The binary millisecond pulsar PSR J1023+0038 – II. Optical spectroscopy

Abstract

We present time-resolved optical spectroscopy of the "redback" binary millisecond pulsar system PSR J1023+0038 during both its radio pulsar (2009) and accretion disc states (2014 and 2016). We provide observational evidence for the companion star being heated during the disc-state. We observe a spectral type change along the orbit, from ∼G5 to ∼F6 at the secondary star's superior and inferior conjunction, respectively, and find that the corresponding irradiating luminosity can be powered by the high energy accretion luminosity or the spin down luminosity of the neutron star. We determine the secondary star's radial velocity semi-amplitude from the metallic (primarily Fe and Ca) and Hα absorption lines during these different states. The metallic and Hα radial velocity semi-amplitude determined from the 2009 pulsar-state observations allows us to constrain the secondary star's true radial velocity K 2 =276.3±5.6 km s −1 and the binary mass ratio q=0.137±0.003. By comparing the observed metallic and Hα absorption-line radial velocity semi-amplitudes with model predictions, we can explain the observed semi-amplitude changes during the pulsar-state and during the pulsar/disc-state transition as being due to different amounts of heating and the presence of an accretion disc, respectively.

Optical photometry of two transitional millisecond pulsars in the radio pulsar state

“…This fit produces a radial velocity of 𝐾 eff = 287 ± 3 km s −1 , corresponding to a mass ratio of 𝑞 = 7.8 ± 0.05 which is comparable but not consistent with the result from radio timing in Archibald et al (2009). This is as expected, as the radial velocity parameter appears to be consistent with the upper bound of measurements from the spectral lines in Shahbaz et al (2019), and the mass ratio is calculated directly from this velocity. Unlike with our modelling of J1227 the posterior distributions of these model parameters, notably the mass, generally converged within the range of expected literature values.…”

“…Notably, the shape of the residuals in u' were unchanged compared to a fit using unaltered fluxes. We note that the spectra presented in Shahbaz et al (2019) show no evidence for a stellar third light. A continuum emission source such as the synchrotron emission produced in the intrabinary shock (Romani & Sanchez 2016) may be an alternative source of the flux, however this emission follows a negative power law.…”

“…As detailed in section 5, our modelling with both the symmetric and hot spot models produced results that were not reliable, and (Shahbaz et al 2019), and in red is the best-fitting radial velocity curve from Icarus. The red RV curve is calculated at each step in the MultiNest sampler and the 𝜒 2 value from the fit to the data is added to the posterior distribution.…”

“…To help constrain the projected companion radial velocity, 𝐾 2 , of J1023 we combined our photometric data with spectroscopic radial velocity measurements from Shahbaz et al (2019) and McConnell et al (2015). We used radial velocity curves obtained from metallic line spectra captured with the ISIS instrument on the 4.2 m William Herschel Telescope (WHT) in 2016 for Shahbaz et al (2019for McConnell et al (2015. Both radial velocity curves had been produced using broadly the same set of metallic lines, over the same range of wavelengths.…”

“…For 𝐾 2 , there are radial velocity measurements available from Shahbaz et al (2019), however the heating of the companion distorts the radial velocity curve. This results in variable measurements of the radial velocity semi-amplitude.…”