9th of April 2020 |
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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. |