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   73015
   There appears to be a small population of extremely luminous blazars
   above redshift 4; GB1428_4217 at z=4.72, GB1508+5714 at z=4.3 and RXJ
   1028.6-0844 at z=4.28 We have now detected X-rays from a 4th such
   object, PMN0525-3343, which at z=4.41 is the second most distant known.
   It has a ROSAT HRI flux about one half that of GB1428, for which we
   obtained an excellent ASCA spectrum. The results for GB1428 suggest
   that it is an extreme member of the MeV blazar population in which the
   jet is propagating through a high radiation field, possibly connected
   with the nucleus being young. Here we propose to measure the restframe
   2.5 - 55 keV spectrum of PMN0525 with ASCA to further study these
   highly active, young, objects.
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