I Is J0740+6620 an eclipsing system?

[GeorgeDishman]

TL;DR Summary

The inclination of the MSP orbit is 87.38 degrees or nearly edge on. If the system is eclipsing, it would imply the possibility exist of a direct observation of the radius of the NS against the background of the WD which would be a key parameter constraining the EoS.

It was recently announced that J0740+6620 contains what is probably the most massive neutron star found to date:

https://arxiv.org/pdf/1904.06759.pdf
This paper concludes the companion to the MSP is a helium-atmosphere white dwarf:

https://arxiv.org/pdf/1902.11150.pdf
Based on a crude eyeball estimate from the graph here:

https://en.wikipedia.org/wiki/White_dwarf#Mass–radius_relationship_and_mass_limit
the radius of the WD would appear to be around 18% of the Solar radius, or about 125 thousand km.

The semi-major axis of the WD orbit around the NS is ~3.98 light seconds or 1.2 million km which, combined with the orbital inclination of 87.38 degrees, suggests a line of sight separation at conjunction of 28 thousand km or about 1/4 the radius of the WD.

Unless I've got my arithmetic wrong, that seems to suggest the system should be eclipsing and it should be possible to determine the radius of the NS directly from the dimming of the WD during transits. Is that correct?

 

[stefan r]

Phys.org reports the mass at 0.2 solar mass. The radius of a white dwarf is proportional to the inverse cube root of the mass M^(-1/3). It should have a 70% larger radius than a solar mass white dwarf. We can also eyeball the radius in the wikipedia chart you linked to around 0.02 solar radii or ~14,000 km.

 

[GeorgeDishman]

We can also eyeball the radius in the wikipedia chart you linked to around 0.02 solar radii or ~14,000 km.

Thank you, I eyeballed it with a missing zero as just under 0.2 instead of 0.02, my radius was too large by an order of magnitude.

Sadly, it's about 12600km radius but 28000km off the line of sight. :-(