A rare outburst from the stealthy BeXRB system Swift J0549.7-6812

Swift J0549.7 -6812 is a Be/X-ray binary system (BeXRB) in the Large Magellanic Cloud (LMC) exhibiting an ~6 s pulse period. Like many such systems, the variable X-ray emission is believed to be driven by the underlying behaviour of the mass donor Be star. In this paper, we report on X-ray observations of the brightest known outburst from this system, which reached a luminosity of ~8 × 10 ³⁷ erg s ^-1. These observations are supported by contemporaneous optical photometric observations, the first reported optical spectrum, as well as several years of historical data from Optical Gravitational Lens Explorer (OGLE) and Gaia . The latter strongly suggest a binary period of 46.1 d. All the observational data indicate that Swift J0549.7 -6812 is a system that spends the vast majority of its time in X-ray quiescence, or even switched off completely. This suggests that occasional observations may easily miss it, and many similar systems, and thereby underestimate the massive star evolution numbers for the LMC.

astro-ph.HE, X-rays: binaries, stars: emission line, Be

10.1093/mnras/stad1987

1365-2966

3263–3269

Coe, M.J.

04dfb23b-1456-46a3-9242-5cee983471d5

Kennea, J.A.

8a2485ff-bd59-4dd8-b6e3-d02e86bbc344

Monageng, I.M.

ea1baf78-c23c-4531-afbd-640eea0dacb3

Buckley, D.A.H.

ea3b9dda-bd50-4635-8911-0f0f9d2c5ccc

Udalski, A.

320fdc94-b983-41cd-ad0d-0aee565e7142

Evans, P.A.

61311b69-1bbe-487b-8ab2-0ffca51927eb

1 September 2023

Coe, M.J.

04dfb23b-1456-46a3-9242-5cee983471d5

Kennea, J.A.

8a2485ff-bd59-4dd8-b6e3-d02e86bbc344

Monageng, I.M.

ea1baf78-c23c-4531-afbd-640eea0dacb3

Buckley, D.A.H.

ea3b9dda-bd50-4635-8911-0f0f9d2c5ccc

Udalski, A.

320fdc94-b983-41cd-ad0d-0aee565e7142

Evans, P.A.

61311b69-1bbe-487b-8ab2-0ffca51927eb

Coe, M.J., Kennea, J.A., Monageng, I.M., Buckley, D.A.H., Udalski, A. and Evans, P.A. (2023) A rare outburst from the stealthy BeXRB system Swift J0549.7-6812. Monthly Notices of the Royal Astronomical Society, 524 (3), 3263–3269. (doi:10.1093/mnras/stad1987).

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Accepted/In Press date: 29 June 2023

Published date: 1 September 2023

Additional Information: Funding Information: PAE acknowledges UKSA support. JAK acknowledges support from NASA grant NAS5-00136. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. IMM and DAHB were supported by the South African National Research Foundation (NRF). Funding Information: Observations reported in this paper were obtained with the Southern African Large Telescope (SALT), as part of the Large Science Programme on transients 2018-2-LSP-001 funded by the South African National Research Foundation (PI: Buckley). Funding Information: PAE acknowledges UKSA support. JAK acknowledges support from NASA grant NAS5-00136. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. IMM and DAHB were supported by the South African National Research Foundation (NRF). Observations reported in this paper were obtained with the Southern African Large Telescope (SALT), as part of the Large Science Programme on transients 2018-2-LSP-001 funded by the South African National Research Foundation (PI: Buckley). Many thanks to Poshak Gandhi and Yue Zhao for help with the Gaia data access. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int /gaia), processed by the Gaia Data Processing and Analysis Consor- tium (DPAC, ht tps://www.cosmos.esa.int /web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Publisher Copyright: © 2023 The Author(s).

Keywords: astro-ph.HE, X-rays: binaries, stars: emission line, Be