Circumstellar emission in Be/X-ray binaries of the Magellanic Clouds and the Milky Way

Abstract

Aims. We study the optical and near-infrared colour excesses produced by circumstellar emission in a sample of Be/X-ray binaries. Our main goals are exploring whether previously published relations, valid for isolated Be stars, are applicable to Be/X-ray binaries and computing the distance to these systems after correcting for the effects of the circumstellar contamination. Methods. Simultaneous UBVRI photometry and spectra in the 3500−7000 Å spectral range were obtained for 11 optical counterparts to Be/X-ray binaries in the LMC, 5 in the SMC and 12 in the Milky Way. As a measure of the amount of circumstellar emission we used the H α equivalent width corrected for photospheric absorption. Results. We find a linear relationship between the strength of the H α emission line and the component of E(B − V) originating from the circumstellar disk. This relationship is valid for stars with emission lines weaker than EW ≈ −15 Å. Beyond this point, the circumstellar contribution to E(B − V) saturates at a value ≈0.17 mag. A similar relationship is found for the (V − I) near infrared colour excess, albeit with a steeper slope and saturation level. The circumstellar excess in (B − V) is found to be about five times higher for Be/X-ray binaries than for isolated Be stars with the same equivalent width EW(H α ), implying significant differences in the physical properties of their circumstellar envelopes. The distance to Be/X-ray binaries (with non-shell Be star companions) can only be correctly estimated by taking into account the excess emission in the V band produced by free-free and free-bound transitions in the circumstellar envelope. We provide a simple method to determine the distances that includes this effect.

GRO J1008−57: an (almost) predictable transient X-ray binary

“…Using the c parameter of the straight line describing the black-body flux, one finds that the black body starts to contribute significantly once the power law reached a 15-50 keV flux of 10(4) × 10 −11 erg s −1 cm −2 . Integrating the modelflux between 0.01−100 keV, this corresponds to a total luminosity of 13(5) × 10 35 erg s −1 assuming a distance of 5.8 kpc (Riquelme et al 2012).…”

The VLT-FLAMES Tarantula survey

“…As a consequence if we fail to account for the presence of the circumstellar disc, we risk overestimating the bolometric luminosity of VFTS399 because (i) we will not subtract the excess disc continuum emission and (ii) because we will overestimate A131, page 7 of 12 A&A 579, A131 (2015) the interstellar reddening component and hence over correct for this. Riquelme et al (2012) investigated this problem for Be X-ray binaries in the Galaxy and Magellanic Clouds, finding it to be significant for the latter objects, where contributions from circumstellar and interstellar reddening are comparable. In the absence of contemporaneous spectroscopic and photometric observations of VFTS 399 in a discless state we may not determine the interstellar reddening.…”

“…In the absence of contemporaneous spectroscopic and photometric observations of VFTS 399 in a discless state we may not determine the interstellar reddening. Moreover, nebular contamination of the Hα line prevents an equivalent width measurement and from this an estimate of the circumstellar reddening via the relationship defined by Riquelme et al (2012). As a consequence we are forced to adopt a "worst-case" approach, correcting for the maximum contribution from circumstellar reddening reported by Riquelme et al (2012) of E cs (B − V) ∼ 0.17mag.…”

“…Moreover, nebular contamination of the Hα line prevents an equivalent width measurement and from this an estimate of the circumstellar reddening via the relationship defined by Riquelme et al (2012). As a consequence we are forced to adopt a "worst-case" approach, correcting for the maximum contribution from circumstellar reddening reported by Riquelme et al (2012) of E cs (B − V) ∼ 0.17mag. Utilising this value, the photometry from Table 1 and the intrinsic colours of an O9 III star (Martins & Pez 2006) we may infer an interstellar reddening.…”

“…Utilising this value, the photometry from Table 1 and the intrinsic colours of an O9 III star (Martins & Pez 2006) we may infer an interstellar reddening. Then employing the relation from Riquelme et al (2012) between total optical extinction and interstellar reddening − A total V = 3.1E is (B − V) − 0.3 mag − for Be X-ray binaries and an appropriate bolometric correction (from Martins et al 2005) we determine log(L bol /L ) ∼ 4.4 for VFTS 399.…”