The 3.8 keV feature implies a compactness for 4U 1820-30 that overlaps only marginally with NICER EoS contours if maximum mass is capped at 2.3 solar masses, creating tension with touchdown-flux estimates.
The Hartle-Thorne circular geodesics
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abstract
The Hartle-Thorne metric is an exact solution of vacuum Einstein field equations that describes the exterior of any slowly and rigidly rotating, stationary and axially symmetric body. The metric is given with accuracy up to the second order terms in the body's angular momentum, and first order in its quadrupole moment. We give, with the same accuracy, analytic formulae for circular geodesics in the Hartle-Thorne metrics. They describe angular velocity, angular momentum, energy, epicyclic frequencies, shear, vorticity and Fermi-Walker precession. These quantities are relevant to several astrophysical phenomena, in particular to the observed high frequency, kilohertz Quasi Periodic Oscillations (kHz QPOs) in the X-ray luminosity from black hole and neutron star sources. It is believed that kHz QPO data may be used to test the strong field regime of Einstein's general relativity, and the physics of super-dense matter of which neutron stars are made of.
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The mass of the neutron star in 4U 1820-30 revisited
The 3.8 keV feature implies a compactness for 4U 1820-30 that overlaps only marginally with NICER EoS contours if maximum mass is capped at 2.3 solar masses, creating tension with touchdown-flux estimates.