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Swift-XRT spectrum of the first detected double-peaked outburst of GRO J1008-57

ATel #6656; M. Kuehnel, R. Ballhausen, F. Krauss, T. Beuchert (Remeis-Observatory & ECAP/Univ. Erlangen, Germany), E. Laplace (IAA Tuebingen, Germany), P. Kretschmar (ESA/ESAC, Spain), L. Townsend (UCT), V. McBride (UCT/SAAO), K. Pottschmidt (CRESST/UMBC/NASA-GSFC, USA), P. Blay (IAC-Nordic Optical Telescope, Spain), F. Fuerst (Caltech-SRL, USA), M. Nakajima (Nihon U.), T. Mihara (RIKEN), I. Kreykenbohm, and J. Wilms (Remeis-Observatory & ECAP/Univ. Erlangen, Germany)
on 2 Nov 2014; 07:00 UT
Credential Certification: Matthias Kuehnel (matthias.kuehnel@sternwarte.uni-erlangen.de)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 6664, 6819, 6823, 6917, 8547, 10529

The neutron star of the Be X-ray binary (BeXRB) GRO J1008-57 is currently active in X-rays as reported by Nakajima et al. (ATel #6630). The type I outburst observed a month ago (ATel #6465) happened close to periastron. This is expected every ~249.5 days, which is the orbital period of the binary (Kuehnel et al., 2013). But instead of fading into quiescence the flux as observed by MAXI started to increase a second time on MJD 56946 (ATel #6630).

The current orbital phase of the binary is 0.17 (MJD 56962) and it's X-ray flux of ~210 mCrab in Swift-BAT is slightly higher than the peak of the preceding type I outburst. For comparison, the orbital phase of the onset of the type II "giant" outburst in 2012 November was around 0.28 (ATel #4564, #4561). Its flux increased by ~55 mCrab/day, which is about a factor of 4 larger than for the current outburst. Additionally, the time span from the onset of the current outburst until now is comparable to the rising part of the type II outburst in 2012. Thus, we don't expect the source to undergo another "giant" outburst. This is supported by the current flattening of the lightcurve as observed by MAXI and Swift-BAT. Thus, it seems that GRO J1008-57 shows its first detected double-peaked outburst since its discovery in 1993. Such outbursts are known from other BeXRBs, like A 0535+26 (Caballero et al., 2013) or GX 304-1 (Postnov et al., 2014).

We have triggered a Swift ToO observation, which was performed on October 25 (MJD 56955). At this time the observed pulse period of the neutron star was 93.528(20) s. Because of calibration issues below 1 keV we have analyzed the grade 0 XRT-spectrum between 1 and 10 keV. To check for spectral differences of the current outburst to the "normal" type I outbursts we have used the model by Kuehnel et al. (2013 and 2014). It consists of a cutoff power-law with an additional black-body and the parameters are either constant for the source or determined by the 15-50 keV flux. Additionally, the absorption column density, NH, is fitted as well. The fit results in a very good description of the data (red. chi sqr. = 1.11 with 479 dof). We had to fit the spectra of the two good time intervals (GTI) separately, but with a combined NH, because of spectral differences between them, which are explained by a flux change of about -26%. The resulting fit parameters are:
flux (GTI 1) [15-50 keV] = 1.65(6) x 10^-9 erg/s/cm^2
flux (GTI 2) [15-50 keV] = 1.22(4) x 10^-9 erg/s/cm^2
NH = 2.55(8) x 10^22 cm^-2
Fitting an absorbed powerlaw to the data as a consistency check results in the same values within the uncertainties.

The measured NH is consistent with previous findings for the source. The upper limit of the iron K alpha emission line's equivalent width of 93 eV does not show a strong enhancement in reflecting material, such as the Be-disk, compared to other observations of the source. From these facts we conclude that there is no strong increase of material in the system. Since the X-ray continuum can be fitted by the 15-50 keV flux only shows that the accretion process did not change either.

In conclusion, no changes in X-rays are seen in GRO J1008-57, although the current activity is very unusual. The reason might be an enhanced activity of the donor star or a larger than normal Be-disk, which was deformed by the tidal interaction with the neutron star and would lead to a different material distribution compared to other outbursts. Thus, we encourage optical follow-up of the system to study the Be-star and its disk, which would shed light on the mystery of the double-peaked outbursts.

We would like to thank the Swift Team for making these observations possible.

The further outburst evolution can be followed at the BeXRB monitor web page (http://integral.esac.esa.int/bexrbmonitor), which combines data from MAXI, Swift-BAT, and Fermi-GBM. We thank these teams for the continued rapid availability of their data.