We report on the discovery of high-energy (HE; E > 0.1 GeV) and very high-energy (VHE; E > 100 GeV) gamma-ray emission from the blazar RBS 0413 with Fermi Large Area Telescope (LAT) and VERITAS. The spectral energy distribution (SED), including contemporaneous X-ray and optical observations, is presented. Synchrotron self-Compton, external-Compton and lepto-hadronic models are applied to the SED and the results are discussed.
Blazars are active galactic nuclei (AGN) that have their jet axes oriented at a small angle with respect to the observer. They are broadly classified as either flat-spectrum radio quasars (FSRQ) or BL Lacertae objects (BL Lac) according to their optical spectra, and are known to emit non-thermal radiation characterized by a double-peaked SED. The lowenergy peak, covering the optical to UV/X-ray bands, is usually explained as due to synchrotron emission from relativistic electrons in the blazar jets. The origin of the highenergy peak, occurring in the X-ray to γ-ray regime, is still not completely resolved and could be due to leptonic [1] or hadronic [2] processes. RBS 0413 was discovered in the X-ray band (1E 0317.0+1834) during the Einstein Medium Sensitivity Survey and was optically identified as a BL Lac [3]. The object was also detected as a radio emitter by the Very Large Array of the National Radio Astronomy Observatory [4]. It has a redshift of 0.190 [3,5]. Having a "featureless" optical spectrum [6] and an estimated synchrotron peak frequency log(ν peak /1Hz) = 16.99 [7], RBS 0413 is classified as a high-frequency-peaked BL Lac object (HBL; ν peak > 10 15 Hz, [8]). The MAGIC Collaboration observed the object in 2004 -2005 and reported a γ-ray flux upper limit of 4.2 × 10 -12 ergs cm -2 s -1 , at 200 GeV, assuming a Γ = -3.0 power-law spectrum [9]. VERITAS observed the source in 2008-2009 season and obtained a marginal significance of ∼ 3 standard deviations (σ). In 2009, Fermi-LAT reported HE emission from the direction of RBS 0413 [10], triggering new VERITAS observations. In October 2009, VERITAS discovered γ-ray emission from RBS 0413 [11].
VERITAS is a ground-based VHE γ-ray observatory consisting of four atmospheric-Cherenkov telescopes, located at the Fred Lawrence Whipple Observatory (FLWO) near Amado, AZ, USA. It is sensitive to γ rays with energies from 100 GeV up to 30 TeV, with an energy resolution of 15-25%. In summer 2009, one of the telescopes was relocated and a new mirror-alignment system was introduced [12], improving the sensitivity of the array [13]. VERITAS can detect a γ-ray source with a flux of 1% of the Crab Nebula flux in less than ∼ 30 hours, with angular resolution less than 0.1 • and pointing accuracy better than 50". Data analysis consists of a number of steps, including calibration, image parametrization, event reconstruction, background rejection and signal extraction, as described in [14]. Twenty-six hours of good-quality VERITAS data on RBS 0413 taken between Sep 2008 and Jan 2010 resulted in an excess of 108 candidate γ rays and a 5.50 σ detection of the blazar. The spectrum above 250 GeV can be described with a power law: dN/dE = F 0 (E/E 0 ) -α , where F 0 = (1.38 ± 0.52) × 10 -7 TeV -1 m -2 s -1 , α = 3.18±0.68 stat and E 0 = 300 GeV, with a value of the χ 2 per degree of freedom (χ 2 /dof) of 0.14/2, correspond-
Fermi-LAT is a pair-conversion γ-ray detector sensitive to photons in the energy range from below 20 MeV to more than 300 GeV [15]. The present analysis includes the data taken between 4 Aug 2008 and 4 Jan 2011, which covers the entire VERITAS observation interval. “Diffuse class” events with energy between 300 MeV and 300 GeV from a square region of side 20 • centered on RBS 0413 and with zenith angles < 100 • were selected for this analysis. The cut at 300 MeV was used to minimize larger systematic errors at lower energies. The time intervals when the source was close to the Sun (MJD 54954-54974 and 55320-55339) were excluded. The data were analyzed with the LAT Science Tools version v9r20p01 and the post-launch instrument response functions P6 V11 DIFFUSE. The binned maximum likelihood tools were used for significance and flux calculation [16,17]. Sources from the 1FGL catalog [10] located within a square region of side 24 • centered on RBS 0413 were included in the model of the region. The background model includes the standard galactic and isotropic diffuse emission components 2 . A point source positionally consistent with RBS 0413 is detected with a significance of more than 9 σ (test statistic, TS = 89). The photon energy spectrum is well described by a power law. Replacing the power-law model with a log-parabola model does not significantly improve the likelihood fit (the likelihood ratio test rejects the powerlaw model with a significance of only 0.35 σ). The timeaveraged integral flux is I (300 MeV < E < 300 GeV) = (1.64 ± 0.43 stat + 0.31 sys -0.22 sys ) ×10 -9 cm -2 s -1 . The spectral index is 1.57 ± 0.12 stat +0.11 sys -0.12 sys . The spectral points were calculated using the procedure presented in [10]. In the energy range 100-300 GeV, no detection was obtained (TS < 9) and an upper limit at the 95% confidence level was derived. No evidence for variability was found when the data were analyzed using ∼ 6month wide time bins (see Figure 2 bottom panel).
The VERITAS detection triggered a Swift [18] target-ofopportunity observation of RBS
This content is AI-processed based on open access ArXiv data.
