2011A&A...533A.101M

Optical observations of pulsars are crucial to studying the neutron star properties from the structure and composition of the interior to the properties and geometry of the magnetosphere. Historically, X and γ-ray observations have paved the way to pulsar optical identifications. The launch of the Fermi Gamma-ray Space Telescope opened new perspectives in the optical-to-γ-ray studies of neutron stars with the detection of more than 80 γ-ray pulsars. Here, we aim to search for optical emission from two Fermi pulsars that are interesting targets on the basis of their spin-down age, energetics, and distance. PSR J1357-6429 is a Vela-like pulsar (P=166.1ms; τ=7.31kyr), at a distance of ∼2.4kpc with a rotational energy loss rate {dot}(E)∼3x10³⁶erg/s. PSR J1048-5832 is also a Vela-like (P=123.6ms; τ=20.3kyr) pulsar at a distance of ∼2.7kpc and with a {dot}(E)∼2x10³⁶erg/s. The two pulsars and their pulsar wind nebulae (PWNe) are also detected in X-rays by Chandra and XMM-Newton. No deep optical observations of these two pulsars have been reported so far. We used multi-band optical images (V,R,I) taken with the Very Large Telescope (VLT) and available in the European Southern Observatory (ESO) archive to search for, or put tight constraints on, their optical emission. We re-assessed the positions of the two pulsars from the analyses of all the available Chandra observations and the comparison with the published radio coordinates. For PSR J1357-6429, this yielded a tentative proper motion µ= 0.17±0.055"/yr (70°±15° position angle). We did not detect any candidate counterparts to PSR J1357-6429 and PSR J1048-5832 down to V∼27 and ∼27.6, respectively, although for the former we found possible evidence of a faint, unresolved object at the Chandra position. Our limits imply an efficiency in converting spin-down power into optical luminosity ≲7x10^–7 and ≲6x10^–6, possibly close to that of the Vela pulsar. Observations with the Hubble Space Telescope (HST) are required to identify PSR J1357-6429 against nearby field stars. Owing to the high extinction (A_V∼5) and the presence of a molecular cloud complex, near-infrared observations of PSR J1048-5832 are better suited to spotting its candidate counterpart.