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Title: The puzzling negative orbit-period derivative of the low-mass X-ray binary 4U 1820-30 in NGC 6624

4U 1820-30 is a low-mass X-ray binary near the center of the globular cluster NGC 6624 consisting of, at least, one neutron star and one helium white dwarf. Analyzing 16 yr of data from the Rossi X-ray Timing Explorer (RXTE) allows us to measure its orbital period and its time derivative with unprecedented accuracy to be P = 685.01197 ± 0.00003 s and P-dot / P=−5.3±0.3×10{sup −8} yr{sup −1}. Hence, we confirm that the period derivative is significantly negative at the >17σ level, contrary to theoretical expectations for an isolated X-ray binary. We discuss possible scenarios that could explain this discrepancy and conclude that the center of NGC 6624 most likely contains large amounts of nonluminous matter such as dark remnants. We also discuss the possibility of an IMBH inside NGC 6624, or that a dark remnant close to 4U 1820-30 causes the observed shift.
Authors:
;  [1] ;  [2] ;  [3]
  1. Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn (Germany)
  2. Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027 (United States)
  3. Helmholtz-Institut für Strahlen- und Kernphysik (HISKP), University of Bonn, Nussallee 14-16, D-53115 Bonn (Germany)
Publication Date:
OSTI Identifier:
22370246
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 795; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCURACY; BINARY STARS; GALAXY NUCLEI; HELIUM; MASS; NEUTRON STARS; NONLUMINOUS MATTER; ORBITS; WHITE DWARF STARS; X RADIATION