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An ultracompact x-ray binary in the Globular Cluster M15 (NGC 7078)

An ultracompact x-ray binary in the Globular Cluster M15 (NGC 7078)
An ultracompact x-ray binary in the Globular Cluster M15 (NGC 7078)
We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M15 in the far-ultraviolet (FUV) wave band. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062+/-0.004 mag semiamplitude, and we clearly detect a period of 22.5806+/-0.0002 minutes. We have carried out extensive Monte Carlo simulations, which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M15 X-2 is FUV-bright (FUV~=17 mag) and is characterized by an extremely blue spectral energy distribution (Flambda~lambda-2.0). We also find evidence for an excess of flux between 1500 and 1600 Å and probably between 1600 and 2000 Å that might be due to C IV 1550 Å and He II 1640 Å emission lines. We also show that M15 X-2's X-ray luminosity can be powered by accretion at the rate expected for mass transfer driven by gravitational waves at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by irradiation of the low-mass white dwarf donor if the inclination of the system is ~34°. We conclude that all observational characteristics of M15 X-2 are consistent with its being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.
stars, binaries, close, galaxy, globular clusters, individual, Messier number, M15, individual (M15 X-2), ultraviolet
0004-637X
L105-L108
Dieball, A.
5f595453-aa6b-4067-a3e1-026867fb9f7a
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Zurek, D.R.
4745ee98-520b-4e26-a05a-113c0c563b97
Shara, M.M
7f28952d-ebf4-421e-9680-e06990b58bdc
Long, K.S.
91417b3d-d408-475a-8907-eec131e17c66
Charles, P. A.
0429b380-0754-4dc1-8def-885c7fa6a086
Hannikainen, D.C.
75e8f67a-ed96-44d1-aee8-6ea2ea7889c4
van Zyl, L.
509ae2e9-0be8-4138-8215-08f59b2ee294
Dieball, A.
5f595453-aa6b-4067-a3e1-026867fb9f7a
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Zurek, D.R.
4745ee98-520b-4e26-a05a-113c0c563b97
Shara, M.M
7f28952d-ebf4-421e-9680-e06990b58bdc
Long, K.S.
91417b3d-d408-475a-8907-eec131e17c66
Charles, P. A.
0429b380-0754-4dc1-8def-885c7fa6a086
Hannikainen, D.C.
75e8f67a-ed96-44d1-aee8-6ea2ea7889c4
van Zyl, L.
509ae2e9-0be8-4138-8215-08f59b2ee294

Dieball, A., Knigge, C., Zurek, D.R., Shara, M.M, Long, K.S., Charles, P. A., Hannikainen, D.C. and van Zyl, L. (2005) An ultracompact x-ray binary in the Globular Cluster M15 (NGC 7078). The Astrophysical Journal, 634 (1), L105-L108. (doi:10.1086/498712).

Record type: Article

Abstract

We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M15 in the far-ultraviolet (FUV) wave band. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062+/-0.004 mag semiamplitude, and we clearly detect a period of 22.5806+/-0.0002 minutes. We have carried out extensive Monte Carlo simulations, which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M15 X-2 is FUV-bright (FUV~=17 mag) and is characterized by an extremely blue spectral energy distribution (Flambda~lambda-2.0). We also find evidence for an excess of flux between 1500 and 1600 Å and probably between 1600 and 2000 Å that might be due to C IV 1550 Å and He II 1640 Å emission lines. We also show that M15 X-2's X-ray luminosity can be powered by accretion at the rate expected for mass transfer driven by gravitational waves at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by irradiation of the low-mass white dwarf donor if the inclination of the system is ~34°. We conclude that all observational characteristics of M15 X-2 are consistent with its being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.

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More information

Published date: 20 November 2005
Keywords: stars, binaries, close, galaxy, globular clusters, individual, Messier number, M15, individual (M15 X-2), ultraviolet

Identifiers

Local EPrints ID: 26872
URI: http://eprints.soton.ac.uk/id/eprint/26872
ISSN: 0004-637X
PURE UUID: 58cef29e-46bc-4355-9a09-14d3ec2576f6

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Date deposited: 24 Apr 2006
Last modified: 15 Jul 2019 19:13

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