MAXI/GSC detection of enhanced X-ray activity from GRO J1008-57 after its type I outburst

ATel #10529; E. Laplace (University of Tuebingen), M. Nakajima (Nihon University), T. Mihara (RIKEN), Y. Moritani (Kavli IPMU), A. Santangelo (University of Tuebingen)
on 24 Jun 2017; 16:02 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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The Be/X-ray binary system GRO J1008-57 has recently displayed a regular type I outburst (ATel#10418). We report that an enhanced X-ray activity is detected with MAXI/GSC after the end of its type I outburst MJD 57922, with an average flux of 20 mCrab.

This enhanced activity is analogous to the peculiar X-ray activity observed before the last two type II (or giant) outbursts (October 2012 and December 2014). In 2012, strong flaring behavior was observed after the type I outburst (ATel #4355), followed by a type II outburst (ATel #4561, #4573). Similarly, in 2014/2015, a type I outburst, and then another, brighter outburst were observed (ATel #6630, #6656), followed by an enhanced X-ray flux phase (ATel#6819, #6823) and later by a type II outburst (ATel #6917). In addition, we noticed that the pre-outburst flares were observed from both the present and the previous type I outbursts preceding type II outbursts. The peak position of these pre-outburst flares are constant at the orbital phase of 0.94 (P_orb=249.48 d, T₀=54424.71(MJD) ; Kühnel et al. 2013). The figure which compare the present outburst profile and the previous ones are displayed in the following address, http://a8095.mascat.nihon-u.ac.jp/nakaji/misc/lc_groj1008.gif.

The time-scale between the current observations and the previous peculiar X-ray activity is t ~2.5 yr. We note that this is close to the time-scale between the flares in 2012 and 2014, which is of t ~ 2.3 yr. This indicates the possibility that a long-term recurrence exists in the source, such as the one reported recently for EXO 2030+375 (Laplace et al. 2017).

Due to these phenomenological similarities, if the same physical mechanisms are at work, we expect to observe a type II outburst during the current orbit (in the next ~230 d, orbital ephemeris of Kühnel et al. 2013). As, the quasi-cyclic recurrence of giant outbursts is possibly related to the Be disc structural change (e.g. Laplace et al. 2017), optical and infrared observations is highly encouraged to know Be disc status.

The evolution of the X-ray light-curve can be followed at the dedicated BeXRB monitor page: http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_GROJ1008-57.html combining data from MAXI (http://maxi.riken.jp/top/index.php?cid=1&jname=J1009-582), Fermi/GBM (http://gammaray.msfc.nasa.gov/gbm/science/pulsars/lightcurves/groj1008.html), and Swift/BAT (http://swift.gsfc.nasa.gov/results/transients/GROJ1008-57/).