Pavel Abolmasov

Supercritically accreting stellar mass black holes as ultraluminous X-ray sources

We derive the luminosity‐temperature relation for the supercritically accreting black holes (BHs) and compare it to the data on ultraluminous X-ray sources (ULXs). At super-Eddington accretion rates, an outflow forms within the spherization radius. We construct the accretion disc model accounting for the advection and the outflow, and compute characteristic disc temperatures. The bolometric luminosity exceeds the Eddington luminosity LEdd by a logarithmic factor 1 + 0. 6l n˙ m (where ˙ m is the accretion rate in Eddington units) and the wind kinetic luminosity is close to LEdd. The apparent luminosity for the face-on observer is 2‐7 times higher because of geometrical beaming. Such an observer has a direct view of the inner hot accretion disc, which has a peak temperature Tmax of a few keV in stellar mass BHs. The emitted spectrum extends as a power law FE ∝ E −1 down to the temperature at the spherization radius Tsp ≈ ˙ m −1/2 keV. We associate T max with a few keV spectral components and Tsp with the soft, 0.1‐0.2 keV components observed in ULXs. An edge-on observer sees only the soft emission from the extended envelope, with the photosphere radius exceeding the spherization radius by orders of magnitude. The dependence of the photosphere temperature on luminosity is consistent with that observed in the super-Eddington accreting BHs SS 433 and V4641 Sgr. Strong outflows combined with the large intrinsic X-ray luminosity of the central BH explain naturally the presence of the photoionized nebulae around ULXs. An excellent agreement between the model and the observational data strongly argues in favour of ULXs being supercritically accreting, stellar mass BHs similar to SS 433, but viewed close to the symmetric axis.

Spectroscopy of optical counterparts of ultraluminous X-ray sources

Here we present the results of panoramic and long-slit observations of eight ULX nebular counterparts performed with the 6m SAO telescope. In two ULX nebulae (ULXNe) we detected for the first time signatures of high excitation ([O III]λ5007 / Hβ > 5). Two of the ULXs were identified with young (T ∼ 5–10 Myr) massive star clusters. Four of the eight ULXNe show bright high-excitation lines. This requires existence of luminous (∼ 1038 ÷ 1040 erg s−1) UV/EUV sources coinciding with the X-ray sources. The other 4 ULXNe require shock excitation of the gas with shock velocities of 20–100 km s−1. However, all the studied ULXNe spectra show signatures of shock excitation, but even those ULXNe where the shocks are prevailing show presence of a hard ionizing source with a luminosity of at least ∼ 1038 erg s−1. Most likely shock waves, X-ray and EUV ionization act simultaneously in all the ULXNe, but they may be roughly separated in two groups: shock-dominated and photoionization-dominated ULXNe. The ULXs have to produce strong winds and/or jets (∼ 1039 erg s−1) for powering their nebulae. Both the wind/jet activity and the existence of a bright UV source are consistent with the suggestion that ULXs are high-mass X-ray binaries with supercritical accretion disks of the SS433 type.

Optical observations of type-IIP supernova 2004dj: Evidence for asymmetry of the 56Ni ejecta

Photometric and spectroscopic observations of the nearby type-IIP supernova 2004dj are presented. The 56Ni mass in the envelope of SN 2004dj was estimated from the light curve to be ≈0.02M⊙. This estimate is confirmed by modeling the Hα luminosity. The Hα emission line exhibits a strong asymmetry characterized by the presence of a blue component in the line with a shift of −1600 km s−1 at the early nebular phase. A similar asymmetry was found in the Hβ, [O I], and [Ca II] lines. The line asymmetry is interpreted as being the result of asymmetric 56Ni ejecta. The Hα profile and its evolution are reproduced in the model of an asymmetric bipolar 56Ni structure for a spherical hydrogen distribution. The mass of the front 56Ni jet is comparable to that of the central component and twice that of the rear 56Ni jet. We point out that the asymmetric bipolar structure of 56Ni ejecta is also present in SN 1999em, a normal type-IIP supernova.

Modelling the optical spectrum of Romano’s star★

We consider the luminous blue variable (LBV) star V532 in M33, also known as Romano’s star, in two different spectral states: in the optical minimum of 2007/2008 and during a local brightening in 2005. Optical spectra of low and moderate resolution are modelled using the non-local thermodynamic equilibrium model atmosphere code CMFGEN. All the observed properties of the object in the minimum are well described by a late WN (nitrogen-sequence Wolf-Rayet) star model with a relatively high hydrogen abundance (H/He = 1.9), while the spectrum during the outburst corresponds to the spectral class WN11 and is similar to the spectrum of P Cyg. The atmosphere is enriched in nitrogen by about a factor of 6 in both states. Most of the heavy-element abundances are consistent with the chemical composition of M33. Bolometric luminosity is shown to vary between the two states by a factor of ∼1.5. This makes V532 another example of an LBV that shows variations in its bolometric luminosity during an outburst.

Resolving the inner structure of QSO discs through fold-caustic-crossing events

Though the bulk of the observed optical flux from the discs of intermediate-redshift lensed quasars is formed well outside the region of strong relativistic boosting and light-bending, relativistic effects have important influence on microlensing curves. The reason is in the divergent nature of amplification factors near fold caustics increasingly sensitive to small spatial size details. Higher-order disc images produced by strong light bending around the black hole may affect the amplification curves, making a contribution of up to several percent near maximum amplification. In accordance with theoretical predictions, some of the observed highamplification events possess fine structure. Here we consider three putative caustic crossing events, one by SBS J1520+530 and two events for individual images of the Einstein’s cross (QSO J2237+0305). Using relativistic disc models allows to improve the fits, but the required inclinations are high, i & 70 . Such high inclinations apparently contradict the absence of any strong absorption that is likely to arise if a disc is observed edge-on through a dust torus. Still, the high inclinations are required only for the central parts of the disc, that allows the disc itself to be initially tilted by 60::90 with respect to the black hole and aligned toward the black hole equatorial plane near the last stable orbit radius. For SBS J1520+530, an alternative explanation for the observed amplification curve is a superposition of two subsequent fold caustic crossings. While relativistic disc models favour black hole masses 10 10 M (several times higher than the virial estimates) or small Eddington ratios, this model is consistent with the observed distribution of galaxies over peculiar velocities only if the black hole mass is . 3 10 8 M .

Broad band variability of SS433: Accretion disk at work?

We present broad band power spectra of variations of SS433 in radio, optical and X-ray spectral bands. We show that at frequencies lower than 10 -5 Hz the source demonstrates the same variability pattern in all these bands. The broad band power spectrum can be fitted by one power law down to frequencies ∼10 -7 Hz with flattening afterwards. Such a flattening means that on time scales longer than ∼10 7 s the source variability becomes uncorrelated. This naturally leads to the appearance of quasi-poissonian flares in the source light curve, which have been regularly observed in radio and optical spectral bands. The radio flux power spectrum appears to have a second break at Fourier frequencies ∼10 -5 Hz which can be caused by the smearing of the intrinsic radio variability on timescale of the light-crossing time of the radio emitting region. We find a correlation of the radio and optical fluxes of SS433 and the radio flux is delayed by about ∼2 days with respect to the optical one. Power spectra of optical and X-ray variabilities continue with the same power law from 10 -7 Hz up to ∼0.01-0.05 Hz. The broad band power spectrum of SS433 can be interpreted in terms of self-similar accretion rate modulations in the accretion disk proposed by Lyubarskii (1997, MNRAS, 292, 679) and elaborated by Churazov et al. (2001, MNRAS, 321, 759). We discuss a viscous time-scale in the accretion disk of SS433 with reference to the observed broad band power spectrum.We obtained a broadband radio spectrum of the galactic compact object SS433 through the multi-wavelength campaign using the Giant Meter Radio Telescope (GMRT) and the Effelsberg 100-m Radio Telescope. The observations took place from 2005 January 03 to 2005 February 08 using eight different frequencies. We observed a flare which appeared to have started prior to January 18 and lasted till February 08. The light curves show a progressively larger time-delay as the radio frequency goes down. The peak of the 6 cm lightcurve is delayed by ∼ 3 days with respect to the peak of the 9mm lightcurve. There is an indication of the flattening of the spectrum and possibly a turnover at ∼ 1.5GHz. If we interpret this to be due to synchrotron self-absorption, the required magnetic field would become too large, however, free-free absorption by hot thermal surrounding medium formed due to stellar winds is still a possibility. All sky monitor (ASM) aboard Rossi X-ray Timing Experiment (RXTE) showed very high X-ray count on 2005 January 25 when the flare was well underway. This may be due to slamming of the radio ‘bullets’ with previously ejected, relatively slowly moving material.

Microlensing evidence for super-Eddington disc accretion in quasars

Microlensing by the stellar population of lensing galaxies provides an important opportunity to spatially resolve the accretion disc structure in strongly lensed quasars. Disc sizes estimated this way are on average larger than the predictions of the standard Shakura-Sunyaev accretion disk model. Analysing the observational data on microlensing variability allows to suggest that some fraction of lensed quasars (primarily, smaller-mass objects) are accreting in super-Eddington regime. Super-Eddington accretion leads to formation of an optically-thick envelope scattering the radiation formed in the disc. This makes the apparent disc size larger and practically independent of wavelength. In the framework of our model, it is possible to make self-consistent estimates of mass accretion rates and black hole masses for the cases when both amplification-corrected fluxes and radii are available.

Optical Spectroscopy of the Environment of a ULX in NGC 7331

Optical photometric and spectroscopic data are presented that show an association of an ultraluminous X-ray source in NGC 7331 with a young star cluster of mass M = (1.1 ± 0.2) × 105 M☉ and age tc = 4.25 ± 0.25 Myr. If the ULX is part of the bright stellar cluster, then the mass of the progenitor of the compact accretor must have been 40-50 M☉ in order to already have evolved through the supernova stage into a compact object. The companion star is also probably an evolved massive star. The emission-line spectrum of the nebula surrounding the cluster can be interpreted as being a result of photoionization by the cluster OB stars with an additional source of shock excitation producing strong [S II], [O I], and N II lines. This additional source appears to be as much as 5 times more powerful than the supernovae and stellar winds in the cluster can provide for. Additional mechanical energy input associated with the ULX itself can help explain the residual shock-excited line luminosities of the emission region.

Super-Eddington accretion on to a magnetized neutron star

Most of ultraluminous X-ray sources are thought to be objects accreting above their Eddington limits. In the recently identified class of ultraluminous X-ray pulsars, accretor is a neutron star and ...

The Eastern filament of W50

We present new spectral (FPI and long-slit) data on the Eastern optical filament of the well known radionebula W50 associated with SS433. We find that on sub-parsec scales different emission lines are emitted by different regions with evidently different physical conditions. Kinematical properties of the ionized gas show evidence for moderately high (V ∼ 100 km s–1) supersonic motions. [O III]λ 5007 emission is found to be multi-component and differs from lowerexcitation [S II]λ 6717 line both in spatial and kinematical properties. Indirect evidence for very low characteristic densities of the gas (n ∼ 0.1 cm–3) is found. We propose radiative (possibly incomplete) shock waves in low-density, moderately high metallicity gas as the most probable candidate for the power source of the optical filament. Apparent nitrogen overabundance is better understood if the location of W50 in the Galaxy is taken into account (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)