The transparency of the Universe to Very High Energy photons - PowerPoint PPT Presentation

The transparency of the Universe to Very High Energy photons Barbara De Lotto - University of Udine & INFN Outline: • Photon propagation • Observations of distant AGN • Physics interpretation • Conclusions Vulcano Workshop 2010 – Frontier Objects in Astrophysics and Particle Physics – May 2010 1

VHE γ -rays have opened a new window on the Universe -quasar quasar μ - μ 2010 GRBs AGN Pulsars PWN Spectral characteristics Scientific targets of blazars Cosmic background radiation SNRs origin of cosmic rays cosmology cosmology dark matter space time

Intergalactic absorption of VHE photons (Heitler 1960) 2 4 2 m e c β = − maximal for ε : 1 ( ) ε − θ E 1 cos Around the TeV region cross section maximized for infrared/optical photons (Extragalactic Background Light) 3

Extragalactic Background Light • Thermal emission produced by stars and partly absorbed/re-emitted by dust during the entire history of the Universe � two components • Several models try to describe the EBL Spectral Energy Distribution: main differences depending on how the evolution in time and frequency is n( ε ,z) Finke & Razzaque treated � arXiv0905.1115v2

Attenuation − τ Φ ≡ Φ × ( E , z ) τ ( , ) ( ) E z E e optical depth obs em ( ε , z ) spectral energy density of background photons (EBL) n ε 5

(An approximation) • Neglecting evolutionary effects for simplicity Coppi & Aharonian ApJ 1997 ⎛ ⎞ Λ (Mpc) Λ ∝ 1 D τ ( E , D ) ≈ ⎜ ⎟ ⎝ ⎠ Λ ( E ) σ D − Φ obs ( E , D ) ≈ Φ em ( E ) × e Λ ( E ) For γ -rays energies above a few TeV, the distance they can propagate ≤ 100 Mpc � most of the VHE Universe seems not visible to us 6

Consequences • Since Λ becomes < R Hubble for E > 100 GeV: – The observed flux should be ~ E -2 exponentially suppressed at VHE � the observed spectrum should be steeper than the emitted one. – The observed flux should be exponentially suppressed at large distances � very far-away sources should τ = 1 become invisible as energy increases γ – ray horizon: τ (E,z) = 1 Fazio & Stecker 1970 7

The far away 38 Sources … VHE Universe … PKS 0447-439 z=0.20 HESS 2009 1ES 1011+496 z=0.21 MAGIC 2007 1ES 0414+009 z=0.29 HESS & Fermi 2009 z=0.31 ± 0.08 MAGIC 2009 S5 0716+71 1ES 0502+675 z=0.34 VERITAS 2009 3C 66A z=0.44 VERITAS 2009 3C 279 z=0.54 MAGIC 2008 8

The distant quasar 3C 279 Science 320 (2008) 1752 • Flat spectrum radio quasar at z = 0.54 • Very bright and strongly variable – Brightest EGRET AGN – Gamma-ray flares in 1991 and 1996. Fast time variation (~ 6hr in 1996 flare) • MAGIC observations – 10 h between Jan.-April 2006 – clear detection on 23 rd Feb. at 6.2 σ First FSRQ in TeV γ -rays Major jump in redshift 9

Follow up observations of 3C 279 • New observations after optical outburst in Jan. 2007 � new flare detected E. Carmona – HEAD2010 Hard spectrum confirmed Most distant object ever detected at VHE - two flares (Feb. 2006 and Jan. 2007) 10

Implications on Extragalactic Background Light blazar IACT γ VHE γ EBL e + e - e - The measurement of spectral features permits to constrain EBL models: = 4.1 ± • Power law Γ 0.7, measured up to 0.5 TeV � Spectrum sensitive to 0.2 - 2 μ m • Assume minimum reasonable index Γ em = 1.5 � Upper limit close to lower limit from galaxy count Emission harder than expected � Universe more transparent Γ to γ -rays than expected 11

Could it be seen? • At ~ 0.5 TeV, flux attenuated by 2 orders of magnitude even for the lowest EBL model: • Possible explanations: – from standard ones • very hard emission mechanisms with intrinsic slope < 1.5 (Stecker 2008) • Very low EBL – to possible evidence for new physics … 12

Is there a new land just behind the horizon?

Oscillation to an Axion-Like Particle in the presence of magnetic fields. Several interpretations: � during the propagation in the intergalactic medium [DeAngelis,Roncadelli& Mansutti, PLB2008, PRD2008]: ( ) a = 1 ⋅ L E B γ a M • available constraints in the parameters m, M: CAST exp. & astrophysical arguments: M > 10 10 GeV, m < 0.01 eV (PDG 08) • Intergalactic magnetic field: domain-like structure with strength ~ 0.5 nG, coherence length λ ~ 10 Mpc, random orientation in each domain RESULTS: Small mass , small coupling (within limits) naturally explain the enhancement at large E m << 10 − 10 eV (maximal mixing) 10 11 GeV < M < 10 13 GeV 14

… continue � Simet, Hooper & Serpico, PRD 08 � Sanchez-Conde et al., PRD 09 • Conversion at the emission • Mixing inside the source and in the intergalactic magnetic field • Milky Way acts as a converter to photons simultaneously considered in the same framework

Experimental input from more sources at high z: MAGIC S5 0716+714 • 3 rd MAGIC discovery after optical ToO ApJ 704 (2009) L129 Optical light curve: KVA telescope, La Palma � MAGIC discovery 23 rd – 25 th April, Atel 29 th • • On 28 th Swift reports high flux (0.3-10 KeV) • MAGIC flux(>400 GeV) ≈ 25% Crab, Γ = 3.45 ± 0.54 3 rd low-peaked VHE blazar after BL Lac & W • comae Host galaxy detected: z = 0.31 ± 0.08 => 3 rd • farthest VHE emitter for which the spectral index has been measured 16

Spectral characterstics of observed AGN: a synoptic view …and more new sources are coming into the game (VERITAS, HESS, MAGIC) spectral index PKS 0447-439 z=0.20 HESS 2009 1ES 1011+496 z=0.21 MAGIC 2007 1ES 0414+009 z=0.29 HESS & Fermi 2009 z=0.31 ± 0.08 MAGIC 2009 S5 0716+71 1ES 0502+675 z=0.34 VERITAS 2009 3C 66A z=0.44 VERITAS 2009 redshift z 3C 279 z=0.54 MAGIC 2008 17

Where do we stand ? • Recent gamma observations might present substantial challenges to the conventional models to explain the observed source spectra and/or EBL density. – MAGIC 3C279 at z=0.54; VERITAS detection above 0.1 TeV from 3C66A (z=0.44): EBL-corrected spectrum harder than 1.5 (Acciari+, ApJ09); • TeV photons coming from 3C 66A? (Neshpor+98; Stepanyan+02). Difficult to explain with conventional EBL and physics. – The lower limit on the EBL at 3.6 μ m was recently revised upwards by a factor ∼ 2, suggesting a more opaque Universe (Levenson+08). � Some sources at z = 0.1 − 0.2 seem to have harder intrinsic energy spectra than previously anticipated (Krennrich+08). – Spectral indices don’t grow with increasing distance: selection bias? • While it is still possible to explain the above points with conventional physics, the axion/photon oscillation could naturally explain these puzzles 18

Other possible explanations related to new physics ⎡ ⎤ ⎛ ⎞ 2 c 2 p 2 = E 2 1 + ξ E + O E ⎢ ⎥ ⎜ ⎟ ⎢ ⎝ ⎠ ⎥ E s E s ⎣ ⎦ • Kifune 2001: Violation of the Lorentz invariance “a la Coleman-Glashow”: the absorption mean free path of VHE γ – rays is altered by orders of magnitude from those conventionally estimated. but we should keep in mind that 19

Conclusions • VHE γ –rays have opened a new window in the Universe • The observation of VHE γ –rays from extragalactic sources (like blazars) has entered a new era, thanks to the new generation of ground based Cherenkov telescopes (and Fermi) • To disentangle attenuation due to EBL from intrinsic properties of blazars, and eventually probe certain aspects of fundamental new physics, the detection of more sources at different redshifts is essential • We have the tool: increased sensitivity of the upgraded current telescopes (like MAGIC) in synergy with Fermi, and possibly a future Cherenkov Telescope Array system (CTA project). 20

”A textbook example of the merging of particle physics and astronomy into the modern field of astroparticle physics” 21

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Imaging Air Cherenkov Technique Imaging Air Cherenkov Technique Gamma ray Cherenkov light Image of particle Particle ~ 10 km shower in telescope camera shower Cherenkov light ~ 1 o - reconstruct: arrival direction, energy ~ 120 m - reject hadron background statistically in the analysis

• Axions were postulated to solve the strong CP problem in the 70s. Good Dark Matter candidates (axions with masses ≈ meV- μ eV could account for • the total Dark Matter content). • They are expected to oscillate into photons (and viceversa) in the presence of magnetic fields: with Photon/axion oscillations are the main vehicle used at present in axion searches (ADMX, CAST…). M 11 : coupling constant 15 ⋅ B G ⋅ s pc inverse (g αγ /10 11 ≥ 1 GeV) Some astrophysical environments M 11 B G : magnetic field (G) fulfill the mixing requirements s pc : size region (pc) AGNs, IGMFs