Revealing the transition of Cen X-3: Chandra Pointed observation from low hard to high soft state

ATel #16077; Jose Miguel Torrejon, Graciela Sanjurjo Ferrin, Jose Joaquin Rodes Roca (Universidad de Alicante), Michael A. Nowak (Washington University), Norbert Schulz (Massachusetts Institute of Technology), Konstantin Postnov (Sternberg Astronomical Institute), Lida Oskinova (Potsdam University)
on 9 Jun 2023; 07:50 UT
Credential Certification: Jose Miguel Torrejon (jmt@ua.es)

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Cen X-3 is one of the brightest disc-fed high-mass X-ray binaries with evolved donors. It also shows pronounced long-term variability: hard-low states and soft-high states. During hard-low states, large scale structures emerging from the eclipse can be detected while, during high-soft states, long and pronounced dips (of about 1000 s) have been observed, caused by the interaction of the NS magnetosphere with the accretion disk. In order to analyze the source during a low-hard state, a Chandra TOO observation was performed, covering approximately phases 0.15-0.65. Unexpectedly, the source changed state in phase 0.3, and consequently only 30% of the observation took place when the source was in the desired low-hard state. On the other hand, we could observe the interesting transition from low-hard to high-soft state, which is sudden and sharp. The spectra of the source during this observation show a very high and variable absorption (from 2.5 to 150 x 10^22 cm-2), most of the observation being much higher than the absorption previously reported, both for observations made in the low-hard state and in the high-soft state (both around 3 x 10^22 cm-2). The NS spin pulse shows the expected Doppler evolution around the orbit and was detected only during phases [0.4 - 0.6]. This Doppler evolution is compatible with an orbital inclination of 76 degrees, in agreement with previous reports. A spin-up evolution of -0.0006 s/yr is observed. References: 2021 MNRAS.501.5892S "X-ray variability of the HMXB Cen X-3: evidence for inhomogeneous accretion flows" Sanjurjo-Ferrin, G.; Torrejon, J. M.; Postnov, K.; Oskinova, L.; Rodes-Roca, J. J.Bernabeu, G.