[Prex] 2.27 solar mass neutron star?

[Jay Benesch]

https://arxiv.org/abs/1805.08799

Peering into the dark side: magnesium lines establish a massive neutron 
star in PSR J2215+5135
Manuel Linares (UPC, IAC), Tariq Shahbaz, Jorge Casares (IAC, ULL)
(Submitted on 22 May 2018)

     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 (TN=5660+260−380 K) and 
bright/hot (TD=8080+470−280 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 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 radial velocity curves and the three-band light curves, we 
find a center-of-mass velocity of K2=412.3±5.0 km s−1 and an orbital 
inclination i=63.9∘+2.4−2.7. Our model is able to reproduce the observed 
fluxes and velocities without invoking irradiation by an extended 
source. We measure masses of M1=2.27+0.17−0.15 M⊙ and M2=0.33+0.03−0.02 
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.

Comments: 	Published in The Astrophysical Journal on May 23d, 2018. 14 
pages, 13 figures, 2 tables. See animated video at this https URL
https://youtu.be/l41y_GqJq7c