1. T HE E XTRAGALACTIC S KY AS S EEN AT V ERY H IGH E NERGIES Elisa Prandini Dipartimento di Fisica & INFN Padova prandini@pd.infn.it prandini@pd.infn.it Dipartimento di Astronomia, Padova, 23 rd June 2011

2. O UTLINE VHE  -ray observations: a recent discipline Observation technique: IACTs The VHE sky map: characteristics ▫EBL and the opacity Highlight results (in a MWL context) ▫Starburst Galaxies ▫Radio Galaxies: M 87 ▫Blazars Outlook 2

3. T HE ENERGETIC RANGE I will adopt the following convention: ▫H IGH E NERGY (HE):  -rays between 0.2 to 100 GeV ▫V ERY H IGH E NERGY (VHE):  -rays above 100 GeV Fermi/LAT Cherenkov telescopes 3

4. I N 1996 THIS WAS THE VHE MAP : 3 sources : two blazars: Mkn 421 and Mkn 501 a supernova remnant: the Crab Nebula 4

5. N OW, IN 2011: 107 sources: ▫46 extragalactic and 61 galactic 5

6. W ORLDWIDE MAIN C HERENKOV T ELESCOPES MAGIC H.E.S.S. VERITAS 6

7. T HE DETECTION TECHNIQUE Imaging Atmospheric Cherenkov Telescopes 7

8. G ROUND B ASED : WHY ? For dimensional reasons! Satellites are simply too small to detect such faint fluxes 8 Fermi/LAT, the most recent HE  -ray satellite, has “only” an effective area of 1 m 2

9. T HE IACT TECHNIQUE Gamma ray enters the atmosphere Electromagnetic shower Emission of Cherenkov light into a cone of ~1 deg aperture ▫Optical waveband ▫Short flash (~ns) 9 ~ 1 o Čerenkov light cone ~ 120 m ~ 10 km telescopes VHE gamma ray Atmospheric “shower” of secondary particles IACTs observe in the optical range!

10. D ETECTION TECHNIQUE 10

11. I MAGING From each image, we have to understand: 1. If it is a gamma 2. The incoming direction 3. Its energy  And determine the spectrum emitted by the observed source 11 IACTs register IMAGES Real and simulated data

12. A N EXAMPLE : MAGIC 12 SIGNAL TRANSPORT IP E CECE NE T ACQUISITION SYSTEM MIRRORS STRUCTURE CAMERA

13. MAGIC: 13  Energy threshold 60 GeV  Energy Resolution ~20%  FOV 3.5 o  Angular Resolution ~0.1 o  Sensitivity (5  in 50 hours) ~1% Crab Nebula flux (> 100 GeV) MAGIC I (2004) MAGIC II (2009)

14. A CLOSER LOOK INTO THE VHE SKY MAP 14 H.E.S.S. MAGIC & VERITAS From: TeVCat http://tevcat.uchicago.edu/

15. IACT S O BSERVABLES 1.The signal (if any) 2.The significance-map 3.The differential energy flux 4. The timing evolution (LC) 15 PKS 2155-304 HESS PKS 1222+21 MAGIC

16. O UR “ STANDARD ” CANDLE : THE C RAB N EBULA 16 Good agreement between IACTs and overlap at lower energies (Fermi/LAT)

17. 44 AGNs: ▫41 blazars ▫3 radio galaxies (Cen A, NGC 1275, and M 87) 2 starburst galaxies (NGC 253 and M82) 17

18. A CTIVE G ALACTIC N UCLEI Super-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 galaxies ▫Blazars  BL Lac & FSRQ Galaxies with an exceptional rate of supernova explosions ▫Cosmic rays 18 S TARBURST G ALAXIES

19. 44 AGNs: ▫41 blazars ▫3 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

20. A T LOWER ENERGIES (0.1-300 G E V) 1451 sources (1FGL): 120 Galactic 701 Extragalactic ▫295 BL Lac ▫278 FSRQ ▫120 Other/uncertain AGN ▫6 Normal galaxies ▫2 Starburst Galaxies 630 unknown 20

21. A N OBSTACLE FOR VHE LIGHT : THE E XTRAGALACTIC B ACKGROUND L IGHT 21 Dominguez et al. (2011) x x x Absorption: dF/dE OBS = (dF/dE EM ) e -  diffuse light VHE photon + diffuse light  electron-positron pair production  VHE  EBL  e + e - 21

22. 22  -  OPACITY Absorption: dF/dE OBS = (dF/dE EM ) e -  EBL Model Dominguez et al. (2011) The E NERGY T HRESHOLD plays a key role!

23. E XAMPLES 23 Absorption: dF/dE OBS = (dF/dE EM ) e -  Absorption: dF/dE OBS = (dF/dE EM ) e -  Mkn 501 z=0.034 Mkn 501 z=0.034 1ES 1218+304 z=0.182 1ES 1218+304 z=0.182 3C 279 z=0.536 3C 279 z=0.536

24. 24 T HE EFFECT OF EBL ON VHE SPECTRA The HE regime is almost not affected by the absorption!

25. T HEREFORE : VHE astrophysics is a challenging science! ▫Complicated detection technique ▫Few objects seem able to emit up to these energies ▫Opacity constrain the observations Many results thanks to the last generation of Cherenkov telescopes ▫The VHE extragalactic sky is being populated ▫C OOPERATION is a winning strategy! 25

26. O PEN QUESTIONS VHE emitters Physical processes at the basis of VHE emission Characteristics of the emitting region 26 S TRATEGIES  Observe new objects (ToO alerts!)  Long term observations of known objects  MWL/multi-messengers campaigns S TRATEGIES  Observe new objects (ToO alerts!)  Long term observations of known objects  MWL/multi-messengers campaigns L ET ’ S SEE SOME RESULTS …

27. Starburst Galaxies VERITAS 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 ACTIVITY  SUPERNOVAE 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 27

28. Radio Galaxies M87 (H.E.S.S. 2004) ▫ variable emission Cen A (H.E.S.S. 2009) NCG 1275 ▫ (MAGIC ATel Oct 2010) 28 E > 400 GeV

29. J OINT HESS-MAGIC-VERITAS CAMPAIGN OF M 87 (S CIENCE 2009) 29 VHE nucleus Peak flux knot HST-1 nucleus X-ray Radio HST-1 Core Knot D Knot A Colours: 0.2 - 6 keV (Chandra) Contours: 8 GHz radio (VLA) Colours: 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 R s to the BH) Chandra jet

30. B LAZARS : A CLOSER LOOK INTO THEIR SPECTRAL CHARACTERISTICS Two bump structure: ▫Synchrotron radiation ▫High energy emission (inverse Compton or hadronic processes?) Variable emission FSRQ: shows evidences for accretion disc and absorption lines BL Lac: lines are very faint/absent ▫Difficult to measure z 30 The large majority of VHE emitters are HBL

31. B LAZARS SPECTRA Are usually well described by simple POWER LAWS of index, in dN/dE representation, between -4 to -2 31 Mazin & Raue 2007

32. B LAZARS SPECTRA Are usually well described by simple POWER LAWS of index, in dN/dE representation, between -4 to -2 Can be strongly variable (down to minute scale) but APERIODIC Are usually well described by simple POWER LAWS of index, in dN/dE representation, between -4 to -2 Can be strongly variable (down to minute scale) but APERIODIC 32

33. B LAZARS SPECTRA Are usually well described by simple POWER LAWS of index, in dN/dE representation, between -4 to -2 Can be strongly variable (down to minute scale) but APERIODIC C ORRELATIONS studies are not conclusive… ▫Especially with X-rays and optical ▫Fermi/LAT observations are crucial! Are usually well described by simple POWER LAWS of index, in dN/dE representation, between -4 to -2 Can be strongly variable (down to minute scale) but APERIODIC C ORRELATIONS studies are not conclusive… ▫Especially with X-rays and optical ▫Fermi/LAT observations are crucial! 33 Mkn 501 Fermi+MAGIC+VERITAS 2010

34. M ODELING B LAZARS EMISSION For 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)

35. 35 SSC 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). S MOKING GUN : strong gamma-rays - optical/X-ray correlation during flares (high states) SSC 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). S MOKING GUN : strong gamma-rays - optical/X-ray correlation during flares (high states) M ODELING B LAZARS EMISSION For BL Lac objects, in general the simplest emission model (1 zone Synchrotron Self Compton) fits quite well the data.

36. 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). 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). M ODELING B LAZARS EMISSION For 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

37. T HE CASE OF FSRQ PKS 1222+21 37 The second most distant TeV emitter (z ~ 0.432) FSRQ One night of detection: ▫17 th June 2010 Rapid variations! No cut-off observed: ▫Emitting region constrained to lie outside the BLR MAGIC Coll., ApJ Letters 2011, 730 L8

38. T HE CASE OF FSRQ PKS 1222+21 38 The second most distant TeV emitter (z ~ 0.432) FSRQ One night of detection: ▫17 th June 2010 Rapid variations! No cut-off observed: ▫Emitting region constrained to lie outside the BLR Challenge for Blazar emission models

39. A ND WHAT ABOUT GRB S ? 39 All IACTs have a program to observe GRBs ▫Fast alert ▫Automatic pointing In particular, MAGIC is the best instrument thanks to its design: ▫Very light structure ▫Energy threshold

40. MAGIC FAST MOVEMENT 40

41. A ND WHAT ABOUT GRB S ? All IACTs have a GRBs program to observe GRBs ▫Fast alert ▫Automatic pointing In particular, MAGIC is the best instrument thanks to its design: ▫Very light structure ▫Energy threshold 41 For the moment… no signal

42. T HE FUTURE MWL campaigns More powerful detectors Cherenkov Telescope Array … Fermi/LAT band? 42

43. F INAL R EMARKS The 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 modeling One of the main process responsible for VHE  -ray attenuation is the interaction with EBL ▫A limit for the detection ▫It can be also used for limiting the EBL itself or giving an estimate on a Blazar distance! 43 THANKS!