Discovery of new gamma-ray pulsars with AGILE

Using gamma-ray data collected by the Astrorivelatore Gamma ad Immagini LEggero (AGILE) satellite over a period of almost one year (from 2007 July to 2008 June), we searched for pulsed signals from 35 potentially interesting radio pulsars, ordered according to $F_{\gamma}\propto \sqrt{\dot{E}} d^{-2}$ and for which contemporary or recent radio data were available. AGILE detected three new top-ranking nearby and Vela-like pulsars with good confidence both through timing and spatial analysis. Among the newcomers we find pulsars with very high rotational energy losses, such as the remarkable PSR B1509-58 with a magnetic field in excess of 10^13 Gauss, and PSR J2229+6114 providing a reliable identification for the previously unidentified EGRET source 3EG 2227+6122. Moreover, the powerful millisecond pulsar B1821-24, in the globular cluster M28, is detected during a fraction of the observations. Four other promising gamma-ray pulsar candidates, among which is the notable J2043+2740 with an age in excess of 1 million years, show a possible detection in the timing analysis only and deserve confirmation.

💡 Research Summary

The paper reports the results of a systematic search for gamma‑ray pulsations using data from the AGILE satellite collected between July 2007 and June 2008. The authors selected 35 radio pulsars that are expected to be bright in gamma rays based on the empirical scaling Fγ ∝ √Ė d⁻², where Ė is the spin‑down power and d the distance. Contemporary radio ephemerides were obtained through a dedicated monitoring campaign with the Jodrell Bank and Nançay telescopes, ensuring accurate timing solutions over the entire AGILE observing interval.

AGILE’s Gamma‑Ray Imaging Detector (GRID) provides a large field of view (≈60° radius), good timing accuracy (<200 µs), and sensitivity in the 30 MeV–30 GeV band. Events with energies above 50 MeV (G‑class) and a combined G+L sample were extracted within a radius optimized for each source’s point‑spread function. For each target, epoch‑folding was performed over a frequency grid defined by the 3σ radio ephemeris uncertainties, with a frequency step of 0.5/T (T being the total exposure time). The folded 10‑bin light curves were examined with Pearson’s χ², the Z²ₙ test, and the H‑test to assess the significance of any periodic signal, while a maximum‑likelihood spatial analysis (ALIKE) was used to confirm source detection and to estimate fluxes.

Three new gamma‑ray pulsars were detected with high confidence. PSR J2229+6114 (Ė≈2×10³⁶ erg s⁻¹, d≈3 kpc) shows a 6σ Z²₂ detection (Z²₂≈45.9) and a spatially coincident source with a flux of ≈2.6×10⁻⁷ ph cm⁻² s⁻¹ (E>100 MeV), providing the first firm identification of the previously unidentified EGRET source 3EG 2227+6122. PSR B1509‑58, a young Vela‑like pulsar with a magnetic field >10¹³ G and Ė≈1.8×10³⁸ erg s⁻¹, is also clearly detected both in timing and imaging, extending the known high‑energy emission of this object. The millisecond pulsar PSR B1821‑24, located in the globular cluster M28, is detected only during a subset of the observations, suggesting either intrinsic variability or sensitivity limits imposed by the short exposure windows.

In addition to these firm detections, four other pulsars (including the old, high‑latitude PSR J2043+2740 with a characteristic age >1 Myr) exhibit tentative periodic signals at the 3–4σ level in the timing analysis but lack a corresponding spatial detection. These candidates merit further observation to confirm their gamma‑ray nature.

The study highlights that AGILE’s wide field of view and continuous monitoring capability enable simultaneous coverage of many radio pulsars, making it especially effective for discovering new gamma‑ray pulsars. Detection efficiency depends strongly on the predicted gamma‑ray flux, exposure time, and background complexity, particularly near the Galactic plane. The results increase the sample of known gamma‑ray pulsars, providing valuable data for testing emission models (e.g., outer‑gap versus slot‑gap) and for constraining the geometry of high‑energy beams. The authors conclude that continued AGILE observations, complemented by data from Fermi‑LAT, will allow a more detailed characterization of pulsar gamma‑ray spectra, light‑curve morphology, and variability, thereby advancing our understanding of particle acceleration in neutron star magnetospheres.