Enhanced optical activity 12 d before X-ray activity, and a 4 d X-ray delay during outburst rise, in a low-mass X-ray binary
Monthly Notices of the Royal Astronomical Society, vol. 498, pp. 3429–3439
Abstract
ABSTRACT X-ray transients, such as accreting neutron stars, periodically undergo outbursts, thought to be caused by a thermal-viscous instability in the accretion disc. Usually outbursts of accreting neutron stars are identified when the accretion disc has undergone an instability, and the persistent X-ray flux has risen to a threshold detectable by all sky monitors on X-ray space observatories. Here, we present the earliest known combined optical, UV, and X-ray monitoring observations of the outburst onset of an accreting neutron star low-mass X-ray binary (LMXB) system. We observed a significant, continuing increase in the optical i′-band magnitude starting on July 25, 12 d before the first X-ray detection with Swift/XRT and NICER (August 6), during the onset of the 2019 outburst of SAX J1808.4−3658. We also observed a 4 d optical to X-ray rise delay, and a 2 d UV to X-ray delay, at the onset of the outburst. We present the multiwavelength observations that were obtained, discussing the theory of outbursts in X-ray transients, including the disc instability model, and the implications of the delay. This work is an important confirmation of the delay in optical to X-ray emission during the onset of outbursts in LMXBs, which has only previously been measured with less sensitive all sky monitors. We find observational evidence that the outburst is triggered by ionization of hydrogen in the disc.
Authors 15
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A J Goodwin corresponding
Affiliation as printed
School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
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Affiliation as printed
Center for Astro, Particle and Planetary Physics, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
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Affiliation as printed
School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
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New York University Abu Dhabi · Brera Astronomical Observatory
Affiliation as printed
Center for Astro, Particle and Planetary Physics, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
Center for Astro, Particle and Planetary Physics, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE; INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
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Affiliation as printed
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands
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South African Astronomical Observatory
Affiliation as printed
South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa
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Eureka Scientific · Space Research Organisation Netherlands
Affiliation as printed
Eureka Scientific, Inc., 2452 Delmer Street, Oakland, CA 94602, USA
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, NL-3584 CA Utrecht, the Netherlands
Eureka Scientific, Inc., 2452 Delmer Street, Oakland, CA 94602, USA; SRON Netherlands Institute for Space Research, Sorbonnelaan 2, NL-3584 CA Utrecht, the Netherlands
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Affiliation as printed
Center for Astro, Particle and Planetary Physics, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
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Affiliation as printed
Eureka Scientific, Inc., 2452 Delmer Street, Oakland, CA 94602, USA
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Affiliation as printed
Department of Physics, University of Alberta, CCIS 4-183, Edmonton, AB T6G 2E1, Canada
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South African Astronomical Observatory
Affiliation as printed
South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa
Southern African Large Telescope Foundation, PO Box 9, Observatory 7935, Cape Town, South Africa
South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa; Southern African Large Telescope Foundation, PO Box 9, Observatory 7935, Cape Town, South Africa
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Astronomical Observatory of Capodimonte
Affiliation as printed
INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Napoli, Italy
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Astronomical Observatory of Rome · National Institute for Astrophysics
Affiliation as printed
INAF – Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monte Porzio Catone (Roma), Italy
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Liverpool John Moores University · Cardiff University
Affiliation as printed
Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK; Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
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Affiliation as printed
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands
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