VLT and Suzaku observations of the Fermi pulsar PSR J1028-5819

Abstract

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 γ-ray pulsars having since been detected. While most Fermi pulsars had previously been observed in the X-rays with Chandra and XMM-Newton, optical observations with 8 m-class telescopes exist for only a tiny fraction of them. Aims. We aim to search for optical emission from the Fermi pulsar PSR J1028-5819 (P = 91.4 ms). With a spin-down age τ ∼ 90 kyr and a rotational energy loss rate of Ä-∼ 8.3 × 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 8 m-class telescopes. Methods. Owing 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. Owing to the large uncertainty in the pulsar's radio position and the presence of a bright (V = 13.2) early F-type star at 4″ (star A), we are unable to 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 find that the X-ray spectrum of the pulsar's candidate counterpart is best-fit by a power-law with spectral index Γ X = 1.7 ± 0.2 and an absorption column density N H < 10 21 cm -2, which would support the proposed X-ray identification. Moreover, we find possible evidence 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 distance of PSR J1028-5819. 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 will be more able to distinguish the optical emission of PSR J1028-5819 from the halo of star A and confirm the existence of the candidate PWN. © 2012 ESO.