The Be/neutron star system Swift J004929.5-733107 in the Small Magellanic Cloud–X-ray characteristics and optical counterpart candidates

Author:

Coe M J1ORCID,Kennea J A2,Evans P A3ORCID,Townsend L J45ORCID,Udalski A6,Monageng I M47ORCID,Buckley D A H45ORCID

Affiliation:

1. Physics and Astronomy, The University of Southampton, Southampton SO17 1BJ, UK

2. Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA

3. Astronomy Research Group, School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK

4. South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa

5. Southern African Large Telescope, PO Box 9, Observatory, 7935 Cape Town, South Africa

6. Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, PL-00-478 Warszawa, Poland

7. Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa

Abstract

ABSTRACT Swift J004929.5-733107 is an X-ray source in the Small Magellanic Cloud (SMC) that has been reported several times, but the optical counterpart has been unclear due to source confusion in a crowded region of the SMC. Previous works proposed [MA93] 302 as the counterpart, however we show here, using data obtained from the S-CUBED project, that the X-ray position is inconsistent with that object. Instead we propose a previously unclassified object which has all the indications of being a newly identified Be star exhibiting strong H α emission. Evidence for the presence of significant I-band variability strongly suggest that this is, in fact, a Be type star with a large circumstellar disc. Over 18 yr worth of optical monitoring by the OGLE project reveal a periodic modulation at a period of 413 d, probably the binary period of the system. A SALT optical spectrum shows strong Balmer emission and supports a proposed spectral classification of B1-3 III-IVe. The X-ray data obtained from the S-CUBED project reveal a time-averaged spectrum well fitted by a photon index Γ = 0.93 ± 0.16. Assuming the known distance to the SMC, the flux corresponds to a luminosity ∼1035 erg s−1. All of these observational facts suggest that this is confirmed as a Be star–neutron star X-ray binary (BeXRB) in the SMC, albeit one with an unusually long binary period at the limits of the Corbet Diagram.

Funder

National Science Centre

UKSA

National Science Foundation, United Arab Emirates

NRF

UCT

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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