Updated Orbital Ephemeris and Detection of Superhump Modulation in X-ray Band for the Ultra-Compact Low Mass X-ray Binary 4U 1820-30

Abstract

The 4U 1820-30 is a ultra-compact low mass X-ray binary (LMXB) near the center of the globular cluster NGC 6624. Its negative orbital period derivative, observed from the phase evolution of its sinusoidal-like orbital variation, contradicts the positive value obtained from the theoretical prediction. In this paper, we present the analysis of the 4U 1820-30 orbital modulation from light curves obtained from the {\it Neutron star Interior Composition ExploreR (NICER)} observations from 2017 to mid 2022. Combined with historical records, the orbital derivative is measured from the orbital phase evolution between 1976 and 2002 is $\dot{P}/P=(-5.21\pm 0.13)\times 10^{-8}$ yr${}^{-1}$. No significant second order orbital period derivative is detected with a 2$\sigma$ upper limit of $|\ddot{P}|<5.48\times 10^{-22}$ s s${}^{-2}$. We discuss the possible intrinsic orbital period derivative of 4U 1820-30 and suggest that this binary system may have a significant mass outflow similar to some other LMXBs. In addition, a periodic modulation with a period of $691.6\pm 0.7$ s, which is consistent with the superhump period discovered in the far ultraviolet band of the {\it Hubble Space Telescope}, was also detected in in the X-ray light curves collected by {\it NICER}. We conclude that such modulation is probably caused by a period of $0.8\pm 0.1$ day apsidal precession of accretion disk similar to the SU UMa type dwarf novae and some LMXBs. However we cannot exclude the possibility that it is induced by a hierarchical third star orbiting around the binary system.

27 pages, 10 figures, accepted for publication by Astrophysical Journal

References in corpus (16)

• Orbital Evolution of an Accreting Millisecond Pulsar: Witnessing the Banquet of a Hidden Black Widow?
• The dynamics of eccentric accretion discs in superhump systems
• Evidence for an intermediate-mass black hole in the globular cluster NGC 6624
• The superorbital variability and triple nature of the X-ray source 4U 1820-303
• New ephemeris of the ADC source 2A 1822-371: a stable orbital-period derivative over 30 years
• Timing of the 2008 Outburst of SAX J1808.4-3658 with XMM-Newton: A Stable Orbital Period Derivative over Ten Years
• Mass models of NGC 6624 without an intermediate-mass black hole
• On the timing properties of SAX J1808.4-3658 during its 2015 outburst
• IR Monitoring of the Microquasar GRS 1915+105: Detection of Orbital and Superhump Signatures
• Dependence of the orbital modulation of X-rays from 4U 1820-303 on the accretion rate
• Parameterization studies of the properties of the X-ray dips for Low Mass X-ray binary X1916-053
• The Puzzling Negative Orbit-Period Derivative of the Low-Mass X-Ray Binary 4U 1820-30 in NGC 6624
• Long-term coherent timing of the accreting millisecond pulsar IGR J17062-6143
• Evidence of a non-conservative mass transfer in the ultra-compact X-ray source XB 1916-053
• On the peculiar long-term orbital evolution of the eclipsing accreting millisecond X-ray pulsar SWIFT J1749.4-2807
• A Circumbinary Disk Scenario For The Negative Orbital-Period Derivative Of The Ultracompact X-ray Binary 4u1820-303