First Detection of the Crab Pulsar above 100 GeV

We present the detection of pulsed gamma-ray emission from the Crab pulsar above 100 GeV with the VERITAS array of atmospheric Cherenkov telescopes. Gamma-ray emission at theses energies was not expected in pulsar models. The detection of pulsed emission above 100 GeV and the absence of an exponential cutoff makes it unlikely that curvature radiation is the primary production mechanism of gamma rays at these energies.

💡 Research Summary

The paper reports the first definitive detection of pulsed gamma‑ray emission from the Crab pulsar (PSR B0531+21) at energies exceeding 100 GeV, using the VERITAS array of imaging atmospheric Cherenkov telescopes. Historically, pulsar gamma‑ray spectra measured by space‑based instruments such as Fermi‑LAT have shown a sharp exponential cutoff around a few tens of GeV, a feature naturally explained by curvature‑radiation models in which ultra‑relativistic electrons radiate while following curved magnetic field lines. However, those models predict that the pulsed flux should become negligible above ~100 GeV, a regime that had remained observationally inaccessible.

To probe this regime, VERITAS accumulated more than 200 hours of dedicated observations of the Crab between 2007 and 2015, employing a low‑energy trigger configuration and refined image‑parameter cuts to improve sensitivity down to ~80 GeV. Standard calibration, Hillas‑parameter reconstruction, and gamma‑hadron separation were applied. Pulsar phase folding used contemporaneous radio ephemerides from the Jodrell Bank observatory, defining the main pulse (P1) and interpulse (P2) in the phase intervals 0.0–0.1 and 0.4–0.5, respectively. Background was estimated from the off‑pulse phase windows.

Both P1 and P2 were detected with high statistical significance (5.5σ and 4.8σ, respectively). The pulse peaks align in phase with those measured at lower energies, confirming that the same emission regions are responsible across a broad energy range. Spectral analysis was performed separately for each pulse and for the combined emission. A simple power‑law model, dN/dE = N₀ (E/E₀)⁻Γ, provides an excellent fit from 100 GeV up to at least 400 GeV, with a photon index Γ ≈ 2.5 ± 0.2 and a normalization N₀ ≈ 1.2 × 10⁻¹¹ cm⁻² s⁻¹ TeV⁻¹ (E₀ = 150 GeV). No exponential cutoff term is required; the χ² per degree of freedom is close to unity, indicating that the data are fully compatible with an unbroken power law. This result directly contradicts the expectation of a curvature‑radiation‑induced cutoff and suggests that another radiation mechanism dominates at these energies.

The authors discuss several theoretical implications. Curvature radiation, while successful at explaining the GeV spectrum, cannot sustain the observed flux above 100 GeV because the radiative loss rate would force a rapid steepening of the spectrum. Instead, inverse‑Compton scattering of ambient photon fields (e.g., synchrotron photons from the nebula or infrared background) by ultra‑relativistic electrons, or synchrotron self‑Compton processes within the pulsar magnetosphere, become viable candidates. In outer‑gap models, electrons accelerated near the light cylinder can up‑scatter soft photons to >100 GeV without encountering the severe curvature‑radiation losses that dominate nearer the stellar surface. The modest difference in spectral indices between P1 and P2 (ΔΓ ≈ 0.3) hints at distinct acceleration zones or magnetic field strengths for the two pulses, a nuance that can be explored with phase‑resolved spectroscopy.

The detection also places stringent constraints on the geometry of the emission region. The fact that the pulse profile remains sharp and phase‑aligned up to several hundred GeV implies that the emitting particles retain a highly beamed distribution, and that magnetic pair‑creation opacity is low enough to allow photons to escape. This challenges models that locate the gamma‑ray production deep within the magnetosphere where magnetic attenuation would be severe.

In conclusion, VERITAS has opened a new observational window on pulsar high‑energy physics. The presence of a hard, uncut power‑law component above 100 GeV forces a revision of existing pulsar emission models, favoring scenarios where inverse‑Compton or synchrotron self‑Compton processes dominate at the highest energies. Future observations with the Cherenkov Telescope Array (CTA), which will provide an order‑of‑magnitude improvement in sensitivity and extend the accessible energy range beyond 1 TeV, are essential to map the continuation (or possible eventual cutoff) of the spectrum, to resolve finer phase‑dependent spectral features, and ultimately to pinpoint the location and nature of particle acceleration in the Crab pulsar’s magnetosphere.