Browsing by Author "Udalski, A"

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    SXP 7.92: a recently rediscovered be/x-ray binary in the small magellanic cloud, viewed edge on
    (2017) Bartlett, E S; Coe, M J; Israel, G L; Clark, J S; Esposito, P; D’Elia, V; Udalski, A
    We present a detailed optical and X-ray study of the 2013 outburst of the Small Magellanic Cloud Be/X-ray binary SXP 7.92, as well as an overview of the last 18 years of observations from OGLE (Optical Gravitational Lensing Experiment), RXTE, Chandra and XMM-Newton. We revise the position of this source to RA(J2000) = 00:57:58.4, Dec(J2000) = −72:22:29.5 with a 1σ uncertainty of 1.5 arcsec, correcting the previously reported position by Coe et al. by more than 20 arcmin. We identify and spectrally classify the correct counterpart as a B1Ve star. The optical spectrum is distinguished by an uncharacteristically deep narrow Balmer series, with the Hα line in particular having a distinctive shell profile, i.e. a deep absorption core embedded in an emission line. We interpret this as evidence that we are viewing the system edge on and are seeing self-obscuration of the circumstellar disc. We derive an optical period for the system of 40.0 ± 0.3 d, which we interpret as the orbital period, and present several mechanisms to describe the X-ray/optical behaviour in the recent outburst, in particular the ‘flares'and ‘dips’ seen in the optical light curve, including a transient accretion disc and an elongated precessing disc.
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    The 2016 super-Eddington outburst of SMC X-3: X-ray and optical properties and system parameters
    (2017) Townsend, L J; McBride, V A; Buckley, D A H; Evans, P A; Udalski, A
    On 2016 July 30 (MJD 57599), observations of the Small Magellanic Cloud by Swift/XRT found an increase in X-ray counts coming from a position consistent with the Be/X-ray binary pulsar SMC X-3. Follow-up observations on 2016 August 3 (MJD 57603) and 2016 August 10 (MJD 57610) revealed a rapidly increasing count rate and confirmed the onset of a new X-ray outburst from the system. Further monitoring by Swift began to uncover the enormity of the outburst, which peaked at 1.2 x 1039 erg/s on 2016 August 25 (MJD 57625). The system then began a gradual decline in flux that was still continuing over 5 months after the initial detection. We explore the X-ray and optical behaviour of SMC X-3 between 2016 July 30 and 2016 December 18 during this super-Eddington outburst. We apply a binary model to the spin-period evolution that takes into account the complex accretion changes over the outburst, to solve for the orbital parameters. Our results show SMC X-3 to be a system with a moderately low eccentricity amongst the Be/X-ray binary systems and to have a dynamically determined orbital period statistically consistent with the prominent period measured in the OGLE optical light curve. Our optical and X-ray derived ephemerides show that the peak in optical flux occurs roughly 6 days after periastron. The measured increase in I-band flux from the counterpart during the outburst is reflected in the measured equivalent width of the H-alpha line emission, though the H-alpha emission itself seems variable on sub-day time-scales, possibly due to the NS interacting with an inhomogeneous disc.