Diffuse and targeted searches for ultra-high-energy photons using the hybrid detector of the Pierre Auger Observatory Marcus Niechciol a for the Pierre Auger Collaboration b a Department Physik, Universität Siegen, 57068 Siegen, Germany b Observatorio Pierre Auger, Av. San Martín Norte 304, 5613 Malargüe, Argentina E-mail: auger_spokespersons@fnal.gov Full author list: http://www.auger.org/archive/authors_icrc_2017.html [Photo by S. Saffi, Univ. of Adelaide]
Windows to the Universe Highest-energy photons detected: ≈ 10 14 eV [ESO] [H.E.S.S.] Visible light γ -rays 10 -3 1 10 4 10 12 10 18 ? E γ [eV]: X-rays Radio • Cosmic rays: energies up to 3 × 10 20 eV observed • UHE photons and cosmic rays are intimately connected Observation may help to → answer some of the most pressing questions about [ESA] [NRAO] cosmic rays 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 2 / 12
Pierre Auger Observatory • Two independent and complementary detector systems § SD : array of 1660 water Cherenkov detectors (covering 3000 km 2 ) § FD : 27 fluorescence telescopes, overlooking the SD § Hybrid data combine information from the FD and the SD elevation [deg] 30 FD 25 20 15 10 SD 5 0 120 110 100 90 80 70 60 50 40 30 azimuth [deg] Signal [VEM] Stage: 4.5 )] 2 /Ndf: 8.5/ 8 2 χ χ /Ndf= 162.4/214 2 dE/dX [PeV/(g/cm 12 candidates non-triggering 2 10 10 removed 8 6 10 4 2 0 1 -2 500 1000 1500 2000 2500 3000 r [m] 400 600 800 1000 1200 2 [The Pierre Auger Collaboration, NIM A 798 (2015) 172-213] slant depth [g/cm ] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 3 / 12
Photon-induced air showers • Main characteristics of photon-induced air showers compared to those initiated by hadrons: § Delayed shower development (multiplicity of electromagnetic processes smaller than that of hadronic processes) → larger depth of the shower maximum X max § Lack of muons (photo-nuclear interactions suppressed compared to electromagnetic interactions) ] 〉 -2 µ CONEX v2r5p40, QGSJET-II-04 CONEX v2r5p40, QGSJET-II-04 N [g cm 1100 Preshower 〈 9 10 Preshower effect effect LPM effect 〉 1000 LPM effect max 8 10 X Photon 〈 Iron 900 Photon Proton Iron 7 10 Proton 800 Proton Proton 6 10 700 5 Photon Photon 10 Iron Iron 600 4 10 500 3 10 16 17 18 19 20 21 16 17 18 19 20 21 10 10 10 10 10 10 10 10 10 10 10 10 E [eV] E [eV] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 4 / 12
Search for a diffuse photon flux: observables • Experimental observables : § X max (FD related) § Parameter S b (SD related) Signal in surface detector i [G. Ros et al, Astropart. Phys. 35 (2011) 140] ✓ r i N ◆ b Parameter chosen for the best photon/hadron separation X Distance between surface detector i and the shower axis S b = S i r 0 r 0 = 1000 m as reference distance i → S b exploits the different lateral distributions of photon- and hadron-induced showers § Number of triggered surface detectors N stat (SD related) [The Pierre Auger Collaboration, JCAP 04 (2017) 009] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 5 / 12
Search for a diffuse photon flux: MVA • The three observables are combined in a boosted decision tree (BDT) • Training of the BDT using simulated samples of photon- and proton-induced air showers for signal and background • Background contamination 0.14 % at 50 % signal efficiency • Apply the analysis to data collected between 01/2005 and 12/2013 § 8178 events with E γ > 10 18 eV after cuts for good geometry and profile reconstruction § Three events pass the photon candidate cut 0.18 Entries Photon (training) Photon (test sample) Proton (training) Proton (test sample) 0.16 18 Median cut data (E > 10 eV) γ 0.14 Photon candidate cut 0.12 at the median of the photon distribution 0.1 0.08 0.06 0.04 0.02 0 0.4 0.2 0 0.2 0.4 0.6 − − BDT response [The Pierre Auger Collaboration, JCAP 04 (2017) 009] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 6 / 12
Search for a diffuse photon flux: results • Number of candidates compatible with the background expectation : determine upper limits on the integral photon flux Number of candidate events at 95 % C.L. N 0 . 95 ( E γ > E 0 ) Φ 0 . 95 γ UL ( E γ > E 0 ) = E γ ( E γ > E 0 | E � Γ γ ) Integrated exposure assuming a power law spectrum ] → Several sources of systematic -1 upper limits 95% CL Z-burst GZK proton I yr 1 TD GZK proton II uncertainties have been studied, e.g. -1 sr SHDM I detector, interaction model, -2 SHDM II [ km Y 2010 HP 2000 spectrum A 2002 1 − 10 0 > E Hy 2011 → Upper limits in the EeV range γ TA 2015 Integral photon flux E Hy 2016 +syst. improved by at least a factor of 4 2 − 10 → Severe constraints for top-down models → Some GZK scenarios are in reach − 3 10 SD 2015 18 19 20 10 10 10 E [eV] 0 • Upper limits to the integral photon fraction assuming the Auger energy spectrum: 0.1 %, 0.15 %, 0.33 %, 0.85 % and 2.7 % at E 0 = 1, 2, 3, 5 and 10 EeV [The Pierre Auger Collaboration, JCAP 04 (2017) 009] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 7 / 12
Targeted photon search • Photons are not deflected in magnetic fields: the signature of a photon point-source is an accumulation of events from the direction of the source • Select photon-like events using a BDT with 5 input observables • FD-related : X max , χ 2 of a Greisen fit to the longitudinal profile, ratio of E Greisen and E FD • SD-related : S b , ratio of early and late signals in the detectors • Selection cut in the BDT response β is optimized for each target direction by minimizing the upper limit, taking into account the expected background • Apply the analysis to data collected between 01/2005 and 12/2013 • Energy range : 10 17.3 – 10 18.5 eV (average angular resolution in this energy range: 0.7°) • Use top-hat counting with radius 1° for each target direction 10 photon (testing sample) photon (training sample) proton (testing sample) proton (training sample) MC simulations (1/ N ) d N / d ß 1 -1 10 Background map obtained using the scrambling method [G.L. Cassiday et al., Nucl. Phys. B (Proc. Suppl.) 14 A (1190) 291] -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 ß [The Pierre Auger Collaboration, ApJ 837 (2017) L25] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 8 / 12
Targeted photon search: target classes • Restrict analysis to pre-defined target classes (reduce the trial factor) • Galactic set : contains e.g. different classes of pulsars and X-ray binaries • Similar to the targeted neutron search previously published by Auger [The Pierre Auger Collaboration, ApJ 789 (2014) L34] • Extragalactic set : include nearby extragalactic targets • Centaurus A ( d = 3.8 Mpc): core region • Large Magellanic Cloud ( d = 50 kpc): three powerful γ -ray emitters [The H.E.S.S. Collaboration, Science 347 (2015) 406] 90 msec Pulsars γ -ray Pulsars 60 Low-mass X-ray Binaries High-mass X-ray Binaries H.E.S.S. Pulsar Wind Nebulae 30 H.E.S.S. other identified sources H.E.S.S. unidentified sources Microquasars 180 120 60 GC 300 240 180 Magnetars Galactic Center -30 Large Magellanic Cloud Centaurus A (Core) -60 -90 [The Pierre Auger Collaboration, ApJ 837 (2017) L25] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 9 / 12
Targeted photon search: results combined p-values Combined analysis Most significant source candidate (unweighted and weighted) UL [km � 2 yr � 1 ] f 0 . 95 Class N P P w p p ⇤ msec PSRs 67 0.14 0.57 0.010 0.476 0.043 Flux upper limit γ -ray PSRs 75 0.98 0.97 0.007 0.431 0.045 LMXB 87 0.74 0.13 0.014 0.718 0.046 p-values (unpenalized and HMXB 48 0.84 0.33 0.040 0.856 0.036 penalized) H.E.S.S. PWN 17 0.90 0.92 0.104 0.845 0.038 H.E.S.S. other 16 0.52 0.12 0.042 0.493 0.040 H.E.S.S. UNID 20 0.45 0.79 0.014 0.251 0.045 Microquasars 13 0.48 0.29 0.037 0.391 0.045 Magnetars 16 0.89 0.30 0.115 0.858 0.031 Gal. Center 1 0.59 0.59 0.471 0.471 0.024 LMC 3 0.62 0.52 0.463 0.845 0.030 Cen A 1 0.31 0.31 0.221 0.221 0.031 • No target class reveals compelling evidence for sources emitting photons with EeV energies No evidence for one outstanding target in any target class • [The Pierre Auger Collaboration, ApJ 837 (2017) L25] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 10 / 12
Targeted photon search: galactic center • Interpretation of H.E.S.S. PeVatron results for the galactic center region [The H.E.S.S. Collaboration, Nature 531 (2016) 476] -11 10 ] -1 s -2 x flux [TeV cm -12 10 -13 10 2 E -14 10 H.E.S.S. measurement 1 confidence band of the best-fit spectra σ -15 10 Auger photon GC limit ( Γ =2.32 0.11) ± H.E.S.S. extrapolation ( =2.32 0.11) Γ ± H.E.S.S. extrapolation ( Γ =2.32 and E = 2.0 EeV) cut -16 10 3 5 6 -1 2 4 7 10 1 10 10 10 10 10 10 10 E [TeV] • Constrain the naive extrapolation to EeV energies • Upper limit on the cutoff energy of 2 EeV [The Pierre Auger Collaboration, ApJ 837 (2017) L25] 18.07.2017 Marcus Niechciol (Pierre Auger Collaboration) | ICRC 2017 (Busan) | CRI183 11 / 12
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