We present the identification of the companion star to the intermediate mass binary pulsar J1439-5501 obtained by means of ground-based deep images in the B , V and I bands , acquired with FORS2 mounted at the ESO-VLT .
The companion is a massive white dwarf ( WD ) with B = 23.57 \pm 0.02 , V = 23.21 \pm 0.01 and I = 22.96 \pm 0.01 , located at only \sim 0.05 \arcsec from the pulsar radio position .
Comparing the WD location in the ( B , B - V ) and ( V , V - I ) Color-Magnitude diagrams with theoretical cooling sequences we derived a range of plausible combinations of companion masses ( 1 \stackrel { < } { { } _ { \sim } } M _ { COM } \stackrel { < } { { } _ { \sim } } 1.3 ~ { } M _ { \odot } ) , distances ( d \stackrel { < } { { } _ { \sim } } 1200 pc ) , radii ( \stackrel { < } { { } _ { \sim } } 7.8 ~ { } 10 ^ { -3 } ~ { } R _ { \odot } ) and temperatures ( T = 31350 ^ { +21500 } _ { -7400 } ) .
From the PSR mass function and the estimated mass range we also constrained the inclination angle i \stackrel { > } { { } _ { \sim } } 55 ^ { \circ } and the pulsar mass ( M _ { PSR } \stackrel { < } { { } _ { \sim } } 2.2 M _ { \odot } ) .
The comparison between the WD cooling age and the spin down age suggests that the latter is overestimated by a factor of about ten .