Peering into the Dark Side: Magnesium Lines Establish a Massive Neutron Star in PSR J2215+5135

Linares, M.; Shahbaz, T.; Casares, J.
Bibliographical reference

The Astrophysical Journal, Volume 859, Issue 1, article id. 54, 14 pp. (2018).

Advertised on:
New millisecond pulsars (MSPs) in compact binaries provide a good opportunity to search for the most massive neutron stars. Their main-sequence companion stars are often strongly irradiated by the pulsar, displacing the effective center of light from their barycenter and making mass measurements uncertain. We present a series of optical spectroscopic and photometric observations of PSR J2215+5135, a “redback” binary MSP in a 4.14 hr orbit, and measure a drastic temperature contrast between the dark/cold (T N = 5660{}-380+260 K) and bright/hot (T D = 8080{}-280+470 K) sides of the companion star. We find that the radial velocities depend systematically on the atmospheric absorption lines used to measure them. Namely, the semi-amplitude of the radial velocity curve (RVC) of J2215 measured with magnesium triplet lines is systematically higher than that measured with hydrogen Balmer lines, by 10%. We interpret this as a consequence of strong irradiation, whereby metallic lines dominate the dark side of the companion (which moves faster) and Balmer lines trace its bright (slower) side. Further, using a physical model of an irradiated star to fit simultaneously the two-species RVCs and the three-band light curves, we find a center-of-mass velocity of K 2 = 412.3 ± 5.0 km s‑1 and an orbital inclination i = 63.°9{}-2.7+2.4. Our model is able to reproduce the observed fluxes and velocities without invoking irradiation by an extended source. We measure masses of M 1 = 2.27{}-0.15+0.17 M ⊙ and M 2 = 0.33{}-0.02+0.03 M ⊙ for the neutron star and the companion star, respectively. If confirmed, such a massive pulsar would rule out some of the proposed equations of state for the neutron star interior.
Related projects
Black hole in outburst
Nature and Evolution of X-Ray Binaries

Accreting black-holes and neutron stars in X-ray binaries provide an ideal laboratory for exploring the physics of compact objects, yielding not only confirmation of the existence of stellar mass black holes via dynamical mass measurements, but also the best opportunity for probing high-gravity environments and the physics of accretion; the most

Muñoz Darias