Blazars as astrophysical and cosmological probes.

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Fa oltà di S ienze Matemati he, Fisi he e Naturali

Dipartimento di Fisi a e Matemati a

Dottorato di Ri er a in Astronomia e Astrosi a

BLAZARS AS ASTROPHYSICAL AND

COSMOLOGICAL PROBES

Supervisor:

Prof. Fran es o Haardt

Massimo Cavadini

Matri ola 610606

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Contents

1 Introdu tion 7

1.1 Status of the VHE astrophysi s . . . . . . . . . . . . . . . . . 7

1.2

γ

^-ray ^spa
e ^teles
opes ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ⁷

1.3 Groundbased dete tors . . . . . . . . . . . . . . . . . . . . . 10

2 Extragala ti

γ

^-ray ^ba
kground ¹³ 2.1 Introdu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 Blazars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.1 Generalfeatures. . . . . . . . . . . . . . . . . . . . . . 14

2.2.2 The BlazarSED . . . . . . . . . . . . . . . . . . . . . 16

2.2.3 Emissionme hanisms . . . . . . . . . . . . . . . . . . . 17

2.2.4 Blazarsin

γ

^-ray ^band^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ¹⁹

2.3 The Fermi-LAT EGB. . . . . . . . . . . . . . . . . . . . . . . 23

2.4 The Blazar ontributiontoEGB . . . . . . . . . . . . . . . . . 23

2.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.6 Star-forminggalaxy omponent . . . . . . . . . . . . . . . . . 31

2.7 Dark Matter omponent . . . . . . . . . . . . . . . . . . . . . 32

2.8 Dis ussion and Con lusions . . . . . . . . . . . . . . . . . . . 33

3 The intergala ti magneti eld 39 3.1 Introdu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.2 Observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

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3.2.2 Constraintsfrom CMB anisotropies . . . . . . . . . . . 44

3.2.3 Costraints fromBig Bang Nu leosynthesis . . . . . . . 44

3.2.4 Constraintsfrom

γ

^-ray observations . . . . . . . . . . . 45

3.3 Time delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.4

γ

^-ray ^data ^of ^1ES ^0229+200 ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ⁵⁰

3.5 Model for as ade radiation . . . . . . . . . . . . . . . . . . . 51

3.6 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.7 Con lusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4 Extragala ti Ba kground Light 61 4.1 Introdu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.2 Observations and measurements . . . . . . . . . . . . . . . . . 63

4.3 Theoreti al Models . . . . . . . . . . . . . . . . . . . . . . . . 67

4.4 Our model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.4.1 Comovingemissivity . . . . . . . . . . . . . . . . . . . 70

4.4.2 Syntheti Galaxy Spe tra . . . . . . . . . . . . . . . . 70

4.4.3 Star-formation Rate History . . . . . . . . . . . . . . . 71

4.4.4 Redshift-Metalli itydistribution . . . . . . . . . . . . . 73

4.4.5 Dust absorption and re-emission . . . . . . . . . . . . . 74

4.4.6 Extragala ti ba kground lightmodel . . . . . . . . . . 80

4.4.7 Comparison with other models. . . . . . . . . . . . . . 81

4.4.8

γ

^-ray ^opti
al ^depth ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ^. ⁸¹

4.5 Con lusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

5 Summary and on lusions 87

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Prefa e

In the last three years the number of extragala ti

γ

^-ray ^sour
es ^in
reased

dramati allythanks to AGILE and Fermi

γ

^-ray ^teles
opês ând ^to ^new îm-

provements in the ground based Cherenkov dete tors. Blazars, radio loud

A tive Gala ti Nu lei (AGN) with a relativisti jet pointing toward the

Earth, result tobe the most ommonsour es inthe extragala ti

γ

^-ray ^sky.

In the GeV band up to one thousand sour es have been dete ted in the ex-

tragala ti sky, allowing statisti alstudies of blazar sour es.

γ

^-ray astrophysi s has signi ant onne tions with other apparentlyfar and dierentbran hesofastrophysi sand osmology.

γ

^-ray^photons^are^absorbed

by lower energy opti al and infrared radiation as they travel toward the

Earth. Thestudyoftheabsorbed spe traofblazarsallowstoput onstraints

ontheintergala ti magneti eld (IGMF),onthe intensity of Extragala ti

Ba kgroundLightandalsoonthe rossse tionandmassofannihilatingdark

matter (DM) parti les. During my PhD I have omputed the ontribution

of blazarsto the Extragala ti

γ

^-ray ^Ba
kground ^(EGB)^and ^I ^have ^derived

an upper limit on the role played by annihilating DM. Moreover studying

the as ade generated by the absorption of

γ

^-ray ^photons ^by ^EBL ^I ^have

derived new lower limits on the IGMF intensity. Finally I have proposed a

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Chapter 1

Introdu tion

1.1 Status of the VHE astrophysi s

The term

γ

^-ray astrophysi s is applied to photons that span 14 orders of magnitude, between

0.5 × 10 ⁶

^e^V ^to

∼ 10 ²⁰

^e^V. ^The ^lower ^bound ^is ^due ^to

the ele tron/positron pair annihilation while the upper bound hara terizes

the energy of photons produ ed by the highest energy parti le observed in

osmi rays.

γ

^-ray astrophysi s is divided in six areas: low (LE: below 30 MeV), high (HE: 30 MeV-30 GeV), very high (VHE: 30 GeV-30 TeV), ultra

high(UHE:30TeV-30PeV)and nallyextremelyhigh(EHE:above30PeV)

energies. InthisthesiswereferonlytoVHE

γ

^-rayastrophysi s. Inthisrange of energies, observations are performed by orbiting teles opes (30 MeV-100

GeV) and by ground based dete tors (100 GeV-20 TeV). In the following

se tions we review briey the features of these two lasses of dete tors and

their evolution. A urate review arti les an be found in Aharonian& Volk

(2001),Enomoto etal. (2003) and Aharonian(2004).

1.2

γ

^-ray ^spa
e ^teles
opes

The

γ

^-ray ^satellites âre ^based ôn ^the ônversion ôf ^the ^primary ^photons ^to

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of the se ondary ele trons with tra king dete tors and their energy with a

total-absorption alorimeter. Thiste hniqueallows there onstru tion ofthe

arrival dire tionand energy of the primary

γ

^-rays. ^The ^energy ^resolution ^is

mainlydue to the absorbing apability of the alorimeter.

The rst signi ant

γ

^-ray observational results appeared in the 70s due to the satellitesSAS-2 (Fi htel,Simpson, &Thompson 1978)and COSB (e.g.

Bignami&Hermsen1983). SAS-2 dete ted four pointsour eswhile COS-B

mission in reased the number of sour es to 25 one of whi h was identied

with the quasar 3C 273 that was the rst extragala ti

γ

^-ray ^sour
es ^de-

te ted.

The EGRET, as part of the Compton Gamma Ray Observatory mission,

during nine years of operations (1991-2000) dete ted 271 sour es of whi h

66 extragala ti (Hartman et al. 1999). The large majority of these ex-

tragala ti sour es were blazars. Moreover the teles ope has provided the

rst reliablemeasure ofthe Extragala ti

γ

^-ray^ba
kground^(EGB)ⁱⁿ^the²⁰

MeV-30GeV band (Strong, Moskalenko, &Reimer 2004).

On 2008June 11the Gamma-rayLargeAreaSpa eTeles ope(GLAST)was

laun hedtoimprovethe previousEGRET observations. Shortlyafterenter-

ing its s ienti operating mission, on 2008 August, GLAST was renamed

Fermi Gamma-ray Spa e Teles ope. The main instrument onboard Fermi

is the Large Area Teles ope (LAT), apair onversion teles ope overing the

energy band from20 MeV up to 300 GeV (e.g. Atwood etal. 2009). In the

sky-survey mode, LATobserves the entire sky every 3hours.

After threeyears ofobservations (September2011)Fermi-LAThas dete ted

861 extragala ti sour eswith high onden e allowingastatisti alstudy of

extragala ti

γ

^-ray ^sour
es^and ^providing^a^strong improvement inthe EGB (see se tion 2.2.4 for further details and Fig. 1.1). Fig. 1.2 shows the om-

parisonbetween the diuse omponentof EGBdete ted by EGRET and by

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Figure 1.1: Lo ations of the sour es in the Clean Sample of the 2LAC (see

hapter2). Red: FSRQs, blue: BLLa s, magenta: non-blazarAGNs, green:

AGNs of unknown type (fromThe Fermi ollaboration2011).

Figure 1.2: The omparison between the EGRET and Fermi-LAT

γ

^-ray

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1.3 Ground based dete tors

Athigher energies, above100 GeV,satellite-baseddete tors are not e ient

be auseofthelowuxesinvolved. Groundbaseddete torsrepresentthebest

waytomeasure

γ

^-ray ^photonsⁱⁿ^theûpper^VHE^band. Îtîs^well^known ^that

VHE

γ

^-rayâre âbsorbed ^by ^theÊarth âtmosphereând^produ
e êxtended ât-

mospheri showersofhadronparti le. Thusarrayofparti le(muon,ele tron,

hadron) dete tors used in the traditional osmi rays experiments were rst

beenusedastoolstoindire tlydete t

γ

^-rays^photons ônÊarth. ^The^rstêx-

perimentbuiltwiththis aimwasCASA-MIA (Borioneetal. 1994). Imaging

Atmospheri Cherenkov Teles opes (IACTs) use a more e ient te hnique.

They dete t the Cherenkov light produ ed by parti les in atmosphere gen-

erated by

γ

^-ray^photons ûsing ânôpti
al^teles
ope^that ^fo
us ^the ^Cherenkov

lightof ashowerintoapixelized amera. ThefourmajorIACT experiments

at are MAGIC (Colin et al. 2009), HESS (Chaves 2009), CANGAROO III

(Kushida et al. 2003) and VERITAS (Holder 2007). These four ollabora-

tionsare involvedintheambitiousproje t alledCherenkov Teles opeArray

(CTA). The idea is to build an array of IACTs with a lower threshold and

a better sensitivity. Up to now (September 2011) 46 extragala ti sour es

have been dete ted by IACTs above 300 GeV, (Fig. 1.3) of whi h the large

majority are BL-La s.

Fig. 1.4 shows the so- alled Kifune plot in whi h the evolution of sour e

dete tion is displayed.

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-180o +180o

+90o

-90

o Mkn 421

Mkn 501

2344+514 1959+650

2155-304 1426+428

M87

BL Lac

2005-489 1218+304

1553+113 Mkn 180

1101-232

2356-309

0548-322

0229+200

0347-121 1011+496

3C 279

0152+017 0806+524

W Comae

0716+714

3C 66A

Cen A 0710+591

1424+240

NGC 253 M82

0521+211

0317+185

0414+009 0502+675

0447-439 1510-089

0648+152

IC 310

1222+216

AP Lib

2001+435

1440+122

2247+381 NGC 1275

SHBL J001355.9-185406

1215+303

-ray sources γ

Extragalactic VHE

>100 GeV) (E

γ

2011-01-08 - Up-to-date plot available at http://www.mpp.mpg.de/~rwagner/sources/

Redshift z

0 0.1 0.2 0.3 0.4 0.5

Figure 1.3: The VHE (

>

³⁰⁰ ^Ge^V) ^maps ^for extragala ti sour ed dete ted by IACTs (From http://www.mpp.mpg.de/ rwagner/sour es/)

Figure 1.4: The Kifune plot: the number of sour e as fun tion of time for

X-ray (green line),

γ

^-ray ^(blue ^line) ^and ^very ^high ^energy

γ

^-ray ^(red-line)

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Extragala ti

γ

^-ray ^ba
kground ¹³

Chapter 2

Extragala ti

γ

^-ray ^ba
kground

2.1 Introdu tion

The EGB represents a fas inating hallenge sin e his rst dete tion by SAS

2 satellite above 30 MeV (Fi htel, Simpson, & Thomson 1978). The

γ

^-ray

teles ope EGRET, improving the SAS 2 dete tion, measure a isotropi

γ

^-

rayemissioninthe 30MeV-30GeV range. The spe trumofthe diuseEGB

omponent, that is the emission due to unresolved sour esand/or truly dif-

fusepro esses, anbetoverthe entire band with apowerlawwith photon

spe tralindex

Γ ∼ 2.1±0.03

^(Sreekumarêtâl. ^1998;^Strong^Moskalenkoând

Reimer 2004). This value is similar to the average photon index of blazars

dete tedby EGRET.This resultandthe fa tthat blazarsare themost om-

mon obje tsin the

γ

^-ray ^sky, ^led ^the ^ommunity ^to ^propose ^models ^able ^to

explain the EGB shape in terms of blazar emission (Padovani et al. 1993;

Ste ker Salamon & Malkan 1993; Chiang et al. 1995; Ste ker

&

^Salamon

1996;Mu ke

&

^Powl^2000; ^Dermerêtâl. ^2007;Înoue

&

^T^otani^2009,^Ste
ker

&Venters 2010; Venters & Pavlidou 2011)

As the new EGB measure performed by Fermi-LAT is more steeper and

withalowerintensitythanthe EGRET EGB(see se tion2.3), dierentthe-

oreti almodels havebeen proposed. In parti ularthe emission omingfrom

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of the EGB (e.g. Dermer 2007 fora review).

Other omponents invoked to a ount for the EGB are lusters of galaxies

(Berezinsky, Blasi

&

^Ptuskin ^1997), ^Gamma^ray ^Bursts ^(Dermer ²⁰⁰⁷⁾ ^and

Pulsars (Fau her-Giguere

&

^Loeb ^2010). ^T^ruly ^diuse ^omponent ^ould ^be

produ ed by the ele tromagneti as ades due tothe intera tion between

γ

^-

ray photonsfromblazarsandthe EBLgeneratedbygalaxiesoverthe osmi

history (Coppi and Aharonian1997; see hapter 3 for further details). Also

Ultra High Cosmi Rays intera ting with the CMB generate a pair as ade

whi h emits photon inthe

γ

^-ray ^band ^(e.g. ^Berezinsky êtâl. ^2011). ^Tô ^the

EGB an also ontribute exoti diuse sour es as de aying or annihilating

Dark Matter (DM) (see the se tion2.7).

In this hapter we show the ontribution of blazars (either FSRQsand BL-

La s) to the Fermi-LAT EGB. To fully a ount for the total EGB star-

forming galaxy omponent is needed. We add this omponent to our blazar

model and tting the Fermi-LAT EGB we put an upper limit on the mass

of annihilating DM parti les.

Inthese tion2.2areviewofblazars,theirfeaturesandtheiremissionme ha-

nismsisproposedwhilethefeaturesoftheEGBmeasuredby Fermi isshown

inthese tion2.3. Thenwewillshowthe ontributionofblazar(se tion2.4),

the resultsobtained (se tion2.5), the star-forminggalaxies(se tion2.6) and

the DM (se tion 2.7) omponent . Dis ussion and on lusions are shown in

se tion 2.8.

2.2 Blazars

2.2.1 General features

It is well established that galaxies host intheir enter asupermassive (from

3

×10 ⁶

^to³

×10 ⁹

^M

^⊙

⁾^bla
k ^hole ^(hereafter^SMBH) ^whose ^mass ^orrelates

with the velo ity dispersion and the luminosity of the galaxy bulge. These

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Extragala ti

γ

^-ray ^ba
kground ¹⁵

rarese

&

^F^ord ^2005).

Asmallper entage ofgalaxies (

∼ 1%

⁾^alled^A
tive^Gala
ti^Nu
lei^(AGNs)

shows a SMBH a tive intheir enters be ause ofthe releaseof gravitational

energyof the gas surroundingtheSMBH due toa retion,asradiationfrom

IR to X-ray band. Sin e the gas temperature rea hes

∼ 10 ⁵

^K^the ^energy ^is

emittedasUV-Xrayradiation. Likelyafra tion ofthis energyis re-emitted

asIR radiationby dust around the SMBH.

Onlyasmallfra tionofallAGNs(

∼ 10%

⁾^shows signi ativeemissioninra- dioband. Thissub lassofAGNs, alledradio-loudAGNs,showthepresen e

ofajetofmatterpropagatingouttokp orMp fromthe enter. Althoughit

isnot fully explained,the radio-laud/radio-quitedivision seems tobe linked

with the spin of the SMBH (Rees 1984).

Radio-loudAGNs an be divided in extended radio sour es (those shown a

resolved stru turewhenobserved withasingle radioteles ope)and ompa t

sour es.

Histori ally, extended radio sour es have been lassied in two lasses by

Fanaro and Riley (1974) in terms of the separation between the brightest

parts of their radio lobes: Fanaro-Riley type I (FRI) and Fanaro-Riley

type II (FRII) sour es.

FRI radio galaxies show symmetri radio jets with high brightness near the

galaxy ore, de reasing in outer regions. On the ontrary, FRII sour es dis-

play two well distin t bright lobes atdistan es of the order of even Mp far

fromthe ore,theso- alledhotspots. Thejets onne tingthelobesareoften

too faint tobe dete ted.

FurthermoreFRI sour es la k strong emissionlines that instead are observ-

able inFRII sour es.

Thephysi sunderlyingthe FRI/FRIIdistin tionsis basedonthespeedand

propagation of the jet that is likely linked to dierent regimes (radiatively

e ient/ine ient) ofthe a retion ow onthe SMBH (Ghisellini

&

^Celotti

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radio galaxies are highlyrelativisti .

If the jet of the radio-galaxy pointto the observer, the observer sees a om-

pa t sour e, with high variability and polarization in radio band, with a

unresolved ore. This kind of radiosour es are alled blazars.

Blazars are radio-loud AGNs with:

•

^high variability atallfrequen ies;

•

^high ^opti
al ^and ^radio polarization(up to

20%

^);

•

^presen
e ôf âômpa
t ^radio ôre.

Basedontheir opti alspe tra,blazars aredivided intwo lasses: Flat Spe -

trum Radio Quasars (FSRQs) and BL-La ertae obje ts (BL-La s). In on-

trast withFSRQs, BL-La sshowabsen e(EquivalentWidth

<

⁵^Å) ^of^emis-

sion lines intheir opti alspe tra.

In the urrent uni ation paradigm for AGNs, where the dierent lassi-

ation of AGNs is based on dierent viewing angle of the a reting SMBH,

radiogalaxies aretheparentpopulationsof blazars(Urry

&

^Padovani^1995).

In this s heme relativisti ee ts amplifythe non-thermal beamed emission

jet, pointing to the observer, produ ing the pe uliar features of blazarspe -

tra. A ording to Urry &Padovani(1995) FRIradio galaxies should be the

parent population of BL-La s while FRII sour es of FSRQs. Although the

observational eviden es of these predi tions are not simple, morphologi al

and environmental studiesof radio AGNs(e.g. M Lure et al. 1999) and the

re ent dete tion of

γ

^-rays êmission ^from ^radio ^galaxies ^(Abdo êt âl. ^2010d)

seem to validate the relationbetween blazarsand radio galaxies.

2.2.2 The Blazar SED

Thanks toEGRET, it has been possible to des ribe the whole Spe tral En-

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Extragala ti

γ

^-ray ^ba
kground ¹⁷

•

^the ^double ^humped ^shape ^of ^the ^SED hara terized by the rst peak due to the syn hrotron emissionof ele trons in the jet and the se ond

peak made by IC s attering of jet ele trons on a low energy photon

eld whosenature willbe explained later;

•

^the ^total ênergeti ôutput ^largely ^dominated ^by ^the ^high ênergy ôm-

ponent

Fossatietal. (1998) onstru tedaverageSEDsbinningtheobje tsa ording

to their radio luminosity (at 5 GHz) irrespe tive of their opti al lassi a-

tions and tting the SEDs with an analyti parametrization. It turns out

that for ea hbolometri luminosity the SED shows two distin t bumps,the

rstpeaking between

10 ¹³ − 10 ¹⁷

^Hz,^while^the^se
ond^between

10 ²¹ − 10 ²⁴

^Hz.

In reasingthe bolometri luminositythe two peaksshiftto lowerenergy fol-

lowingtheso alledblazarsequen e (Fig.2.1). The theoreti alexplanation

of the phenomenologi al blazar sequen e has been given by Ghisellini et

al. (1998) in terms of dierent radiative ooling suered by the emitting

ele trons inthe jet with dierent power.

2.2.3 Emission me hanisms

As pointed out before, blazars are identied as radio-loud AGNs with a rel-

ativisti jet pointing along the observer's line of sight. The population of

relativisti ele trons in the jet is responsible of the pe uliar blazar shape.

As demonstrated by the high degree of polarization, the rst peak is due to

syn hrotronemission omingfromrelativisti ele tronsand magneti eldin

the jet. More ompli ated is the explanation of the se ond bump at higher

energy. Themost ommontheoreti almodelsarehadroni andleptoni mod-

els.

In the leptoni models the same ele trons responsible for syn hrotron emis-

sionup-s atterviaInverseCompton(IC)alowerenergyphotoneld. Dier-

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Figure2.1: TheaverageSEDs omputedbyFossatietal. (1998)asdisplayed

in Donato etal. (2001)

Blazars as astrophysical and cosmological probes.

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

0.5 × 10 6

∼ 10 20

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

-90

-ray sources γ

Extragalactic VHE

>100 GeV) (E

>

γ

γ

γ

γ

γ

γ

Γ ∼ 2.1±0.03

γ

&

&

&

&

&

γ

γ

×10 6

×10 9

⊙

γ

&

∼ 1%

∼ 10 5

∼ 10%

&

•

•

20%

•

<

&

γ

γ

•

•

10 13 − 10 17

10 21 − 10 24

0.5 × 10 ⁶

∼ 10 ²⁰

×10 ⁶

×10 ⁹

^⊙

∼ 10 ⁵

10 ¹³ − 10 ¹⁷

10 ²¹ − 10 ²⁴