A Tev Source in the 3C 66A/B Region

PROCEEDINGS OF THE 31st ICRC, ŁOD´ Z´ 2009 1 A TeV source in the 3C 66A/B region Manel Errando∗, Elina Lindfors†, Daniel Mazin∗, Elisa Prandini‡ and Fabrizio Tavecchio§ for the MAGIC Collaboration ∗IFAE, Edifici Cn., Campus UAB, E-08193 Bellaterra, Spain †Tuorla Observatory, Turku University, FI-21500 Piikkio,¨ Finland ‡Universita` di Padova and INFN, I-35131 Padova, Italy §INAF National Institute for Astrophysics, I-00136 Rome, Italy Abstract. The MAGIC telescope observed the re- As pointed aout in [6], the often referred redshift of gion around the distant blazar 3C 66A for 54.2hr 0.444 for 3C 66A is uncertain. It is based on a single in 2007 August–December. The observations re- measurement of one emission line only [7], while in sulted in the discovery of a γ-ray source cen- later observations no lines in the spectra of 3C 66A tered at celestial coordinates R.A. = 2h23m12s were reported [8]. Based on the marginally resolved host and decl.= 43◦0.′7 (MAGIC J0223+430), coincid- galaxy, a photometric redshift of ∼ 0.321 was inferred ing with the nearby radio galaxy 3C 66B. A [9]. In this paper we report the discovery of VHE γ- possible association of the excess with the blazar ray emission located 6.′1 away from the blazar 3C 66A 3C 66A is discussed. The energy spectrum of and coinciding with the radio galaxy 3C 66B in 2007. MAGIC J0223+430 follows a power law with Detailed results and discussion can be found in [10]. −11 a normalization of (1.7 ± 0.3stat±0.6syst) ×10 TeV−1 cm−2 s−1 at 300GeV and a photon index II. OBSERVATIONS AND DATA ANALYSIS Γ = −3.10 ± 0.31stat±0.2syst. 3C 66A underwent an optical outburst in 2007 August, Keywords: gamma rays: observations — BL Lac- as monitored by the Tuorla blazar monitoring program. ertae objects: individual (3C 66A) — galaxies: indi- The outburst triggered VHE γ-ray observations of the vidual (3C 66B) source with the MAGIC telescope following the Target of Opportunity program, which resulted in discoveries I. INTRODUCTION of new VHE γ-ray sources in the past. As of today, there are 26 known extragalactic very MAGIC has a standard trigger threshold of 60GeV, an high energy (VHE, defined here as E > 100 GeV) angular resolution of ∼ 0.◦1 and an energy resolution γ-ray sources. All of them are active galactic nuclei above 150GeV of ∼ 25% (see [11] for details). The (AGNs) with relativistic jets. With the exception of the MAGIC data analysis is described in detail in [10], [11]. radio galaxy M 87 and CenA all detected sources are Data were taken in the false-source tracking (wobble) blazars, whose jets (characterized by a bulk Lorentz mode pointing alternatively to two different sky direc- factor Γ ∼ 20) point, within a small angle (θ ∼ 1/Γ), to tions, each at 24′ distance from the 3C 66A catalog posi- the observer. The spectral energy distribution (logarithm tion. The zenith distance distribution of the data extends of the observed energy density versus logarithm of the from 13◦ to 35◦. Observations were made in 2007 Au- photon energy) of AGNs typically show a two-bump gust, September, and December and lasted 54.2hr, out of structure. Various models have been proposed for the which 45.3hr passed the quality cuts based on the event origin of the high-frequency bump, the most popular rate after image cleaning. An additional cut removed the being inverse Compton scattering of low-frequency pho- events with total charge less than 150 photoelectrons tons. There have been several suggestions for the origin (corresponding to an energy of ∼ 90 GeV) in order to of the low-frequency seed photons that are up-scattered assure a better background rejection. to γ-ray energies: they may be produced within the Just before the start of the observation campaign jet by synchrotron radiation (synchrotron self-Compton ∼ 5% of the mirrors on the telescope were replaced, or SSC mechanism[1]) or come from outside the jet worsening the optical point-spread function (PSF). As a (external Compton or EC mechanism[2]). Relativistic consequence, a new calibration of the mirror alignment effects boost the observed emission as the Doppler factor system became necessary, which took place within the depends on the angle to the line of sight. In case the jet observation campaign and improved the PSF again. The angle to the line of sight is large, models that depend less sigma of the Gaussian PSF (40% light containment) critically on beaming effects are needed [3]. The VHE γ- was measured to be 3.′0 in 2007 August 12-14, 2.′6 ray emission of AGNs might also be of hadronic origin in 2007 August 15-26 and 2.′1 in 2007 September through the emission from secondary electrons [4]. and December. To take this into account, data were 3C 66A and 3C 66B are two AGNs separated by just analyzed separately for each period and the results were 6′ in the sky. 3C 66B is a large Fanaroff–Riley-I-type combined at the end of the analysis chain. However, the (FRI) radio galaxy, similar to M 87, with a redshift realignment resulted in a mispointing, which was taken of 0.0215 [5], whereas 3C 66A is a distant blazar. care of by a new pointing model applied offline using 2 ERRANDO et al. A TEV SOURCE IN THE 3C 66A/B REGION ° 43.6 5 2000 3C 66A 3C 66B 1800 4 43.4 events / 2 1600 3 1400 43.2 2 ce 1200 can 43.0 1 ifi 1000 DEC (deg) Sign 800 0 42.8 600 Nexc = 567.9 N = 6538.1 -1 bkd 400 On/Off normalization = 0.33 42.6 σ -2 200 Significance = 6.0 0 02h26m 02h25m 02h24m 02h23m 02h22m 02h21m 02h20m 0 10 20 30 40 50 60 70 80 90 RA (h) |Alpha| (deg) Fig. 1. Left plot: Significance map for γ-like events above 150 GeV in the observed sky region. The green cross corresponds to the fitted maximum excess position of MAGIC J0223+403. The probability of the true source to be inside the green circles is 68.2%, 95.4% and 99.7% for the inner, middle and outer contour, respectively. The catalog positions of 3C 66A and 3C 66B are indicated by a white square and a black dot, respectively. Right plot: |Alpha| distribution after all cuts evaluated with respect to the position of MAGIC J0223+430. A γ-ray excess with a significance of 6.0 σ is found, which corresponds to a post-trial significance of 5.4 σ. starguider information [12]. Considering the additional To calculate the significance of the detection, an uncertainty caused by the offline corrections, we esti- |ALPHA| distribution was produced (see Figure 1, left), mate the systematic uncertainty of the pointing accuracy where ALPHA is the angle between the major axis of to be 2′ [13]. the shower image ellipse and the source position in the camera. A signal of 6.0 σ significance (pre-trial, at III. RESULTS the position of 3C 66B) and 5.4 σ (post-trial, using 30 Figure 1 (left plot) shows a significance map produced independent trials) has been calculated. from the signal and background maps, both smoothed ′ Figure 2 shows the light curve of MAGIC J0223+430 with a Gaussian of σ =6 (corresponding to the γ-PSF), together with the flux of 3C 66A in optical wavelengths. for photon energies between 150GeV and 1TeV. Loose As we integrate over γ-ray events from a wide sky region 2 background rejection cuts are applied to keep a large (∼ 0.07 deg ), we cannot exclude that 3C 66A con- number of gamma-like events. The center of gravity of tributes to the measured signal. The integral flux above the γ-ray emission is derived from Figure 1 by fitting a 150GeV corresponds to (7.3 ± 1.5) × 10−12 cm−2 s−1 bell-shaped function of the form (2.2% of the Crab Nebula flux) and is the lowest ever (x − x¯)2 + (y − y¯)2 detected by MAGIC. The γ-ray light curve is consistent F (x, y)= A · exp − (1) 2σ2 with a constant flux within statistical errors. These errors, however, are large, and some variability of the for which the distribution of the excess events is assumed signal cannot be excluded. to be rotationally symmetric, i.e., σ = σ = σ. The x y For the energy spectrum of MAGIC J0223+430, loose fit yields reconstructed coordinates of the excess center h m s ◦ ′ cuts are made to keep the γ-ray acceptance high. The of R.A. = 2 23 12 and decl.= 43 0. 7. The detected ′ differential energy spectrum was unfolded and is shown excess, which we name MAGIC J0223+430, is 6. 1 in Figure 3. The spectrum can be well fitted by a power away from the catalog position of 3C 66A, while the −1 −2 −1 ′ law which gives a differential flux (TeV cm s ) distance to 3C 66B is 1. 1. We made a study to estimate of: statistical uncertainty of the reconstructed position. The probabilities are shown in Figure 1 by the green contours corresponding to 68.2%, 95.4%, and 99.7% for the inner, dN −11 −3.1±0.3 middle, and outer contour, respectively. Using this study = (1.7 ± 0.3) × 10 (E/300 GeV) dE dA dt we find that the measured excess coincides with the cata- (2) log position of 3C 66B. The origin of the emission from The quoted errors are statistical only.

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