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The spectral type of the optical counterpart to the high-mass X-ray binary IGR J06074+2205

ATel #2085; Pablo Reig & Andreas Zezas (FORTH/University of Crete)
on 15 Jun 2009; 15:38 UT
Credential Certification: Pablo Reig (pablo.reig@uv.es)

Subjects: Optical, Binary, Neutron Star

Spectroscopic observations of the optical counterpart to the high-mass X-ray binary IGR J06074+2205 were obtained from the 1.3m telescope of the Skinakas observatory in Crete (Greece) and from the 1.5m telescope of the Fred Lawrence Whipple Observatory at Mt. Hopkins (Arizona) on several occasions throughout 2006-2008.These observations allowed us to identify the spectral type of this source and study its long-term spectral variability. The blue-end spectrum (4000-5000 A) of IGR J06074+2205 is dominated by hydrogen and neutral helium, clearly indicating an early-type star (O or B). The Balmer series lines from Hbeta up to Heta are seen in absorption. No or very weak HeII lines (at 4541 A and 4686 A) are present, which implies a type later than B0. The weak MgII 4481 indicates a type earlier than B2, whereas the strong CIII+OII blend indicates a type earlier than B1.5. SiIII 4552-68-75 is also present. On the main sequence SiIII have their maximum strength at type B0.5. As for the luminosity class, the fact that the relative strength of the SiIII 4552-68-75 complex and the CIII+OII blends are comparable to that of nearby HeI lines favours a main sequence classification. Likewise, the relative weak strength of OII lines also indicates a luminosity class V star. In summary, we assign a spectral type B0.5V to IGR J06074+2205, although a slightly later type, i.e. B1V, cannot be ruled out. Our monitoring of IGR J06074+2205 also reveals that the Halpha line is highly variable, both in strength and shape. V/R variability on timescales of months is clearly seen, indicating global changes in the structure of the equatorial disc.