2013MNRAS.428.3636R

PSR J1048-5832 is a Vela-like (P = 123.6 ms; τ ∼ 20.3 kyr) γ-ray pulsar detected by Fermi, at a distance of ∼ 2.7 kpc and with a rotational energy loss rate {dot}E_SD ∼2 ×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 to 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 ∼ 26.7; R ∼ 25.8) imply a - 0.13 ≲ (V - R)0 ≲ 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 ≳ 26.4; V ≳ 27.6) with the extrapolation of the X-ray and γ-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 γ rays to the optical.