[Paper Review] VERITAS Observations of the BL Lac Object PG 1553+113 Between May 2010 and May 2011

M. Orr|arXiv (Cornell University)|Oct 20, 2011

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

VERITAS conducted deep observations of the high-frequency peaked BL Lac object PG 1553+113 from May 2010 to May 2011, achieving a 39σ detection—the most significant VHE gamma-ray detection of this source to date. The analysis yields an upper limit on its redshift of z < 0.5 at 95% confidence, constraining its distance and enhancing its utility for probing the extragalactic background light (EBL).

ABSTRACT

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 &gt; 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 &lt; 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.

Motivation & Objective

• To measure the very-high-energy (VHE) gamma-ray spectrum of the BL Lac object PG 1553+113 using VERITAS observations.
• To use the observed VHE spectrum to constrain the source's redshift, given its uncertain value despite a lower limit of z > 0.4.
• To assess the potential of PG 1553+113 as a probe for constraining the extragalactic background light (EBL) through gamma-ray absorption effects.
• To evaluate the source's suitability for EBL studies by comparing its VHE spectral index with Fermi-LAT data at lower energies.

Proposed method

• VERITAS observed PG 1553+113 for 50 hours of live time between May 2010 and May 2011 using wobble-mode observations with 0.5° source offset.
• Imaging atmospheric-Cherenkov telescope data were calibrated using nightly flasher runs and cleaned using signal-to-noise cuts on individual pixels.
• Hillas parameters were extracted from cleaned images and used to reconstruct shower direction and core position via stereoscopic reconstruction.
• Gamma-ray energies were estimated using time- and season-specific Monte Carlo simulations for energy reconstruction.
• Background rejection was achieved using the reflected-regions method with seven background regions, and detection significance was calculated using the test statistic from Li & Ma (1983).
• The intrinsic spectral index was inferred by comparing the observed VHE spectrum with theoretical models of EBL absorption, assuming a power-law form and no intrinsic hardening above 300 GeV.

Experimental results

Research questions

• RQ1 What is the very-high-energy gamma-ray flux and spectral index of PG 1553+113 as measured by VERITAS over a 12-month period?
• RQ2 Can the observed VHE spectrum of PG 1553+113 be used to place a robust upper limit on its redshift, given the uncertainty in its true distance?
• RQ3 To what extent does the observed spectral softening in PG 1553+113's VHE spectrum indicate absorption due to the extragalactic background light (EBL)?
• RQ4 How well does PG 1553+113 perform as a probe for constraining the intensity and spectral shape of the EBL compared to other known VHE blazars?

Key findings

• VERITAS achieved a 39σ detection significance for PG 1553+113, the most significant VHE detection of this source to date.
• The time-averaged VHE spectrum of PG 1553+113 is well described by a power law with a spectral index of Γ = 4.41 ± 0.14 between 175 GeV and 500 GeV.
• The integral flux above 200 GeV is consistent across 2010 and 2011, showing no evidence of annual variability.
• An upper limit of z < 0.5 is derived for PG 1553+113 at the 95% confidence level, assuming no intrinsic spectral hardening above ~300 GeV.
• Combined with the existing lower limit of z > 0.4, this constrains the redshift to 0.4 < z < 0.5, making PG 1553+113 a prime candidate for EBL studies.
• PG 1553+113 is ranked among the most effective sources in the VERITAS catalog for constraining the EBL, with a value of α = 1.8–2.1 × 10⁻³ Mpc⁻¹, indicating strong sensitivity to EBL intensity.