Using a 16.2 ~ { } \mathrm { hr } radio observation by the Australia Telescope Compact Array ( ATCA ) and archival Chandra data , we found > 5 \sigma radio counterparts to 4 known and 3 new X-ray sources within the half-light radius ( r _ { \mathrm { h } } ) of the Galactic globular cluster NGC 6397 .
The previously suggested millisecond pulsar ( MSP ) candidate , U18 , is a steep-spectrum ( S _ { \nu } \propto \nu ^ { \alpha } ; \alpha = -2.0 ^ { +0.4 } _ { -0.5 } ) radio source with a 5.5 GHz flux density of 54.7 \pm 4.3 ~ { } \mu \mathrm { Jy } .
We argue that U18 is most likely a ‘ ‘ hidden '' MSP that is continuously hidden by plasma shocked at the collision betwen the winds from the pulsar and companion star .
The nondetection of radio pulsations so far is probably the result of enhanced scattering in this shocked wind .
On the other hand , we observed the 5.5 GHz flux of the known MSP PSR J1740-5340 ( U12 ) to decrease by a factor of > 2.8 during epochs of 1.4 GHz eclipse , indicating that the radio flux is absorbed in its shocked wind . If U18 is indeed a pulsar whose pulsations are scattered , we note the contrast with U12 ’ s flux decrease in eclipse , which argues for two different eclipse mechanisms at the same radio frequency . In addition to U12 and U18 , we also found radio associations for 5 other Chandra X-ray sources , four of which are likely background galaxies .
The last , U97 , which shows strong \mathrm { H } \alpha variability , is mysterious ; it may be either a quiescent black hole low-mass X-ray binary , or something more unusual .