Kavli Affiliate: Ronald A. Remillard
| First 5 Authors: Yuhan Yao, S. R. Kulkarni, K. C. Gendreau, Gaurava K. Jaisawal, Teruaki Enoto
| Summary:
Here, we present MAXI, SWIFT, NICER, NuSTAR and Chandra observations of the
X-ray transient AT2019wey (SRGA J043520.9+552226, SRGE J043523.3+552234). From
spectral and timing analyses we classify it as a Galactic low-mass X-ray binary
(LMXB) with a black hole (BH) or neutron star (NS) accretor. AT2019wey stayed
in the low/hard state (LHS) from 2019 December to 2020 August 21, and the
hard-intermediate state (HIMS) from 2020 August 21 to 2020 November. For the
first six months of the LHS, AT2019wey had a flux of $sim 1$ mCrab, and
displayed a power-law X-ray spectrum with photon index $Gamma = 1.8$. From
2020 June to August, it brightened to $sim 20$ mCrab. Spectral features
characteristic of relativistic reflection became prominent. On 2020 August 21,
the source left the "hard line" on the rms–intensity diagram, and transitioned
from LHS to HIMS. The thermal disk component became comparable to the power-law
component. A low-frequency quasi-periodic oscillation (QPO) was observed. The
QPO central frequency increased as the spectrum softened. No evidence of
pulsation was detected. We are not able to decisively determine the nature of
the accretor (BH or NS). However, the BH option is favored by the position of
this source on the $Gamma$–$L_{rm X}$, $L_{rm radio}$–$L_{rm X}$, and
$L_{rm opt}$–$L_{rm X}$ diagrams. We find the BH candidate XTE J1752-223 to
be an analog of AT2019wey. Both systems display outbursts with long plateau
phases in the hard states. We conclude by noting the potential of SRG in
finding new members of this emerging class of low luminosity and long-duration
LMXB outbursts.
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