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Detection of a cyclotron line with RXTE in the current outburst of EXO2030+375

ATEL # 877; Colleen A. Wilson, Mark H. Finger (NSSTC)
on 20 Aug 2006; 22:33 UT
Password Certification: Colleen A. Wilson (Colleen.Wilson@nasa.gov)

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

We have regularly monitored the transient Be/X-ray pulsar EXO 2030+375 with pointed RXTE observations since the beginning of its current giant (type II) outburst. As reported by Corbet & Levine (Atel#843) this outburst began in mid June. The flux rose (see Atel#861 and #868) until the end of July to a count rate of 1600 counts/s per PCU (2-60 keV) and during August it varied substantially around this constant level.

Since the beginning of the outburst, the spin frequency has changed by 6.6(3)x10E-5 Hz, and is currently changing at the rate of 1.4(2)x10E-11 Hz/s. The spin frequency was 24.084(3) mHz on MJD 53967.5.

RXTE observations on August 15 showed a 2-80 keV flux of 3.3x10E-8 erg/cm^2/s. The corresponding luminosity is 2.0x10E+38 erg/s, using a distance of 7.1 kps (Wilson et al. 2002, ApJ, 570, 287). The 2-20 keV flux (2.2x10E-8 erg/cm^2/s) is about 70% of the peak flux observed in the first EXOSAT observations near the peak of the discovery outburst.

In several recent observations we see evidence for a cyclotron resonance scattering feature near 10 keV. For example, in a joint fit of the PCA and HEXTE pulse height spectra for August 15 the data can be fit with an absorbed power law with a high energy cut-off (highecut), an iron line, and a Gaussian cyclotron line (gabs). The powerlaw has a photon index of 1.53(2) with a cutoff energy at 12.4(4) keV, a folding energy of 27.4(5) keV. The cyclotron line energy is 10.1(2) keV, with a Gaussian width of 3.3(2) keV and peak depth of 1.1(1). This feature is significant at the 7.4 sigma level.


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