Different twins in the millisecond pulsar recycling scenario: Optical polarimetry of PSR J1023+0038 and XSS J12270-4859^⋆,⋆⋆

¹ Università dell’Insubria, Dipartimento di Scienza e Alta Tecnologia, 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 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090- GE Amsterdam, The Netherlands
⁴ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates

Received: 25 February 2016
Accepted: 30 April 2016

Abstract

We present the first optical polarimetric study of the two transitional pulsars PSR J1023+0038 and XSS J12270-4859. This work is focused on the search for intrinsically linearly polarised optical emission from the two systems. To this aim, we carried out multiband optical (BVRi) and near-infrared (NIR; JHK) photo-polarimetric observations of the two systems using the ESO New Technology Telescope (NTT) at La Silla (Chile), equipped with the EFOSC2 and the SOFI instruments. The system XSS J12270-4859 was observed during its radio-pulsar state; we did not detect a significant degree of polarisation in any of the bands, with 3σ upper limits, for example, of 1.4% in the R-band. We built the NIR−optical averaged spectral energy distribution (SED) of the system, which could be described well by an irradiated black body with radius R_∗ = 0.33 ± 0.03 R_⊙ and albedo η = 0.32 ± 0.05, without the need for further components. Thus, we excluded the visible presence of an extended accretion disc and/or of relativistic jets. The case was different for PSR J1023+0038, which was in its accretion phase during our campaign. We measured a linear polarisation of 1.09 ± 0.27% and 0.90 ± 0.17% in the V and R bands, respectively. The phase-resolved polarimetric curve of the source in the R band reveals a hint of a sinusoidal modulation at the source 4.75 h orbital period, peaked at the same orbital phase as the light curve. The measured optical polarisation of PSR J1023+0038 could, in principle, be interpreted as electron scattering with free electrons, which can be found in the accretion disc of the system or even in the hot corona that sorrounds the disc itself, or as synchrotron emission from a jet of relativistic particles or an outflow. However, the NIR-optical SED of the system built from our dataset did not suggest the presence of a jet. We conclude that the optical linear polarisation observed for PSR J1023+0038 is possibly due to Thomson scattering with electrons in the disc, as is also suggested from the possible modulation of the R-band linear polarisation at the system orbital period.