[Paper Review] VERITAS Observations of Magnetars

R. Guénette|arXiv (Cornell University)|Aug 5, 2009

Astrophysics and Cosmic Phenomena2 references5 citations

TL;DR

VERITAS conducted high-energy gamma-ray observations of two magnetars, 4U 0142+61 and 1E 2259+586, during quiescent states to test predictions of non-thermal TeV emission from magnetic field decay. No significant emission was detected, setting 99% confidence level upper limits of 0.9% and 2.5% of the Crab Nebula flux above 400 GeV, respectively, constraining models of hadronic or electromagnetic TeV emission during outbursts.

ABSTRACT

Magnetars are rotating neutron stars with extremely strong magnetic fields (~ 10^14-10^15 G). X-ray and soft gamma-ray observations have revealed the existence of non-thermal particle populations which may suggest emission of very high energy photons. VERITAS, an array of four 12m imaging atmospheric Cherenkov telescopes, is designed to observe gamma-ray emission between 100 GeV and 30 TeV. Here we present the results of VERITAS observations of two magnetars, 4U 0142+61 and 1E 2259+586.

Motivation & Objective

• To test theoretical predictions of steady very-high-energy (VHE) gamma-ray emission from magnetars due to magnetic field decay.
• To search for VHE emission during quiescent states when no X-ray flares are active, as a baseline for future Target of Opportunity observations during outbursts.
• To constrain models predicting TeV emission via hadronic interactions or electromagnetic cascades during giant X-ray flares.
• To establish flux upper limits for future comparison with potential burst-induced VHE emission.
• To assess the detectability of magnetar VHE emission using imaging atmospheric Cherenkov telescopes like VERITAS.

Proposed method

• VERITAS, an array of four 12-meter imaging atmospheric Cherenkov telescopes, observed both magnetars at energies between 100 GeV and 30 TeV.
• Observations were conducted during dark time and low moonlight, with source positions offset by 0.5° in wobble mode for 1E 2259+586 and near-pointing for 4U 0142+61.
• Data selection applied quality cuts, requiring at least 500 digital counts per image, central image centroid within 1.5°, and image shape constraints (reduced width/length between -1.2 and 0.5).
• Background estimation used the 'ring background' model, with significance calculated via the Li and Ma method (equation 17).
• Flux upper limits were computed using the Helene method for a power-law spectrum with index -2.5, expressed in units of the Crab Nebula flux.
• Energy thresholds were set above 400 GeV, and upper limits were derived at the 99% confidence level.

Experimental results

Research questions

• RQ1 Is steady VHE gamma-ray emission detectable from magnetars during quiescent states, as predicted by some non-thermal emission models?
• RQ2 Can VERITAS detect TeV emission from magnetars during major X-ray outbursts, as theorized in electromagnetic or hadronic interaction models?
• RQ3 What are the flux upper limits for VHE emission from magnetars, assuming a power-law spectrum with index -2.5?
• RQ4 How do the observed upper limits constrain theoretical models of magnetar emission mechanisms?
• RQ5 Can the SNR CTB 109 be probed for point-source VHE emission from its central magnetar 1E 2259+586?

Key findings

• No significant gamma-ray emission was detected from 4U 0142+61, with a 99% confidence level upper limit of 8.68 × 10⁻¹³ cm⁻² s⁻¹ for energies above 400 GeV, equivalent to 0.9% of the Crab Nebula flux.
• For 1E 2259+586, the 99% C.L. flux upper limit is 2.49 × 10⁻¹² cm⁻² s⁻¹ above 400 GeV, corresponding to 2.5% of the Crab Nebula flux.
• The significance of the on-source event count for 4U 0142+61 was -2.9σ, indicating no excess over background.
• The significance for 1E 2259+586 was 1.5σ, also consistent with background fluctuations.
• These results constrain models predicting TeV emission during X-ray flares, as a flux of ~100× Crab would be easily detectable during a 0.3s burst.
• The upper limits are applicable to both the magnetars and the surrounding SNR CTB 109 for 1E 2259+586, given the source's central location.