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تسجيل مستخدم جديدLow/Hard State Spectra of GRO J1655-40 Observed with Suzaku
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The Galactic black-hole binary GRO J1655$-$40 was observed with Suzaku on 2005 September 22--23, for a net exposure of 35 ks with the X-ray Imaging Spectrometer (XIS) and 20 ks with the Hard X-ray Detector (HXD). The source was detected over a broad and continuous energy range of 0.7--300 keV, with an intensity of $sim$50 mCrab at 20 keV. At a distance of 3.2 kpc, the 0.7--300 keV luminosity is $ sim 5.1 times 10^{36}$ erg s$^{-1}$ ($sim 0.7$ % of the Eddington luminosity for a 6 $M_{odot}$ black hole). The source was in a typical low/hard state, exhibiting a power-law shaped continuum with a photon index of $sim 1.6$. During the observation, the source intensity gradually decreased by 25% at energies above $sim 3$ keV, and by 35% below 2 keV. This, together with the soft X-ray spectra taken with the XIS, suggests the presence of an independent soft component that can be represented by emission from a cool ($sim 0.2$ keV) disk. The hard X-ray spectra obtained with the HXD reveal a high-energy spectral cutoff, with an e-folding energy of $sim 200$ keV. Since the spectral photon index above 10 keV is harder by $sim 0.4$ than that observed in the softer energy band, and the e-folding energy is higher than those of typical reflection humps, the entire 0.7--300 keV spectrum cannot be reproduced by a single thermal Comptonization model, even considering reflection effects. Instead, the spectrum (except the soft excess) can be successfully explained by invoking two thermal-Comptonization components with different $y$-parameters. In contrast to the high/soft state spectra of this object in which narrow iron absorption lines are detected with equivalent widths of 60--100 eV, the present XIS spectra bear no such features beyond an upper-limit equivalent width of 25 eV.
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zsprung gap. Usually in such systems the donors radius expansion drives a near-Eddington or super-Eddington mass transfer rate which would sustain a persistently bright accretion disk. We show that GRO J1655-40 is close to a narrow parameter range where disk instabilities can occur. This range corresponds to a short-lived evolutionary stage where the secondarys radius expansion stalls (or reverses), with a correspondingly lower mass transfer rate. If GRO J1655-40 belongs to this class of transients the predicted accretion rates imply large populations of luminous persistent and transient sources, which are not seen in X-rays. The transient nature of the related system GRS 1915+105 may reflect spectral variations in a bolometrically persistent source rather than a genuine luminosity increase.
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