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Discovery of a Very High Energy Gamma-Ray Signal from the 3C 66A/B Region

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Discovery of a Very High Energy Gamma-Ray Signal from the 3C 66A/B Region Discovery of a very high energy gamma-ray signal from the 3C 66A/B region Daniel Mazin1, M. Errando1, E. Lindfors2, E. Prandini3 and F. Tavecchio4 for the MAGIC Collaboration 1: IFAE, Edifici Cn., Campus UAB, E-08193 Bellaterra, Spain 2: Tuorla Observatory, Turku University, FI-21500 Piikkiö, Finland 3: Università di Padova and INFN, I-35131 Padova, Italy 4: INAF National Institute for Astrophysics, I-00136 Rome, Italy D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.1 Contents 1) Motivation of the observations 2) Data taken 3) The analysis 4) Results • γ-ray signal, sky map • Light curve • Differential spectrum 5) Possible interpretations 6) Conclusions D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.2 Motivation of the observations Simple one-zone SSC • 3C 66A is a low frequency BL Lac at z=0.444 (Miller et al. 1978) • The redshift is uncertain (Finke et al. 2008) with a lower limit of z > 0.096 • Redshift is crucial due to the absorption by the EBL • Very promising TeV candidate! • Claimed detection by the Crimean observatory (Neshpor et al. 1998) • Upper limits set by STACEE, Whipple and HEGRA D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.3 Data taken with MAGIC • Scheduled observations in summer 2007 • Observations intensified triggered by the optical brightness of 3C 66A: • Hint in August 2007 data: extension of the observations until December 2007 • In total: 54.2 hours of data (45.3hr after quality cuts) at zenith angles between 13° and 35° D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.4 3C66A and 3C66B 3C66B 3C66A 3C 66A and 3C 66B are two AGNs separated by just 6 arcmin in the sky. 3C 66B is a large Fanaroff–Riley-I-type (FRI) radio galaxy, similar to M 87, with a redshift of 0.0215 (Stull et al. 1975), whereas 3C 66A is a blazar with uncertain redshift. D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.5 Data analysis Standard data analysis using Random forest and timing information of the events 1) Optical PSF changed (improved) during the observations: 3 sub - Crab nebula sky map: samples of data resulting in a higher systematic uncertainty of 2 arcmin (normally 1’) 2) Cuts optimized on 5% Crab Nebula signal 3) Mispointing cross-checked by different methods D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.6 Results: sky map Position of the excess RA: 2h23m12s DEC: 43°0.7´ D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.7 Results: sky map 1) 6.0 σ excess, 5.4 σ after 30 trials: clear detection 2) Not at the position of 3C66A 3) Excess coincides with 3C 66B: nearby radio galaxy 4) MC studies showed that the position of 3C66A can be excluded with 95.6% (statistically only) and with 85.4% including systematics 5) Gave a MAGIC name: MAGIC J0233+430 D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.8 Results: light curve 1) Emission above 150 GeV is compatible with a constant flux of 2.2% Crab flux: Φ(>150GeV) = (7.3 ±1.5) × 10-12 cm-2 s-1 2) No obvious correlation with the optical variability of 3C 66A D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.9 Results: energy spectrum 1) The differential energy spectrum extends from 100 GeV up to 2 TeV 2) Well described by a power law with a photon index of Γ = -3.1 ± 0.3 D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.10 Interpretation: 1 If the excess is entirely from 3C 66A: • Reminder: this is excluded with with 95.6% (statistically only) and with 85.4% including systematics using the sky map • Spectrum is too hard for a source at the distance of 3C 66A (z=0.444) • Spectrum extends up to 2 TeV: challenges all EBL models or the AGN physics! • Alternative: 3C 66A is located at z < 0.24 (<0.17) for an intrinsic spectrum slope Γ>2/3 (>1.5) D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.11 Interpretation: 2 1) The excess is entirely from 3C 66B: • Would be a second TeV radio galaxy after M87 • See Tavecchio & Ghisellini 2009 for the modeling • Structured jet model (Ghisellini et al. 2005) assuming a fast inner core (spine) and a slower surrounding (layer) • TeV emission observed by MAGIC is due to the layer • Similar parameters as in M87 (TG08) but: • The TeV output of 3C 66B is an order of magnitude larger than in M87 • Variations in TeV band are expected to reproduce variations in the X-rays D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.12 Interpretation: remarks 1) Given the VERITAS result from 2008 (detection of 3C66A, Acciari et al. 2009, arxiv:0901.4527), 3C66B is variable by at least factor of 2 between 2007 and 2008 observation seasons 2) It cannot be excluded that at lower energies (E < 150 GeV) the excess of MAGIC J0233+430 is contaminated by 3C 66A D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.13 Conclusions 1) TeV gamma ray source MAGIC J0233+430 is detected Aliu et al., ApJ 692 (2009) L29-L33 2) Integral flux of 2.2% of the Crab Nebula above 150 GeV -> weakest source ever detected by MAGIC so far 3) Position compatible with 3C 66B, whereas 3C 66A is improbable (85.4%) 4) New data in winter 2008 - 2009 are being analyzed D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.14 BACKUPBA D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.15 Interpretations: 2 1) The excess is entirely from 3C 66B: D. Mazin: Discovery of a TeV γ-ray source in the 3C66 A/B region with MAGIC p.16.
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