Confirmation of the cyclotron line at 50 keV from GX 304-1 by Swift/BAT
ATel #2815; T. Sakamoto (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
on 24 Aug 2010; 18:14 UT
Credential Certification: Takanori Sakamoto (Taka.Sakamoto@nasa.gov)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
We report the confirmation of the cyclotron absorption feature from
GX 304-1 using Swift/BAT data. We collected the BAT survey data
(Detector Plane Histograms) from August 12 to August 17 when GX 304-1
was in the BAT field of view. We selected the data when GX 304-1 is
the dominant source in the BAT image, and used the mask weighted technique
to construct a spectrum.
Since we constructed the spectrum using the mask-weighting technique,
we only selected the data when GX 304-1 is the dominant source in
the BAT image. The total exposure of the summed spectrum is 43.4 ksec.
The spectral fitting energy range is from 14 keV to 90 keV.
The BAT spectrum is well fit by NPEX (Negative and Positive power-law
with EXponential) and the cyclotron absorption model. The best fit
parameters of the cyclotron feature are E0 = 53 +/- 2 keV,
Width = 13 (-3/+5) keV, and Depth = 1.2 +/- 0.2 (chi2/dof = 11.96/28).
These values are consistent with the discovery of this feature from
the RXTE data (Atel#2796). A fit to the spectrum using the NPEX model
alone yields to the reduced-chi2 of 3.57 (chi2/dof = 110.73/31).
Swift BAT has been triggered on-board twice by the recent outburst from
GX 304-1 (Atel #2779, Atel #2796). The initial trigger was at 00:34:58 UT
on August 12, 2010 (trigger=431454). The energy flux during this episode
based on the spectrum created by the 1043 sec event data in the 14-90 keV
band using the NPEX model is 1.0 (-0.3/+0.5) e-8 erg/cm2/s. The 2nd trigger
happened at 09:03:14 UT on August 15, 2010 (trigger=431711). The flux during
this 2nd trigger is 2.2 (-0.6/+1.8) e-8 erg/cm2/s in the 14-90 keV band based
on the spectrum created from the 578 sec event data. The quoted errors are at
the 90% confidence level.
GX 304-1 is currently in the decay phase of the outburst in the hard X-rays
according to the BAT Hard X-ray Transient Monitor. The peak daily averaged
count rate was 0.307 +/- 0.011 ct/s/cm2 (15-50 keV) or about 1400 mCrab on
MJD 55424 (August 16, 2010). The light curve of GX 304-1 based on the BAT Hard
X-ray Transient Monitor is available at:
http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/GX304-1/
