We report on the first NuSTAR observation of the transitional millisecond pulsar binary XSS J12270-4859 during its current rotation-powered state , complemented with a 2.5 yr-long radio monitoring at Parkes telescope and archival XMM-Newton and Swift X-ray and optical data .
The radio pulsar is mainly detected at 1.4 GHz displaying eclipses over \sim 40 \% of the 6.91 h orbital cycle .
We derive a new updated radio ephemeris to study the 3-79 keV light curve that displays a significant orbital modulation with fractional amplitude of 28 \pm 3 \% , a structured maximum centred at the inferior conjunction of the pulsar and no cycle-to-cycle or low-high-flaring mode variabilities .
The average X-ray spectrum , extending up to \sim 70 keV without a spectral break , is well described by a simple power-law with photon index \Gamma = 1.17 \pm 0.08 giving a 3-79 keV luminosity of 7.6 _ { -0.8 } ^ { +3.8 } \times 10 ^ { 32 } erg s ^ { -1 } , for a distance of 1.37 _ { -0.15 } ^ { +0.69 } kpc .
Energy resolved orbital light curves reveal that the modulation is not energy dependent from 3 keV to 25 keV and is undetected with an upper limit of \sim 10 \% above 25 keV .
Comparison with previous X-ray XMM-Newton observations in common energy ranges confirms that the modulation amplitudes vary on timescales of a few months , indicative of a non-stationary contribution of the intrabinary shock formed by the colliding winds of the pulsar and the companion .
A more detailed inspection of energy resolved modulations than previously reported gives hints of a mild softening at superior conjunction of the pulsar below 3 keV , likely due to the contribution of the thermal emission from the neutron star .
The intrabinary shock emission , if extending into the MeV range , would be energetically capable alone to irradiate the donor star .