2016A&A...589A..34G

Context. 4U 1323-619 is a low mass X-ray binary system that shows type I X-ray bursts and dips. The most accurate estimation of the orbital period is 2.941923(36)h and a distance from the source that is lower than 11kpc has been proposed.
Aims. We aim to obtain the orbital ephemeris, the orbital period of the system, as well as its derivative to compare the observed luminosity with that predicted by the theory of secular evolution.
Methods. We took the advantage of about 26yrs of X-ray data and grouped the selected observations when close in time. We folded the light curves and used the timing technique, obtaining 12 dip arrival times. We fit the delays of the dip arrival times both with a linear and a quadratic function.
Results. We locate 4U 1323-619 within a circular area centred at RA(J2000)=201.6543° and Dec(J2000)=-62.1358° with an associated error of 0.0002°, and confirm the detection of the IR counterpart already discussed in literature. We estimate an orbital period of P=2.9419156(6) h compatible with the estimations that are present in the literature, but with an accuracy ten times higher. We also obtain a constraint on the orbital period derivative for the first time, estimating P=(8±13)x10^–12s/s. Assuming that the companion star is in thermal equilibrium in the lower main sequence, and is a neutron star of 1.4M_☉, we infer a mass of 0.28±0.03M_☉ for the companion star. Assuming a distance of 10kpc, we obtained a luminosity of (4.3±0.5)x10³⁶erg/s, which is not in agreement with what is predicted by the theory of secular evolution. Using a 3D extinction map of the K_s radiation in our Galaxy, we obtain a distance of 4.2^+0.8_–0.7kpc at 68% confidence level. This distance implies a luminosity estimation of (0.8±0.3)x10³⁶erg/s, which is consistent with the adopted scenario in which the companion star is in thermal equilibrium.