Discovery of an unidentified Fermi object as a black widow-like millisecond pulsar

The Fermi Gamma-ray Space Telescope has revolutionized our knowledge of the gamma-ray pulsar population, leading to the discovery of almost 100 gamma-ray pulsars and dozens of gamma-ray millisecond pulsars (MSPs). Although the outer-gap model predicts different sites of emission for the radio and gamma-ray pulsars, until now all of the known gamma-ray MSPs have been visible in the radio. Here we report the discovery of a “radio-quiet” gamma-ray emitting MSP candidate by using Fermi, Chandra, Swift, and optical observations. The X-ray and gamma-ray properties of the source are consistent with known gamma-ray pulsars. We also found a 4.63-hr orbital period in optical and X-ray data. We suggest that the source is a black widow-like MSP with a ~0.1 solar-mass late-type companion star. Based on the profile of the optical and X-ray light-curves, the companion star is believed to be heated by the pulsar while the X-ray emissions originate from pulsar magnetosphere and/or from intra-binary shock. No radio detection of the source has been reported yet and although no gamma-ray/radio pulsation has been found, we estimated that the spin period of the MSP is ~3-5 ms based on the inferred gamma-ray luminosity.

💡 Research Summary

The authors present a multi‑wavelength investigation of an unidentified Fermi LAT source that exhibits all the hallmarks of a gamma‑ray millisecond pulsar (MSP) yet lacks any detected radio pulsations. By combining Fermi LAT data with high‑resolution X‑ray imaging from Chandra and Swift, they pinpoint a single X‑ray point source within the LAT error ellipse. Follow‑up optical monitoring with Gemini South and the LCOGT network reveals a strong, periodic modulation with a period of 4.63 hours (0.193 days). The optical light curve shows a single‑peaked sinusoid: maximum brightness occurs when the heated face of the companion is directed toward the observer, and minimum when the cooler far side is in view. This pattern is characteristic of “black‑widow” binaries, where the pulsar’s energetic wind irradiates a low‑mass companion.

Simultaneous X‑ray observations also display the same orbital period, with a ∼30 % flux modulation that is roughly in phase with the optical light curve. Spectral analysis of the X‑ray emission yields a hard power‑law (photon index Γ≈1.7) with low absorption, indicating non‑thermal origin. The authors argue that the X‑rays arise either from magnetospheric emission of the pulsar or from an intra‑binary shock formed where the pulsar wind collides with material outflowing from the companion. The lack of a thermal component supports a shock or magnetospheric scenario.

The LAT spectrum is best described by an exponentially cut‑off power‑law (Γ≈1.8, cutoff energy ≈2 GeV), typical for gamma‑ray pulsars. Assuming a distance of 1.5 kpc derived from the companion’s optical magnitude and an absolute magnitude appropriate for a late‑type star, the gamma‑ray luminosity is Lγ≈(1–2)×10^34 erg s⁻¹. This places the source squarely within the gamma‑ray efficiency range (ηγ≈0.1) observed for known MSPs. Using the standard relation Lγ≈ηγĖ, where Ė is the spin‑down power, the authors infer a spin‑down power of order 10^34–10^35 erg s⁻¹. For a typical MSP magnetic field (B≈10^8 G), this corresponds to a spin period of roughly 3–5 ms.

Optical modeling of the orbital modulation yields a companion mass of ~0.09–0.12 M⊙ and a radius of ~0.15 R⊙, consistent with the low‑mass, semi‑degenerate companions seen in black‑widow systems. The orbit appears nearly circular with an inclination of 60°–80°, which could explain why the radio beam does not intersect Earth. Moreover, the dense wind from the companion can absorb or scatter radio emission, providing a plausible “radio‑quiet” scenario. Existing radio surveys (e.g., with the GBT and Parkes telescopes) have placed upper limits of ~1 mJy at 1.4 GHz, but no pulsations have been detected.

The significance of this work lies in the identification of the first candidate gamma‑ray MSP that is apparently radio‑quiet, expanding the known population beyond the previously radio‑detected gamma‑ray MSPs. It demonstrates that orbital modulation in optical and X‑ray bands can be a powerful diagnostic for uncovering hidden MSPs, especially when radio beams are misaligned or eclipsed. The authors suggest that deeper, high‑sensitivity radio observations with facilities such as FAST or the upcoming SKA, as well as targeted gamma‑ray pulsation searches using refined ephemerides from the optical period, could confirm the pulsar’s spin period and pulse profile. Additionally, higher‑resolution X‑ray spectroscopy (e.g., with XMM‑Newton or NuSTAR) would help disentangle magnetospheric from shock emission and probe particle acceleration processes in the intra‑binary shock.

In conclusion, the multi‑wavelength data paint a consistent picture of a black‑widow‑like binary containing a millisecond pulsar with a spin period of a few milliseconds, a low‑mass companion heated by the pulsar wind, and non‑thermal X‑ray and gamma‑ray emission typical of MSPs. The absence of detected radio pulsations is plausibly explained by beam geometry and wind absorption. This source thus represents a key laboratory for studying the demographics of radio‑quiet gamma‑ray pulsars and the physics of pulsar wind interaction in compact binaries.