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On the binary nature of the γ-ray sources AGL J2241+4454 (= MWC 656) and HESS J0632+057 (= MWC 148)

Authors

  • J. Casares,

    Corresponding author
    1. Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
    2. Departamento de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, E-38271 La Laguna, Tenerife, Spain
      E-mail: jorge.casares@iac.es (JC); mribo@am.ub.es (MR); iribas@ieec.uab.es (IR); jmparedes@ub.edu (JMP); vilardell@ice.cat (FV); ignacio@dfists.ua.es (IN)
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  • M. Ribó,

    Corresponding author
    1. Departament d’Astronomia i Meteorologia, Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, E-08028 Barcelona, Spain
      E-mail: jorge.casares@iac.es (JC); mribo@am.ub.es (MR); iribas@ieec.uab.es (IR); jmparedes@ub.edu (JMP); vilardell@ice.cat (FV); ignacio@dfists.ua.es (IN)
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  • I. Ribas,

    Corresponding author
    1. Institut de Ciències de l’Espai – (IEEC-CSIC), Campus UAB, Facultat de Ciències, Torre C5 - parell - 2a planta, E-08193 Bellaterra, Spain
      E-mail: jorge.casares@iac.es (JC); mribo@am.ub.es (MR); iribas@ieec.uab.es (IR); jmparedes@ub.edu (JMP); vilardell@ice.cat (FV); ignacio@dfists.ua.es (IN)
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  • J. M. Paredes,

    Corresponding author
    1. Departament d’Astronomia i Meteorologia, Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, E-08028 Barcelona, Spain
      E-mail: jorge.casares@iac.es (JC); mribo@am.ub.es (MR); iribas@ieec.uab.es (IR); jmparedes@ub.edu (JMP); vilardell@ice.cat (FV); ignacio@dfists.ua.es (IN)
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  • F. Vilardell,

    Corresponding author
    1. Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain
      E-mail: jorge.casares@iac.es (JC); mribo@am.ub.es (MR); iribas@ieec.uab.es (IR); jmparedes@ub.edu (JMP); vilardell@ice.cat (FV); ignacio@dfists.ua.es (IN)
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  • I. Negueruela

    Corresponding author
    1. Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain
      E-mail: jorge.casares@iac.es (JC); mribo@am.ub.es (MR); iribas@ieec.uab.es (IR); jmparedes@ub.edu (JMP); vilardell@ice.cat (FV); ignacio@dfists.ua.es (IN)
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Errata

This article is corrected by:

  1. Errata: Erratum: On the binary nature of the γ-ray sources AGL J2241+4454 (=MWC 656) and HESS J0632+057 (=MWC 148) Volume 426, Issue 1, 796, Article first published online: 20 September 2012

E-mail: jorge.casares@iac.es (JC); mribo@am.ub.es (MR); iribas@ieec.uab.es (IR); jmparedes@ub.edu (JMP); vilardell@ice.cat (FV); ignacio@dfists.ua.es (IN)

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.

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