We have recently proposed that the ultra-high energy cosmic rays ( UHECRs ) observed above the GZK limit could be mostly protons accelerated in reconnection sites just above the magnetosphere of newborn millisecond pulsars originated by accretion induced collapse ( AIC-pulsars ) .
Although the expected rate of AIC sources in our own Galaxy is very small ( \sim 10 ^ { -5 } yr ^ { -1 } ) , our estimates have shown that the observed total flux of UHECRs could be obtained from the integrated contribution from AIC-pulsars of the whole distribution of galaxies located within a distance which is unaffected by the GZK cutoff ( \sim 50 Mpc ) .
We presently examine the potential acceleration mechanisms in the magnetic reconnection site and find that first-order Fermi acceleration can not provide sufficient efficiency .
To prevent synchrotron losses , only very small deflection angles of the UHECRs would be allowed in the strong magnetic fields of the pulsar , which is contrary to the requirements for efficient Fermi acceleration .
This leaves the one-shot acceleration via an induced electric field within the reconnection region as the only viable process for UHECR acceleration .
We formulate the constraints on both the magnetic field topology and strength in order to accelerate the particles and allow them to freely escape from the system .
Under fast reconnection conditions , we find that AIC-pulsars with surface magnetic fields 10 ^ { 12 } G < B _ { \star } \lesssim 10 ^ { 15 } G and spin periods 1 ms \lesssim P _ { \star } < 60 ms , are able to accelerate particles to energies \geq 10 ^ { 20 } eV , but the magnetic field just above the Alfvén surface must be predominantly toroidal for the particles to be allowed to escape from the acceleration zone without being deflected .
Synchrotron losses bring potentially important constraints on the magnetic field geometry of any UHECR accelerators involving compact sources with strong magnetic fields .