Optical Modulation in the X-Ray Binary 4U 1543-624 Revisited

TL;DR

This study confirms the ultracompact nature of the X-ray binary 4U 1543-624 by measuring its orbital period through optical photometry, and estimates its distance and inclination based on observed modulation.

Contribution

The paper provides the first high-cadence optical confirmation of the 18-minute orbital period in 4U 1543-624, solidifying its classification as an ultracompact X-ray binary.

Findings

Confirmed the 18.20-minute orbital period.

Estimated the distance to be 6.0-6.7 kpc.

Determined the inclination angle to be 34°-61°.

Abstract

The X-ray binary 4U 1543$-$624 has been provisionally identified as an ultracompact system with an orbital period of $\simeq$18~min. We have carried out time-resolved optical imaging of the binary to verify the ultra-short orbital period. Using 140\,min of high-cadence $r'$-band photometry we recover the previously-seen sinusoidal modulation and determine a period $P=18.20\pm0.09$\,min. In addition, we also see a 7.0$\times 10^{-4}$\,mag\,min$^{-1}$ linear decay, likely related to variations in the source's accretion activity. Assuming that the sinusoidal modulation arises from X-ray heating of the inner face of the companion star, we estimate a distance of 6.0--6.7\,kpc and an inclination angle of 34$^{\circ}$--61$^{\circ}$ (90\% confidence) for the binary. Given the stability of the modulation we can confirm that the modulation is orbital in origin and 4U 1543$-$624 is an ultracompact X-ray binary.