The MAVERIC survey: a hidden pulsar and a black hole candidate in ATCA radio imaging of the globular cluster NGC 6397
Using a 16.2 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 suggest...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Journal Article |
| Language: | English |
| Published: |
OXFORD UNIV PRESS
2020
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| Subjects: | |
| Online Access: | http://purl.org/au-research/grants/arc/FT140101082 http://hdl.handle.net/20.500.11937/80178 |
| Summary: | Using a 16.2 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
$54.7\pm 4.3~\mathrm{\mu Jy}$. We argue that U18 is most likely a "hidden" MSP
that is continuously hidden by plasma shocked at the collision between 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 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 H$\alpha$ variability, is mysterious; it may be
either a quiescent black hole low-mass X-ray binary, or something more unusual. |
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