We argue that the recent groundbreaking discovery by Badenes et al .
( 2009 ) of a nearby ( D \approx 50 pc ) white dwarf-neutron star ( or black hole ) binary ( SDSS 1257+5428 ) with a merger timescale \lesssim 500 Myr implies that such systems are common ; we estimate that there are of order \sim 10 ^ { 6 } in the Galaxy .
Although subject to large uncertainties , the nominal derived merger rate is \Gamma _ { MW } \sim 5 \times 10 ^ { -4 } yr ^ { -1 } in the Milky Way , just \sim 3 - 6 and \sim 20 - 40 times less than the Type Ia and core-collapse supernova ( SN ) rates , respectively .
This implies that the merger rate is \sim 0.5 - 1 \times 10 ^ { 4 } Gpc ^ { -3 } yr ^ { -1 } in the local universe , \sim 5000 - 10000 times more than the observed ( beaming-uncorrected ) long-duration gamma-ray burst ( GRB ) rate .
We estimate the lower limit on the rate in the Galaxy to be \Gamma _ { MW } \gtrsim 2.5 \times 10 ^ { -5 } yr ^ { -1 } at 95 % confidence .
We briefly discuss the implications of this finding for the census of long- and short-duration GRBs and their progenitors , the frequency of tight binary companions to Type-Ib/c SN progenitors , the origin of ultra-high energy cosmic rays ( UHECRs ) , the formation of rapidly rotating neutron stars and \sim 2 - 3 M _ { \odot } black holes , the census of faint Ia-like SNe , as well as for upcoming and current transient surveys ( e.g. , LOSS , PTF , LSST ) , and for high- ( LIGO ) and low-frequency ( LISA ) gravitational wave searches .