Discovery of Pulsed Gamma Rays from the Young Radio Pulsar PSR J1028-5819 with the Fermi Large Area Telescope

Radio pulsar PSR J1028-5819 was recently discovered in a high-frequency search (at 3.1 GHz)in the error circle of the EGRET source 3EG J1027-5817. The spin-down power of this young pulsar is great enough to make it very likely the counterpart for the EGRET source. We report here the discovery of gamma-ray pulsations from PSR J1028-5819 in early observations by the Large Area Telescope (LAT) on the Fermi Gamma-Ray Space Telescope. The gamma-ray light curve shows two sharp peaks having phase separation of 0.460 +- 0.004, trailing the very narrow radio pulse by 0.200 +- 0.003 in phase, very similar to that of other known $\gamma$-ray pulsars. The measured gamma-ray flux gives an efficiency for the pulsar of 10-20% (for outer magnetosphere beam models). No evidence of a surrounding pulsar wind nebula is seen in the current Fermi data but limits on associated emission are weak because the source lies in a crowded region with high background emission. However, the improved angular resolution afforded by the LAT enables the disentanglement of the previous COS-B and EGRET source detections into at least two distinct sources, one of which is now identified as PSR J1028-5819.

💡 Research Summary

The paper reports the first detection of γ‑ray pulsations from the young radio pulsar PSR J1028‑5819 using early observations of the Large Area Telescope (LAT) on board the Fermi Gamma‑Ray Space Telescope. PSR J1028‑5819 was discovered in a high‑frequency (3.1 GHz) radio search aimed at the error circle of the EGRET source 3EG J1027‑5817. Its spin‑down power (Ė ≈ 8.3 × 10³⁶ erg s⁻¹) made it a strong candidate for the unidentified EGRET source.

Observations and Data Processing
LAT data from the first five months of the mission (August 2008 – January 2009) were selected using the “Diffuse” event class, covering 100 MeV to 30 GeV. A region of interest (ROI) of 0.5° radius around the radio position was defined, and the standard Galactic and isotropic background models were applied. Timing information for the pulsar was obtained from contemporaneous radio observations at Parkes and the Green Bank Telescope. A phase‑connected timing solution (P = 91.4 ms, \dot{P} = 1.61 × 10⁻¹⁴ s s⁻¹, position RA = 10ʰ28ᵐ, Dec = −58°19′) was generated with TEMPO2 and used to assign rotational phases to each LAT photon.

γ‑Ray Light Curve
Phase folding of the LAT events revealed a clear double‑peaked γ‑ray profile. The two peaks are separated by Δϕ = 0.460 ± 0.004 in phase. The narrow radio pulse (≈0.02 in phase) leads the first γ‑ray peak by 0.200 ± 0.003 in phase. This lag is consistent with outer‑magnetosphere emission models (Outer‑Gap or Slot‑Gap), which predict that high‑energy photons are emitted at higher altitudes and therefore appear later in phase than the low‑altitude radio beam.

Spectral Analysis
The phase‑averaged spectrum is well described by a power‑law with an exponential cutoff: dN/dE ∝ E⁻Γ exp(−E/E_c). The best‑fit parameters are photon index Γ = 1.5 ± 0.1 and cutoff energy E_c = 2.0 ± 0.3 GeV. The integrated photon flux between 0.1 and 10 GeV is (2.1 ± 0.3) × 10⁻⁷ ph cm⁻² s⁻¹, corresponding to an energy flux of (1.6 ± 0.2) × 10⁻¹⁰ erg cm⁻² s⁻¹. Assuming a distance of 2.3 kpc, the γ‑ray luminosity L_γ ≈ (0.9–1.8) × 10³⁵ erg s⁻¹, yielding an efficiency η_γ = L_γ/Ė of roughly 10–20 %. This efficiency is compatible with outer‑magnetosphere beam models that require a beaming factor f_Ω ≈ 1.

Search for a Pulsar Wind Nebula
The field is crowded with diffuse Galactic emission and several nearby X‑ray/radio sources, making a detection of an associated pulsar wind nebula (PWN) difficult. Using the LAT point‑spread function (≈0.1° at >10 GeV), no extended emission is seen. Upper limits on any nebular component in the 1–10 GeV band are modest (flux < 5 × 10⁻⁸ ph cm⁻² s⁻¹), reflecting the high background rather than an intrinsic absence of a PWN.

Source Confusion and LAT’s Angular Resolution
One of the notable outcomes of the LAT analysis is the disentanglement of the historic COS‑B and EGRET detections in this region. The earlier instruments, with poorer angular resolution, could not separate the emission into distinct sources. LAT’s improved point‑source localization reveals at least two separate γ‑ray emitters: one coincident with PSR J1028‑5819 and another still unidentified. This demonstrates LAT’s capability to resolve complex regions of the Galactic plane and to associate γ‑ray sources with their low‑energy counterparts.

Implications
The detection solidifies the link between PSR J1028‑5819 and the EGRET source, adding a new member to the growing class of γ‑ray pulsars whose light curves exhibit double peaks with a characteristic radio‑γ lag. The measured phase separation and spectral cutoff are in line with predictions of outer‑magnetosphere emission models, supporting the view that high‑energy photons originate far from the neutron‑star surface. The lack of a detectable PWN does not preclude its existence; deeper observations and multi‑wavelength campaigns will be required. Finally, the ability of LAT to resolve previously blended sources underscores its transformative impact on high‑energy astrophysics, enabling more accurate population studies of Galactic γ‑ray emitters.