REFINED ORBITAL SOLUTION AND QUIESCENT VARIABILITY IN THE BLACK HOLE TRANSIENT GS 1354-64 (= BW Cir)

Abstract

In Casares et al. we presented the first radial velocity curve of the companion star to BW Cir which demonstrates the presence of a black hole in this historical X-ray transient. But these data were affected by aliasing and two possible periods at 2.5445 days and 2.5635 days were equally possible. Here we present new spectroscopic data that enable us to break the 1-year aliasing and confirm 2.5445 days as the correct orbital period. We also present R-band photometry over 14 years, which reveals the presence of important flaring activity dominating the light curves.

Simultaneous optical/X-ray study of GS 1354-64 (=BW Cir) during hard outburst: evidence for optical cyclo-synchrotron emission from the hot accretion flow

“…Since GS 1354-64 is close to the poorly located Cen X−2, a soft X-ray transient, discovered in 1967 with a peak X-ray flux of ∼ 8 Crab, it is likely they are the same source (Kitamoto et al 1990). Using the optical spectroscopy of GS 1354-64, Casares et al (2009) presented dynamical evidence for a 7.9 ± 0.5 M⊙ BH. With P orb = 2.54 days (Casares et al 2009), the donor is a G0-5 III star (Casares et al 2004) of mass 1.1 ± 0.1 M⊙ (from the rotational broadening of the companion's absorption spectrum).…”

“…We use the finding chart provided by Casares et al (2009) including field stars for performing relative photometry. For comparison we used nearby 'Star D' as listed in Table 1 of Casares et al (2009), which has an R magnitude of 17.361.…”

“…Using the optical spectroscopy of GS 1354-64, Casares et al (2009) presented dynamical evidence for a 7.9 ± 0.5 M⊙ BH. With P orb = 2.54 days (Casares et al 2009), the donor is a G0-5 III star (Casares et al 2004) of mass 1.1 ± 0.1 M⊙ (from the rotational broadening of the companion's absorption spectrum). Interestingly, during quiescence, GS 1354-64 shows strong optical variability (Casares et al 2009) with R filter falling by 4 magnitudes (∼ 17 to ∼ 21) over a period of 14 years.…”

“…With P orb = 2.54 days (Casares et al 2009), the donor is a G0-5 III star (Casares et al 2004) of mass 1.1 ± 0.1 M⊙ (from the rotational broadening of the companion's absorption spectrum). Interestingly, during quiescence, GS 1354-64 shows strong optical variability (Casares et al 2009) with R filter falling by 4 magnitudes (∼ 17 to ∼ 21) over a period of 14 years. Casares et al (2009), subsequently constrained the source distance and disc inclination angle to be 25 kpc and 79 • respectively.…”

“…Interestingly, during quiescence, GS 1354-64 shows strong optical variability (Casares et al 2009) with R filter falling by 4 magnitudes (∼ 17 to ∼ 21) over a period of 14 years. Casares et al (2009), subsequently constrained the source distance and disc inclination angle to be 25 kpc and 79 • respectively.…”