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Evidence of the presence of a Be circumstelar disk in the Be/X-ray binaries KS 1947+ 300 and Cep X-4

ATel #6265; M. Ozbey-Arabaci (METU), A. Camero-Arranz (IEEC/ICE), J. Fabregat (University of Valencia), H. Bilal Ozcan (METU), and V. Peris (University of Valencia) on behalf of a larger collaboration
on 24 Jun 2014; 17:31 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Ascension Camero-Arranz (camero@ice.cat)

Subjects: Optical, X-ray, Binary, Pulsar

Referred to by ATel #: 6439, 11791

We report on photometric and spectroscopic optical observations of the Be/X-ray binaries KS 1947+300 and Cep X-4, obtained with the TUG Faint Object Spectrograph and Camera (TFOSC) mounted on the focal plane of the 1.5-m Russian-Turkish Telescope (RTT150) at TÜBİTAK National Observatory (Antalya, Turkey) between 2014 June 18-20 (MJD 56826.933-56828.067), and with the spectrograph located at the 51-cm telescope of the Observatorio de Aras de los Olmos of the University of Valencia on 2014 June 3 (MJD 56811.097).

For the optical counterpart of KS 1947+300 (2MASS J19493548+3012317) the Hα line was found in emission with an equivalent width (EW) of -9.0±2.0 Å on 2014 June 3 (MJD 56811.097), and in single-peaked emission having an EW of -15.2±1.33 Å and a full width at half maximum (FWHM) of 9.33±0.21 Å on 2014 June 18 (MJD 56826.994). Comparing with the last reported Hα measurement of -19.5±0.5 Å on 2001 July (Riquelme et al., 2012, A&A, 539, 114), just after the 2001 X-ray activity of the system, the decretion disk of Be companion seems to be still present but smaller. On the other hand, there is no X-ray activity currently being reported from the compact object since 2014 May 8 (see ATels #6123 and #6129). We also measured the B magnitude of the optical companion as 15.08±0.02 on 2014 June 18 (MJD 56826.978).

In contrast, Cep X-4 is currently active in X-rays (see ATels #6212 and #6243). Its optical counterpart V490 Cep showed a strong single-peaked Hα emission line in the spectra obtained on the nights of 2014 June 18 (MJD 56826.933) and 20 (MJD 56828.067). The average EW and FWHM values of the line were measured to be -52.45±0.72 Å and -10.6±0.72 Å respectively. This is a strong indication of the formation of a disk around this Be star. The average photometric magnitudes of this source obtained simultaneously with the B/V/R filters on June 19 (MJD 56827.842) were: B=15.70±0.01 mag, V=14.14±0.01 mag, and R=14.50±0.01 mag.