1A 1118-616 exhibits strong evidence for a 17.0 year period
ATel #17615; M. J. Coe (Southampton University, UK)
on 20 Jan 2026; 14:44 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)
Subjects: Binary, Neutron Star, Transient
Referred to by ATel #: 17690
The Be/X-ray system 1A 1118-616 is currently undergoing a rare major outburst (Nakajima et al ATel #17613). From its initial discovery it is known to exhibit a 404s pulse period, but the binary period is less clear. Staubert et al (2011) proposed that this period could be 24.0d based upon a detailed study of the outburst that occurred in January 2009. However, this latest X-ray outburst is the 4th one detected and a strong alternative binary period is now emerging.
The first outburst peaked on JD2442406 (Eyles et al 1975), the second on JD2448620 (Coe et al 1994), the third peaked on JD 2454844 (Mangano 2009) and the 4th is predicted to peak on JD2461062 (Nakajima 2026). The intervals between each of these outbursts is almost exactly 17.0 years.
Though neither 24.0d nor 17.0y fit comfortably on the Corbet Diagram with a 404s pulse period, the latest outburst suggests strongly that there is a fundamental period of 17.0 years involved in the behaviour of this system. It must be either the true binary period, or perhaps evidence for a triple system.
