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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
Password Certification: Takanori Sakamoto (Taka.Sakamoto@nasa.gov)

Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients

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/


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R. E. Rutledge , Editor
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