[Paper Review] Observation of the Supernova Remnant IC 443 with VERITAS

VERITAS Collaboration, T. B. Humensky|ArXiv.org|Sep 27, 2007

Astrophysics and Cosmic Phenomena3 references3 citations

TL;DR

VERITAS detected very-high-energy (VHE) gamma-ray emission from the supernova remnant IC 443 at a significance of 6.0σ post-trials, with a flux of approximately 3–4% of the Crab Nebula above 200 GeV. The emission is spatially coincident with the dense molecular cloud and OH masers, suggesting a hadronic origin from cosmic-ray interactions with the cloud, rather than association with the nearby pulsar wind nebula.

ABSTRACT

Shell-type supernova remnants (SNRs) accelerate particles at the shock front between the expanding remnant and the swept-up interstellar medium. If these particles include protons and nuclei, very-high-energy gamma-ray emission may result from the decay of pions produced in interactions between cosmic rays and the local insterstellar medium. For SNRs that are interacting with a nearby molecular cloud, such as IC 443, the enhanced matter density provides a target medium that can amplify the gamma-ray emission. IC 443 also contains the pulsar wind nebula (PWN) CXOU J061705.3+222127. PWNe are the most plentiful galactic sources of very-high-energy gamma rays, which are produced in the shock formed at the collision of the pulsar wind with the ambient medium. VERITAS is an array of four 12-m telescopes dedicated to gamma-ray astronomy in the energy band above 100 GeV. Located on Mt. Hopkins in southern Arizona, VERITAS operated during the 2006-2007 season in 2-, 3-, and 4-telescope observation modes. In this talk, results from three-telescope observations of the composite supernova remnant IC 443 during the 2006-2007 season are discussed.

Motivation & Objective

• To detect very-high-energy gamma-ray emission from the composite supernova remnant IC 443 using the VERITAS telescope array.
• To determine the spatial and spectral morphology of the VHE emission and assess its association with the pulsar wind nebula (PWN) or molecular cloud.
• To investigate the origin of the VHE emission—whether hadronic (proton-proton interactions) or leptonic (inverse Compton or PWN emission)—through multiwavelength coincidence.
• To validate the robustness of the analysis in the presence of bright stars that could contaminate the data.
• To contribute to the understanding of cosmic-ray acceleration in supernova remnants and the role of molecular clouds in amplifying gamma-ray emission.

Proposed method

• VERITAS observed IC 443 using three 12-m telescopes in wobble mode, with the source offset by 0.5° from the telescope pointing center to reduce background.
• Data were collected over 15.9 hours with quality cuts applied to weather, hardware stability, and livetime corrections.
• Pixel smoothing was applied to recover signal in pixels suppressed by high PMT currents due to bright stars (HIP 29655 and HIP 30343).
• Gamma/hadron separation used the mean-scaled width and length parameters to distinguish gamma-ray showers from background cosmic rays.
• The ring background model was employed for background estimation, suitable for slightly extended or offset sources.
• Source significance and flux were determined by comparing the gamma-ray rate to that of the Crab Nebula at similar zenith angles (11°).

Experimental results

Research questions

• RQ1 Is there a detectable very-high-energy gamma-ray signal from IC 443 above 200 GeV?
• RQ2 Where is the VHE emission spatially located relative to the Pulsar Wind Nebula (PWN) and the molecular cloud?
• RQ3 Does the observed VHE emission originate from hadronic interactions in the dense molecular cloud or from leptonic processes in the PWN?
• RQ4 How do bright stars in the field affect the gamma-ray analysis, and can their impact be mitigated?
• RQ5 Is the VHE emission consistent with a power-law spectrum, and what is its flux relative to the Crab Nebula?

Key findings

• VERITAS detected a significant VHE gamma-ray excess from IC 443 with a pre-trial significance of 7.1σ and post-trial significance of 6.0σ.
• The flux of the emission is approximately 3–4% of the Crab Nebula flux above 200 GeV, consistent with the MAGIC detection.
• The centroid of the VHE emission is located at right ascension 06h16m, declination +22°30′, which is consistent with the MAGIC position.
• The emission is spatially offset by about 0.15° from the Pulsar Wind Nebula (PWN) at J061705.3+222127, arguing against a direct association with the PWN.
• The VHE emission is spatially coincident with the densest region of the molecular cloud and with OH maser emission, supporting a hadronic origin from cosmic-ray interactions with the cloud.
• Systematic tests, including observations of a bright-star field without known gamma-ray sources, confirmed that the bright stars did not produce a false excess, validating the analysis.