The two BL Lac objects, 1ES 1215+303 and 1ES 1218+304, separated by 0.8 degree, were observed with the MAGIC Cherenkov telescopes in 2010-2011. The January 2011 observations resulted in the first detection above 100 GeV of 1ES 1215+303 (known also as ON-325) which has been flagged as a promising Very High Energy (VHE, E > 100 GeV) gamma-ray source candidate by the Fermi-LAT Collaboration in October 2010. The January 2011 observations were triggered by the high optical state of the source, as reported by the Tuorla blazar monitoring program. A comparison with the 2010 data suggests that 1ES 1215+303 was also flaring in VHE gamma-rays. In addition, the Swift Target of Opportunity (ToO) observations in X-rays showed that the flux was almost doubled with respect to previous observations (December 2009). Instead, 1ES 1218+304 is a well known VHE gamma-ray emitter located in the same field of view, which was then simultaneously observed with MAGIC. In this contribution we present preliminary results of the MAGIC observations of these two VHE gamma-ray emitting AGNs.
A. The MAGIC Telescopes MAGIC consists of two 17 m dish Imaging Air Cherenkov Telescopes located in the Canary Island of La Palma, at 2200 m a.s.l. (Figure 1). The stereoscopic system has been in operation since fall 2009 and has a sensitivity of 0.8% Crab Nebula flux above ∼ 300 GeV in 50 h of observations, and a trigger threshold of 50 GeV, which is the lowest among the existing IACTs. The MAGIC cameras have a field of view of 3.5 • . Details on the performance of the MAGIC stereoscopic system are presented in [1].
MAGIC has been successfully performing optically triggered target of opportunity (ToO) observations of active galactic nuclei (AGN) since the beginning of its science observations. The optical triggers have been provided by the Tuorla blazar monitoring program [19]. The observations are performed with the KVA 35 cm telescope located on La Palma, but remotely operated from Tuorla Observatory. The longterm monitoring program consists of > 50 blazars that are considered to be good candidates to emit Very High Energy (VHE, E > 100 GeV) γ-rays based on their X-ray and γ-ray properties. The ToO observations with MAGIC have resulted so far in the discovery of many new VHE γ-ray emitting sources, most recently B3 2247+381 (ATel#2910) and 1ES 1215+303 (ATel#3100, this contribution). However, in many cases it has not been possible to confirm if the sources were in a high VHE γ-ray state during the observations and therefore the connection between the optical and VHE γ-ray states has remained an open question. In the first days of January 2011 1ES 1215+303 was observed to be in a high optical state (see Fig- ure 2) which triggered the MAGIC observations of the source. Here we present preliminary results of these observations together with previous observations carried out in 2010, where the source was in a lower optical state.
BL Lac objects belong to a subclass of AGN where the relativistic jet is pointing very close to our line of sight, causing flux enhancement and fast variability in all wavebands. Their spectral energy distribution (SED) is characterized by a typical double bump shape. The first and second bumps are associated respectively to synchrotron and inverse Compton emissions. 1ES 1215+303 (also known as ON 325) is a high energy peaking BL Lac object. Two values can be found for its redshift in the literature: z=0.130, and z=0.237 [20]. However, since none of the references given for the redshift show the optical spectra, it is difficult to judge which is the most correct value. Nevertheless, 1ES 1215+303 has a bright host galaxy of magnitude R = 16.24 (see e.g. [2]), which can be used for estimating the redshift. Following the methodology of [3] we derive z=0.13±0.04 (Nilsson, 2011, priv. comm.), favoring the lower redshift value. 1ES 1215+303 was classified as promising candidate TeV blazar in [4] and has been observed several times in VHE γ-rays prior to the observations presented here, providing only upper limits (Whipple:
). The source was also present in the Fermi-LAT bright AGN catalog [7], showing variable flux and a hard spectrum (Γ = -1.89 ± 0.06). In the Fermi-LAT band, 1ES 1215+303 is an exceptional source. The source underwent a large outburst in late 2008 and in the first bright AGN catalog [8] it was the only high energy peaking source that showed significant variability. 1ES 1215+303 has also been flagged as a promising VHE γ-ray source candidate by the Fermi-LAT collaboration in October 2010. 1ES 1218+304 is another high-peaking BL Lac object located only 0.8 • away from 1ES 1215+303. It has a redshift of 0.182 and was first detected to emit VHE γ-rays by MAGIC in 2005 [9]. In 2009 VER-ITAS reported fast variability from the source, the peak flux reaching ∼20% of the Crab Nebula flux [10]. In the Fermi-LAT one year catalog [11] the source is flagged non-variable. Since the measured VHE spectrum of 1ES 1218+304 is particularly hard for its redshift, an intrinsic SED with an inverse Compton peak above 1 TeV is expected. The γ-ray emission is thus strongly interacting with the extragalactic backgroung light (EBL) before reaching us, making the source a good candidate to probe the EBL [12,13] or the extragalactic magnetic field [14].
1ES 1215+303 and 1ES 1218+304 were observed by the MAGIC telescopes in January-February 2010, May-June 2010, and January-February 2011, for a total observation time of 48 hr. The observations were carried out in the so-called wobble mode [15] around 1ES 1215+303, with four pointing positions 0.4 • away from this source. 1ES 1218+304 was not the primary target of these observations, but it was always inside the MAGIC camera field of view, allowing its study at VHE as well. The data were taken in dark night and moderate moon conditions, and at zenith angles between 1 and 40 degrees. For the analysis, the data were divided into two samples corresponding to two observing epochs, i.e. 2010 and 2011. The data were analyzed using the stan- dard MAGIC s
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