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A far-ultraviolet variable with an 18-minute period in the globular cluster NGC 1851

A far-ultraviolet variable with an 18-minute period in the globular cluster NGC 1851
A far-ultraviolet variable with an 18-minute period in the globular cluster NGC 1851
We present the detection of a variable star with an 18.05 minute period in far-ultraviolet (FUV) images of the globular cluster NGC 1851 taken with the Hubble Space Telescope (HST). A candidate optical counterpart lies on the red horizontal branch or the asymptotic giant branch star of the cluster, but it is statistically possible that this is a chance superposition. This interpretation is supported by optical spectroscopt obtained with HST/STIS: the spectrum contains none of the strong emission lines that would be expected if the object was a symbiotic star (i.e. a compact accretor fed by a giant donor). We therefore consider two other possibilities for the nature of FUV variable: (i) an intermediate polar (i.e. a compact binary containing an accreting magnetic white dwarf), or (ii) an AM CVn star (i.e. an interacting double-degenerate system). In the intermediate polar scenario, the object is expected to be an X-ray source. However, no X-rays are detected at its location in ? 65 ksec of Chandra imaging, which limits the X-ray luminosity to LX ? 1032 erg s?1. We therefore favour the AM CVn interpretation, but a FUV spectrum is needed to distinguish conclusively between the two possibilities. If the object is an AM CVn binary, it would be the first such system known in any globular cluster.
0035-8711
1-10
Zurek, D.R.
0de59032-c43c-4c8f-89a2-b451488b86db
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Maccarone, T.J.
5d8ee27f-a174-424c-b3b7-a6739bfeea50
Pooley, D.
a5ace8d5-12a1-442a-997e-c6fdbd35661c
Dieball, A.
5f595453-aa6b-4067-a3e1-026867fb9f7a
Long, K.S.
91417b3d-d408-475a-8907-eec131e17c66
Shara, M.
1ff52820-d17b-432e-90f0-777540daea67
Sarajedini, A.
840c3737-1a63-46a1-b327-e636bb070736
Zurek, D.R.
0de59032-c43c-4c8f-89a2-b451488b86db
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Maccarone, T.J.
5d8ee27f-a174-424c-b3b7-a6739bfeea50
Pooley, D.
a5ace8d5-12a1-442a-997e-c6fdbd35661c
Dieball, A.
5f595453-aa6b-4067-a3e1-026867fb9f7a
Long, K.S.
91417b3d-d408-475a-8907-eec131e17c66
Shara, M.
1ff52820-d17b-432e-90f0-777540daea67
Sarajedini, A.
840c3737-1a63-46a1-b327-e636bb070736

Zurek, D.R., Knigge, C., Maccarone, T.J., Pooley, D., Dieball, A., Long, K.S., Shara, M. and Sarajedini, A. (2016) A far-ultraviolet variable with an 18-minute period in the globular cluster NGC 1851. Monthly Notices of the Royal Astronomical Society, 1-10. (doi:10.1093/mnras/stw1190).

Record type: Article

Abstract

We present the detection of a variable star with an 18.05 minute period in far-ultraviolet (FUV) images of the globular cluster NGC 1851 taken with the Hubble Space Telescope (HST). A candidate optical counterpart lies on the red horizontal branch or the asymptotic giant branch star of the cluster, but it is statistically possible that this is a chance superposition. This interpretation is supported by optical spectroscopt obtained with HST/STIS: the spectrum contains none of the strong emission lines that would be expected if the object was a symbiotic star (i.e. a compact accretor fed by a giant donor). We therefore consider two other possibilities for the nature of FUV variable: (i) an intermediate polar (i.e. a compact binary containing an accreting magnetic white dwarf), or (ii) an AM CVn star (i.e. an interacting double-degenerate system). In the intermediate polar scenario, the object is expected to be an X-ray source. However, no X-rays are detected at its location in ? 65 ksec of Chandra imaging, which limits the X-ray luminosity to LX ? 1032 erg s?1. We therefore favour the AM CVn interpretation, but a FUV spectrum is needed to distinguish conclusively between the two possibilities. If the object is an AM CVn binary, it would be the first such system known in any globular cluster.

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Accepted/In Press date: 16 May 2016
e-pub ahead of print date: 19 May 2016
Organisations: Astronomy Group

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Local EPrints ID: 396286
URI: http://eprints.soton.ac.uk/id/eprint/396286
ISSN: 0035-8711
PURE UUID: 42670dbb-cd07-4d80-b1ec-53c6349c031e

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Date deposited: 07 Jun 2016 14:09
Last modified: 09 Dec 2019 19:36

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