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MAXI/GSC detection of a brightening of GRO J1008-57

ATel #10617; E. Laplace (University of Tuebingen), M. Nakajima (Nihon University), T. Mihara (RIKEN), Y. Moritani (Kavli IPMU), A. Santangelo (University of Tuebingen)
on 3 Aug 2017; 10:21 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

Subjects: Far-Infra-Red, Optical, X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 10626, 10902

The MAXI/GSC nova-alert system (Negoro et al. 2016) has detected a brightening of the Be/X-ray binary system GRO J1008-57. On August 3rd , 2017 (MJD 57968), the MAXI/GSC flux increased to a value of 81 +- 14 mCrab in the 4-10 keV energy band. After its type I outburst (ATel #10418, #10466), a peculiar enhanced X-ray activity was observed at an approximately constant value of 20 mCrab in the 2 - 20 keV band (ATel #10529, #10533). Optical observations revealed an unprecedentedly high value of the equivalent width of the H alpha line, indicating an exceptionally large circumstellar disk around the Be star (ATel #10561).

The 4-10 keV flux increase rate of the current steep brightening (~ 0.020 photons/cm^2/sec/day) agrees with that of the previous giant outbursts (0.025 photons/cm^2/sec/day). In addition, the morphological similarities to the events in 2012 and 2014 strongly suggest the formation of the third recorded giant outburst of this source, predicted in ATel #10529. This would also confirm that the source shows the same long-term recurrence as EXO 2030+375.

We strongly encourage multi-wavelength observations to better understand the physical mechanisms at work.

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/).