The Extragalactic Sky as Seen at Very High Energies

1. The Extragalactic Sky as Seen at Very High Energies Elisa Prandini DipartimentodiFisica & INFN Padovaprandini@pd.infn.it DipartimentodiAstronomia, Padova, 23rd June 2011

2. Outline • VHE g-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

3. The energetic range • I will adopt the following convention: • High Energy (HE): g-rays between 0.2 to 100 GeV • Very High Energy (VHE): g-rays above 100 GeV Fermi/LAT Cherenkov telescopes

4. In 1996 this was the VHE map: • 3 sources : • two blazars: Mkn 421 and Mkn501 • a supernova remnant: the Crab Nebula

5. Now, in 2011: • 107 sources: • 46 extragalactic and 61 galactic

6. Worldwide main Cherenkov Telescopes MAGIC VERITAS H.E.S.S.

7. The detection technique Imaging Atmospheric Cherenkov Telescopes

8. Ground Based: why? • For dimensional reasons! • Satellites are simply too small to detect such faint fluxes Fermi/LAT, the most recent HE g-ray satellite, has “only” an effective area of 1 m2

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

10. Detection technique

11. Imaging IACTs register images Fromeachimage, wehavetounderstand: 1. Ifitis a gamma 2. The incoming direction 3.Itsenergy  And determine the spectrum emitted by the observed source Real and simulated data

12. ACQUISITION SYSTEM STRUCTURE SIGNAL TRANSPORT An example: MAGIC CAMERA MIRRORS IPE IPE NET CE

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

14. A closer look into the VHE sky map MAGIC & VERITAS H.E.S.S. From: TeVCat http://tevcat.uchicago.edu/

15. PKS 1222+21 MAGIC IACTsObservables • The signal (if any) • The significance-map • The differential energy flux • The timing evolution (LC) PKS 2155-304 HESS

16. Our “standard” candle: the Crab Nebula Good agreement between IACTs and overlap at lower energies (Fermi/LAT)

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

18. Starburst Galaxies Active Galactic Nuclei • 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

19. 44 AGNs: • 41 blazars • 3radio galaxies (CenA, NGC 1275, and M 87) • 2 starburst galaxies (NGC 253 and M82) One of the key parameter is the distance!

20. At lower energies (0.1-300 GeV) 1451 sources (1FGL): • 120 Galactic • 701 Extragalactic • 295 BL Lac • 278 FSRQ • 120 Other/uncertain AGN • 6 Normal galaxies • 2 Starburst Galaxies • 630 unknown

21. An obstacle for VHE light: the Extragalactic Background Light x x x VHE photon + diffuse light  electron-positron pair production VHEEBLe+e- Absorption: dF/dEobs= (dF/dEem) e-t Dominguez et al. (2011) 21

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

23. Examples 1ES 1218+304 z=0.182 Mkn 501 z=0.034 3C 279 z=0.536 Absorption: dF/dEobs= (dF/dEem) e-t

24. The effect of EBL on VHE spectra The HE regime is almost not affected by the absorption!

25. Therefore: • 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 • Cooperation is a winning strategy!

26. Open questions • VHE emitters • Physical processes at the basis of VHE emission • Characteristics of the emitting region Strategies •  Observe new objects (ToO alerts!) • Long term observations of known objects • MWL/multi-messengers campaigns Let’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- starwinds 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

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

29. Joint HESS-MAGIC-VERITAS campaign of M 87 (Science 2009) The M87 radio-galaxy Jet VHE Colours: 0.2 - 6 keV (Chandra) Contours: 8 GHz radio (VLA) HST-1 Core Knot A Chandra Knot D knot HST-1 X-ray nucleus • Shared monitoring HESS, MAGIC VERITAS • Confirmed day-scale variability at VHE • Evidence of central origin of the VHE emission (60 Rs to the BH) nucleus Radio Peak flux jet

30. Blazars: 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 The large majority of VHE emitters are HBL

31. Blazars spectra • Are usually well described by simple power laws of index, in dN/dE representation, between -4 to -2 Mazin & Raue 2007

32. Blazars 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

33. Blazars 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 • Correlations studies are not conclusive… • Especially with X-rays and optical • Fermi/LAT observations are crucial! Mkn 501 Fermi+MAGIC+VERITAS 2010

34. Modeling Blazars emission • For BL Lac objects, in general the simplest emission model (1 zone Synchrotron Self Compton) fits quite well the data. MWL campaign Mkn 421 (2008-2010)

35. Modeling Blazars emission • For BL Lac objects, in general the simplest emission model (1 zone Synchrotron Self Compton) fits quite well the data. 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). Smoking gun: strong gamma-rays-optical/X-ray correlation during flares (high states)

36. Modeling Blazars 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 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).

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

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

39. And what about GRBs? • All IACTshave 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

41. And what about GRBs? • All IACTshave 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 For the moment… no signal

42. The future • MWL campaigns • More powerful detectors Cherenkov Telescope Array … Fermi/LAT band?

43. Final Remarks • The VHE extragalactic sky counts 46 sources (quite a lot w.r.t. 15 years ago…) • IACTsare 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 g-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! THANKS!