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Discovery of cyclotron-line feature at 76 keV from Be/X-ray binary pulsar, GRO J1008-57

ATel #4759; T. Yamamoto, T. Mihara, M. Sugizaki (RIKEN), M. Sasano, K. Makishima (Univ. of Tokyo), M. Nakajima (Nihon U.)
on 24 Jan 2013; 11:30 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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

We report the discovery of a cyclotron-line feature from the Be/X-ray binary pulsar GRO J1008-57. The source re-entered an active season since 2005 February (Atel#413), and exhibited a series of thirteen normal outbursts (Atel#647, #1298, #1304, #1619, #1995, #2313, #2769, #3254, #3796, #3805, #4319) in the orbital period of 249.46 d (Atel#4564). On 2012-11-05 (MJD 56236), the MAXI/GSC detected an onset of a giant outburst (Atel#4561) at the orbital phase of ~0.3 from the periastron passage. According to the MAXI/GSC and the Swift/BAT light curves, we requested Suzaku ToO observation. The observation was performed at the outburst peak from 2012-11-20 (MJD 56251) 14:44 to 2012-11-22 5:21 with an exposure of 50 ks. The MAXI/GSC and Swift/BAT fluxes were 400 +- 11 mCrab in 4-10 keV and 900 +- 30 mCrab in 15-50 keV, respectively. The flux with Suzaku was 6.8 x 10-9 erg cm-2 s-1 in 2-10 keV, and 2.65 x 10-8 erg cm-2 s-1 in 1-100 keV. The luminosity was 8.0 x 1037 erg s-1 in 1-100 keV at the 5 kpc distance (Coe et al. 1994).

Our preliminary analysis of the Suzaku HXD/PIN and GSO data revealed that the spectrum can be reproduced by an NPEX with a cyclabs model with a reduced chi squared (dof) = 0.94 (127). The derived parameters of the cyclotron-line feature are Ecyc = 75.5 (+2.5, -1.5) keV, W = 4.0 (+5.4, -4.0) keV, and D = 1.6 (+9.4, -0.6) with errors of 90% confidence limits. The feature existed throughout the entire phase of the apparent 93.6257 s spin period. The Ecyc were in 74-78 keV among the devided spin phases of rise, peak, fall, and valley of the pulse shape.

In the previous observation of a giant outburst emerged in 1993 July, the marginal detection of the cyclotron-line feature at about 88 keV was reported with CGRO/OSSE and BATSE (Grove et al. 1995, Shrader et al. 1999). There was no feature in 40-50 keV range in the spectra obtained with the RXTE/HEXTE in 2005 and the INTEGRAL/IBIS and JEM-X in 2004-2005 (Coe et al. 2007). Suzaku spectrum also shows no signature of cyclotron-line feature in 40-50 keV range. Thus, the feature at 76 keV is considered as the fundamental cyclotron line. It corresponds to a magnetic field strength of 6.6 x 1012 G, which is the highest among binary X-ray pulsars ever observed.

Figures of the spectrum and pulse profile are available at http://maxi.riken.jp/news/en/