VERITAS, an array of imaging atmospheric-Cherenkov telescopes, studies blazars in the energy range between ~100 GeV and ~30 TeV. With its excellent sensitivity at these energies, and ever-deepening source exposures, VERITAS is in a position to probe distant blazars for clear absorption signatures in their very-high-energy gamma-ray spectra due to interactions with the extragalactic background light (EBL). We discuss results from recent VERITAS observations of PG 1553+113 (z > 0.4) which have resulted in the most significant very-high-energy detection ever obtained for this source. The most recent VERITAS spectral measurements are used to place an upper limit on the source redshift of z < 0.5 at the 95% confidence level. Also discussed are the prospects of using these observations, along with those of other hard- spectrum blazars, to place constraints on the EBL.
Deep Dive into VERITAS Observations of the BL Lac Object PG 1553+113 Between May 2010 and May 2011.
VERITAS, an array of imaging atmospheric-Cherenkov telescopes, studies blazars in the energy range between ~100 GeV and ~30 TeV. With its excellent sensitivity at these energies, and ever-deepening source exposures, VERITAS is in a position to probe distant blazars for clear absorption signatures in their very-high-energy gamma-ray spectra due to interactions with the extragalactic background light (EBL). We discuss results from recent VERITAS observations of PG 1553+113 (z > 0.4) which have resulted in the most significant very-high-energy detection ever obtained for this source. The most recent VERITAS spectral measurements are used to place an upper limit on the source redshift of z < 0.5 at the 95% confidence level. Also discussed are the prospects of using these observations, along with those of other hard- spectrum blazars, to place constraints on the EBL.
PG 1553+113 is a high-frequency peaked BL Lac object (HBL) [1,2,3] discovered by Green et al. (1986) [4]. Evidence of very-high-energy (VHE) gamma-ray emission from this source was first detected by HESS in 2005 [5]. This was later confirmed by observations made with the MAGIC telescope in 2005 and 2006 [6]. Due to its featureless spectrum, the redshift of PG 1553+113 remains uncertain. However, constraints on the redshift have been continually narrowing with improved optical measurements and limits from VHE observations (e.g., [7,8,5,9]). Recent measurements using the Hubble Space Telescope/Cosmic Origins Spectrograph place a lower limit on the redshift of PG 1553+113 of z > 0.4 [10]. Statistical arguments are also presented placing a 1σ upper limit of z ≤ 0.58.
These limits suggest that PG 1553+113 is one of the most distant sources ever detected at VHE, and perhaps the most distant source of all. This makes it an incredibly interesting source for studying the extragalactic background light (EBL). As VHE gamma rays traverse the distance between their point of origin and the Earth, they can interact with the diffuse infrared radiation field known as the EBL [11,12]. The optical/infrared EBL consists of the progressive emission of galaxies and active galactic nuclei (AGN), and the absorption and re-radiation of this emission by dust (see [13,14] for a review). The level of gamma-ray absorption due to the EBL increases with both gamma-ray energy and source distance. Consequently, observed VHE spectra are increasingly softened, with respect to their intrinsic emission, as the source becomes more distant. This effectively produces a gamma-ray horizon, beyond which gamma rays of a particular energy are fully absorbed.
Very-high-energy observations of distant blazars are a valuable tool for constraining the EBL. As such, PG 1553+113 has been an important component in the long-term blazar science program of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaboration. The VERITAS observatory is an array of four 12-meter imaging atmospheric-Cherenkov telescopes (IACTs) located at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona [15]. Each reflector comprises 350 hexagonal mirrors following the Davies-Cotton design. The focal plane camera consists of 499 photomultiplier tubes covering a 3.5 • field of view. Its large collection area (∼ 10 5 m 2 ), in conjunction with the stereoscopic imaging of air showers, allows VERITAS to detect veryhigh-energy gamma rays between energies of 100 GeV and 30 TeV with an energy resolution of ∼ 15-25% and an angular resolution of ∼ 0.1 • . A source with a flux 1% that of the Crab Nebula can be detected by VERITAS in ∼ 25 hours with a statistical significance of 5 standard deviations (σ). PG 1553+113 was observed by VERITAS between May 2010 and May 2011 for a total of nearly 72 hours. Excluding bad-weather runs, and some 3-telescope data not yet analyzed, the dataset presented here consists of 50 hours of live time. All observations were performed in wobble mode, with the source offset from the center of the field of view by 0.5 • . This allows for simultaneous background estimation and source observation. Four wobble directions are used (North, South, East, West) and are alternated from run to run.
The standard VERITAS analysis chain begins with the calibration of camera gains using nightly flasher runs. Each camera image is then cleaned, with cuts being placed on each pixel according to its signal-to-noise ratio for a given event. The cleaned images are then characterized, using the standard Hillas parameters [16], by calculating the moments of the light distributions. Cuts on these image parameters are placed to maximize the rejection of the cosmic-ray background while minimizing the loss of gamma rays. Images from a given event passing the requisite cuts are then combined stereoscopically to reconstruct the direction of the incident gamma ray in the camera plane and the core location of the air shower in the ground plane. The energies of reconstructed events are estimated using Monte Carlo simulations. Separate simulations are used for data taken during the summer (May-October) and winter (November-April) months, with each set of simulations using atmospheric models appropriate for that time of year. For the analysis of PG 1553+113, a circular region of radius 0.14 • , centered around the source direction, was used to calculate the number of ON counts in the data. The OFF counts were determined using the reflected-regions background model [17] with 7 background regions. The detection significance was calculated following Equation 17 of [18].
The VERITAS observations of PG 1553+113 between May 2010 and May 2011 resulted in a detection significance of 39σ, the most significant VHE detection ever obtained for this source. The significance map of these observations is shown in Figure 1. The time-averaged energy spectrum for the full observa
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