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تسجيل مستخدم جديدA new X-ray nova MAXI J1910-057 (= Swift J1910.2-0546) and mass-accretion inflow
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We report on a long-term monitoring of a newly discovered X-ray nova, MAXI J1910-057 (= Swift J1910.2-0546), by MAXI and Swift. The new X-ray transient was first detected on 2012 May 31 by MAXI Gas Slit Camera (GSC) and Swift Burst Alert Telescope (BAT) almost simultaneously. We analyzed X-ray and UV data for 270 days since the outburst onset taken by repeated MAXI scans and Swift pointing observations. The obtained X-ray light curve for the inital 90 days is roughly represented by a fast-rise and exponential-decay profile. However, it re-brightened on the ~110 days after the onset and finally went down below both GSC and BAT detec- tion limits on the 240 day. All the X-ray energy spectra are fitted well with a model consisting of a multi-color-disk blackbody and its Comptonized hard tail. During the soft-state periods, the inner-disk radius of the best-fit model were almost constant. If the radius represents the innermost stable circular orbit of a non-spinning black hole and the soft-to-hard transitions occur at 1-4% of the Eddington luminosity, the mass of the compact object is estimated to be > 2.9Mo and the distance to be > 1.70 kpc. The inner-disk radius became larger in the hard / hard-intermediate state. This suggests that the accretion disk would be truncated. We detected an excess of the UV flux over the disk blackbody component extrapolated from the X-ray data, which can be modelled as reprocessed emission irradiated by the inner disk. We also found that the UV light curve mostly traced the X-ray curve, but a short dipping event was observed in both the UV and the X-ray bands with a 3.5-day X-ray time lag. This can be interpreted as the radial inflow of accreting matter from the outer UV region to the inner X-ray region.
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Context. The disc instability model (DIM) successfully explains why many accreting compact binary systems exhibit outbursts, during which their luminosity increases by orders of magnitude. The DIM correctly predicts which systems should be transient
and works regardless of whether the accretor is a black hole, a neutron star or a white dwarf. However, it has been known for some time that the outbursts of X-ray binaries (which contain neutron-star or black-hole accretors) exhibit hysteresis in the X-ray hardness-intensity diagram (HID). More recently, it has been shown that the outbursts of accreting white dwarfs also show hysteresis, but in a diagram combining optical, EUV and X-ray fluxes. Aims. We examine here the nature of the hysteresis observed in cataclysmic variables and low-mass X-ray binaries. Methods. We use the Hameury et al. (1998) code for modelling dwarf nova outbursts, and construct the hardness intensity diagram as predicted by the disc instability model. Results. We show explicitly that the standard DIM - modified only to account for disc truncation - can explain the hysteresis observed in accreting white dwarfs, but cannot explain that observed in X-ray binaries. Conclusions. The spectral evidence for the existence of different accretion regimes / components (disc, corona, jets, etc.) should be based only on wavebands that are specific to the innermost parts of the discs, i.e. EUV and X-rays, which is a difficult task because of interstellar absorption. The existing data, however, indicate that an EUV/X-ray hysteresis is present in SS Cyg.
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