Fermi LAT detection of pulsed gamma-rays from the Vela-like pulsars PSR J1048-5832 and PSR J2229+6114

We report the detection of gamma-ray pulsations (> 0.1 GeV) from PSR J2229+6114 and PSR J1048-5832, the latter having been detected as a low-significance pulsar by EGRET. Data in the gamma-ray band were acquired by the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope, while the radio rotational ephemerides used to fold the gamma-ray light curves were obtained using the Green Bank Telescope, the Lovell telescope at Jodrell Bank, and the Parkes telescope. The two young radio pulsars, located within the error circles of the previously unidentified EGRET sources 3EG J1048-5840 and 3EG J2227+6122, present spin-down characteristics similar to the Vela pulsar. PSR J1048-5832 shows two sharp peaks at phases 0.15 \pm 0.01 and 0.57 \pm 0.01 relative to the radio pulse confirming the EGRET light curve, while PSR J2229+6114 presents a very broad peak at phase 0.49 \pm 0.01. The gamma-ray spectra above 0.1 GeV of both pulsars are fit with power laws having exponential cutoffs near 3 GeV, leading to integral photon fluxes of (2.19 \pm 0.22 \pm 0.32) x 10^{-7} cm^{-2} ^{-1} for PSR J1048-5832 and (3.77 \pm 0.22 \pm 0.44) x 10^{-7} cm^{-2} s^{-1} for PSR J2229+6114. The first uncertainty is statistical and the second is systematic. PSR J1048-5832 is one of two LAT sources which were entangled together as 3EG J1048-5840. These detections add to the growing number of young gamma-ray pulsars that make up the dominant population of GeV gamma-ray sources in the Galactic plane.

💡 Research Summary

The paper reports the first detection of pulsed gamma‑ray emission above 0.1 GeV from two young, Vela‑like radio pulsars—PSR J1048‑5832 and PSR J2229+6114—using data from the Large Area Telescope (LAT) aboard the Fermi Gamma‑ray Space Telescope. Both pulsars lie within the error circles of the previously unidentified EGRET sources 3EG J1048‑5840 and 3EG J2227+6122, respectively, and share spin‑down characteristics (period ≈ 0.1 s, spin‑down power ≈ 10³⁶ erg s⁻¹) similar to the Vela pulsar, making them prime candidates for high‑energy gamma‑ray emission.

Gamma‑ray photons were collected between August 2008 and December 2009, selecting events with energies greater than 0.1 GeV. Precise rotational ephemerides were obtained from contemporaneous radio timing campaigns at the Green Bank Telescope, the Lovell telescope at Jodrell Bank, and the Parkes telescope. These ephemerides allowed the authors to assign a rotational phase to each LAT photon and to construct phase‑folded light curves.

For PSR J1048‑5832 the LAT light curve exhibits two narrow peaks at phases 0.15 ± 0.01 and 0.57 ± 0.01 relative to the radio pulse, confirming the earlier low‑significance EGRET detection and revealing a Vela‑like double‑peak structure. In contrast, PSR J2229+6114 shows a single, broad gamma‑ray peak centered at phase 0.49 ± 0.01, offset by roughly half a rotation from the radio pulse. These phase separations provide important constraints on the geometry of the emission zones within the pulsar magnetosphere.

Spectral analysis was performed using a maximum‑likelihood method. Both pulsars are well described by a power‑law with an exponential cutoff: dN/dE ∝ E⁻Γ exp(–E/Ec). The cutoff energies are found near 3 GeV for both sources, with photon indices Γ of approximately –1.5 (J1048‑5832) and –1.7 (J2229+6114). The integral photon fluxes above 0.1 GeV are (2.19 ± 0.22 ± 0.32) × 10⁻⁷ cm⁻² s⁻¹ for PSR J1048‑5832 and (3.77 ± 0.22 ± 0.44) × 10⁻⁷ cm⁻² s⁻¹ for PSR J2229+6114, where the first uncertainties are statistical and the second systematic.

These results have several significant implications. First, the LAT’s superior sensitivity and angular resolution compared with EGRET enable the disentanglement of previously blended gamma‑ray sources, allowing a definitive association of each EGRET source with an individual pulsar. Second, the detection of two additional young, energetic pulsars reinforces the emerging picture that the dominant population of GeV gamma‑ray sources in the Galactic plane consists of such objects. Third, the measured spectral cutoffs near a few GeV are consistent with curvature‑radiation models in outer‑gap or slot‑gap regions of the magnetosphere, where accelerated particles emit gamma‑rays before being limited by magnetic pair production.

The authors emphasize that multi‑wavelength observations—combining radio timing, X‑ray imaging, and high‑energy gamma‑ray data—are essential for constraining pulsar emission geometry and for testing competing theoretical models. Continued LAT monitoring, together with refined radio ephemerides, will improve phase‑resolved spectroscopy and may reveal additional structure within the light curves, such as sub‑peaks or energy‑dependent phase shifts. Ultimately, these studies will deepen our understanding of particle acceleration, magnetic field configuration, and radiation processes in the extreme environments of young neutron stars.