The ultracompact low-mass X-ray binary 4U 1820 - 30 situated in the globular cluster NGC 6624 has an orbital period of only \approx 11.4 min which likely implies a white dwarf companion .
The observed X-ray bursts demonstrate a photospheric radius expansion phase and therefore are believed to reach the Eddington luminosity allowing us to estimate the mass and the radius of the neutron star ( NS ) in this binary .
Here we re-analyse all Rossi X-ray Timing Explorer observations of the system and confirm that almost all the bursts took place during the hard persistent state of the system .
This allows us to use the recently developed direct cooling tail method to estimate the NS mass and radius .
However , because of the very short , about a second , duration of the cooling tail phases that can be described by the theoretical atmosphere models , the obtained constraints on the NS radius are not very strict .
Assuming a pure helium NS atmosphere we found that the NS radius is in the range 10–12 km , if the NS mass is below 1.7 M _ { \odot } , and in a wider range of 8–12 km for a higher 1.7–2.0 M _ { \odot } NS mass .
The method also constrains the distance to the system to be 6.5 \pm 0.5 kpc , which is consistent with the distance to the cluster .
For the solar composition atmosphere , the NS parameters are in strong contradiction with the generally accepted range of possible NS masses and radii .