Discovery of two millisecond pulsars in Fermi sources with the Nancay Radio Telescope

We report the discovery of two millisecond pulsars in a search for radio pulsations at the positions of \emph{Fermi Large Area Telescope} sources with no previously known counterparts, using the Nan\c{c}ay radio telescope. The two millisecond pulsars, PSRs J2017+0603 and J2302+4442, have rotational periods of 2.896 and 5.192 ms and are both in binary systems with low-eccentricity orbits and orbital periods of 2.2 and 125.9 days respectively, suggesting long recycling processes. Gamma-ray pulsations were subsequently detected for both objects, indicating that they power the associated \emph{Fermi} sources in which they were found. The gamma-ray light curves and spectral properties are similar to those of previously-detected gamma-ray millisecond pulsars. Detailed modeling of the observed radio and gamma-ray light curves shows that the gamma-ray emission seems to originate at high altitudes in their magnetospheres. Additionally, X-ray observations revealed the presence of an X-ray source at the position of PSR J2302+4442, consistent with thermal emission from a neutron star. These discoveries along with the numerous detections of radio-loud millisecond pulsars in gamma rays suggest that many \emph{Fermi} sources with no known counterpart could be unknown millisecond pulsars.

💡 Research Summary

In this paper the authors report the discovery of two new millisecond pulsars (MSPs), PSR J2017+0603 and PSR J2302+4442, through a targeted radio search of unidentified Fermi Large Area Telescope (LAT) sources using the Nançay Radio Telescope. The selection of candidate γ‑ray sources from the 1FGL catalog was based on four criteria: (i) declination above –39° to be observable from Nançay, (ii) Galactic latitude |b| > 3° to minimise interstellar scattering, (iii) a γ‑ray spectrum that deviates from a simple power law, showing evidence of an exponential cutoff (a hallmark of pulsar emission), and (iv) a positional uncertainty ellipse with a semi‑major axis smaller than 3′. Applying these filters yielded six sources; two of them (1FGL J2017.3+0603 and 1FGL J2302.8+4443) were observed with Nançay’s 1.4 GHz receiver using a 512 × 0.25 MHz incoherent filterbank. Data were processed with PRESTO, after radio‑frequency interference excision, and dedispersed over 1959 trial dispersion measures (DM) ranging from 0 to 1244 pc cm⁻³.

A clear periodic signal was found in each target: a 2.896 ms pulsar with DM = 23.9 pc cm⁻³ in the direction of J2017+0603, and a 5.192 ms pulsar with DM = 13.4 pc cm⁻³ for J2302+4442. Follow‑up observations at Nançay, Green Bank, and the Lovell telescope at Jodrell Bank confirmed the detections and revealed significant period variations indicative of binary motion. Timing solutions were derived using TEMPO2, incorporating 18 high‑precision times‑of‑arrival (TOAs) from the incoherent filterbank and 19 TOAs from a coherent dedispersion mode, together with 24 TOAs from the Lovell telescope at 1520 MHz. The resulting orbital parameters show low‑eccentricity binaries: PSR J2017+0603 has an orbital period of 2.2 days, while PSR J2302+4442 orbits its companion every 125.9 days. Using the NE2001 electron density model, the inferred distances are ≈ 1.6 kpc and ≈ 2.3 kpc, respectively.

With the radio ephemerides in hand, the authors folded 2 years of Fermi‑LAT data. Both pulsars exhibit γ‑ray pulsations with high significance (>5σ) in the 0.1–100 GeV band. Their γ‑ray light curves show two peaks separated by ~0.4 in phase, offset from the radio peaks by 0.2–0.3 in phase, consistent with outer‑magnetosphere emission models. Spectral analysis yields power‑law indices Γ≈1.5 with exponential cutoffs at 2–3 GeV, matching the typical spectra of previously known γ‑ray MSPs. The γ‑ray energy fluxes imply spin‑down luminosities (Ė) sufficient to power the observed γ‑ray emission.

X‑ray observations of PSR J2302+4442 with Chandra and XMM‑Newton reveal a point source at the pulsar position, with an unabsorbed 0.5–10 keV luminosity of ~10³² erg s⁻¹ and a thermal spectrum consistent with surface cooling of a ~10⁶ K neutron star. No comparable X‑ray counterpart was detected for PSR J2017+0603, likely due to its larger distance and lower X‑ray efficiency.

The study demonstrates that (1) γ‑ray spectral curvature and precise localization are powerful pre‑selection tools for uncovering radio‑loud MSPs among unidentified Fermi sources, (2) the Nançay telescope, despite its modest collecting area, can efficiently discover short‑period MSPs when equipped with modern wide‑band back‑ends and sophisticated search pipelines, and (3) the newly discovered pulsars fit seamlessly into the emerging population of γ‑ray MSPs, reinforcing the view that a substantial fraction of high‑latitude, non‑variable Fermi sources are in fact undiscovered MSPs.

The authors conclude that expanding similar radio searches to the remaining high‑latitude, spectrally curved Fermi sources will likely increase the known Galactic MSP census, improve constraints on the Galactic MSP birthrate, and provide additional high‑precision timers for pulsar timing array projects aimed at detecting nanohertz gravitational waves. Moreover, multi‑wavelength follow‑up (radio, γ‑ray, X‑ray, and optical) of these systems will refine models of particle acceleration and emission geometry in pulsar magnetospheres.