Initial RXTE Observations of the 2003 Outburst of XTE J1550-564

ATel #135; Jon M. Miller (CfA) and Jeroen Homan (Oss. di Brera, Merate), on behalf of a larger collaboration
on 1 Apr 2003; 17:06 UT
Credential Certification: Jon M. Miller (jmm@space.mit.edu)

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At present, we have obtained four observations of the current outburst of XTE J1550-564 with RXTE. The observations occurred on 27 March (2.2 ksec, 3.5 ksec), 29 March (2.9 ksec), and 30 March (1.9 ksec). Strong flaring, on several time scales, was observed in the light curves of all four observations. In the combined power spectrum of the four observations this shows up as three broad components; around 0.02 Hz (fractional rms amplitude of 28%), 0.5 Hz (25% rms), and 2.5 Hz (25% rms). These components are present in each of the four observations. The integrated rms variability between 0.001 and 100 Hz is 45 %, which is typical for the low/hard state. The lowest frequency component in our power spectrum could be related to the optical 1-2 min variability detected by Woudt et al. (IAUC. 8102) on 27 March. The short optical flare they reported may in turn be related to one of the short (<16 s) flares, which we observed during our 27 March RXTE observations. We examined background-subtracted spectra from the PCU-2 and HEXTE-A detectors (in the 3-25 keV and 20-250 keV bands, respectively; we also assumed N_H = 9.0 E+21 cm^-2). The time-averaged spectra are extremely hard, also consistent with the low/hard state. The source is easily detected out to 250 keV even in these short exposures. We find no evidence for an accretion disk component; this may only indicate that the disk is relatively cool in this stage of the outburst. Simple power-law and cut-off power-law models do not give acceptable fits. A model consisting of an Fe K line (the "Laor" model; E = 6.4 keV, EW = 200 eV) and neutral reflection ("pexrav"; Gamma = 1.4, E_fold = 400 keV, Omega/2*pi = 0.3) gives acceptable fits. The unabsorbed 3-250 keV flux in the most recent observation is 7.1 E-9 erg/cm2/s.