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  1. OA@INAF
  2. PRODOTTI RICERCA INAF
  3. 1 CONTRIBUTI IN RIVISTE (Journal articles)
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Please use this identifier to cite or link to this item: http://hdl.handle.net/20.500.12386/31275
Title: Enhanced optical activity 12 days before X-ray activity, and a 4 day X-ray delay during outburst rise, in a low-mass X-ray binary
Authors: Goodwin, A. J.
Russell, D. M.
Galloway, D. K.
Baglio, M. C.
Parikh, A. S.
Buckley, D. A. H.
Homan, J.
Bramich, D. M.
in 't Zand, J. J. M.
Heinke, C. O.
Kotze, E. J.
DE MARTINO, Domitilla 
PAPITTO, ALESSANDRO 
Lewis, F.
Wijnands, R.
Issue Date: 2020
Journal: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 
Number: 498
Issue: 3
First Page: 3429
Abstract: X-ray transients, such as accreting neutron stars, periodically undergo outbursts, thought to be caused by a thermal-viscous instability in the accretion disk. Usually outbursts of accreting neutron stars are identified when the accretion disk 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 system. We observed a significant, continuing increase in the optical i'-band magnitude starting on July 25, 12 days 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 day optical to X-ray rise delay, and a 2 day 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 disk 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 low mass X-ray binaries, which has only previously been measured with less sensitive all sky monitors. We find observational evidence that the outburst is triggered by ionisation of hydrogen in the disk.
URI: http://hdl.handle.net/20.500.12386/31275
URL: http://arxiv.org/abs/2006.02872v2
https://academic.oup.com/mnras/article/498/3/3429/5897366
ISSN: 0035-8711
DOI: 10.1093/mnras/staa2588
Bibcode ADS: 2020MNRAS.498.3429G
Fulltext: open
Appears in Collections:1.01 Articoli in rivista

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