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
Date Issued
2020
Author(s)
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.
•
•
•
Lewis, F.
•
Wijnands, R.
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.
Volume
498
Issue
3
Start page
3429
Issn Identifier
0035-8711
Ads BibCode
2020MNRAS.498.3429G
Rights
open.access
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