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9th of April 2020
A hidden pulsar and a black hole candidate in NGC 6397
by Zhao et al.
Zhao et al. have used a 16 hr ATCA observation together with archival Chandra X-ray data to find radio counterparts to new and known X-ray sources in the Galactic globular cluster NGC 6397. The previously suggested millisecond pulsar (MSP) candidate, U18, is a steep-spectrum radio source with a 5.5 GHz flux density of 55 μJy. It is argued 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 non-detection of radio pulsations to date 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, radio associations were found for 5 other Chandra X-ray sources, four of which are probably background galaxies. The last, U97, is mysterious and may be a quiescent black hole low-mass X-ray binary. More details are given in the paper, to be published in Monthly Notices of the Royal Astronomical Society.

As April 10--13 is the Easter long weekend, the next ADAP will be Tuesday April 14.

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