Polarimetric and spectroscopic optical observations of the ultra-compact X-ray binary 4U 0614+091^⋆,⋆⋆

¹ Università dell’Insubria, Dipartimento di Fisica, via Valleggio 11, 22100 Como, Italy
e-mail: cristina.baglio@brera.inaf.it
² INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (Lc), Italy
³ Università di Milano-Bicocca, Dipartimento di Fisica G. Occhialini, Piazza della Scienza 3, 20126 Milano, Italy
⁴ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates
⁵ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain

Received: 23 July 2014
Accepted: 7 October 2014

Abstract

Aims. We present a polarimetric and spectroscopic study of the persistent ultra-compact X-ray binary 4U 0614+091 aimed at searching for the emission of a relativistic particle jet and at unveiling the orbital period P_orb of the system.

Methods. We obtained r-band polarimetric observations with the Telescopio Nazionale Galileo (TNG) equipped with the PAOLO polarimeter and with the Nordic Optical Telescope (NOT) equipped with the ALFOSC instrument, covering ~2 h and ~0.5 h observations, respectively. We carried out low resolution spectroscopy of the system using the ESO Very Large Telescope equipped with FORS1 for ~1.5 h (16 spectra covering the range 4300−8000 Å).

Results. The polarimetric analysis performed starting from the TNG dataset revealed a polarisation degree in the r-band of 3% ± 1%. From the NOT dataset, due to the lower signal-to-noise ratio, we could obtain only a 3σ upper limit of 3.4%. From the joining of a spectroscopic and photometric analysis, through the study of the equivalent width variations of the CII 7240 Å line and the r-band light curve, we could find a hint of a ~45 min periodicity.

Conclusions. A polarisation degree P of ~3% in the r-band is consistent with the emission of a relativistic particle jet, which is supposed to emit intrinsically linearly polarised synchrotron radiation. Since no variations of P with time have been detected, and the accretion disc of the system does not contain ionised hydrogen, scattering by free electrons in the accretion disc has been rejected. The period of ~45 min obtained through the analysis of the system light curve and of the equivalent width variations of the selected spectral line is probably linked to the presence of a hot spot or a superhump in the accretion disc, and lead to an orbital period ≳1 h for the binary system.