Orbital Dynamics of Candidate Transitional Millisecond Pulsar 3FGL J1544.6-1125: An unusually face-on system

TL;DR

This study provides the orbital parameters of the candidate transitional millisecond pulsar 3FGL J1544.6-1125, revealing an unusually face-on orientation and implications for pulsar beaming and emission mechanisms.

Contribution

First detailed orbital solution for 3FGL J1544.6-1125, highlighting its face-on inclination and potential effects on pulsar population studies.

Findings

Orbital period of 0.2415 days consistent with a K dwarf donor.

Exceptionally low radial velocity semi-amplitude indicating a face-on system.

Coherent emission line variations suggest asymmetric shock near the inner disk.

Abstract

We present the orbital solution for the donor star of the candidate transitional millisecond pulsar 3FGL J1544.6-1125, currently observed as an accreting low-mass X-ray binary. The orbital period is $0.2415361(36)$ days, entirely consistent with the spectral classification of the donor star as a mid to late K dwarf. The semi-amplitude of the radial velocity curve is exceptionally low at $K_2=39.3\pm1.5$ km s$^{-1}$, implying a remarkably face-on inclination in the range 5--8$^{\circ}$, depending on the neutron star and donor masses. After determining the veiling of the secondary, we derive a distance to the binary of $3.8\pm0.7$ kpc, yielding a 0.3--10 keV X-ray luminosity of $6.1\pm1.9\times10^{33}\,{\rm erg\,s^{-1}}$, similar to confirmed transitional millisecond pulsars. As face-on binaries rarely occur by chance, we discuss the possibility that Fermi-selected samples of transitional milli-second pulsars in the sub-luminous disk state are affected by beaming. By phasing emission line strength on the spectroscopic ephemeris, we find coherent variations, and argue that these variations are most consistent with emission from an asymmetric shock originating near the inner disk.