The extragalactic g-ray sky

The Extragalactic Sky as Seen at Very High EnergiesElisa Prandini

Dipartimento di Fisica & INFN Padova prandini@pd.infn.it

Dipartimento di Astronomia, Padova, 23rd June 2011

OutlineVHE g-ray observations: a recent disciplineObservation technique: IACTsThe VHE sky map: characteristicsEBL and the opacityHighlight results (in a MWL context)Starburst GalaxiesRadio Galaxies: M 87BlazarsOutlook

2The energetic rangeI will adopt the following convention:High Energy (HE): g-rays between 0.2 to 100 GeVVery High Energy (VHE): g-rays above 100 GeVFermi/LATCherenkov telescopes3In 1996 this was the VHE map:

3 sources : two blazars: Mkn 421 and Mkn 501 a supernova remnant: the Crab Nebula4Now, in 2011:107 sources:46 extragalactic and 61 galactic

5Worldwide main Cherenkov Telescopes

MAGICH.E.S.S.VERITAS

6The detection techniqueImagingAtmosphericCherenkovTelescopes

7Ground Based: why?For dimensional reasons!Satellites are simply too small to detect such faint fluxes

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Fermi/LAT, the most recent HE g-ray satellite, has only an effective area of 1 m2The IACT techniqueGamma ray enters the atmosphereElectromagnetic showerEmission of Cherenkov light into a cone of ~1 deg apertureOptical wavebandShort flash (~ns)

9~ 1oerenkov light cone~ 120 m~ 10 kmtelescopesVHE gamma rayAtmospheric shower of secondary particles

IACTs observe in the optical range!Detection technique

10Imaging

From each image, we have to understand:1. If it is a gamma2. The incoming direction 3. Its energy And determine the spectrum emitted by the observed source 11IACTs register imagesReal and simulated data11An example: MAGIC12

SIGNAL TRANSPORTIPEIPECENETACQUISITION SYSTEMMIRRORS

STRUCTURE CAMERA12MAGIC:13

Energy threshold 60 GeVEnergy Resolution ~20%FOV 3.5oAngular Resolution ~0.1oSensitivity (5 s in 50 hours) ~1% Crab Nebula flux (> 100 GeV)

MAGIC I (2004)MAGIC II (2009)A closer look into the VHE sky map14

H.E.S.S.MAGIC & VERITASFrom: TeVCat http://tevcat.uchicago.edu/

IACTs ObservablesThe signal (if any)The significance-mapThe differential energy flux The timing evolution (LC)15

PKS 2155-304HESSPKS 1222+21MAGICOur standard candle: the Crab Nebula16

Good agreement between IACTs and overlap at lower energies (Fermi/LAT)44 AGNs:41 blazars3 radio galaxies (Cen A, NGC 1275, and M 87)2 starburst galaxies (NGC 253 and M82)17

Active Galactic NucleiSuper-massive black hole accreting matter. In some cases: two narrow jet with accelerated particles (radio loud objects)Spectrum emitted: is strongly dependent on the viewing angle to the observer. For radio loud sources:Radio galaxiesBlazars BL Lac & FSRQGalaxies with an exceptional rate of supernova explosionsCosmic rays18

Starburst Galaxies

44 AGNs:41 blazars3 radio galaxies (Cen A, NGC 1275, and M 87)2 starburst galaxies (NGC 253 and M82)One of the key parameter is the distance!19

At lower energies (0.1-300 GeV)1451 sources (1FGL):

120 Galactic701 Extragalactic295 BL Lac278 FSRQ120 Other/uncertain AGN6 Normal galaxies2 Starburst Galaxies630 unknown

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An obstacle for VHE light: the Extragalactic Background Light21Dominguez et al. (2011)

xxxAbsorption:

dF/dEobs= (dF/dEem) e-t VHE photon + diffuse light electron-positron pair productionVHEEBL e+e-2122

g-g opacityAbsorption:

dF/dEobs= (dF/dEem) e-t EBL Model Dominguez et al. (2011)The Energy Threshold plays a key role!

Examples23

Absorption:

dF/dEobs= (dF/dEem) e-t Mkn 501 z=0.0341ES 1218+304z=0.1823C 279z=0.53624The effect of EBL on VHE spectra

The HE regime is almost not affected by the absorption!Therefore:VHE astrophysics is a challenging science!Complicated detection techniqueFew objects seem able to emit up to these energiesOpacity constrain the observationsMany results thanks to the last generation of Cherenkov telescopesThe VHE extragalactic sky is being populatedCooperation is a winning strategy!

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Open questionsVHE emittersPhysical processes at the basis of VHE emissionCharacteristics of the emitting region26Strategies

Observe new objects (ToO alerts!) Long term observations of known objects MWL/multi-messengers campaignsLets see some resultsStarburst GalaxiesVERITAS detection of M 82 at E>700 GeV (Science 2009). Cosmic-ray density of 250 eV cm-3 in the starburst core of M 82 (500 times the average Galactic density). This result strongly supports that cosmic-ray acceleration is tied to star formation activitysupernovae and massive- star winds are the dominant accelerators.H.E.S.S. detection of NGC 253 (Science 2009)Cosmic-ray density 3 orders of magnitude larger than that in the Milky way center

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Radio GalaxiesM87 (H.E.S.S. 2004) variable emissionCen A (H.E.S.S. 2009)NCG 1275 (MAGIC ATel Oct 2010)28

E > 400 GeVJoint HESS-MAGIC-VERITAS campaign of M 87 (Science 2009)29

VHEnucleusPeak fluxknot HST-1nucleusX-rayRadio

HST-1CoreKnot DKnot AColours: 0.2 - 6 keV (Chandra)Contours: 8 GHz radio (VLA)The M87 radio-galaxy Jet Shared monitoring HESS, MAGIC VERITAS Confirmed day-scale variability at VHE Evidence of central origin of the VHE emission (60 Rs to the BH)ChandrajetBlazars: a closer look into their spectral characteristicsTwo bump structure:Synchrotron radiationHigh energy emission (inverse Compton or hadronic processes?)Variable emissionFSRQ: shows evidences for accretion disc and absorption linesBL Lac: lines are very faint/absentDifficult to measure z

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The large majority of VHE emitters are HBLBlazars spectraAre usually well described by simple power laws of index, in dN/dE representation, between -4 to -231

Mazin & Raue 2007Blazars spectraAre usually well described by simple power laws of index, in dN/dE representation, between -4 to -2Can be strongly variable (down to minute scale) but aperiodic

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Blazars spectraAre usually well described by simple power laws of index, in dN/dE representation, between -4 to -2Can be strongly variable (down to minute scale) but aperiodicCorrelations studies are not conclusiveEspecially with X-rays and opticalFermi/LAT observations are crucial!

33Mkn 501Fermi+MAGIC+VERITAS 2010Modeling Blazars emissionFor BL Lac objects, in general the simplest emission model (1 zone Synchrotron Self Compton) fits quite well the data.34

MWL campaign Mkn 421 (2008-2010)

35SSC model: the low energy photons present in the jet (synchrotron bump) are up-scattered by the same electrons emitting them and form the high energy bump (leptonic origin).

Smoking gun: strong gamma-rays - optical/X-ray correlation during flares (high states)

Modeling Blazars emissionFor BL Lac objects, in general the simplest emission model (1 zone Synchrotron Self Compton) fits quite well the data.36

FSRQ: more polluted ambient. The high energy bump, according to leptonic models, is due to IC of

synchrotron photons in the jet + photons outside the jet (EC=external Compton).Modeling Blazars emissionFor BL Lac objects, in general the simplest emission model (1 zone Synchrotron Self Compton) fits quite well the data.For FSRQ additional components are necessary to describe the SED

The case of FSRQPKS 1222+2137The second most distant TeV emitter (z ~ 0.432)FSRQOne night of detection:17th June 2010Rapid variations!No cut-off observed: Emitting region constrained to lie outside the BLR

MAGIC Coll.,ApJ Letters 2011, 730 L8The case of FSRQPKS 1222+2138The second most distant TeV emitter (z ~ 0.432)FSRQOne night of detection:17th June 2010Rapid variations!No cut-off observed: Emitting region constrained to lie outside the BLR

Challenge for Blazar emission models

And what about GRBs?39All IACTs have a program to observe GRBsFast alertAutomatic pointingIn particular, MAGIC is the best instrument thanks to its design:Very light structureEnergy threshold

MAGIC fast movement

40And what about GRBs?All IACTs have a GRBs program to observe GRBsFast alertAutomatic pointingIn particular, MAGIC is the best instrument thanks to its design:Very light structureEnergy threshold

41For the moment no signalThe futureMWL campaignsMore powerful detectorsCherenkovTelescopeArray

Fermi/LAT band?42

Final RemarksThe VHE extragalactic sky counts 46 sources (quite a lot w.r.t. 15 years ago)IACTs are working to uncover it, with the help of other instruments (especially Fermi/LAT)VHE Blazars are relatively nearby objects, mainly HBL + few FSRQ whose emission is challenging for modelingOne of the main process responsible for VHE g-ray attenuation is the interaction with EBLA limit for the detectionIt can be also used for limiting the EBL itself or giving an estimate on a Blazar distance! 43THANKS!

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this workFranceschini+ 08Gilmore+ 10Aharonian+ 06Mazin & Raue 07 - realisticMazin & Raue 07 - extremeAlbert+ 08Schlegel+ 98Hauser+ 98Finkbeiner+ 00Lagache+ 00Gardner+ 00Gorjian+ 00Cambresy+ 01Madau & Pozzetti 01Metcalfe+ 03Chary+ 04Fazio+ 04; Franceschini+ 08Xu+ 05Matsumoto+ 05Frayer+ 06Bernstein+ 07Levenson & Wright 08Matsuura+ 10Hopwood+ 10Bethermin+ 10Berta+ 10Keenan+ 10