90 research outputs found
Understanding ULX Nebulae in the Framework of Supercritical Accretion
For a long time, the well-known supercritically accreting binary SS433 is
being proposed as a prototype for a class of hypothetical bright X-ray sources
that may be identified with the so-called Ultraluminous X-ray sources (ULXs) in
nearby galaxies or at least with part of them. Like SS433, these objects should
be associated with optical nebulae, powered by both radiation of the central
source and its wind or jet activity. Indeed, around many ULXs, bright optical
nebulae (ULX Nebulae, ULXNe) are found. Here, we use SS433 as a prototype for
the power source creating the nebulae around ULXs. Though many factors are
important such as the structure of the host star-forming region and the
possible supernova remnant formed together with the accreting compact object,
we show that most of the properties of ULXNe may be explained by an SS433-like
system evolving for up to about one million years in a constant density
environment. The basic stages of evolution of a ULX Nebula include a
non-spherical HII-region with a central cavity created by non-radiative shock
waves, an elongated or bipolar shock-powered nebula created by jet activity and
a large-scale quasi-spherical bubble.Comment: 23 pages, 7 figures; accepted for publication by New Astronom
Optically Thick Outflows of Supercritical Accretion Discs: Radiative Diffusion Approach
Highly supercritical accretion discs are probable sources of dense optically
thick axisymmetric winds. We introduce a new approach based on diffusion
approximation radiative transfer in a funnel geometry and obtain an analytical
solution for the energy density distribution inside the wind assuming that all
the mass, momentum and energy are injected well inside the spherization radius.
This allows to derive the spectrum of emergent emission for various inclination
angles. We show that self-irradiation effects play an important role altering
the temperature of the outcoming radiation by about 20% and the apparent X-ray
luminosity by a factor of 2-3. The model has been successfully applied to two
ULXs. The basic properties of the high ionization HII-regions found around some
ULXs are also easily reproduced in our assumptions.Comment: 32 pages, 9 figures, 1 table; accepted to PAS
Stable accretion from a cold disc in highly magnetized neutron stars
The aim of this paper is to investigate the transition of a strongly
magnetized neutron star into the accretion regime with very low accretion rate.
For this purpose we monitored the Be-transient X-ray pulsar GRO J1008-57
throughout a full orbital cycle. The current observational campaign was
performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV)
between two subsequent Type I outbursts in January and September 2016. The
expected transition to the propeller regime was not observed. However, the
transitions between different regimes of accretion were detected. In
particular, after an outburst the source entered a stable accretion state
characterised by the accretion rate of ~10^14-10^15 g/s. We associate this
state with accretion from a cold (low-ionised) disc of temperature below ~6500
K. We argue that a transition to such accretion regime should be observed in
all X-ray pulsars with certain combination of the rotation frequency and
magnetic field strength. The proposed model of accretion from a cold disc is
able to explain several puzzling observational properties of X-ray pulsars.Comment: 8 pages, 3 figures, 1 table, accepted by A&
Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
We derive the luminosity-temperature relation for the super-critically
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 L_Edd by a logarithmic
factor 1+0.6 ln mdot (where mdot is the accretion rate in Eddington units) and
the wind kinetic luminosity is close to L_Edd. 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 T_max of a few keV in stellar-mass BHs. The emitted spectrum
extends as a power-law F_E ~ E**{-1} down to the temperature at the
spherization radius T_sp ~ mdot**(-1/2) keV. We associate T_max with a few keV
spectral components and T_sp 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 super-critically
accreting, stellar-mass BHs similar to SS 433, but viewed close to the symmetry
axis.Comment: 8 pages, 5 figures; heavily revised version; accepted to MNRA
Spitzer Observations of MF 16 Nebula and the associated Ultraluminous X-ray Source
We present Spitzer Infrared Spectrograph (IRS) observations of the
ultra-luminous X-ray source (ULX) NGC 6946 X-1 and its associated nebula MF 16.
This ULX has very similar properties to the famous Holmberg II ULX, the first
ULX to show a prominent infrared [O IV] emission line comparable to those found
in AGN. This paper attempts to constrain the ULX Spectral Energy Distribution
(SED) given the optical/UV photometric fluxes and high-resolution X-ray
observations. Specifically, Chandra X-ray data and published Hubble optical/UV
data are extrapolated to produce a model for the full optical to X-ray SED. The
photoionization modeling of the IR lines and ratios is then used to test
different accretion spectral models. While either an irradiated disk model or
an O-supergiant plus accretion disk model fit the data very well, we prefer the
latter because it fits the nebular parameters slightly better. In this second
case the accretion disk alone dominates the extreme-UV and X-ray emission,
while an O-supergiant is responsible for most of the far-UV emission.Comment: 26 pages, 7 figures, accepted by Ap
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