VLT polarimetry observations of the middle-aged pulsar PSR B0656+14

Abstract

Context. Optical polarisation measurements are key tests for different models of the pulsar magnetosphere. Furthermore, comparing the relative orientation of the phase-averaged linear polarisation direction and the pulsar proper motion vector may unveil a peculiar alignment, clearly seen in the Crab pulsar. Aims. Our goal is to obtain the first measurement of the phase-averaged optical linear polarisation of the fifth brightest optical pulsar, PSR B0656+14, which also has a precisely measured proper motion, and to verify a possible alignment between the polarisation direction and the proper motion vector. Methods. We carried out observations with the Very Large Telescope (VLT) to measure the phase-averaged optical polarisation degree (PD) and position angle (PA) of PSR B0656+14. Results. We measured a PD of 11.9% ± 5.5% and a PA of 125.8 • ± 13.2 • , measured east of north. Albeit of marginal significance, this is the first measurement of the phase-averaged optical PD for this pulsar. Moreover, we found that the PA of the phase-averaged polarisation vector is close to that of the pulsar proper motion (93.12 • ± 0.38 • ). Conclusions. Deeper observations are needed to confirm our polarisation measurement of PSR B0656+14, whereas polarisation measurements for more pulsars will better assess possible correlations of the polarisation degree with the pulsar parameters.

Evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star RX J1856.5−3754

“…According to our experience with polarisation measurements of such faint targets (see, e.g. Mignani et al 2015), PSF photometry is more robust than aperture photometry. In particular, we fitted the model PSF to the RX J1856.5−3754 intensity profile within an area of 10 pixel radius (2.…”

“…With X-ray polarimetry still moving its first steps and dedicated polarimetric missions, such as XIPE 1 (Soffitta et al 2013) and IXPE (Weisskopf et al 2013), having just been proposed, other energy ranges must be explored. Besides the X-rays, all M7 have been detected in the optical/near-UV (Kaplan et al 2011) where well-tested techniques, even for objects as faint as INSs, can be exploited, as shown by the polarisation measurements in rotationpowered pulsars (RPPs), for which we refer to Mignani et al (2015) for an exhaustive summary. Whether the optical/near-UV emission is thermal and arises from the cooling surface is not entirely clear yet.…”

Multi-Wavelength Polarimetry of Isolated Neutron Stars

“…As of now, there are five RPPs for which a measurement of the optical polarisation degree has been obtained, albeit with different level of significance owing to the different object brightness (see, [15] for a recent compilation and reference list). After the Crab, a robust measurement (8.1%±0.7%) has been obtained for the 11 kyr-old Vela pulsar (V=23.6), with the HST [13].…”

“…The observed optical polarisation is also a factor of few lower than expected from predictions based on most pulsar magnetosphere models, although such discrepancy can be due to over-simplification in the simulation codes or larger than expected depolarisation effects in the pulsar magnetosphere (see, [12] for a discussion). Bearing its current limitations in mind, the starting sample can be investigated for possible correlations between the polarisation degree and the pulsar properties [15]. For instance, a hint of a correlation between PD and the pulsar spindown age, and of anti-correlation with the spin-down energy, can be seen, suggesting that the PD is higher for older and ess energetic pulsars.…”

Optical linear polarization of 74 white dwarfs with the RoboPol polarimeter

“…There are also optical polarization studies of isolated neutron stars including pulsars (e.g. S lowikowska et al 2009;Lundqvist et al 2011;Moran et al 2013Moran et al , 2014Mignani et al 2015) and magnetars (e.g. Wang et al 2015), as well as neutron stars in high mass X-ray binaries (e.g.…”