[Paper Review] Observation of selected SNRs with the MAGIC Cherenkov Telescope

E. Carmona, M. T. Costado+2 more|arXiv (Cornell University)|Jul 6, 2009

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This study reports on two years of Very High Energy (VHE) gamma-ray observations of selected Supernova Remnants (SNRs) using the MAGIC Cherenkov telescope, focusing on Tycho and nine radio-selected SNRs. Despite high sensitivity and optimized selection criteria, no significant VHE emission was detected above 270 GeV, with upper limits ranging from 5% to 25% of the Crab Nebula flux.

ABSTRACT

Supernova Remnants (SNRs) are believed to be the acceleration sites of galactic cosmic rays. As such they are expected to produce Very High Energy (VHE) gamma-rays through hadronic and/or electromagnetic scenarios, hence they are natural targets for observations with ground-based Imaging Atmospheric Cherenkov Telescopes (IACTs). Currently, VHE emission has been detected from several SNRs, making them one of the most abundant types of established galactic VHE sources. The MAGIC telescope, located in the Canary island of La Palma, has been performing observations of several SNRs over the last two years. Parameters like age, distance, radio flux, or possible EGRET association (i.e., criteria matching those already used for previous successful IACT SNR detections), were used to select candidate targets. Here we summarize the results of the past two years of observations.

Motivation & Objective

• To test the hypothesis that young SNRs are sources of VHE gamma-rays via hadronic or electromagnetic particle acceleration mechanisms.
• To search for VHE gamma-ray emission from the Tycho SNR and nine radio-selected SNRs using the MAGIC Cherenkov telescope.
• To evaluate the impact of source distance, size, and position on detection sensitivity, especially for extended or off-center sources.
• To compare observational results with theoretical predictions of gamma-ray emission from SNRs, particularly for Tycho.
• To establish stringent upper limits on VHE flux for candidate SNRs based on observational data

Proposed method

• Target selection based on radio flux (≥49 Jy), spectral index (≤0.6), distance (≤7 kpc), and age (≤50,000 yr), emulating known VHE-emitting SNRs.
• Observations conducted in wobble mode with the MAGIC telescope at La Palma, using a 17 m diameter reflector and 2 GHz FADC-triggered data acquisition system.
• Data analysis using time-image cleaning with boundary thresholds of 8/4 pe for Moon observations and 6/3 pe for dark conditions, applied to reduce background.
• Calculation of integral flux upper limits (3σ) above 270 GeV for point-like sources at the center of wobble positions, assuming Crab Nebula normalization.
• Skymap reconstruction and significance evaluation to search for extended or off-center emission, with comparison to known sources like W51.
• Sensitivity estimation accounting for source offset: e.g., 30 hours for 5% Crab flux at 0.5° offset, compared to 8 hours for on-axis point sources.

Experimental results

Research questions

• RQ1 Is the Tycho SNR detectable in VHE gamma-rays with the current sensitivity of the MAGIC telescope, given its predicted emission levels?
• RQ2 Do radio-selected SNRs with properties similar to known VHE emitters (e.g., IC443, RXJ1713) show detectable VHE gamma-ray emission?
• RQ3 How does source offset from the telescope's pointing center affect the detectability of extended or off-center VHE sources?
• RQ4 What constraints do the upper limits on VHE flux place on the distance to the Tycho SNR, given theoretical predictions of emission?
• RQ5 To what extent do the observed skymaps and significance maps support or contradict previous VHE detections, such as the MILAGRO signal from W51?

Key findings

• No significant VHE gamma-ray emission (>5σ) was detected from the Tycho SNR or any of the nine radio-selected SNRs during the observation period.
• Integral flux upper limits (3σ) above 270 GeV range from 1.60×10⁻¹¹ ph cm⁻² s⁻¹ (11% Crab) for HB-9 to 3.56×10⁻¹¹ ph cm⁻² s⁻¹ (25% Crab) for CTB-80.
• The most sensitive upper limit was 6.68×10⁻¹² ph cm⁻² s⁻¹ (5% Crab) for W66, observed under dark conditions with 11.4 hours of effective time.
• For sources offset by 0.5° from the observation center, detection times required for 5% Crab flux increased to ~30 hours, highlighting sensitivity loss for off-axis sources.
• The skymap for W51 shows a ~3σ significance region coinciding with the MILAGRO source J1923.0+1411, but the flux is below the 9% Crab upper limit, indicating no significant detection.
• The lack of detection in Tycho, despite theoretical predictions, suggests the distance may be larger than commonly assumed—potentially exceeding 3.5 kpc—undermining standard models if emission is hadronic.