gamma}-ray blazar spectra

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by  S. Baklagin

XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 1
Indications for a cascade component in γ-ray blazar spectra
S. Baklagin, T. Dzhatdoev, E. Khalikov
Federal State Budget Educational Institution of Higher Education M.V. Lomonosov Moscow State University,
Skobeltsyn Institute of Nuclear Physics (SINP MSU),
1(2), Leninskie gory, GSP-1, Moscow 119991,
Russian Federation; e-mail: [email protected], [email protected]
A. Kircheva
Federal State Budget Educational Institution of Higher Education M.V. Lomonosov Moscow State University,
Department of Physics, 1(2), Leninskie gory, GSP-1, Moscow 119991,
7 Russian Federation and Federal State Budget Educational Institution
1 of Higher Education M.V. Lomonosov Moscow State University,
0 Skobeltsyn Institute of Nuclear Physics (SINP MSU), 1(2),
2 Leninskie gory, GSP-1, Moscow 119991, Russian Federation

n
a A. Lyukshin
J Federal State Budget Educational Institution of Higher Education M.V. Lomonosov Moscow State University,

3 Department of Physics, 1(2), Leninskie gory, GSP-1, Moscow 119991, Russian Federation

] Wepresentaverybriefoverviewofsomerecentγ-rayobservationsofselectedblazarstorevealan
E
indicationforaconsiderableorevendominantcontributionofsecondaryγ-raysfromelectromagnetic
H cascades to the observable spectra in the 1–500 GeV energy range.
.
h
p
I. INTRODUCTION ics on primary γ-ray energy would be crucial while
-
o using CMB photons at E0 <100 TeV as a probe of
r the γγ →e+e− process.
t The Universe is filled by light. There are two prin-
s
a cipalsourcesofsuchphotons—thecosmicmicrowave In the present conference contribution we empha-
[ background(CMB)andtheextragalacticbackground size on the potential importance of the development

light (EBL). While CMB properties are quite well of electromagnetic cascades on the EBL/CMB to the
1
measuredandtheoretically understood for more than interpretationofobservationsinthehighenergy(HE,
v
1 20 years (e.g. [1]), many EBL models with widely E=100 MeV–100 GeV) and the very high energy
4 different parameters were developed [2]–[10]; for in- (VHE, E=100GeV–100TeV) ranges. This shortpa-
7 stance,thetotalEBLintensityforthesemodelsdiffers per does not pretend to claim great originality; most
0 by more than a factor of two [11]. Comparatively re- of its basic ideas were already discussed and most of
0 cently, however,some of these EBL models startedto its main results were already presented in [15]–[19].
.
1 converge,atleastinthe 0.5–10nm wavelengthregion
0 (e.g., [4] and [9]).
7
High-energy γ-rays with primary energy E0 >
1
0.25(1[eV]/ǫ)[TeV] get absorbed [12]–[13] (γγ →
:
v e+e−) on EBL and CMB photons with energy
Xi ǫ, reaching maximum cross section at E0 ≈ II. EXTRAGALACTIC γ-RAY
PROPAGATION: ANOMALIES AND MODELS
(1[eV]/ǫ)[TeV] [9]. Therefore, the improved reliabil-
r
a ity of EBL models allows for more detailed studies of
thisfundamentalquantumelectrodynamicsprocessin
A. Absorption-Only Model
the γ-ray energy range of about 500 GeV– 10 TeV,
and of extragalactic γ-ray propagation effects in gen-
eral. Soon after the discovery of the first TeV extra-
UsingtheCMBforsuchstudiesis,inprinciple,also galacticγ-raysource[20],thefirstγ-astronomicalcon-
possible [14],but atpresentis notfeasible due to sev- straints on the EBL density were obtained [21]–[22],
eralfactors. Indeed,the typicalmeanfreepathLofa accountingforonlythepair-productionprocessasthe
∼1015 eV=1 PeV γ-rayonthe CMB is∼10kpc; for main cause of transformation of the primary γ-ray
such high energies there is still no discoveredsources, spectrum (for very distant sources, it is necessary to
while using lower energies is not convenient either as include adiabatic losses). This model we call “the
in the E0 <100 TeV energy region the L(E0) depen- absorption-only model”. Since 1993, almost every
dence is very strong: L falls for about an order of year several papers appear, assuming the absorption-
magnitude for every 10 % of E decrease. Therefore, only model, sometimes at a rate of a dozen a year or
extremely well knowledge of experimental systemat- more.
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2 XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016
FIG. 1. Some constraints on theEGMF strength in voids(a figure from [19]).
B. Anomalies constraints: 10−14 G < B <2·10−12 G and 10−17
G < B <3·10−16 G. The first case corresponds to
In whatfollows,anysignificantdeviations fromthe the absorption-only model regime: for such a strong
absorption-onlymodel are called “anomalies”,even if EGMFsecondaryelectronsare,asarule,stronglyde-
theydonotfallbeyondtheconventionalphysics. Sev- flected and delayed so that cascade photons do not
eralsuchanomalieswerereported[23]–[27],withasta- contribute to the point-like image of the source. On
tistical significance not overwhelming in every single the otherhand, forthe caseofthe secondoptionsuch
case, but together they indicate that the absorption- a contribution is still possible, at least in the VHE
only model is incomplete and must be modified in energyrange. ThelowerboundontheB value,10−17
some way (see [15] for more details). G, is highlyuncertain[28]-[29]. Very recently apaper
[30] appeared, disfavouring a narrow range of values
around B= 10−14 G for Lc= 1 Mpc.
C. Extragalactic Magnetic Field
Secondary electrons and positrons (hereafter sim- D. Electromagnetic Cascade Model
ply “electrons”) produced in the γγ → e+e− pro-
cess radiate cascade γ-rays by means of the Inverse Let us assume for a moment the following sim-
Compton (IC) process; these γ-rays may contribute ple two-phase model of the EGMF. LSS voids with
totheobservablespectrumofanextragalacticsource. B = 0 fill a fraction of the total volume 0 < V < 1,
Therefore, besides the properties of background pho- whiletherestisoccupiedbythecomparativelystrong
tonfields(EBL/CMB),anotherveryimportantfactor (B > 10−14 G) EGMF with Lc ∼ 1 Mpc. Figure 2
isthestrengthandstructureoftheextragalacticmag- shows severalfits to the observedspectral energy dis-
netic field (EGMF). Some selected constraints on the tribution (SED) of blazar 1ES 0229+200([31] for the
EGMF strength B in voids of the large scale struc- four lowest-energy bins shown in black and [32] for
ture (LSS), mostof them assuming correlationlength other bins) assuming this model with various values
of the field Lc= 1 Mpc,are shownin Figure 1. There of V. Cascade component dominates at low ener-
are two regions of B values that satisfy most of these gies(E <300–500GeV). Detailsofsimulationscorre-
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XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 3
FIG. 2. Several fits (solid curves) to the spectrum of blazar 1ES 0229+200 [31]–[32] obtained in the framework of the
intergalactic electromagnetic cascade model. Black curve denotes V= 1, green curve — V= 0.6, blue — V= 0.4, cyan
— V= 0.3, magenta — V= 0.2. Primary (intrinsic) spectra for each case are also shown by the same colors (dashed
curvesin theright part of thegraph). Fulluncertainty for the four lowest-energy bins is shown by black dashed lines.
FIG. 3. The main spectral signatures of theelectromagnetic cascade model (a figurefrom [35]).
spondtothe caseofFigure15of[15]. Reasonablefits delay of cascade electrons while travelling the EGMF
intheenergyrange30GeV–10TeV maybeobtained [33]–[34]orbyadditional(withrespecttoIC)electron
forthecaseofV between0.4and0.6. However,below energy losses [29].
30GeV themodelintensityexceedstheobservedone. Figure3depictsthemainspectralsignaturesofthe
Thismaybeexplainedeitherbythedeflectionand/or electromagnetic (EM) cascade model: 1) high-energy
eConf C16-09-04.3
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cutoff,similartotheabsorption-onlymodel,2)anan- model [27].
kleattheintersectionoftheprimaryandcascadecom- Thus, the EM cascade model appears to be the sim-
ponents (in fact, this signature is also visible in Fig- plestextragalacticγ-raypropagationscenariothatco-
ure2),3)alow-energycutoffforthecaseofanon-zero herently explains all these anomalies. Other, more
EGMF value (“magnetic cutoff”), and 4) a “second exotic models that could account for a part of these
ankle” at the low-energy region of the spectrum. anomalies do exist [37]–[38]; the main difficulties of
these scenarios were discussed by us in [15],[18].
E. Explanation of anomalies in the EM Cascade
Model III. CONCLUSIONS
We argue that all above-mentioned anomalies may Recent observations of some blazars in the 1 GeV
be interpreted within the framework of the EM cas- –10TeV energyregionseemtoindicate thatthesec-
cade model, namely: ondary component of cascade γ-rays may constitute
1. The apparent excess of observed γ-rays in the a considerable contribution to the observable flux in
highest-energybins claimedin[23]–[24]wasdiscussed the E =1–500GeV energy range.
by [15]. A prominent ankle (signature 2 in Figure 3)
may account for this effect.
2. The unusual spectral hardening towards lower en-
ergies[25]isexplainedbytheexistenceofa“magnetic
ACKNOWLEDGMENTS
cutoff”(signature3inFigure3),astheauthorsof[25]
themselves note (however, see [36] and [25] itself for
This work was supported by the RFBR Grant 16-
other possible explanations).
32-00823. T.D. acknowledges the support of the Stu-
3. The same spectral feature (a prominent magnetic
dents and ResearchersExchangeProgramin Sciences
cutoff)mayaccountfortheresultof[26],asdiscussed
(STEPS),the Re-InventingJapanProject,JSPS,and
in [35].
the hospitality of the University of Tokyo ICRR.
4. Finally,“halos”observedaroundsome blazarsalso
may be explained in the context of the EM cascade
nd
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