VLT and Suzaku observations of the Fermi pulsar PSR J1028 5819

Context. The launch of the Fermi Gamma-ray Space Telescope in 2008 opened new perspectives in the multi-wavelength studies of neutron stars, with more than 100 -ray pulsars now detected. While most Fermi pulsars have been already observed in the X-rays with Chandra and XMM-Newton, optical observations with 8m-class telescopes only exist for a tiny fraction of them. Aims. Here, we aim at searching for optical emission from the Fermi pulsar PSR J1028 5819 (P = 91.4 ms). With a spin-down age � = 92.1 kyr and a rotational energy loss rate u E � 8.43 × 10 35 erg s −1 , PSR J1028 5819 can be considered a transition object between the young, Vela-like pulsars and the middle-aged ones. At a distance of � 2.3 kpc and with a relatively low hydrogen column density, PSR J1028 5819 is a good potential target for 8m-class telescopes. Methods. Due to its recent discovery, no optical observations of this pulsar have been reported so far. We used optical images taken with the Very Large Telescope (VLT) in the B and V bands to search for the optical counterpart of PSR J1028 5819 or constrain its optical brightness. At the same time, we used an archival Suzaku observation to confirm the preliminary identification of the pulsar’s X-ray counterpart obtained by Swift. Results. Due to the large uncertainty on the pulsar’s radio position and the presence of a bright (V = 13.2) early F-type star at < 4 ′′ (Star A), we could not detect its counterpart down to flux limits of B � 25.4 and V � 25.3, the deepest obtained so far for PSR J1028 5819. From the Suzaku observations, we found that the X-ray spectrum of the pulsar’s candidate counterpart is best-fit by a power-law with spectral index X = 1.7 ± 0.2 and an absorption column density NH < 10 21 cm −2 , which would support the proposed X-ray identification. Moreover, we found possible evidence for the presence of diffuse emission around the pulsar. If real, and associated with a pulsar wind nebula (PWN), its surface brightness and angular extent would be compatible with the expectations for a � 100 kyr old pulsar at the PSR J1028 5819 distance. Conclusions. A far more accurate radio position for PSR J1028 5819 is necessary to better determine its position relative to Star A. Future high-spatial resolution observations with both the HST and Chandra are better suited to disentangle the optical emission of PSR J1028 5819 against the halo of Star A and to confirm the existence of the candidate PWN.