We report the discovery of the orbital period of the ultracompact low-mass X-ray binary (LMXB) 4U 1543624 using time-resolved optical photometry taken with the 6.5 m Clay (Magellan II) telescope in Chile. The light curve in the Sloan band clearly shows a periodic sinusoidal modulation at minutes with a fractional r 18.2 0.1 semiamplitude of 8%, which we identify as the binary period. This is the second shortest orbital period among all the known LMXBs, and it verifies the earlier suggestion of 4U 1543624 as an ultracompact binary based on X-ray spectroscopic properties. The sinusoidal shape of the optical modulation suggests that it arises from X-ray heating of the mass donor in a relatively low-inclination binary, although it could also be a superhump oscillation, in which case the orbital period is slightly shorter. If the donor is a CO white dwarf as previously suggested, its likely mass and radius are around 0.03 and , respectively. For conservative mass M 0.03 R , , transfer onto a 1.4 neutron star and driven by gravitational radiation, this implies an X-ray luminosity of M , ergs s 1 and a source distance of ≈7 kpc. We also discuss optical photometry of another LMXB, the 36 6.5 # 10 candidate ultracompact binary 4U 1822000. We detected significant optical variability on a timescale of about 90 minutes, but it is not yet clear whether this was due to a periodic modulation.
CITATION STYLE
Wang, Z., & Chakrabarty, D. (2004). The Orbital Period of the Ultracompact Low-Mass X-Ray Binary 4U 1543-624. The Astrophysical Journal, 616(2), L139–L142. https://doi.org/10.1086/426787
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