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Discovery of the cyclotron line at 51 keV from GX 304-1

ATEL # 2796; T.Mihara, T.Yamamoto, M.Sugizaki (RIKEN), K.Yamaoka (AGU) and the MAXI team
on 17 Aug 2010; 13:50 UT
Password Certification: Tatehiro Mihara (mihara@crab.riken.jp)

Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients
Referred to by ATEL #: 2815

We report the discovery of a cyclotron line from the binary X-ray pulsar GX 304-1. After the report of the outburst onset (ATel#2779), the source flux has reached ~700 mCrab in 4-10 keV on August 16, 2010. The outburst is still continuing. According to the MAXI and Swift/BAT light curves we requested RXTE and Suzaku TOO observations. Suzaku observation was done in Aug.13 16:10 - 23:00 UT for 20ks. The RXTE observations were performed for four times. The 2-10 keV flux (ergs/cm^2/s) with dates (UT) are
8.8e-9 in Aug.13 03:31-04:18,
8.9e-9 in Aug.13 04:43-06:35,
9.5e-9 in Aug.14 01:36-04:33, and
10.3e-9 in Aug.15 01:58-04:43.
The RXTE/HEXTE spectra were fitted with NPEX and cycloabs model with reduced chi2 = 0.85. The best-fit parameters with 90% errors are Ea = 50.8 +- 0.5 keV, W = 8.2 +- 1.4 keV, and tau = 0.76 +- 0.05 for the Aug.15 data. It is improved from the NPEX model without cycloabs (reduced chi2 = 11.8).


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