The reported GX 1+4 orbit and absence of eclipses permit a neutron star with two solar masses

We test the mass limits reported for the X-ray binary GX 1+4. We use the public giant velocity orbit, the adopted giant mass and gravity ranges, and absence of a geometric eclipse. A model with a compact mass of 2.00 solar masses, a giant mass of 1.15 solar masses, and log g of 0.5 has an inclination of 50.54 degrees. Its minimum projected separation behind the giant is 436.3 solar radii. The giant radius is 99.9 solar radii. The model therefore avoids eclipse. At fixed giant properties, larger compact masses give lower inclinations and larger projected distances. The stated equations give no finite upper compact mass. At the central orbit, the extreme grazing mass thresholds are 1.236 and 1.448 solar masses. Allowed masses continue above these thresholds. A fit to all 17 public velocities reproduces the reported orbit and gives the same conclusion. A conservative bound also proves that every model with a compact mass of 2.00 solar masses in the stated sensitivity box avoids eclipse. This box is not a joint confidence region. The result tests the reported inference; it does not measure the true neutron star mass. The package includes code, input data, figures, source records, and reproduction instructions.

GX 1+4 · V2116 Ophiuchi · neutron star mass · mass function · eclipse geometry · radial velocity · symbiotic X-ray binary · reproducible research