Spectroscopic observations of the peculiar low-mass X-ray binary XSS J12270-4859

ATel #5651; D. de Martino (INAF/Naples), J. Casares Velazquez (IAC), E. Mason (INAF/Trieste), M. Kotze (SAAO), D. A.H. Buckley (SALT), J.-M. Bonnet-Bidaud (CEA/Saclay), T. Belloni (INAF/Brera), M. Mouchet (Paris), M. Falanga (ISSI/Bern)
on 11 Dec 2013; 18:28 UT
Credential Certification: Domitilla de Martino (demartino@oacn.inaf.it)

Tweet

The enigmatic low-mass X-ray binary XSS J12270-4859 associated to the Fermi/LAT Gamma ray source 1FGL 1227.9-4852/2FGL 1227.7-4853 (de Martino et al. 2010, A&A 515, A25; Hill et al. 2011, MNRAS 415, 235; de Martino et al. 2013, A&A 550, A89) was extensively observed from radio to gamma rays but its orbital period is still unknown. Pretorius (2009, MNRAS, 395, 386) did not find a period in time resolved optical observations. A 4.5hr photometric period was claimed by de Martino et al. (2010). XSS J12270-4859 was observed from 2012 March 30 at 00:14UT to April 1 at 09:28UT with the ESO NTT telescope equipped with EFOSC2 and Gr.#19 covering the range 4445 - 5110 Ang at 0.33 Ang/px and 0.67 Ang/px. A total of 73 1200s exposure spectra were obtained during the three consecutive nights. Further coordinated observations were acquired from 2012 March 28 at 19:12UT to April 2 at 19:57UT with the SALT telescope equipped with RSS Gr. PG2300 (2300 l/mm) covering the range 4000 - 5000 Ang. A total of 65 300s exposure spectra were obtained at SALT. The average spectrum of XSS J12270-4859 of 2012 March-April is at similar flux level as that observed on March 22, 2006 by Masetti et al. (2006, A&A 459, 21) showing emission features from Balmer, Helium and Bowen blend lines. However, the source is found to be highly variable during the runs showing strong emissions that develop into deep absorptions on timescales of hours. Additional metallic absorption features are detected that show velocity shifts. Cross-correlation with mid-late type template spectra gives an average spectral type F2V. Strong changes in the spectral type are detected along the orbital cycle, suggesting an irradiated donor star. Combining the measures of radial velocities of absorption features from both NTT and SALT spectra we find the true orbital period is 6.91(4)hr. Hbeta emission line radial velocities are found to be antiphased to those of absorptions. We infer a mass ratio of 0.53(3). We also find a strong variability of Hbeta equivalent widths with the orbital period. Phasing of spectra along the orbital period show that emission lines disappear for about one-third of the orbital cycle (about 2hr) indicating peculiar geometry of accretion flow. XSS J12270-4859 has been reported to have faded in the optical and X-rays between 2012 and 2013 (Bassa et al. 2013 ATel #5647) and that spectroscopy on 2013 November 8 using the ESO NTT, spanning about half hour, shows the source with weak absorption features. Given the peculiar variability of its emission lines, the source could have been found during those portions of the orbital cycle where emission features disappear. Further X-ray and optical observations are needed to assess if state changes have occurred in this enigmatic X-ray binary. * Based on observations collected at the European Southern Observatory, La Silla, Chile with Program: 088.D-0311 (PI:de Martino) and at the Southern African Large Telescope (SALT), South Africa, with Program: 2011-3-RSA_OTH-025 (PI: Buckley).