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تسجيل مستخدم جديدLarge optical modulations during 2018 outburst of MAXI J1820+070 reveal evolution of warped accretion disc through X-ray state change
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The black-hole X-ray transient MAXI J1820+070 (=ASSASN-18ey) discovered in March 2018 was one of the optically brightest ever seen, which has resulted in very detailed optical outburst light-curves being obtained. We combine them here with X-ray and radio light-curves to show the major geometric changes the source undergoes. We present a detailed temporal analysis which reveals the presence of remarkably high amplitude (>0.5 mag) modulations, which evolve from the superhump (16.87 h) period towards the presumed orbital (16.45 h) period. These modulations appear ~87d after the outburst began, and follow the Swift/BAT hard X-ray light-curve, which peaks 4 days before the radio flare and jet ejection, when the source undergoes a rapid hard to soft state transition. The optical modulation then moves closer to the orbital period, with a light curve peak that drifts slowly in orbital phase from ~0.8 to ~0.3 during the soft state. We propose that the unprecedentedly large amplitude modulation requires a warp in the disc in order to provide a large enough radiating area, and for the warp to be irradiation-driven. Its sudden turn-on implies a change in the inner disc geometry which raises the hard X-ray emitting component to a height where it can illuminate the warped outer disc regions.
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The microquasar MAXI J(1820+070) went into outburst from mid-March until mid-July 2018 with several faint rebrightenings afterwards. With a peak flux of approximately 4 Crab in the (20-50) keV, energy range the source was monitored across the electro
magnetic spectrum with detections from radio to hard X-ray frequencies. Using these multi-wavelength observations, we analyzed quasi-simultaneous observations from 12 April, near the peak of the outburst ((sim 23) March). Spectral analysis of the hard X-rays found a (kT_e sim 30 ) keV and ( tau sim 2) with a texttt{CompTT} model, indicative of an accreting black hole binary in the hard state. The flat/inverted radio spectrum and the accretion disk winds seen at optical wavelengths are also consistent with the hard state. Then we constructed a spectral energy distribution spanning (sim 12) orders of magnitude using modelling in texttt{JetSeT}. The model is composed of an irradiated disk with a Compton hump and a leptonic jet with an acceleration region and a synchrotron-dominated cooling region. texttt{JetSeT} finds the spectrum is dominated by jet emission up to approximately (10^{14}) Hz after which disk and coronal emission dominate. The acceleration region has a magnetic field of ( B sim 1.6 times 10^4 ) G, a cross section of (R sim 2.8 times 10^{9} ) cm, and a flat radio spectral shape naturally obtained from the synchroton cooling of the accelerated electrons. The jet luminosity of (> 8 times 10^{37} ) erg/s ((> 0.15L_{Edd})) compared to an accretion luminosity of ( sim 6 times 10^{37}) erg/s, assuming a distance of 3 kpc. Because these two values are comparable, it is possible the jet is powered predominately via accretion with only a small contribution needed from the Blanford-Znajek mechanism from the reportedly slowly spinning black hole.
We report on a multi-epoch campaign of rapid optical/X-ray timing observations of the superbright 2018 outburst of MAXI J1820+070, a black hole low-mass X-ray binary system. The observations spanned 80 days in the initial hard-state, and were taken w
ith NTT/ULTRACAM and GTC/HiPERCAM in the optical (ugriz filters at time resolutions of 8--300 Hz) and with ISS/NICER in X-rays. We find (i) a growing anti-correlation between the optical and X-ray lightcurves, (ii) a steady, positive correlation at an optical lag of 0.2 s (with a longer lag at longer wavelengths) present in all epochs, and (iii) a curious positive correlation at textit{negative} optical lags in the last, X-ray softest epoch, with longer wavelengths showing a greater correlation and a more negative lag. To explain these we postulate the possible existence of two synchrotron-emitting components; a compact jet and a hot flow. In our model, the significance of the jet decreases over the outburst, while the hot flow remains static (thus, relatively, increasing in significance). We also discuss a previously discovered quasi-periodic oscillation and note how it creates coherent optical time lags, stronger at longer wavelengths, during at least two epochs.
We study X-ray spectra from the outburst rise of the accreting black-hole binary MAXI J1820+070. We find that models having the disk inclinations within those of either the binary or the jet imply significant changes of the accretion disk inner radiu
s during the luminous part of the hard spectral state, with that radius changing from $>$100 to $sim$10 gravitational radii. The main trend is a decrease with the decreasing spectral hardness. Our analysis requires the accretion flow to be structured, with at least two components with different spectral slopes. The harder component dominates the bolometric luminosity and produces strong, narrow, X-ray reflection features. The softer component is responsible for the underlying broader reflection features. The data are compatible with the harder component having a large scale height, located downstream the disk truncation radius, and reflecting mostly from remote parts of the disk. The softer component forms a corona above the disk up to some transition radius. Our findings can explain the changes of the characteristic variability time scales, found in other works, as being driven by the changes of the disk characteristic radii.
We describe the first complete polarimetric dataset of the entire outburst of a low-mass black hole X-ray binary system and discuss the constraints for geometry and radiative mechanisms it imposes. During the decaying hard state, when the optical flu
x is dominated by the non-thermal component, the observed polarization is consistent with the interstellar values in all filters. During the soft state, the intrinsic polarization of the source is small, $sim 0.15$ per cent in $B$ and $V$ filters, and is likely produced in the irradiated disc. A much higher polarization, reaching $sim 0.5$ per cent in $V$ and $R$ filters, at position angle of $sim 25^circ$ observed in the rising hard state coincides in time with the detection of winds in the system. This angle coincides with the position angle of the jet. The detected optical polarization is best explained by scattering of the non-thermal (hot flow or jet base) radiation in an equatorial wind.
Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated.
Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, $sim$0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between $sim$0.3% and 0.7%, and the polarisation position angle is between $sim10deg-30deg$. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.
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