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Astron. Astrophys. 362, L53-L56 (2000)

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3. Results

The X-ray flux history of the XTE J2012+381 outburst based on RXTE/PCA and ASM data in 1.3-12 keV energy range is shown in Fig. 1 (presented ASM intensities provided by the RXTE/ASM team, http://xte.mit.edu/ASM_lc.html). The evolution of the source flux in the soft X-ray band (1.3-12 keV as well as in the 3-20 keV energy band) was characterized by the fast initial rise to a level of [FORMULA] mCrab  1 on a time scale of [FORMULA] a week followed by a [FORMULA]3 day long maximum and relatively slow decay, interrupted by secondary maximum [FORMULA]30 days after the beginning of the outburst. The subsequent outburst evolution was unusual because of long and powerful tertiary peak (close to 150 days from the beginning of the outburst).

[FIGURE] Fig. 1. Light curve of the 1998 outburst of the X-ray transient XTE J2012+381. Open circles and filled circles represent the data of All Sky Monitor (ASM/RXTE, 1.3-12.2 keV) and Proportional Counter Array (PCA/RXTE, 3-20 keV) respectively.

The energy spectra of the X-ray transient were quite typical - a dominant soft thermal component with a rather weak power law tail.

Three PCA spectra (3-20 keV) are shown in Fig. 2. These spectra demonstrate once again that the soft spectral component in the spectra of X-ray Novae rise not instantly, but slightly after the power law component. Similar behavior was observed in Nova Muscae 1991 (Ebisawa et al. 1994), KS 1730-312 (Trudolyubov et al. 1996), GRS 1739-278 (Borozdin et al. 1998) and XTE J1748-288 (Revnivtsev, Trudolyubov & Borozdin 2000). The sequence of typical spectra is presented in Fig. 2. In Fig. 2, one can see that during the first observation the soft component is not very strong (May 27, 1998), the second spectrum already has much more dominant soft component (June 1, 1998) and the third spectrum shows the weakening of the power law component (July 7, 1998).

[FIGURE] Fig. 2. Typical energy spectra of XTE J2012+381 during 3 consequent phases. One can see that the strength of the soft component rises while the strength of the power law component decreases.

A broad band spectrum from both the PCA and HEXTE, averaged over observations #2-#17 is presented in Fig. 3. Low fluxes in the HEXTE limited our analysis to energies [FORMULA] 80 keV.

[FIGURE] Fig. 3. Averaged broad band energy spectrum of XTE J2012+381 during observations #2-#17 according to the data of PCA (3-20 keV) and HEXTE (20-80 keV) instruments.

As it is clearly seen from Fig. 4, the average power density spectrum of the source is dominated by Very Low Frequency noise (VLFN) component, reasonably approximated by a simple power law model ([FORMULA]) with a slope [FORMULA] (0.001-0.5 Hz frequency range). We have not detected any statistically significant source flux variability at the frequencies higher than [FORMULA] 1 Hz. The 2 [FORMULA] upper limit on the possible Lorentzian component at [FORMULA]500-1000 Hz is approximately 2% for [FORMULA] and 1% for [FORMULA]. In Fig. 5 the fractional rms amplitude of variability is shown as a function of the photon energy. In spite of a poor statistics (very low amplitude of X-ray variability), one can see that there is an indication on the rise of the rms with energy. Such dependence is quite typical for the emission of black hole candidates in the Very High State (VHS)(see discussion of such behavior in Churazov, Gilfanov & Revnivtsev 2000).

[FIGURE] Fig. 4. Power spectrum of XTE J2012+381 averaged over observations [FORMULA] - [FORMULA] (PCA data, 3-20 keV energy range).

[FIGURE] Fig. 5. The dependence of fractional rms variability of X-ray flux (0.001- 1 Hz) of XTE J2012+381 on the photon energy.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000