Spectroscopic and photometric observations confirm XSS J12270-4859 in a low state

ATel #5747; J. Casares Velazquez (IAC), D. de Martino (INAF/Naples),E. Mason (INAF/Trieste), P. D'Avanzo, S. Campana, D. Fugazza, S. Covino and T. Belloni (INAF/Brera), T. Munoz-Darias (Univ. of Oxford), R. Cornelisse (IAC), L. Nicastro (INAF/Bologna)
on 9 Jan 2014; 15:26 UT
Credential Certification: Domitilla de Martino (demartino@oacn.inaf.it)

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We have observed the low-mass X-ray binary and gamma-ray source XSS J12270-4859/1FGL 1227.9-4852/2FGL 1227.7-4853 (de Martino et al. 2010, A&A 515, A25) on 2013 Dec 14.31, 15.32, 17.30 and 18.26 using the ESO NTT telescope. We employed EFOSC2 and Gr.#19 covering the range 4435-5109 Ang at 2.3 Ang resolution. Three 900s spectra and four 1200s spectra were obtained over the 4 nights. The spectra show metallic absorptions typical of mid/late G stars with no evidence for emission lines in any of them. Cross-correlation analysis with a G5V template reveals radial velocity excursions consistent with the 6.91h orbital period reported in ATel #5651. The spectra cover orbital phases 0, 0.24-0.29 and 0.66-0.84.

We also report on optical g and r band photometry using the La Silla INAF-REM telescope on 2013 Dec 13.29, 14.29 and 15.26 with the ROSS2 camera for 1.9h, 0.7h and 0.8h respectively. The latter two runs are almost simultaneous to the NTT spectra. Fifty 120s exposures were acquired in each filter. Night average magnitudes are g=18.89(5), g=18.69(6), g= 18.70(8) and r=18.32(4), r=18.02(4) and r=18.12(7), respectively. We also detect large amplitude variations, Δg= 0.61+/-0.03mag and Δr= 0.69 +/- 0.04mag, over each night. Light curves folded at the 6.91h orbital period indicate a maximum at orbital phases between 0.3-0.7 consistent with the superior conjunction of the secondary star.

The absence of emission lines at all phases in the optical spectra and the further decrease in brightness by about 0.7mag with respect to March 2013 reported by Monard 2013 (see ATel #5647) allow us to confirm that the binary is undergoing a low state, with no detectable sign of accretion, reminiscent of the "missing link" pulsar binary PSR J1023+0038 (Atel #5513).

* Based on observations collected at the European Southern Observatory, La Silla, Chile with Programs: 092.D-0588 and DDT-REM:28903