Constraining the optical emission from the double pulsar system J0737-3039

We present the first optical observations of the unique system J0737-3039 (composed of two pulsars, hereafter PSR-A and PSR-B). Ultra-deep optical observations, performed with the High Resolution Camera of the Advanced Camera for Surveys on board the Hubble Space Telescope could not detect any optical emission from the system down to m_F435W=27.0 and m_F606W=28.3. The estimated optical flux limits are used to constrain the three-component (two thermal and one non-thermal) model recently proposed to reproduce the XMM-Newton X-ray spectrum. They suggest the presence of a break at low energies in the non-thermal power law component of PSR-A and are compatible with the expected black-body emission from the PSR-B surface. The corresponding efficiency of the optical emission from PSR-A’s magnetosphere would be comparable to that of other Myr-old pulsars, thus suggesting that this parameter may not dramatically evolve over a time-scale of a few Myr.

💡 Research Summary

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The authors present the first deep optical imaging of the unique double‑pulsar system J0737‑3039, which consists of the recycled millisecond pulsar PSR‑A (P ≈ 22 ms) and its slower companion PSR‑B (P ≈ 2.7 s). Observations were carried out with the High‑Resolution Camera (HRC) of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). Two broadband filters were used: F435W (approximately Johnson B) and F606W (approximately V). After standard CALACS processing, precise astrometry was achieved by tying the HRC field to Gaia DR2 reference stars, yielding an absolute positional accuracy of ~0.03 arcsec. Source detection was performed with DAOPHOT‑style PSF fitting, adopting a 5σ threshold. No source was found at the pulsar position in either filter. The resulting 5σ upper limits are m_F435W = 27.0 mag and m_F606W = 28.3 mag (AB system). After correcting for a modest line‑of‑sight extinction (A_V ≈ 0.2 mag, E(B‑V) ≈ 0.06) and assuming a distance of 600 pc, these limits correspond to flux densities of ≈ 2.5 × 10⁻³⁰ erg cm⁻² s⁻¹ Hz⁻¹ (B‑band) and ≈ 1.1 × 10⁻³⁰ erg cm⁻² s⁻¹ Hz⁻¹ (V‑band).

The authors then confront these limits with the three‑component spectral model that successfully fits the XMM‑Newton X‑ray data (Pellizzoni et al. 2008). That model comprises two thermal black‑body components—one representing a hot spot on PSR‑A (kT ≈ 0.2 keV, radius ≈ 0.5 km) and the other the cooler surface of PSR‑B (kT ≈ 0.04 keV, radius ≈ 10 km)—plus a non‑thermal power‑law (PL) component attributed to magnetospheric emission from PSR‑A (photon index Γ ≈ 2.5, normalisation set by the 0.5–10 keV flux). Extrapolating the PL unchanged into the optical would predict fluxes well above the HST limits (by more than an order of magnitude). Consequently, the optical non‑detections require a low‑energy break in the PL spectrum. By forcing the PL to intersect the measured upper limits, the authors infer that the break must occur at energies ≤ 0.1 keV, or that the PL slope must flatten to Γ ≲ 1.5 in the optical regime. This is consistent with theoretical expectations that pulsar magnetospheric emission steepens at high energies and flattens at low energies due to changes in particle acceleration and radiation mechanisms.

The thermal components are fully compatible with the optical limits. The black‑body from PSR‑B, with T ≈ 0.5 MK and a radius comparable to a neutron‑star surface, yields an expected optical flux roughly a factor of ten below the HST upper bounds, explaining why it remains undetected. The hot spot on PSR‑A contributes negligibly in the optical because its Rayleigh‑Jeans tail is extremely faint at these wavelengths.

From the upper limits the authors estimate the optical emission efficiency of PSR‑A’s magnetosphere, η_opt = L_opt/Ė, where Ė ≈ 5.8 × 10³³ erg s⁻¹ is the spin‑down power. Using the most conservative flux limit (the V‑band) they obtain η_opt ≲ 10⁻⁶. This value is comparable to those measured for isolated pulsars of similar age (∼ 1 Myr), such as PSR B0656+14 and PSR B1055‑52, suggesting that the optical efficiency does not decline dramatically over a few Myr of evolution.

The paper discusses the broader implications of these findings. First, the required low‑energy break provides a new observational constraint on pulsar magnetospheric models, indicating that the particle energy distribution or radiation geometry must change below ∼ 0.1 keV. Second, the consistency of the PSR‑B black‑body with the limits supports the picture that the companion’s surface is heated only modestly, likely by spin‑down power from PSR‑A and by mutual interaction within the binary. Third, the similarity of η_opt to that of older isolated pulsars hints that the conversion of rotational energy into optical photons is relatively stable over the first few million years of a pulsar’s life.

Finally, the authors outline prospects for future work. Deeper optical or near‑UV imaging with next‑generation facilities (e.g., JWST NIRCam or the upcoming 30‑m class ground‑based telescopes equipped with adaptive optics) could push the limits down by another magnitude, potentially detecting the faint Rayleigh‑Jeans tail of PSR‑B’s surface emission. Simultaneous multi‑wavelength campaigns, combining high‑time‑resolution X‑ray/γ‑ray observations with optical monitoring, would enable phase‑resolved studies of the magnetospheric component and could directly locate the spectral break. Such observations would refine our understanding of particle acceleration in pulsar magnetospheres and of the thermal evolution of neutron‑star surfaces in tight binary systems.