A signature of the donor star in the extra-galactic X-ray binary LMC X−2

Abstract

Two nights of phase‐resolved medium‐resolution Very Large Telescope spectroscopy of the extra‐galactic low‐mass X‐ray binary LMC X−2 have revealed a 0.32 ± 0.02 d spectroscopic period in the radial velocity curve of the He iiλ4686 emission line that we interpret as the orbital period. However, similar to previous findings, this radial velocity curve shows a longer term variation that is most likely due to the presence of a precessing accretion disc in LMC X−2. This is strengthened by He iiλ4686 Doppler maps that show a bright spot that is moving from night to night. Furthermore, we detect narrow emission lines in the Bowen region of LMC X−2, with a velocity of Kem= 351 ± 28 km s−1, that we tentatively interpret as coming from the irradiated side of the donor star. Since Kem must be smaller than K2, this leads to the first upper limit on the mass function of LMC X−2 of f(M1) ≥ 0.86 M⊙ (95 per cent confidence), and the first constraints on its system parameters.

Characterizing X-ray binary long-term variability

“…MS 1603.6+2600 has P orb ∼ 1.85 h and LMC X‐2 has P orb ∼ 8.16 h (Liu et al 2007), and as LMXBs with P orb < 1 d, they are not expected to produce warped accretion discs (OD01). However, with q ∼ 0.14 for LMC X‐2 (Cornelisse et al 2007), tidal disc precession is likely. MS 1603.6+2600, also known as UW CrB, has a short P orb and likely shares properties with the other short‐period systems, such as having a q < 0.25, making tidal disc precession likely.…”

“…MS 1603.6+2600 has P orb ∼1.85 hours and LMC X-2 has P orb ∼8.16 hours (Liu, van Paradijs & van den Heuvel 2007) and as LMXBs with P orb <1 d, they are not expected to produce warped accretion discs (OD01). However, with q ∼0.14 for LMC X-2 (Cornelisse et al 2007), tidal disc precession is likely. MS 1603.6+2600 a.k.a.…”

XMM-NEWTONANDCHANDRAOBSERVATIONS OF THE M31 GLOBULAR CLUSTER BLACK HOLE CANDIDATE XB135: A HEAVYWEIGHT CONTENDER CUT DOWN TO SIZE

“…It is also extremely bright for a NS XB, ∼0.5-2 L Edd where L Edd is the Eddington limit. It has a known counterpart, and a mass ratio ≤0.4 (Cornelisse et al 2007).…”

The X-ray stellar population of the LMC

“…This can be explained by the different star formation history of the two galaxies since LMXBs are associated with an older stellar population than HMXBs (Liu et al 2005). LMC X-2 is a binary of period 8h (Cornelisse et al 2007), similar in many respects to Sco X-1. A monitoring has indicated that, during the bright X-ray states, the optical lightcurve lags 20s behind the X-ray lightcurve (McGowan et al 2003).…”

“…LMC X-2 is a binary of period 8 h (Cornelisse et al 2007), similar in many respects to Sco X-1. A monitoring has indicated that, during the bright X-ray states, the optical lightcurve lags 20 s behind the X-ray lightcurve (McGowan et al 2003).…”