PSR J1048 - 5832 is a Vela-like ( P=123.6 ms ; \tau \sim 20.3 kyr ) \gamma -ray pulsar detected by Fermi , at a distance of \sim 2.7 kpc and with a rotational energy loss rate \dot { E } _ { SD } \sim 2 \times 10 ^ { 36 } erg s ^ { -1 } .
The PSR J1048 - 5832 field has been observed with the Very Large Telescope ( VLT ) in the V and R bands .
We used these data to determine the colour of the object detected closest to the Chandra position ( Star D ) and confirm that it is not associated with the pulsar .
For the estimated extinction along the line of sight , inferred from a re-analysis of the Chandra and XMM-Newton spectra , the fluxes of Star D ( V \sim 26.7 ; R \sim 25.8 ) imply a -0.13 \la ( V - R ) _ { 0 } \la 0.6 .
This means that the PSR J1048 - 5832 spectrum would be unusually red compared to the Vela pulsar .
Moreover , the ratio between the unabsorbed optical and X-ray flux of PSR J1048 - 5832 would be much higher than for other young pulsars .
Thus , we conclude that Star D is not the PSR J1048 - 5832 counterpart .
We compared the derived R and V-band upper limits ( R \ga 26.4 ; V \ga 27.6 ) with the extrapolation of the X and \gamma -ray spectra and constrained the pulsar spectrum at low-energies .
In particular , the VLT upper limits suggest that the pulsar spectrum could be consistent with a single power-law , stretching from the \gamma -rays to the optical .