The long-term evolution of the X-ray pulsar XTE J1814-338: A receding jet contribution to the quiescent optical emission?

Abstract

Aims. We present a study of the quiescent optical counterpart of the accreting millisecond X-ray pulsar XTE J1814-338 that is aimed at unveiling the different components, which contribute to the quiescent optical emission of the system. Methods. We carried out multiband (BVR) orbital phase-resolved photometry of the system using the ESO Very Large Telescope (VLT) that is equipped with the FORS2 camera, covering about 70% of the 4.3 hour orbital period. Results. The optical light curves are consistent with a sinusoidal variability that are modulated with an orbital period with a semiamplitude of 0.5−0.7 mag. They show evidence of a strongly irradiated companion star, which agrees with previous findings for this system. However, the observed colours cannot be accounted for by the companion star alone, suggesting the presence of an accretion disc during quiescence. The system seems to be fainter in all analysed bands compared to previous observations. The R band light curve displays a possible phase offset with respect to the B and V band. Through a combined fit of the multi-band light curve performed with a Markov chain Monte Carlo technique, we derive constraints on the companion star, disc fluxes, system distance, and companion star mass. Conclusions. The irradiation luminosity required to account for the observed day-side temperature of the companion star is consistent with the spin-down luminosity of a millisecond radio pulsar. Compared to our data with previous observations, which were collected over 5 years, the flux decrease and spectral evolution of the observed quiescent optical emission cannot be satisfactorily explained with the combined contribution of an irradiated companion star and of an accretion disc alone. The observed progressive flux decrease as the system gets bluer could be due to a continuum component that evolves towards a lower, bluer spectrum. While most of the continuum component is likely due to the disc, we do not expect it to become bluer in quiescence. Hence, we hypothesize that an additional component, such as synchrotron emission from a jet was significantly contributing in the data obtained earlier during quiescence and then progressively fading or moving its break frequency towards longer wavelengths.

Low-mass X-ray binaries: the effects of the magnetic braking prescription

“…XTE J1814-338 is known to show accretion-induced X-ray pulsations . Its donor star also shows evidence of irradiation by an unknown energy source, which may be spin-down energy from a radio pulsar (Baglio et al 2013;Wang et al 2017). EXO 0748-676 has not shown detected X-ray pulsations (despite sensitive RXTE X-ray observations).…”

Disc–jet coupling in low-luminosity accreting neutron stars

“…Cen X-4 has been in quiescence ever since, although the currently variable X-ray emission indicates that it is still actively accreting, albeit at a very low level (L X = 10 32 erg s −1 ; Campana et al , 2004aCackett et al 2010;Bernardini et al 2013). Given the radio detection of the tMSP PSR J1023+0038 in its accretion state (Deller et al 2015), during which it has a similar X-ray luminosity as Cen X-4 in quiescence, and indications of a jet in the quiescent AMXP XTE J1814-338 (Baglio et al 2013), we hypothesise that a jet could still be launched in Cen X-4 even at extremely low X-ray luminosities (L X ≈ 10 −6 L Edd ).…”

System mass constraints for the accreting millisecond pulsar XTE J1814-338 using Bowen fluorescence

“…From fitting multi-band, quiescent optical light curves of J1814 using a model for an irradiated secondary, Baglio et al (2013) noted an apparent discrepancy between the inferred value of M 2 (consistent with the companion being an M-type main-sequence star) and the day-side surface temperature of the companion (∼ 5500 K), which is typical of an earlier spectral type (G or K-type) star (see also D'Avanzo et al 2009). By assuming that the relativistic wind of an active radio pulsar irradiates the companion and increases its surface temperature, an estimate of the irradiation luminosity, L irr = σ SB T 4 day -T 4 night , can be related to the pulsar's spin-down energy as L sd = 4πa 2 L irr / irr , where σ SB is the Stefan-Boltzmann constant, a is the orbital separation and irr is the irradiation efficiency.…”