In the last four years, the Be/X-ray binary EXO 2030+375 has displayed an unusual behaviour: its X-ray activity decreased and type I outbursts were not detected at several periastron passages. At the same time, its pulse-period derivative changed from a constant spin up to almost zero (ATEL #8835). Optical observations revealed a large equivalent width of the H alpha line (ATEL #8927).
We report a shift of the outburst peak phase in the most recent (MJD 57589) type I X-ray outburst of EXO 2030+375. The peak flux of the latest outburst was ~30 mCrab on 2016 July 20 (MJD 57589) for both MAXI/GSC and Swift/BAT. This corresponds to an orbital phase of 0.015 (orbital ephemeris of Wilson et al. 2008) and represents a significant change from the almost constant value of ~0.13 for all the detected type I outbursts since 2007. The peak time changed from ~5 days after the periastron to ~0.7 days after the periastron.
In addition, the latest Fermi/GBM spin frequency measurement (2016 April 25) suggests that the source is now starting to spin down.
All these observations are strikingly similar to the events which occurred from January 1994 to October 1995. The X-ray flux decreased, leading to several undetected outbursts, followed by a slow transition to spin down. In October 1995, a sudden shift of the outburst peak time from ~5 days after the periastron to ~5 days before the periastron was observed (Wilson et al. 2002).
The first giant outburst was detected on MJD 46204 (1985 May). The peak could be 0-30 days earlier than this date. The orbital phase shift occurred at MJD 50040 (1995 October). The time between these events is of 3836 ~ 3866 days. The peak of the second giant outburst was around MJD 53960 (2006 May), which is 3920 days after the phase shift. The time between the second giant outburst and the current phase shift is of 3629 days. All these time spans are similar.
We strongly encourage optical and infra-red observations to better understand the origin of this apparently recurrent events.
The recent outburst compared to the last two brightest outbursts is shown below.
Recent type I outburst of EXO 2030+375 compared to the last two brightest outbursts. Orbit 1 is MJD 50086.967, which is the first periastron passage after the launch of RXTE. The current orbit is 164.
The latest evolution of the outburst can be followed at the BeXRB monitor page: http://integral.esac.esa.int/bexrbmonitor/ which combines data from MAXI/GSC, Swift/BAT and Fermi/GBM. This news has been posted to ATEL #9263.