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

Abstract

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% 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%, 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 × 10 32 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 keV to 25 keV and is undetected with an upper limit of ∼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.

Gamma-ray orbital modulation of the transitioning millisecond pulsar binary XSS J12270-4859

“…3) with a minimum at φ B ≈ 0.75 (IFC) as opposed to the optical (Fig. 1) and X-ray light curves (maximum at IFC; de Martino et al 2020). This is the opposite of the result from a previous gammaray study (0.3-300 GeV; XW15) in which the gammaray maximum was suggested to be at IFC.…”

“…Orbital periods of pulsar binaries can vary slightly over time, and radio timing solutions for J1227 obtained over short time intervals (c.f., >12.5 year of LAT op-eration) require one or two orbital-period derivatives (Roy et al 2015;de Martino et al 2020). They may reflect long-term changes of the orbital motion or a small but unpredictable variation as seen in other pulsar binaries (e.g., ∆P B /P B ≤ 10 −6 for PSR J2039−5617; Clark et al 2021).…”

“…XSS J12270−4859 (4FGL J1228.0−4853; J1227 hereafter) is a transitioning MSP (tMSP) which transitioned from an LMXB to an MSP state late 2012 (∼MJD 56250; Bassa et al 2014). Radio timing studies (Roy et al 2015;de Martino et al 2020) revealed that the source has a 1.69 ms pulsar in a 6.9 hr orbit with a G-type companion (de Martino et al 2015). After the transition, the source was well studied at X-rays, and the characteristic double-peak X-ray light curve was clearly detected with a maximum at the pulsar's inferior conjunction (de Martino et al 2020), as expected in IBS scenarios if the stellar wind is stronger than the pulsar's (e.g., Kandel et al 2019).…”

“…Radio timing studies (Roy et al 2015;de Martino et al 2020) revealed that the source has a 1.69 ms pulsar in a 6.9 hr orbit with a G-type companion (de Martino et al 2015). After the transition, the source was well studied at X-rays, and the characteristic double-peak X-ray light curve was clearly detected with a maximum at the pulsar's inferior conjunction (de Martino et al 2020), as expected in IBS scenarios if the stellar wind is stronger than the pulsar's (e.g., Kandel et al 2019). In the gamma-ray band, the pulsar's pulsations were detected over a relatively short time period after the transition (Johnson et al 2015), and weak orbital modulation (≤3σ) in 1.6 years of > 300 MeV data taken with Fermi Large Area Telescope (LAT; Atwood et al 2009) was suggested (Xing & Wang 2015, ;XW15 hereafter).…”

"Spider" Millisecond Pulsar Binaries as Potential TeV Emitters

“…An example of such a two-humped spectral energy distribution is shown for transitional redback PSR J1227-4853 in Figure 1 (reported here for the first time). PSR J1227-4853 (also known as XSS J12270-4859) is an energetic MSP with an orbital period of 6.91 hr, orbital inclination ∼ 60 • , irradiated companion with mass ∼ 0.3𝑀 and is relatively nearby at distance ∼ 1.4 − 1.6 kpc [17][18][19][20][21]. The system exhibits strong nonthermal orbital modulated emission in the XMM-Newton and NuSTAR bands [20,21] with photon index Γ 𝑋 ≈ 1.2 at a few×10 −13 erg cm −2 s −1 flux in the 3 − 79 keV band.…”