NuSTAR and Parkes observations of the transitional millisecond pulsar binary XSS J12270-4859 in the rotation-powered state

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 ∼ 40 per cent 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 ± 3 per cent, 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 ∼70 keV without a spectral break, is well described by a simple power law with photon index γ = 1.17 ± 0.08 giving a 3-79 keV luminosity of 7.6+3.8 -0.8 × 1032 erg s-1 for a distance of 1.37+0.69 -0.15 kpc. Energy resolved orbital light curves reveal that the modulation is not energy dependent from 3 to 25 keV and is undetected with an upper limit of ∼10 per cent above 25 keV. Comparison with previous X-ray XMM-Newton observations in common energy ranges confirms that themodulation amplitudes vary on time-scales of a fewmonths, indicative of a non-stationary contribution of the intrabinary shock (IBS) 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 IBS emission, if extending into the MeV range, would be energetically capable alone to irradiate the donor star.
DdM, AP, and TMB acknowledge financial support from the Italian Space Agency (ASI) and National Institute for Astrophysics (INAF) under agreements ASI-INAF I/037/12/0 and ASI-INAF n.2017-14-H.0 and from INAF ‘Sostegno alla ricerca scientifica main streams dell’INAF’, Presidential Decree 43/2018. DdM, AP, MB, and AP also acknowledge support from INAF ‘SKA/CTA projects’, Presidential Decree 70/2016. DFT acknowledges support from the Spanish Ministry of Economy, Industry and Competitiveness grants PGC2018-095512-B-I00, SGR2017-1383, and AYA2017-92402-EXP. FCZ is supported by a Juan de la Cierva fellowship and grants from the Spanish Ministry of Economy, Industry and Comeptitiveness grants SGR2017-1383 and PGC2018-095512-B-I00. We also thank the ‘PHAROS’ COST Action (CA16214) for partial support.