[Paper Review] The VERITAS Survey of the Cygnus Region of the Galactic Plane

A. J. Weinstein|arXiv (Cornell University)|Dec 22, 2009

Astronomical Observations and Instrumentation3 references21 citations

TL;DR

The VERITAS survey of the Cygnus region, conducted over 140 hours of observation, achieved a sensitivity of better than 4% of the Crab Nebula flux above 200 GeV for point sources, detecting two VHE gamma-ray sources—TeV J2032+4130 and extended emission VER J019+407—despite expecting 2–3 detections based on H.E.S.S. survey extrapolations, suggesting a distinct VHE source population in Cygnus compared to the Galactic center.

ABSTRACT

The VERITAS IACT observatory has carried out an extensive survey of the Cygnus region between 67 and 82 degrees in galactic longitude and between -1 and 4 degrees in galactic latitude. This region is a natural choice for a Very High Energy (VHE) gamma-ray survey in the Northern Hemisphere, as it contains a substantial number of potential VHE gamma-ray emitters such as supernova remnants, pulsar wind nebulae, high-mass X-ray binaries, and massive star clusters, in addition to a few previously detected VHE gamma-ray sources. It is also home to a number of GeV gamma-ray sources, including no less than four new high-significance sources detected in the first six months of Fermi data. The VERITAS survey, comprising more than 140 hours of observations, reaches an average VHE point-source flux sensitivity of better than 4% of the Crab Nebula flux at energies above 200 GeV. Here we report on preliminary results from this survey, including two source detections, and discuss the prospects for further studies that would exploit the joint coverage provided by VERITAS and Fermi data in this region.

Motivation & Objective

• To conduct an unbiased, large-scale Very High Energy (VHE) gamma-ray survey of the Cygnus region in the Northern Galactic Plane.
• To probe the VHE gamma-ray source population in a region known to host multiple potential emitters such as supernova remnants, pulsar wind nebulae, and high-mass X-ray binaries.
• To compare VERITAS results with Fermi-LAT GeV data to identify multi-wavelength counterparts and study source correlations.
• To assess the sensitivity of the survey to both point-like and extended VHE sources using simulated background fields.
• To investigate discrepancies in source expectations between the Cygnus region and the previously surveyed Galactic center region.

Proposed method

• Conducted a 5° × 15° survey in galactic coordinates (67° ≤ l ≤ 82°, -1° ≤ b ≤ 4°) using a grid of pointed observations with 0.8° and 1.0° spacing in longitude and latitude.
• Used a four-telescope configuration of the VERITAS IACT array, with observations taken at zenith angles between 10° and 35°, averaging 20° across the survey.
• Applied quality cuts to exclude data with unstable triggers, poor weather, or hardware issues, and prioritized moonless nights.
• Performed follow-up observations using the wobble technique at 0.5°–0.7° offsets from potential sources or transient events.
• Simulated background conditions using blank sky fields matched in zenith angle and azimuth to test-pointing fields, with gamma-ray injections at various fluxes and spectral indices.
• Used a 1.7° angular cut-off to define fields contributing to sensitivity at each test point, ensuring accurate background estimation for source detection.

Experimental results

Research questions

• RQ1 What is the VHE gamma-ray source population in the Cygnus region, and how does it compare to that of the Galactic center region surveyed by H.E.S.S.?
• RQ2 How does the VERITAS survey sensitivity vary with source spectral index and spatial extension?
• RQ3 Which GeV sources detected by Fermi-LAT are associated with VHE gamma-ray sources in the Cygnus region?
• RQ4 What is the significance of the lack of high-significance detections in the base survey despite expectations based on prior surveys?
• RQ5 Can joint VERITAS and Fermi data reveal correlations or associations between VHE and GeV sources in this region?

Key findings

• The VERITAS survey achieved an average VHE point-source flux sensitivity of better than 4% of the Crab Nebula flux above 200 GeV.
• Preliminary flux limits were derived as 3% of the Crab Nebula for point sources and 8.5% for extended sources, both below 3σ significance.
• Despite expectations of 2–3 detections at or above 5σ pre-trials based on H.E.S.S. survey extrapolations, no such sources were detected in the base survey.
• Two VHE gamma-ray sources were detected in follow-up observations: TeV J2032+4130, likely associated with Fermi source OFGL J2032.2+4122, and extended emission VER J019+407 near the γ Cygni SNR.
• The absence of strong detections in the base survey, despite high sensitivity, suggests a significant difference in the VHE source population between the Cygnus region and the Galactic center region.
• Joint VERITAS and Fermi data analysis is planned to further explore associations and multi-wavelength correlations in the region.