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
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
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).
