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Keywords:

  • stars: emission-line, Be;
  • stars: individual: MWC 656;
  • stars: individual: MWC 148;
  • gamma rays: stars;
  • X-rays: stars.

ABSTRACT

We present optical spectroscopy of MWC 656 and MWC 148, the proposed optical counterparts of the γ-ray sources AGL J2241+4454 and HESS J0632+057, respectively. The main parameters of the Hα emission line [equivalent width (EW), full width at half-maximum and centroid velocity] in these stars are modulated on the proposed orbital periods of 60.37 and 321 d, respectively. These modulations are likely produced by the resonant interaction of the Be discs with compact stars in eccentric orbits. We also present radial velocity curves of the optical stars folded on the above periods and obtain the first orbital elements of the two γ-ray sources, thus confirming their binary nature. Our orbital solution supports eccentricities e∼ 0.4 and 0.83 ± 0.08 for MWC 656 and MWC 148, respectively. Furthermore, our orbital elements imply that the X-ray outbursts in HESS J0632+057/MWC 148 are delayed ∼0.3 orbital phases after periastron passage, similar to the case of LS I +61 303. In addition, the optical photometric light-curve maxima in AGL J2241+4454/MWC 656 occur ∼0.25 phases passed periastron, similar to what is seen in LS I +61 303. We also find that the orbital eccentricity is correlated with the orbital period for the known γ-ray binaries. This is explained by the fact that small stellar separations are required for the efficient triggering of very high energy radiation. Another correlation between the EW of Hα and orbital period is also observed, similar to the case of Be/X-ray binaries. These correlations are useful to provide estimates of the key orbital parameters Porb and e from the Hα line in future Be γ-ray binary candidates.