Evolution of broad-band SED during outburst rise in NS X-ray Nova Aql X-1

: Meshcheryakov AV, Tsygankov SS, Khamitov IM, Shakura NI, Bikmaev IF, Eselevich MV, Vlasyuk VV, Pavlinsky MN

Publisher: OXFORD UNIV PRESS

: Oxford

: 2018

: Monthly Notices of the Royal Astronomical Society

: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

: MON NOT R ASTRON SOC

: 473

: 3

: 3987

: 4002

: 16

: 0035-8711

: 1365-2966

DOI: https://doi.org/10.1093/mnras/stx2565

: https://academic.oup.com/mnras/article/473/3/3987/4331644#

: https://research.utu.fi/converis/portal/detail/Publication/30311303

The observed evolution of the broad-band spectral energy distribution (SED) in NS X-ray Nova Aql X-1 during the rise phase of a bright Fast-Rise-Exponential-Decay-type outburst in 2013 can be understood in the framework of thermal emission from non-stationary accretion disc with radial temperature distribution transforming from a single-temperature blackbody emitting ring into the multicolour irradiated accretion disc. SED evolution during the hard to soft X-ray state transition looks unusual, as it cannot be reproduced by the standard disc irradiation model with a single irradiation parameter for NUV, Optical and NIR spectral bands. NIR (NUV) band is correlated with soft (hard) X-ray flux changes during the state transition interval, respectively. In our interpretation, at the moment of X-ray state transition UV-emitting parts of the accretion disc are screened from direct X-ray illumination from the central source and are heated primarily by hard X-rays (E > 10 keV), scattered in the hot corona or wind possibly formed above the optically thick outer accretion flow; the outer edge of multicolour disc, which emits in Optical-NIR, can be heated primarily by direct X-ray illumination. We point out that future simultaneous multiwavelength observations of X-ray Nova systems during the fast X-ray state transition interval are of great importance, as it can serve as 'X-ray tomograph' to study physical conditions in outer regions of accretion flow. This can provide an effective tool to directly test the energy-dependent X-ray heating efficiency, vertical structure and accretion flow geometry in transient low-mass X-ray binaries.

stx2565.pdf